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Fertilization and Placenta

by Michael Rindler, Ph.D.

 

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Fertilization

Objectives:

1.         To understand the basics of gametogenesis in males and females including the role of hormones, the importance of accessory cells, and the differences in the meiotic process in males and females.

2.         To learn what are the major steps in fertilization (fusion of the sperm and oocyte).

3.         To gain familiarity with clinical aspects of infertility and in vitro fertilization.

I.         Gametogenesis

            The maturation of the oocytes (eggs) in the ovaries and spermatocytes (sperm) in the testes, necessarily the first step in the production of a viable embryo, is regulated by hormones. In both males and females, steroid hormones, that is those that are produced from cholesterol, are important effectors of development. Testosterone in the male and estrogen and progesterone in the female are synthesized in the gonads themselves. Their synthesis is in turn regulated by polypeptide hormones produced in the pituitary gland, which is located at the base of the brain.

            The pituitary (a.k.a., adenohypophysis) is a master control gland producing a number of important polypeptide hormones (Fig. 2.13). Of paramount importance as far as gametogenesis is concerned are the gonadotropins -- luteinizing hormone (LH) and follicle stimulating hormone (FSH). The release of the pituitary hormones is controlled by a small protein (GnRH or gonadotropin releasing hormone) secreted a nearby gland known as the hypothalamus.

            In the male, spermatogenesis begins in the seminiferous tubules of the testis as cells called spermatogonia divide and start to differentiate (Figs. 1.21, 1.24, 1.25). They undergo meiosis during this process, which is dependent on LH and FSH. LH acts on cells between the tubules, called interstitial or Leydig cells, and induces these cells to produce testosterone, which acts upon the spermatogonia and the Sertoli cells. FSH influences the Sertoli cells of the seminiferous tubules to provide the right environment to foster spermiogenesis. Testosterone, like other steroid hormones, exerts its effects by binding to a receptor that acts in the cell nucleus and activates certain genes important for the proliferation and differentiation of the sperm progenitors. After a sixty four day (approximately) period of maturation in the seminiferous tubules, the now completed spermatocytes, travel to the epididymus where further maturation occurs.

            LH and FSH are also crucial to oogenesis in the female. Here the situation is more complex (Fig. 1.17). At the beginning of the menstrual cycle a number of undifferentiated primordial follicles (3 to 20) spontaneously start to differentiate to become primary (growing) follicles (Fig. 1.18). This event is not under any known hormonal control. Unlike their counterparts in the male, the oogonia (precursor cells) of the adult ovary do not divide. Oogonia are produced by cell division (mitosis) of primordial germ cells during fetal life. In fact, oogonia already begin the process of meiosis, whereby their genetic material is distributed to the gametes, during fetal development as well. The oogonia form what is known as primary oocytes that have replicated their DNA and have four copies of each chromosome instead of the usual two. However, they are blocked from further maturation by the surrounding follicular (granulosa) cells and remain in meiotic prophase I (Fig. 1.16). At puberty, under the influence of the hormone FSH, the follicular cells cooperate with surrounding connective tissue known as thecal cells to synthesize the hormone estrogen (estradiol). Estradiol promotes the differentiation of the primary follicle into a secondary or antral follicle whose principal histological characteristic is the presence of a fluid-filled cavity in the granulosa cell layers known as an antrum (Fig. 1.19). Ordinarily only a single secondary follicle will continue to mature into the massive preovulatory or Graafian follicle, a process that requires LH; the others will degenerate (Fig. 2.2).

            LH is released in greatly increased amounts by the pituitary in a surge ~48 hours prior to ovulation, causing a number of events to occur, including ovulation itself.  These events include the completion of the first meiotic division and the production of progesterone by the follicular cells. For the follicle to undergo ovulation it must allow the oocyte to penetrate the thick capsule surrounding the ovary known as the tunica albiginea, and under the influence of LH proteolytic enzymes are released from the follicle that catalyze the digestion of the tunica (Fig. 2.2).

II.        Hormonal Control of the Ovulatory Cycle

            The estradiol and progesterone produced by the follicles have important effects on the rest of the female reproductive tract as well. They are directly responsible for the cyclic proliferation and degeneration of the endometrial layer of the uterus (Figs. 2.12, 2.13). In the early stages estradiol induces the proliferation of the tissue, while exposure to progesterone beginning about the time of ovulation induces important changes of the epithelium and glands (secretory phase) in preparation for implantation of the fertilized embryo at 5-7 days post-ovulation (Fig. 2.11). The fall of progesterone accompanying the failure of fertilizaton or implantation initiates a process of ischemia (oxygen and nutrient deprivation) whereby blood vessel contraction in the endometrial layer initiates cell death and degeneration (menstruation) of the endometrial layer fourteen days after ovulation.

            The continued presence of the steroid hormone progesterone is essential for the maintenance of the implanted embryo. During the embryonic period (up to nine weeks) of gestation, progesterone is produced in the ovary by the corpus luteum; later on it will be produced primarily in the placenta by the trophoblast cells. The corpus luteum is formed by the follicular and thecal cells that originally surrounded the Graafian follicle. After ovulation, these cells continue to proliferate and differentiate into a hormone-producing organ under the influence of LH. In fact, the production of progesterone and estrogen is absolutely dependent upon the continued stimulation by LH, but the pituitary ceases to release LH soon after ovulation. As the LH levels decrease, the corpus luteum will degenerate, progesterone levels will fall and menstruation will occur unless another source of hormone can be found. It turns out that the trophoblast cells of the embryo responsible for its implantation and the formation of the placenta synthesize another polypeptide hormone called human chorionic gonadotropin (hCG). hCG is similar to LH in its activity and continues to maintain the corpus luteum. It also is excreted in the mother¹s urine and thus is used as a convenient early test for pregnancy.

III.                     Meiosis (for more complete explanation consult Sadler, p. 3-9)

            As previously mentioned, the meiotic process differs considerably in males and females. In the male, meiotic events begin at puberty, and the first and second meiotic divisions required to produce haploid gametes are part of the ~64 day cycle of sperm maturation (Figs. 1.22, 1.23). In the female, however, proliferation of the germ cell precursors to produce oogonia is completed by the fifth month of prenatal development, and the first stage in maturation, the replication of the DNA and the formation of the paired sets of chromosomes, also begins prenatally. Oocyte development is arrested, however, in the prophase of the first meiotic division. This division will not be completed until just prior to ovulation, an event that might occur as many as fifty years later. This lengthy meiotic process is believed to contribute to the increase in chromosomal nondisjunction (Fig. 1.5). Nondisjunction is the failure of chromosomes to separate during meiotic divisions, leading to duplicated chromosomes in the adult. Nondisjunction is correlated with maternal age and there is a significantly increased incidence of Down¹s syndrome (trisomy 21) offspring born to older mothers (although other factors, such as greater tolerance of the uterine wall are perhaps more important). The second meiotic division is not accomplished until after fertilization. Another difference between gametogenesis in the male and the female is the number of viable haploid gametes produced by the meiotic process. In the male, each original spermatogonium that replicates its DNA can produce four haploid spermatocytes. In the female, each oogonium gives rise to a single oocyte. The remainder of the genetic material is extruded in polar bodies, membrane-bounded spheres that lie adjacent to the oocyte. Two polar bodies are produced, one during each meiotic division. The first polar body appears just prior to ovulation and contains a diploid set of chromosomes. The second polar body forms just after fertilization.

IV.       Transport of Gametes

            After ovulation, the oocyte, along with some surrounding follicular cells known as the corona radiata (known as the cumulus oophorus before ovulation) is extruded into the peritoneal cavity. The oviduct (uterine tube), under the influence of steroid hormones, is prepared for the oocyte¹s journey (Fig. 2.4). It secretes a rich fluid and cells lining the duct produce long extensions of their surface known as cilia. In addition, the oviduct begins to contract quite vigorously, sweeping across the ovary and conducting the oocyte down the tube. The contractions, the beating cilia and fluid movements move the egg down the oviduct and in an hour it has travelled about a quarter of the way down the tube to a region known as the ampulla. At this point its movement is quite slow and it will not leave the ampulla (about 1/3 of the way down) until ~30 hours after ovulation. It is also here, in the ampulla, that fertilization occurs. The oocyte will continue moving slowly down the oviduct to the uterus, reaching there about 4 or 5 days after ovulation. If it is fertilized, the embryo by this stage will be beginning blastulation. The ovum will not implant until ~5-7 days after ovulation.

            For the sperm, the journey is much more treacherous. Deposited at the external ostium of the cervix in a viscous seminal fluid (semen) it must not only traverse the cervix and the uterus but make its way up two-thirds of the oviduct as well. Of more than 200 million present in a normal ejaculate, only an estimated 200-300 manage to make it to the site of fertilizaton. The first barrier to sperm movement is the semen itself. Semen is a secretion of male accessory glands, primarily the seminal vesicles and prostate gland. Coagulation proteins in the semen become crosslinked by seminal enzymes causing the ejaculate to jellify, perhaps to help prevent sperm from leaking away from the site of deposition. Within 30 minutes or so, the ejaculate will reliquify, the result of proteolytic enzymes present in semen which break down the coagulate. Semen is also rich in fructose which serves as the energy source for the sperm, and is highly buffered to neutralize the acidic pH of the female reproductive tract. Sperm are not motile at acidic pH. A major barrier appears to be the cervical fluid. Ordinarily a very viscous material, it changes in viscosity under the influence of progesterone at the time of ovulation and becomes more liquid, allowing easier sperm movement. Despite the emphasis given to motility of sperm, even nonmotile sperm can make it to the fertilization site, underscoring the important influence of fluid movements and muscular contractions of the female reproductive tract in this process. Sperm motility, which is important for fertilization itself (see below), is thought to be stimulated by molecules released from the oocyte that also serve as a chemoattractants.

            Although the sperm can reach the ampulla in as little as 30 minutes after coitus, the actual fertilizing event is believed not to occur for several hours due to a phenomenon known as capacitation. As ejaculated, sperm are not fully competent to fertilize the oocyte. They require a period of 5-7 hours in the female reproductive tract first, where alterations in the proteins and lipids of the plasma membrane occur. These alterations somehow prime the sperm for the subsequent acrosomal reaction. Recent studies suggest that sperm can survive and be fertile for as long as a week in the oviduct.

V.        Fertilization

            After reaching the ampulla the spermatocyte must penetrate three oocyte barriers -- the corona radiata, the zona pellucida, and the plasma membrane (Fig. 2.5). The coronal cells are very loosely attached to the zona and are partially disrupted by the action of the oviduct fluid and enzymes. Thus, they probably do not constitute a real barrier. The zona pellucida is an amorphous mass of proteinaceous material surrounding the oocytes, which in histological preparations appears as a clear zone. It is composed primarily of three glycoproteins consisting of a protein backbone and a large number of oligosaccharide side chains. One of these glycoproteins, ZP3, serves as a receptor for a protein on the plasma membrane of the sperm, and induces the acrosome reaction. In fact, the galactose-rich oligosaccharide side chains of ZP3 are also capable of inducing the acrosomal reaction in vitro. A galactosyl transferase present on the sperm plasma membrane has been implicated as a zona binding protein responsible in part for triggering the fusion of the membrane surrounding the acrosomal organelle with that of the plasma membrane of the sperm itself. Fusion of these membranes leads to release of the contents of the acrosome. An influx of calcium ions into the cytoplasm of the sperm as a result of zona binding is involved in the process. Increased cytoplasmic Ca++ concentration regulates fusion processes in many other types of secretory cells in a similar fashion.

            The penetration of the zona pellucida is accomplished by digestion of the layer in the immediate vicinity of the sperm by the released acrosomal enzymes. The best characterized of these enzymes is acrosin, which resembles pancreatic trypsin. Like trypsin, it is synthesized as a higher molecular weight precursor and stored in a secretory granule (in this case, the acrosome) until release. The acrosome itself is kept at an acidic pH where the enzyme is inactive. Upon release into the neutral pH environment of the oviduct, it is converted to a lower molecular weight active form that is capable of cleaving the zona proteins. Other enzymes aid in this process as well. Whiplike actions of the flagellum at this point are important to help the sperm expeditiously penetrate this small opening.

Upon reaching the membrane of the oocyte the sperm plasma membrane on the lateral portion of the head lines up in a closely apposed position and fusion occurs. A metalloprotease protein (fertilin/disintegrin) in the sperm plasma membrane plays a central role in the binding of sperm to oocytes. Its receptor on the oocyte membrane is a member of the integrin family of cell adhesion molecules (a6ß1). The fusogenic proteins have not yet been conclusively identified. After fusion the entire sperm is engulfed into the cytoplasm of the ovum, and a new program of events is triggered.

            The cortical reaction occurs approximately 30 seconds after fertilization. Cortical granules, membrane-bounded and filled with protein material, are located just underneath the plasma membrane of the oocyte . Upon fertilization, these granules fuse with the plasma membrane. The enzymes that are released from the granules crosslink and alter the proteins in the zona pellucida, effectively preventing any further sperm penetration. This is known as the block to polyspermy. If the zona pellucida is removed, the egg can be fertilized by many sperm.

            Actually, a whole series of post fertilization events occur. The most rapid have to do with a depolarization of the oocyte membrane, an increase in the levels of the second messenger IP3 (inositol triphosphate) and the release of calcium from internal stores to induce the cortical reaction. The ionic changes also activate a series of metabolic processes including the initiation of protein synthesis and ATP production. Protein synthesis is necessary for further development but messenger RNA synthesis is not required up to the two-cell stage because sufficient levels are available from maternal (oocyte) stores.

            The nucleus of the spermatozoan as it enters the oocyte is very compact. Its DNA is packaged tightly because it contains protamines rather than the usual histones bound to the DNA. As it enters the cytoplasm of the egg, however, the nucleus greatly expands and it then known as the male pronucleus. The activity for inducing this change is present in the cytoplasm of the oocyte. While this is occurring, the oocyte completes its second meiotic division and releases the second polar body (Figs. 2.6, 2.7). The remaining haploid nucleus becomes surrounded by a nuclear membrane and is now known as the female pronucleus. By 30 minutes, actin filaments in the cytoplasm bring the two pronuclei together where they remain until just prior to the first mitotic division when their nuclear membranes break down and their chromosomes form the metaphase plate. DNA synthesis, necessary for the division of the ovum, begins about 45 minutes after fertilization, and cell division (cleavage) occurs roughly 23 hours later.

VI.       Gamete Wastage

            We produce vastly more gametes than ever are used to fertilize the egg (gamete wastage). The average male ejaculate contains 200 million sperm. The fetal female ovaries may produce 7 million oocytes or more. Many become atretic (i.e., they degenerate) before birth when perhaps 2 million are left. They continue to degenerate during childhood so that by puberty perhaps 50,000 remain. Despite this, during the course of a woman¹s lifetime it is unlikely that more than 500 will undergo ovulation. While the reason for gamete wastage in mammals is obscure, one can speculate that it contributes to the elimination of improperly formed gametes (see below) and thus a low rate of birth defects in organisms producing relatively few offspring.

VII.     Causes of Infertility

A.        Male-related problems account for roughly half of the diagnosable ones. The problems are fairly predictable based on what is known about fertilization:

1.   Problems with sperm. These include low sperm count. A normal ejaculate contains 3.5 ml. of sperm with 60 million/ml. If less than 20 million/ml or less than 2.0 ml. are produced, this could result in poor fertility. Low sperm motility is another problem. Motile sperm are much more effective at fertilization than nonmotile ones. At least 60% should be visibly motile under a microscope. Finally, the number of abnormal sperm should not exceed 40% of the total. It is important to realize that there are many abnormal sperm in a normal ejaculate (Fig. 1.26). These can be identified visually (small heads, two tails, etc.) but should not be too numerous.

2.   Abnormal semen. Semen should contain the appropriate contributions from the male accessory glands. These include coagulation factors and enzymes from the prostate that reliquify the ejaculate within 30 minutes. The pH should be neutral and contain fructose for energy (seminal vesicles). These parameters can be measured and used as an indication of glandular dysfunction.

3.   Congenital defects. Cryptorchidism (undescended testicles) will be discussed in detail later in both the Gross Anatomy and Embryology courses. The testes originate in the posterior abdominal wall and migrate into the scrotum, something that does not occur properly in cryptorchidism. Varicocele, where the left spermatic vein is enlarged resulting in an elevated testicular temperature, is another common problem. In both cases, elevated temperature in the testis interferes with spermiogenesis.

4.   Tubal (ductus deferens) obstruction - sometimes a result of bacterial infections like tuberculosis and gonorrhea.

            The causes of problems related to the number and normality of sperm and even glandular function are largely unknown, although production of appropriate levels of testosterone is crucial. Recent evidence indicates that genes on the Y chromosome are necessary for spermiogenesis -- deletion of a region of the Y chromosome is associated with male infertility in ~10% of all cases. At the current time, hormone therapy is not a very successful technique. Direct injection of sperm into the oocyte cytoplasm is now in common use as a method to circumvent some male infertility problems (see in vitro fertilization below).

B.        In the female, a myriad of problems lie at the root of infertility.

1.     Failure to ovulate. This can be accompanied by a normal menstrual flow (menorrhea) or be amenorrheic. Usually a hormonal imbalance is the cause and this can be detected by measurement of the levels of the gonadotropins and steroid hormones at various times of the cycle. For example, there may be no LH surge at midcycle. The most common remedy for this type of problem is hormone therapy. One such fertility agent is known a clomiphene. This is an estrogen analog that inhibits a feedback regulatory mechanism. As a result, the pituitary produces more FSH and more primary follicles are induced to form secondary ones. In many cases this treatment is sufficient, but in others the normal midcycle surge of LH is still insufficient. To mimic the surge, chorionic gonadotropin (hCG), an LH-like hormone purified from the urine of pregnant women (see Placenta chapter), is injected after the appropriate interval. For women who do not respond to this treatment, another regime may be attempted involving the injection of human menopausal gonadotropin (hMG), a mixture of LH and FSH produced by menopausal women. [During menopause, women's ovaries fail to respond to pituitary hormones adequately and therefore do not make sufficient steroid hormones. The regulatory system, sensing this, tries to compensate by producing high levels of these pituitary hormones, which eventually are excreted into the urine, from which hMG is prepared.] hMG can be used alone or in combination with a single hCG injection to mimic the LH surge. Unfortunately, with clomiphene and especially with hMG it is very difficult to control the dosage and response to these treatments. They are associated with a greatly increased level of polyovulation and multiple births. The periodic release of a single oocyte in the human female is a very delicately controlled process.

2.     Oviduct problems. These account for about 30% of all diagnosable female infertility problems. The oviduct does not always do its job properly and tubal blockage is not uncommon. The causes range from congenital ones to damage due to infections, such as venereal disease. Antibiotics or surgical resection is successful in some cases, in others in vitro fertilization is the only viable remedy at the current time.

3.     Endometriosis. This common condition is an interesting one because it involves tissue that looks and behaves as if it should be in the uterine wall but instead is found in the peritoneum, on the ovary itself or in the oviduct. This tissue is apparently derived from retrograde menstrual flow and proliferates in response to steroid hormones just at the wrong time, clogging or obstructing the oviduct. Hormone treatment or surgical removal of the blockage is successful in most cases.

4.     Uterine problems. These are generally diagnosed by biopsy of the endometrium during the menstrual cycle. In cases where the endometrium is not properly prepared for implantation, the cause is often progesterone insufficiency. If so, it can be remedied by hormone therapy. Congenital malformations of the uterus, which often lead to early abortion, are also common.

5.     Cervical mucous -- the failure of the cervical mucous to change viscosity (known as ferning) at the time of ovulation can be assayed in laboratory tests. Abnormal ferning usually reflects, however, a general hormonal imbalance.

VIII.   In Vitro Fertilization (IVF)

            The pioneer work of Steptoe and Edwards and a handful of other investigators mostly working abroad has led to a test tube baby boom over the last 25 years. It turns out that fertilization in vitro is a relatively straightforward affair because early development will occur in culture media used for growing other types of cells -- that is, a buffered saline solution containing amino acids, vitamins, and bovine serum. Generally the woman is given fertility drugs to induce several follicles to mature at once. Sonography has progressed to the point where the time of ovulation can be predicted accurately. Sonography is also used to guide the collection the mature oocytes from the ovaries at the proper time as an aspirator is inserted through the abdomen just prior to ovulation. Sperm are collected from ejaculates or directly from the epididymus. Fertilization then is initiated either by mixing sperm with the oocytes or, more recently, by micro-injecting a single sperm directly into the oocyte cytoplasm. Embryos are then cultured until the morula or early blastocyst stage, when they can be introduced into the uterus or the oviducts themselves. Embryos are monitored visually to make sure they are normal (there is evidence that the possibility of producing abnormal blastocysts is somewhat increased). Recent advances in technology allow the capture of a single blastomere for use in PCR reactions to ascertain if certain genetic abnormalities are present in the embryo. The remainder of the embryo is usually quite viable after this procedure. Embryos are now often frozen and preserved until an appropriate time for implantation (creating lots of opportunities for lawyers and provoking the recent debate on stem cell research!). Implantation of such blastocysts is an inefficient process (less than 30% successful) so generally two or more are introduced. Of course, the uterus must be properly prepared for implantation, and hormones or fertility drugs are often used for this purpose as well.

            From a biological standpoint, an important consideration is how the egg manages to be fertilized by only a single sperm. Ordinarily only 500 or so reach the fertilization site, and perhaps not all at the same time. But in the test tube millions are present. It is unlikely that the cortical reaction, which takes about 30 seconds, can insure a single entry event under these circumstances. There is also evidence for a so-called rapid block to polyspermy that is thought to be the result of rapid membrane depolarization (the opening of ion channels in the oocyte membrane that in turn change the ionic composition in the cytoplasm) accompanying fusion with the sperm plasma membrane. The details of the depolarization are not completely understood. Obviously, micro-injection technology gets around all of these issues and helps insure that a single egg is fertilized by a single sperm. A second issue is that of capacitation. The sperm used in IVF has not spent any time in the female reproductive tract, so how it is competent to undergo the acrosome reaction?  The serum in the culture fluid is apparently sufficient to induce capacitation, a fact that underscores our poor understanding of the details of this process.

Further ReadingFor those interested in learning more, the following book has been placed on reserve in the library:  Speroff, L., Glass, R. H., and Kase, N.G. (1999)  Clinical Gynecologic Endocrinology and Infertility. Sixth Edition. Williams and Wilkins, Baltimore, MD.

STUDY QUESTIONS

1. Hormonal control of gametogenesis:

What are the gonadotropic hormones?  What are the roles do they play in gametogenesis in the male and female?

Which steroid hormones are synthesized in response to LH and/or FSH in ovaries and testes?  What cells in the gonads make the hormones

What is the LH surge and how does LH influence ovulation?

What is the corpus luteum and what does it do?

What hormones are responsible for the preparation of the endometrium for implantation?  In general, what changes occur in the endometrium during the ovulatory cycle?  What initiates menstruation?  What prevents the corpus luteum from degenerating?

2. Meiosis:

When and where do the meiotic divisions occur during oogenesis? spermiogenesis?

What is nondisjunction?

What is a polar body?

3. Fertilization:

Where does fertilization ordinarily occur and in what time period after ovulation?

What is capacitation?

What is the role of the flagellum of the sperm in fertilization?

What is the acrosome reaction?  What is the major enzyme of the acrosome called?

How does the sperm penetrate the zona pellucida?  What is the role of ZP3?

What prevents polyspermy?

What are pronuclei? 

4. Infertility:

What is the significance of a low sperm count, abnormal, or nonmotile sperm in the semen?

Anovulatory women can frequently be treated with fertility drugs. How do these drugs work and what is their relationship to steroid and gonadotropic hormones?  Why is there a greater incidence of multiple births?

5. In vitro Fertilization (IVF):

How is IVF performed?  Why are women often treated with fertility drugs prior to the procedure?  Why is there a greater incidence of multiple births?


PLACENTA

by Michael Rindler, Ph.D.

Objectives

1.         To study the differentiation of the early embryo into a blastocyst and the formation of the inner cell mass (embryoblast) and trophoblast cell layers.

2.         To learn about the process of implantation and the development of the placenta from fetal trophoblast.

3.         To understand the role of the placenta in gestation as a nutritive, endocrine, and protective organ for the embryo.

4.         To appreciate the consequences of abnormal placental development.

I. Introduction

         The placenta has five basic functions:

1.         It serves as the barrier separating the maternal and fetal circulations.

2.         It transports nutrients from mother to fetus and waste products in the other direction.

3.         It is the major endocrine organ, producing both steroid and polypeptide hormones.

4.         It transfers maternal antibodies to the fetus, providing immunological protection both in utero and in postnatal life.

5.         It initiates parturition (birth).

         The initial formation of the placenta and the trophoblast-mediated invasion of the endometrial decidua (outer layer of the endometrium) begin approximately 6 days after fertilization as the newly formed embryo undergoes implantation. Essential to this process is the formation of the trophoblast layer of cells. In order to understand the origin of the placenta, a description of the differentiation of the early embryo is provided in the next two sections.

II.     Preimplantation Stage (Blastulation)

            Fetal development takes place in the uterus. Implantation of the embryo into the uterine wall occurs on the fifth or sixth day after fertilization in humans. The earliest stages of development take place in the oviduct, where the fertilized ovum, now known as a zygote, undergoes the first cleavages or cell divisions (Fig. 2.11). These divisions are generally equal so that each of the daughter cells is roughly half the size of the original predivision parent. The early divisions are fairly synchronous and little overall growth in the size of the embryo occurs during this stage. The 4-32 cell stage embryo is known as morula (from the Greek meaning mulberry) and each of the cells is called a blastomere (Fig. 2.8). The blastomeres appear identical to one another, and experimental evidence obtained from experiments with mammalian blastomeres indicates that they are still totipotent; that is, they have the ability to form any differentiated structure of the embryo. This property of blastomeres has been exploited in gene transfer experiments. Cultured cells retaining the properties of blastomeres have had their genes altered (knocked out or mutated) and then have been reintroduced into early embryos (to make transgenic animals).

            The formation of the blastocyst, beginning at the 32-64 cell stage in humans, marks the first true differentiation event (Fig. 2.10). The early blastocyst forms from the morula when the cells on the outer surface near to the still intact zona pellucida form a continuous epithelial cell layer known as the trophoblast. The trophoblast layer seals off the interior of the embryo and then pumps in salt, which in turn results in the accumulation of fluid in a chamber called the blastocyst cavity or blastocoele. This is the first of four major cavities to be formed during embryogenesis ­ the others are the intra and extraembryonic coeloms and the amniotic cavity. On one edge of the blastocyst cavity a group of cells remains aggregated together, the embryoblast (inner cell mass), whose appearance is approximately the same as that of the undifferentiated cells of the morula. These cells will give rise to all of the structures of the embryo proper, while the trophoblast will participate in the implantation process and contribute primarily to the formation of the placenta. The trophoblast cells are not only distinguished by their position in the embryo and their epithelial appearance, but have undergone irreversible genetic changes as well (such as X-inactivation, the inactivation of one of the two X chromosomes in female embryos). Their developmental fate is also fixed -- trophoblast cells cannot differentiate back into embryoblasts, while embryoblasts still retain the capability to produce trophoblasts.

            The emergence of the blastocyst starts a phase of rapid growth of the embryo. As the blastocyst matures, the zona pellucida begins to disintegrate. The blastocyst, now known as a late blastocyst emerges or hatches around day 5 through a hole in the zona at its embryonic pole, that is, adjacent to the embryoblast cells. The degradation of the zona is through the action of hydrolytic enzymes produced by the newly emerging syncytiotrophoblast (see below). The late blastocyst, containing roughly 100 or so cells, is very adhesive after hatching and is ready for implantation, which occurs 5-6 days after fertilization.

III.   Implantation

            Fetal development in humans occurs completely within the endometrial layer of the uterine wall (the lining of the external surface of the uterus). The blastocyst must penetrate completely into the endometrium to allow further development to occur. After attachment to the epithelial cell layer of the endometrium at the embryonic pole (Fig. 2.10), the trophoblast cells in this area undergo a differentiation process that results in their transformation into large, multinucleated, amoeba-like cells known as the syncytiotrophoblast (Fig. 3.1). The syncytiotrophoblast is a highly adhesive and invasive cell that is responsible for implantation. It is able to adhere to the epithelial lining of the endometrium using several types of receptors, including the integrins, and penetrate deep into the endometrial stroma. Proteolytic enzymes that can degrade extracellular matrix (the collagen-rich substance surrounding cells) are critically involved. The remainder of the embryo is pulled in behind. The process of implantation is essentially complete by the end of the second week of development. A blood clot known as a closing plug (fibrin coagulum) then seals the site of the initial penetration (Fig. 3.3).

            Syncytiotrophoblast cells do not divide but are instead continuously generated from the remaining trophoblast layer, known now as the cytotrophoblast. Both types of trophoblast will contribute to the formation of the embryonic portion of the placenta. In the earliest stages, the syncytiotrophoblast (SCT) layer will develop spaces within it known as lacunae that participate in nutrient exchange (Fig. 3.3).

Later the SCT will penetrate and surround maternal capillaries, then the veins and finally the arteries. This leads to the establishment of blood filled cavities within the SCT known as maternal sinusoids (Fig. 3.4). The direct contact between maternal blood and the trophoblast layer provides the embryo with a rich source of nutrients, which enter the growing embryo by diffusion prior to the formation of the fetal circulatory system. The nearby maternal decidual cells also provide important nutrient substances to the embryo. Ordinarily, the invasion of blood vessels is perceived by the body as an injury and would induce a wound-healing response accompanied by clotting of the blood. However, a natural anti-clotting agent, tissue factor, is produced by the maternal decidual cells and is thought to prevent clotting from occurring.

         Maternal blood vessels of the uterine wall and fetal vessels in the placenta are relatively deficient in smooth muscle. By contrast, most large blood vessels in the adult have extensive layers of smooth muscle cells surrounding them. Smooth muscle cells are influenced by hormones released into the circulation which induce them to contract or relax, thereby regulating the size of the lumen of the vessels. Placental circulation is designed to be resistant to the actions of vasoconstrictive hormones. So, for example, when epinephrine is released into the maternal circulation due to circumstances such as sudden fright, there is little reduction in blood flow to the fetus since the placental vessels have much less smooth muscle and do not contract much. Nonetheless, the fetus is at risk when there are disturbances in maternal circulation, such as occurs in pre-eclampsia, a common condition characterized by elevated maternal blood pressure.

            The process of early differentiation and implantation is an intricate one, and a sizable fraction of fertilized ova are thought never to implant. Studies conducted over the last 50 years on spontaneously and electively aborted fetuses have given researchers an indication of the rates of defective embryos and fetal wastage in the general population. These studies have led to the conclusion that most spontaneous abortions occur in the first trimester. It is difficult to know precisely what is occurring during the critical first two weeks after fertilization, but it is estimated that perhaps as many as 60% of all embryos have serious defects and that a large proportion of these are weeded out very early in embryogenesis. Thus, it is likely that most of the defective embryos are eliminated before the mother is aware of anything unusual. In addition, many may never implant for unknown reasons.

            Implantation occurs in the uterine wall about 99% of the time, most often in the posterior wall. Ectopic pregnancy, where implantation occurs at abnormal sites can be a life threatening condition to the mother and fetus (Fig. 3.8). Implantation at the cervical opening of the uterus leads to a condition where the placenta seals off the opening, known as placenta previa. In this rare circumstance, the fetus can go to term but usually causes much bleeding. Delivery is often by Caesarean section. More commonly, implantation occurs in the oviduct (tubal pregnancy), probably due to impeded or improper transport of the embryo. If it is in the extrauterine portion, it will generally rupture the tube by the 8th week, resulting in the death of the embryo and severe hemorrhaging and danger of infection for the mother (Fig. 3.9). If implantation occurs in the intrauterine portion of the tube, appropriate placental circulation can be established and the fetus can develop somewhat further, although probably not to term. Implantation at other sites, such as the ovary or cervix, is very rare and usually results in spontaneous abortion. Implantation in the abdominal cavity may also occur very rarely. In this case, the fetus can occasionally go to term, although this type of implantation might compromise the health of the mother by damaging internal organs (Fig. 3.10).

IV. Formation of the Chorionic Villi

            After penetration of the maternal blood vessels by the trophoblast, the chorionic villi form. Primary villi have only syncytiotrophoblast (SCT) on the outside with cytotrophoblast (CT) in the center (Fig. 4.15). At a later stage, extraembryonic mesoderm migrates into the core of the primary villi to form the secondary villi. Differentiation of the extraembryonic mesoderm leads to the formation of tertiary villi, which have the fetal blood vessels inside of them. In humans, the fetal and maternal circulations are always separated by the synctiotrophoblast and this constitutes the fetal-maternal barrier that is so important (Fig. 4.16). Cytotrophoblasts will migrate to the maternal side across a class of tertiary villi called anchoring villi, which extend from the fetal to the maternal side. These cells will form a seal, the outer cytotrophoblastic shell, between the maternal and fetal tissue at the decidual plate. Villi continue to branch forming an extensive network, increasing surface area available for nutrient exchange (Fig. 6.8). In summary, mature villi have, from maternal to fetal side, a layer of SCT, CT directly underneath the SCT, extensive spongy connective tissue, and capillaries.

         During gestation, the placenta will continue to grow and mature. In the first trimester, there are greater barriers to placental transfer since there is an intact cytotrophoblast layer underlying the syncytiotrophoblast, more extensive intravillous stroma and smaller villous vessels (Fig. 6.8). However, by term, exchange is more efficient. The CT layer is absent except for small residual foci of CTs, the stroma is greatly reduced and the placental vasculature is far more extensive.

            Maternal tissue also contributes to the placenta. The functional layer of the uterine wall that is shed at parturition is called the decidua and is identified according to where it is found (Fig. 6.10). The tissue directly apposed to the placenta is called decidua basalis. The superficial layer covering the remainder of the fetus is known as decidua capsularis and the rest of the uterine wall is called decidua parietalis. The capsularis and parietalis will fuse together as the fetus grows to entirely fill the uterine cavity.

            The mature placenta is a pancake-shaped structure roughly 25 cm. in diameter (Fig. 6.14). Also known as the afterbirth, it detaches from the wall of the uterus during parturition and is expelled shortly after the fetus. On the fetal side of the placenta itself, a network of chorionic blood vessels converge onto the umbilical veins and artery. This surface is smooth in appearance due to the covering of embryonic membranes (amnion). The lobulated maternal side resembles 'brown cauliflower' because it is divided into structure/function units known as cotyledons (~50). Each cotyledon is separated from the next by connective tissue (Fig. 6.13). From the fetal side, branches of the umbilical artery and veins enter the highly anastomosing tree of branched villi and form capillary beds within them. On the maternal side, blood from spiral arteries in the endometrium enters the cotyledon and bathes the villi, eventually exiting through maternal veins. The circulation in the placenta is sluggish to allow exchange of nutrients and waste products.

V. Placental Barrier Function

         Primate placentas have what is known as a hemichorial placenta where the maternal and fetal circulations remain completely separated. The barrier function of the placenta is maintained primarily by the SCT (Fig. 6.8). These gigantic cells have hundreds of nuclei within them and are formed by the fusion of mononuclear CT precursors into a syncitium. Between SCT are tight junctional complexes that allow slow passage of ions and small molecules but restrict movement of larger molecules. Obviously, maternal and fetal blood cells do not cross the barrier (except when pieces of fetal tissue break off ­ see below). Thus, to be efficiently transported across the barrier, most nutrients must cross the plasma membranes of the SCT. There is little exchange of large maternal and fetal proteins across the barrier (with the exception of immunoglobulins ­ see below). Once a substance has penetrated the SCT layer there is no particular barrier associated with CT cells, the stromal cells, or fetal capillary vessels, which are porous in this region.

VI. Nutrient Transport

         Modes of placental transfer include:

- simple diffusion (e.g. gases, H20,, steroids, lipid soluble vitamins, thyroxine). Molecules that have a high lipid solubility can cross the barrier by direct diffusion across the lipid bilayers of the SCT. Steroid hormones and their precursors are of particular significance. This is a bidirectional process, with CO2, urea and other waste products diffusing from fetus to mother. In addition, electrolytes and some small molecules like glucose can diffuse through the tight junctions between cells. Diffusional processes in many cases are inadequate. Specific cellular channels (for water, urea, etc.) are present in the cell membranes which significantly enhance the rate of diffusion across the SCT.

- facilitated diffusion (e.g., glucose, amino acids, lactate, I-, Fe, Zn, and water soluble vitamins). These molecules are transported across the two membranes of the SCT by specialized membrane proteins. Some of these transporters do not have directionality but equilibrate maternal and fetal pools. Others use the energy of ATP to concentrate molecules or make use of the electrochemical membrane potential to transport ions across the two lipid bilayers of the SCT. The surface facing the maternal blood has plasma membrane specializations (microvilli) that increase surface area.

-  surface receptor binding and endocytosis (e.g., lipids and cholesterol from maternal LDL, folate, Fe++, Cu++). Molecules that are transported in the bloodstream complexed to proteins are generally taken up by receptor-mediated endocytosis into the trophoblast. The LDL receptor, for example, mediates the uptake of LDL, which contains cholesterol. In endosomes and lysosomes, the nutrient molecules like cholesterol are liberated from the complexes. Some of the molecules are used by the SCT cells themselves, while others are released through the basal surface of the cell where they associate with fetal carrier proteins and enter the fetal circulation. The maternal carrier proteins themselves are not transported across the SCT.

-  receptor-mediated transcytosis for IgG type of immunoglobulins. The fetus and the newborn are incapable of mounting their own immune responses. To protect the newborn until its own systems take over, maternal immunoglobulins (Ig) in the blood are equilibrated with the fetal circulation across the placenta in the third trimester. A specific protein receptor at the microvillar plasma membrane of the SCT binds IgG, and the receptor/ligand complex undergoes endocytosis and re-insertion at the basal membrane, where the IgG molecules are released on the fetal side. The receptors are recycled back to the microvillar surface for reuse. The receptor resembles that involved in IgG uptake by neonatal intestine from maternal colostrum (milk), which is another source of IgG.

Other Fetal Membranes --  To some extent, nutrients, water and gases can also cross the amnion,allowing exchange between the maternal circulation and the amniotic fluid.

VII. Endocrine Functions

Placental Steroidogenesis

         The placental trophoblast cells synthesize the steroid hormones progesterone and estrogen and are the only major source after the first trimester, when the corpus luteum degenerates. Both hormones are required for the maintenance of the maternal decidua. In their absence, the decidua will degenerate and the fetus cannot remain attached to the uterine wall.

         In general, the principle source of progesterone is through de novo synthesis by trophoblast using cholesterol obtained through the uptake of maternal LDL (see Figure 1). The fetal adrenal synthesizes pregnenolone during gestation, which crosses into the placenta for conversion to progesterone, providing an alternative precursor. Estrogen is produced from dehydroepiandrosterone sulfate (DHAS) synthesized by the fetal adrenal gland. Fetal steroids, including cortisol, are produced in response to progressive increases in fetal adrenocorticotropin (ACTH) production [ACTH stimulates adrenal steroid synthesis] across gestation. This axis is also important for initiating parturition (see below).

Placental Peptide Hormones

         SCTs and CTs are a rich source of a large number of peptide hormones required for placental or fetal function. These include:

1)  human Chorionic Gonadotropin (hCG) - a 36,700 dalton glycoprotein which binds to the LH receptor of the ovarian corpus luteum to stimulate massive maternal ovarian progesterone production until 8-10 weeks gestation, at which time placental progesterone production is adequate to maintain the pregnancy. hCG is produced by the SCTs and its synthesis is positively modulated by CT-derived GnRH (gonadotropin releasing hormone) and negatively modulated by CT-derived inhibin and activin. hCG also stimulates the Leydig cells in the fetal testes to produce testosterone, promoting sexual differentiation in male fetuses.

         hCG is first detectable in the maternal circulation around day 10 after ovulation and peaks at 10 weeks. It is also present in the urine and one of the principle molecules used to detect pregnancy. Because Down syndrome pregnancies produce lower amounts of hCG, the assessment of hCG levels in the maternal circulation at 16 weeks gestation is recommended as the primary biochemical screening test for fetal Down syndrome.

2)  human Placental Lactogen (hPL) - a 25,000 dalton glycopeptide with 94% homology to growth hormone and 67% homology to prolactin. It is produced by the SCT. hPL acts as a potent anti-insulin and lipolytic agent in the maternal circulation to increase the availability of glucose, amino acids, free fatty acids and ketones for the fetus. It has the side effect of making the mother mildly 'diabetic' that is, having glucose levels above normal after meals (insulin normally lowers blood glucose).

3)  Corticotropin releasing hormone (CRH) -- a 31 amino acid glycopeptide originally identified in the hypothalamus, where its release into the portal circulation increases pituitary adrenocorticotropin (ACTH) release which, in turn, enhances adrenal cortisol production (see above). CRH has been found to be synthesized in the placenta, fetal membranes (amnion and chorion) and the uterine decidua in amounts that increase dramatically near term.

         While hypothalamic release of CRH is inhibited by glucocorticoids establishing a negative feed-back loop, placental, fetal membrane and decidual synthesis is stimulated by glucocorticoids, creating a potential positive feed-back loop. Newly synthesized placental CRH will stimulate the fetal pituitary and the placenta itself to produce corticotropin (hcACTH, see below), which results in stimulation of adrenal production of the steroid cortisol (a corticosteroid).

         CRH can enhance prostaglandin production by the fetal membranes and decidua to potentially initiate parturition (labor and delivery). Prostaglandins as well as oxytocin stimulate uterine muscle contraction. Rapid increase in circulating CRH, potentially driven by rising fetal and maternal cortisol production, occurs during the final few weeks of pregnancy. This suggests that CRH is part of the biological clock mechanism regulating the onset of parturition. It is not known how cortisol itself inhibits progesterone function, thereby disrupting decidual function. The best hypothesis is that it does so by binding to the same receptors and antagonizing the progesterone response. In other animals, like sheep, cortisol lowers progesterone levels by activating an enzyme that converts androgen precursors to estrogens instead. In human placenta, however, this enzyme is not present.

4)  human chorionic corticotropin (hcACTH). As mentioned above, this hormone stimulates cortisol production. Its synthesis by the placenta is regulated by CRH.

5)  GnRH -- hypothalamic releasing hormone also produced by trophoblast stimulates release of hCG and production of steroid hormones.

VIII. Transplacental Antibody Passage

         The placenta is an immunologically privileged organ. This means that the mother does not ordinarily make antibodies against the trophoblast itself even though it may express foreign, paternally derived antigens on its surface. Part of the explanation for this is that the placenta produces substances which locally suppress the immune response (the nature of these is unknown). In addition, effective immune responses against cells require the presence of so-called histocompatibility antigens produced by all cells. Trophoblast cells are an exception, producing instead their own unique histocompatibility antigen. This antigen not only cannot participate in the immune response, it is thought to suppress the maternal immune response.

         Antibodies are not synthesized by the fetus prenatally. Maternal IgG (which is the major type of antibodies produced) is specifically transported across the placenta, particularly late in gestation. As mentioned above, specific receptors for IgG exist on the microvillar surface of the trophoblast which transport it across the SCT cells. Since the fetus is a separate immunological entity, if the mother is exposed to immunologically unique, paternal-derived fetal antigens, she will generate anti-fetal antigen antibodies. In the case of erythrocyte (red blood cell) antigens such as Rh, maternal antibodies can cross the placenta to cause alloimmune hemolytic anemia (killing of erythrocytes), also known as Rh disease, in the fetus. Rh disease can cause cardiac failure due to fluid accumulation (hydrops fetalis), intrauterine death, or even severe jaundice leading to mental retardation. In most cases, Rh disease does not affect the first pregnancy. Exposure to fetal cells does occur naturally late in gestation when pieces of placental villi break off and enter the maternal circulation, often lodging in the lungs. The fetal red blood cells therein can trigger a maternal immune response, but significant antibody accumulation would take place only very late in gestation and therefore the fetus is not affected significantly. However, in subsequent pregnancies the problem potentially may become much more severe as the maternal immune response is more vigorous. Rh immunoglobulins (a sort of blocking antibody) given to the mother can prevent damage to the fetus.

         A very similar type of antibody response can affect platelets. A mother who does not express the PLA-1 platelet antigen, but whose fetus does, can produce anti-PLA-1 antibodies that can cross the placenta to cause profound alloimmune thrombocytopenia (deficiency of platelets) with fetal intracerebral hemorrhage. Again, this is more common after the first pregnancy.

         Maternal auto-antibodies that cause autoimmune diseases in the mother can also cross the placenta: a) anti-acetylcholine receptor antibodies, leading to neonatal Myasthenia Gravis (impaired muscular function); b) anti-TSH receptor antibodies, leading to fetal and neonatal Graves disease (impaired thyroid function); c) anti-ribonucleoprotein antibodies, leading to congenital heart block; and d) anti-phospholipid antibodies causing fetal or neonatal thrombosis (blood clots).

IX. Transport of Infectious Agents

         A number of viruses, including cytomegalovirus, rubella (German measles), varicella-zoster (chicken pox), measles, and poliovirus, can cross the placenta and infect fetal tissues. Rubella in particular was a significant pathological agent before the advent of vaccines. It was a major cause of craniofacial abnormalities including congenital deafness. Today, the most important virus that can infect the fetus is HIV, which is transmitted to about 1/4 of infants of HIV-infected mothers who are not being treated with anti-viral drugs. AZT and other HIV virus-suppressing drug therapies administered to mothers with HIV have reduced this percentage dramatically in the last few years. Fetal infection is believed to occur around the time of parturition, either through microscopically damaged areas of the placenta that allow maternal blood to enter the fetus or during delivery. Cesarean delivery before the onset of labor is therefore recommended in these cases. Because maternal antibodies are present for the first 2-3 months after birth, conventional AIDS testing, which detects circulating antibodies to HIV in the bloodstream, cannot be used to detect HIV infection in newborns.

         Bacteria and other protozoa do not ordinarily cross the barrier. Exceptions include tre­ponema palladium (syphilis) and toxoplasma gondii that can cause abnormalities of the brain and eyes. Toxoplasma is primarily transmitted by maternal ingestion of uncooked meat.

X. Pathology -- Hydatidiform Mole

         Rarely (perhaps 1:500-1:1000 pregnancies), the process of growth and invasion of the maternal endometrium by trophoblast goes awry. This results in the death of the fetus and the production of tumors (hydatidiform mole) consisting exclusively of trophoblastic tissue. Moles produce abnormally high levels of circulating hCG, which is used to diagnose them. There are two kinds of such moles. One is a generally benign lesion that is not very invasive called partial hydatidiform mole. The more invasive complete variety, a choriocarcinoma, maintains the highly invasive properties of the early SCT and, if untreated, will generally metastasize (migrate to and colonize other sites). It has characteristic swollen villi. Modern chemotherapeutic techniques have improved survival rates for women, even those with invasive or metastatic moles, and most are now curable.

         Recently, it has been found that hydatidiform moles have chromosomal abnormalities. Complete moles are diploid, but both sets of chromosomes are paternal, that is, derived solely from sperm. Most of them have XX sex chromosomes and are believed to arise from monospermic fertilization (the X chromosome is essential, therefore no YY moles can develop). In this case, the oocyte pronucleus is lost and the sperm pronucleus apparently undergoes an initial duplication of its chromosomes without cleavage to produce a diploid nucleus. Occasionally, complete moles are XY, indicating that they arose from dispermic fertilization, but the maternal chromosomes are nonetheless not present. Partial moles are triploid, with two paternal sets of chromosomes and one maternal set. Most of these arise from dispermic fertilization or possibly by a single abnormal diploid sperm.

         The inheritance of paternal chromosomes as a cause of hydratidiform mole raises the puzzling question of why it should matter to the developing oocyte what the origin of the chromosomes is. Developmental biologists have long been able to experimentally produce a similar phenomenon in mouse oocytes by replacing the male or female pronucleus in fertilized ova with another of the opposite origin. This results in diploid embryos with two sets of paternal or maternal chromosomes. Interestingly, those ova having a maternal set form recognizable embryos with greatly reduced placental development. By contrast, those with paternal chromosomes form trophoblast and a mass of placental tissue resembling moles but generally lack embryonic tissue. The results imply that maternal chromosomes regulate embryoblast development while paternal chromosomes are responsible for trophoblast development.

         The manifestation of phenotypically different characteristics depending on whether inheritance is maternal or paternal is known as genetic imprinting. Imprinting is thought to have evolved to control placenta formation but its effects are not limited to early development. The severity and age of onset of a number of genetic diseases differ depending on the parent from whom the mutated gene is inherited. These include Huntington's chorea (Woody Guthrie's disease), neurofibromatosis, and Wilm's tumor (a kidney tumor). In one case when the defective gene is inherited from the father, it causes Prader-Willi syndrome and from the mother a distinct condition called Angelman syndrome. While both syndromes are characterized by mental retardation and by characteristic facial traits, other features differ between the two. With respect to the mechanisms governing imprinting, in some cases it has been shown that two copies of the affected chromosome were inherited from a single parent (uniparental disomy) probably after a trisomic conceptus lost one of the chromosomes in an early division. On the molecular level, differential methylation of genes coupled with cis acting transcriptional enhancers have been shown to be involved, but the detailed mechanisms are not well understood.

XI. Multiple Pregnancies.

         Twinning may occur when there are several oocytes that undergo ovulation (dizygotic = fraternal), as occurs relatively frequently when the mother takes fertility drugs. Less commonly, monozygotic (identical) twins are derived from a single embryo when the embryo separates into two during early embryogenesis. Monozygotic twins formed by an early separation (morula blastomere stage), like dizygotic twins, always have separate placentas and amniotic sacs (Figs. 6.17, 6.18). If implantation is close together, the placentas may fuse together but the vascular systems remain separate. When monozygotic twins are generated later in embryogenesis, at the blastocyst stage, the inner cell mass separates into two and the twins will have a common chorionic cavity and placenta but almost always have separate amniotic cavities (Fig. 6.18). In this case, the circulations in the placenta may be separate or may have anastomoses between them. If the splitting of the inner cell mass occurs very late in embryogenesis, which is rare, the twin embryos will share the same amniotic cavity as well as a common placenta. In very rare cases, the embryos may fail to separate completely and are called conjoined twins (Fig. 6.21). Complications from multiple pregnancies include death of one or more of the fetuses because of compression, lack of adequate nutrients, or unbalanced blood supply, particularly when there are vascular anastomoses. Furthermore, low birth weight and prematurity are very common, putting the newborns at great risk. Infant mortality is high (as much as 20%) for twin pregnancies.


STUDY QUESTIONS:

What is the function of the inner cell mass and the trophoblast layer in the early embryo?

At what stage does implantation take place?

What role does the syncitiotrophoblast play in villus formation? The cytotrophoblast?

What important structures derive from the extra-embryonic mesoderm in the chorion and villi?

What happens to the chorionic cavity? Why does the membrane around the fetus actually consist of two layers?

What are decidual cells?  Spiral arteries? 

What is the difference between primary, secondary, and tertiary villi?

What specific changes occur in the trophoblast and stroma of the placental villi during gestation and what consequences do these changes have on nutrient exchange?

A woman who has already had a successful pregnancy has a subsequent series of miscarriages. She is Rh-, PLA-1-, but her husband is Rh+, PLA-1+. What could be the cause of the problem? 

Approximately 1/4 of HIV-positive, untreated (i.e., no anti-HIV drug regimen) mothers transmit their virus to the fetus, yet all of their children initially test HIV positive. Why is this?

What pathogenic agents can cross the placenta? Most pathogens do not cross. Why not?

What are the major hormones synthesized by the syncytiotrophoblast and cytotrophoblast?  What is the function of hCG, hPL, and CRH?  What is the importance of progesterone?  Why is it produced by the placenta when the corpus luteum is a rich source?

How do nutrients cross the placenta?  Gases?  Steroid hormones?

What is the basis for dizygotic twins?  Monozygotic?  For monozygotic twins what determines how many placentas, chorions, and amnions there will be?

What is a conjoined twin and what is the cause of it?

What is a hydatidiform mole?  What chromosomal anomalies have been found in association with the two types, partial and complete?

A woman of child-bearing age suddenly develops severe lower abdominal pain and calls her physician. She believes she may have missed her last two periods. What specific diagnosis would be important to consider and what tests could be conducted to confirm a potential diagnosis?