I. Development of the Early Embryo -- General Developmental Strategies
II. Formation of the Bilaminar Embryo
III. Formation of the Extraembryonic Coelom
IV. Formation of the Trilaminar Embryo (Gastrulation)
VI. Differentiation of the Mesoderm and the Development of Somites
VII. Development of the Intraembryonic Coelomic Cavity
VIII. Folding of the Embryo
IX. The Fetal Circulatory System
X. Embryonic Induction
1. To describe the early stages of development that give rise to the three germ layers.
2. To understand the basic elements of differentiation of the three germ layers into definitive body structures.
3. To learn about the general molecular mechanisms underlying the organization of the embryonic body plan and differentiation of cells during development.
1. To go over the formation bilaminar embryo, as well as the yolk sac, amniotic cavity, and extraembryonic coelom/chorionic cavity.
2. To be able to recognize the primitive streak and node and other major landmarks, the oropharyngeal and cloacal membranes, allantois, and (later) connecting stalk.
3. To learn about gastrulation and the formation of the three germ layers.
4. To understand what a sacrococcygeal teratoma is.
5. To be able to describe the formation of the prechordal plate and the notochord.
6. To study neurulation and closure of the neural tube anterior and posterior neuropores. To understand the origins of neural tube defects, like anencephaly and spina bifida.
7. To identify paraxial, intermediate and lateral mesoderm and to recognize somites. To learn how sclerotome, myotome, and dermatome form and the structures and tissues derived from each.
8. To understand the formation of the gut tube in the context of transverse folding. To study longitudinal folding, including the formation of foregut and hindgut. Note midgut communication with yolk sac through vitelline duct.
9. To describe the formation of the intraembryonic coelom and to identify its splanchnic/visceral and somatic/parietal portions. To understand how folding changes the position of the heart tubes and septum transversum. To describe the formation of body cavities (pericardial, pleural and peritoneal; covered again in Cardiovascular and GI sections).
10. To go over the formation of the umbilicus from connecting stalk, allantois,, vitelline duct and surrounding tissue. To understand how the fetus becomes surrounded by amniotic cavity.
11. To define an organizer and discuss the role of the primitive node as an organizer.
12. To define induction. Using neural tube formation and differentiation as an example, to elaborate on the role of growth factors, such as shh, in the induction process.
13. To discuss HOX genes and their role in patterning and segmentation.
14. To understand the developmental potential of early embryo cells and concept of a stem cell.
From invertebrates to vertebrates to man, embryological development has certain characteristic stages. after fertilization embryos form a morula and undergo blastulation (the reorganization of the embryo to surround a cavity) as we have already seen in early human embryos. Later they will go through gastrulation (the formation of three embryonic layers), and neurulation (the development of the nervous system). The earliest development in humans is geared more toward providing a safe and nutritionally appropriate environment for growth (the placenta) than it is for the growth of the embryo proper. Once establishment of the placenta is safely underway, the three definitive germ layers are formed, the endoderm, mesoderm, and ectoderm, which will give rise to three classes of structures in the final organism. The endoderm will develop into the gastrointestinal system, the mesoderm into much of the muscle, bone, blood, and connective tissue, and the ectoderm into the skin and nervous system (see your textbook for a complete listing). The next major undertaking by the embryo is to transform itself from a disc shape into the three dimensional form as we know it. Embryonic folding results in the formation of the digestive system and body cavities.
Most organ development of human embryos occurs during the first 8 weeks of gestation, known as the embryonic period (see inside cover pages in Sadler). During the fetal period that succeeds it, the fetus does undergo major development but grows tremendously in size.
In the Placenta chapter, the embryo's earliest, including formation of the early and late blastocyst was discussed. At the late blastocyst stage, the inner cell mass/embryoblast, which will give rise to the entire embryo, begins to differentiate. At first, a layer of cells forms at the edge of the blastocyst cavity, an epithelial layer that now separates the embryoblast from the cavity, and is called the hypoblast (primitive endoderm) (Fig. 3.1/2.1). This cell layer continues to divide and spread around the blastocyst cavity underneath the cytotrophoblast. When the process is complete, the cavity is renamed the primitive yolk sac (Fig. 3.3/2.3). The hypoblast constitutes one of the two layers of the primitive embryo. The other layer, the epiblast or primitive ectoderm, is formed during the appearance of the second important cavity, the amniotic cavity, a fluid-filled sac that appears during the second week in the embryoblast. The embryoblast eventually forms a single layer of cells lining the amniotic cavity. Those at the top of the cavity (amnioblasts) underlie the cytotrophoblast while those at the bottom of the cavity are called the epiblast and overlie the hypoblast. These two cell layers, hypoblast and epiblast, together are known as the bilaminar embryonic disc.
At this time cells of the embryo, thought to delaminate from the cytotrophoblast and/or epiblast, contribute the extraembryonic mesoderm, a disorganized layer between the cytotrophoblast and the yolk sac as well as between the cytotrophoblast and the cells surrounding the amniotic cavity (Fig. 3.4/2.4). Within this newly formed mesoderm, fluid-filled spaces appear as growth continues and these eventually coalesce to form the extraembryonic coelom, later known as the chorionic cavity, the third major cavity of embryonic development. The excavation of the cavity results in the separation of the extraembryonic mesoderm surrounding the amniotic cavity (together called the amnion) and yolk sac (together called the exocoelomic or Heuserπs membrane) from that extraembryonic mesoderm remaining with the trophoblast (together known as the chorion). At this stage the extraembryonic mesoderm surrounding the yolk sac is called extraembryonic visceral or splanchnic mesoderm while the rest is called somatic or parietal mesoderm. The significance of these designations will be clarified when the intraembryonic mesoderm is discussed below.
Once formation of the chorionic cavity is complete, the embryo itself is attached to the cytotrophoblast only by a thin connecting stalk consisting primarily of extraembryonic mesoderm (Fig. 3.6/2.6). The connecting stalk is attached to what will eventually be the caudal pole of the embryo (the tail), and it will go on to form the bulk of the umbilical cord. The primitive yolk sac, until this point quite large, becomes considerably smaller when the ventral portion pinches off, and is thereafter known as the definitive or secondary yolk sac. The remnant of the primitive yolk sac is left in the extraembryonic coelom, where it is known as the exocoelomic cyst.
The bilaminar disc is not uniform in its appearance. Two oval shaped areas on opposite ends of the midline can be distinguished morphologically. In what will be the cranial region, the buccopharyngeal (oropharyngeal) membrane is characterized by hypoblast cells that are very tall. A similar region consisting of cells that are very flattened at the caudal pole is called the cloacal membrane. These regions, which will end up in the oral and anal cavities, respectively, serve as points of reference in the descriptions that follow. Both the buccopharyngeal and cloacal membranes remain bilaminar structures not penetrated by mesoderm tissue, in contrast to the rest of the bilaminar disc. It is not known what determines the polarization of the early embryo,several growth factors (chordin, nodal, Vg1) are secreted by cells at the caudalmost region of the embryo (posterior marginal zone) and others (e.g., cerberus) are synthesized in the cephalic region shortly after the axis is established. These proteins help mediate antero-posterior axis specification (Fig. 4.5/5.5).
The formation of the three definitive germ layers (endoderm, mesoderm, and ectoderm) is of fundamental importance to embryogenesis. The transition from the bilaminar to the trilaminar embryonic disc involves the further differentiation of the epiblast, which at this stage is still totipotent so far as the development of the embryo proper is concerned and will give rise to all three definitive germ layers. At the beginning of the third week, a groove or depression (the primitive streak) arises in the midline of the disc near the caudal end. At the cranialmost pole of the primitive streak, an elevation of the epiblast appears, known as the primitive node. Cells in the primitive node are responsible for initiating gastrulation. Transplantation of an ectopic node, for example, leads to the formation of a second notochord and a second neural tube. This region thus resembles the dorsal blastopore lip (known also as the Spemann organizer) of amphibians. Spemann and Mangold demonstrated about 75 years ago that transplantion of the lip induces gastrulation in the embryos of amphibians. Indeed, the cells of the node produce some of the same growth factors and transcription factors implicated in the action of the blastopore lip, including nodal, chordin, and noggin (Figs. 4.5/5.5, 4.6/5.6, 4.7/5.7).
Cells at the primitive streak invaginate inward, leaving the epiblast layer. Some of the initial wave of cells enter the hypoblast layer (Fig. 4.3/5.3). These cells push aside the hypoblast as they grow and migrate, forming a new layer, the definitive endoderm (endoblast), although the original hypoblast remains as the lining of the remainder of the yolk sac. In later waves of migration, cells entering the streak area migrate between the epiblast and endoblast to form the intraembryonic mesoderm or mesoblast. Other cells from more peripheral regions of the epiblast continue to divide and contribute cells that migrate toward the streak (Fig. 4.11/5.8). The process of mesoblast formation in histological terms involves the actual detachment of the cells from the epiblast layer and a subsequent cell shape change to an elongated, migratory cell that then perambulates wherever it can. The type of loosely organized connective tissue that results is often referred to as mesenchyme. In molecular terms, scientists have found that the expression of certain cell surface adhesion molecules is lost during this process to facilitate the detachment while the cells reorganize their cytoskeletal infrastructure and gain the ability to migrate spontaneously. The mesodermal cells from the primitive streak migrate laterally and cranially, everywhere except in the regions of the cloacal membrane and the buccopharyngeal membrane, which remain bilaminar structures, and in the midline between the node and the buccopharyngeal membrane, where the notochord will develop.
The primitive streak continues to produce mesoblast until the end of the fourth week, although it does not increase in size as the embryo grows and thus becomes progressively smaller relative to the overall dimensions of the trilaminar disc. Rarely, cells of the primitive streak fail to degenerate at the appropriate time and persist as a tumor, known as a teratoma (Fig. 4.14/5.10). Teratomas are generally found in the newborn in the sacrococcygeal region at the base of the spine. Primitive streak tumors such as this ordinarily contain disorganized but still recognizable differentiated tissue. Since the epiblast cells that give rise to it are capable of differentiating into all embryonic structures, these tumors often contain derivatives of all three germ layers. Testifying to the invasive and migratory nature of some of the streak derivatives, teratomas often become malignant (i.e., capable of invading into the underlying tissue) in infancy.
In addition, a number of abnormalities are attributed to inadequate mesoblast formation by the primitive streak. These are referred to collectively as caudal dysplasia and include sirenomelia, where the lower extremities are malformed and appear in a single mermaid-like extension (Fig. 4.13/5.9). There are frequently other defects associated with caudal dysplasia, often in the cranial region as well, suggesting that a more general deficiency in mesoderm formation or differentiation is involved in some cases. Reinforcing this view are experiments involving irradiation of the primitive streak in chick embryos, which can result in similar defects.
In the midline, epiblast cells migrating through the primitive node contribute mesoderm initially to the prechordal plate, a structure containing relatively undifferentiated mesoderm just caudal to the buccopharyngeal membrane (NOTE: this is incorrectly depicted in Sadler and in Moore & Persaud). The prechordal mesoderm contributes later to the formation of the face and deficiencies in its formation give rise to midline facial malformations. The notochordal plate (process) also arises from the node as a column of invaginating cells migrating between the epiblast and hypoblast in the midline and continuing to progress towards the cranial end of the disc, stopping at the prechordal plate. Accompanying the formation of the notochordal plate is tremendous growth and elongation of the cranial portion of the embryo resulting from the proliferation of the epiblast and the migration of cells forming the mesoblast and notochord. The notochord and prechordal plate are known as the axial mesoderm.
Cells at the base of the notochordal plate then fuse with the hypoblast layer (Fig. 4.4/5.4). Since the notochordal plate has a channel or canal in its center connected to the amniotic cavity at the primitive pit, when its floor fuses with the hypoblast, the canal is transiently open to the yolk sac and is called the neurenteric canal. The definitive notochord reforms by invagination from the definitive endoderm by the end of the third week, closing off this connection once again. The vertebral column will ultimately form around the notochord, whose remnants remain as part of the nucleus pulposus in the intervertebral discs. The notochord, despite its limited contribution to the adult organism, is actually the orchestrater of the subsequent development of the overlying ectoblast and the surrounding mesoblast. It produces growth factors that induce the development of the neural plate and the somites.
Beginning in the middle of the third week, the ectoblast (the renamed epiblast) dorsal to the notochord forms a elevated thickening bilaterally known as the neural plate (Fig. 5.2/6.2). The neural folds at the edges of the plate invaginate and form a groove that eventually fuses in the midline to form the neural tube, the precursor of the entire central nervous system (Fig. 5.3/6.3). The neural tube detaches from the overlying ectoderm in the central axis beginning in the midportion of the embryo and extending cranially (Fig. 5.5/6.5). Closure also proceeds in the caudal direction, with the last segments to fuse known as the rostral (anterior) and caudal (posterior) neuropores (Fig. 5.8/6.6). [In the most caudal region, a small portion of the neural tube is contributed by a second site of neurulation known as the caudal eminence. Here, the tube forms from what appears to be mesoderm that hollows out and fuses with the developing neural tube.] Failure of the neural tube to close properly leads to neural tube defects (NTD). In the rostral (cranial) region this can lead to anencephaly where the neural tissue of the brain is exposed on the surface of the cranium (Fig. 8.7/9.7). Outside the cranial region, spina bifida occurs when the neural tube or caudal neuropore fails to seal properly (Figs. 19.15/17.15 and 19.16/17.16). The different manifestations of NTDπs, of which there are several (meningocele, etc.), are not important for the purpose of our discussion.
Just as the neural tube is closing, another group of cells arising more laterally in the ectoblast migrate out of the epithelium and into the interior of the embryo (Fig. 5.3/6.3). These neural crest cells are initially found as a ridge dorsal to the neural tube but soon begin to migrate in a characteristic fashion around the neural tube and all around the embryo. They will form the cells of the peripheral nervous system as well as the sensory ganglia (Table 5.1). In addition, they contribute to many structures in the cranial region, including some cranial nerves, connective tissue, cartilage and bone. These fascinating cells are so intent upon migration that they will do so readily in vitro when placed in a culture dish.
The ectoblast that does not form the nervous system, neural crest or special sense organs forms the epidermis of the skin and associated glands (sweat, mammary, some salivary, etc) and well as the epithelium lining the oral and anal cavities.
5.1: Germ Layer Derivatives
Table 5.1: Germ Layer Derivatives
Ectoderm (in general,
nerves, epidermis, with exceptional structures in head)
Neuroectoderm (in general, nervous system and special sense organs)
Neural Tube CNS, retina, posterior pituitary, pineal body
Placodes lens, olfactory mucosa, inner ear, cranial nerve ganglia
Neural Crest PNS, pigment cells (melanocytes), adrenal medulla
In head and neck-- cranial nerves, connective tissue, skeleton, dermis (skin), C cells of thyroid, truncoconal septum, odontoblasts (teeth), corneal endothelium, pupillary and ciliary muscles (eye)
Surface ectoderm Epidermis, cutaneous and mammary glands, hair, nails, anterior pituitary, thymus stroma, corneal epithelium, enamel (teeth), parotid gland, mucosa of oral cavity, anus, and glans of penis
Mesoderm (in general, connective tissue,
blood, urogenital, muscle, skeleton, body cavities)
Somitic/Paraxial muscles, skeleton (except in limb and parts of head & neck ), dermis (skin), connective tissue
Intermediate kidney, ureters, gonads, uterus, vagina, genital ducts and glands
Lateral connective tissue, muscle of viscera (splanchnic), cardiovascular and lymphatic systems including blood and lymph cells (splanchnic), lining of body cavities, adrenal cortex, bone and cartilage of limb
Endoderm (in general,
derivatives of gut tube and pouches = gut and associated structures)
respiratory mucosa, bladder, urachus, prostate, part of vagina, thyroid follicular cells, tympanic cavity, tonsil and thymus stroma, salivary glands, parathyroid, GI tract including liver, pancreas, and gall bladder
Accompanying the formation of the neural tube at the end of the third week is the differentiation of the mesodermal layer into 4 distinct parts. The axial mesoderm consists of the midline notochord and prechordal plate. On either side of the developing neural fold a condensation of mesoderm can be recognized, the paraxial or somitic mesoderm (Fig. 5.9/6.7). Adjacent to the somitic mesoderm is another condensed portion called the intermediate mesoderm, which will give rise to the urogenital system. The mesoderm that is furthest laterally from the neural tube becomes known as the lateral plate mesoderm which will eventually gives rise to the lining of the body cavities, the cardiovascular system, the visceral musculature and some connective tissue. Each of these mesodermal subtypes derives from cells that originated in specific regions of the original epiblast layer as they migrated through different parts of the primitive streak (Fig. 4.11/5.8).
The somitic mesoderm gives rise to the somites (Figs. 5.5/6.5, 5.9/6.7). These paired structures, recognizable histologically as an aggregated mass of cells, will form the skeletal muscles and most of the bones as well as the dermis of the skin and other connective tissue. The principal characteristic of the somitic mesoderm is its segmentation, which presages the eventual vertebral segmentation. Somite formation begins in the midportion of the embryo proximal to the first region of closure of the neural tube and proceeds caudally until roughly 42-44 pairs are formed by the fifth week (Fig. 5.5/6.5). The first occipital 4 pairs and the last 5 to 7 pairs do not contribute to the formation of the vertebral column but the rest give rise to the axial skeleton. The somites are visible from outside the embryo, so somitic age is often used by scientists as the criterion for comparing embryogenesis in different species. This is particularly important, for our ability to do research on human embryos is quite limited and much of what we know about development is derived by extrapolation from experimental findings in other species.
The somites continue to develop during the fourth and fifth weeks. They first divide into three segments -- the sclerotome, myotome and dermatome (Fig. 5.11/6.10). Each of these three can be identified histologically and each has a different developmental fate. The dermatome gives rise to the connective tissue layer of the skin known as the dermis. Together with the myotome, which gives rise to the muscles of the trunk and limbs, the dermatome, assumes a more lateral position under the skin as the embryo grows. Eventually the cells migrate through the entire organism. Myotomes divide into dorsal and ventral segments that enlarge and migrate, eventually coalescing into muscle masses. Sclerotomal cells detach from the somite but remain in the vicinity of the neural tube where they form the vertebrae and the intervertebral discs. Each sclerotome actually divides in two to form half of each of two adjacent vertebrae (Fig. 8.21/8.23). In this way, each vertebra is derived from parts of two adjacent sclerotomes. Blood vessels, initially running between the somites, are trapped between the two fusing halves of each sclerotome as the vertebra is formed, accounting for their final positions coursing out of the vertebrae themselves. The intervertebral discs are formed from the medial portion of each sclerotome and adjacent notochordal remnants. The segmental nerves, which originally run through the center of each sclerotome, become intervertebral during this process due to the division of each sclerotome. In short, each vertebra is formed from components of two somites, but a spinal nerve continues to supply the muscles derived from the myotome of one somite.
Beginning in the third week in the cranial half of the embryo, small spaces appear in the lateral mesoderm and coalesce into an inverted 'U' shaped cavity whose arms extend laterally, the intraembryonic coelom (cavity) (Fig. 11.1/12.1). The cranialmost part of the 'U' is lateral mesoderm derived from the primitive streak that migrated cranial to the buccopharyngeal membrane. The tube is self-enclosed in the cranial 2/3 of the embryo, but in the midsection it is continuous with the extraembryonic coelom. The lateral mesoderm surrounding the coelom is divided by the cavity into two. The floor of the cavity adjacent to the endoderm is continuous with mesoderm surrounding the yolk sac and is known as the visceral (splanchnic) mesoderm, while the roof is continuous with the parietal (somatic) extra-embryonic mesoderm and underlies the ectoderm. The intraembryonic coelom will give rise to the pleural, pericardial, and peritoneal cavities of the adult, but this requires a new adventure in morphogenesis ≠ embryonic folding.
Folding of the embryo is the process by which a flat three-layered structure is molded into its final embryonic shape, bringing structures such as the future mouth and umbilicus from their original positions on the dorsal aspects of the embryo to the ventral aspect. Of course, folding is not really movement as such but differential growth of the embryo constraining new development into defined directions. Longitudinal folding (cephalocaudal, along the cranial-caudal axis) is accompanied by the growth of the neural tube in the cranial region while lateral (transverse) folding is accompanied by the appearance of the somites and the expansion of the amniotic cavity. Longitudinal folding brings the buccopharyngeal membrane, which anchors the oral cavity, from its original position to a ventral position where it will form the stomodeum (future mouth). In addition, the cranialmost segment of the inverted U-shaped intraembryonic coelom is flexed 180∫ ventrally and back on itself, giving rise to what will be the pericardial cavity in the chest (Fig. 5.16/6.16). Longitudinal folding of the cranial region also is responsible for the ventral movement of two structures that form initially at or near the furthest tip of the embryo -- the cardiogenic area (from which gives rise to the primitive heart tubes) and the septum transversum, which eventually forms the central tendon of the diaphragm. The septum transversum, the cranialmost structure in the embryo, probably arises from extraembryonic mesoderm. However, somitic mesoderm in the cranial region migrates into it during folding, bringing with it innervation from segmental nerves C3-5. Like the coelom itself, the cranialmost structures fold toward ventrally toward the chest and become inverted 180∫, so that the septum transversum, which originates most cranially, ends up beneath the developing heart. The head fold also contributes to the formation and extension of the foregut portion of the gut tube.
In the caudal region a similar sequence of longitudinal folding ensues (Fig. 5.16/6.16). The cloacal membrane, which will form the anal membrane, is moved from its initial dorsal position to a ventral one. In the process, the tail fold contributes to the formation of a pouch containing the hindgut and future cloaca. It also helps give rise to the umbilicus by positioning the connecting stalk on the ventral side of the embryo. The hypoblast in this region has penetrated the connecting stalk. A finger-like projection of hypoblast called the allantois, a largely vestigial structure that served as a urinary bladder in primitive organisms, is present in the stalk. In the human embryo, it is believed to play a crucial role in the induction of future umbilical arteries and veins. Together with the umbilical vessels, the allantois is incorporated into the umbilicus, although the portion of it continuous with the hindgut will eventually become a segment of the primitive urinary bladder.
Lateral folding is somewhat more complex, but in general involves the rolling of the flat endoderm into a round tube, accompanied by the extensive proliferation of the somites (Fig. 5.9/6.7). The coelom surrounding the endoderm is likewise folded toward the midline and the two sides must fuse to seal off the peritoneal cavity, which until this point is open to the chorionic cavity (extraembryonic coelom). The visceral lateral plate mesoderm overlying the endoderm follows it around to form the mesenteries of the gut, while the somatic mesoderm underlying the ectoderm fuses upon itself to close off the body wall (Figs. 5.13/6.12, 5.17/6.17). In this way, the peritoneal cavity is completed and the somatic lateral mesoderm becomes its outer lining, the serous membranes. The remainder of the yolk sac, is not incorporated into the gut but is pushed into the umbilicus where it is known as the vitelline duct (Fig. 5.18/6.18). In the midgut region the yolk sac via the vitelline duct is still continuous with the gut, but the duct will progressively degenerate. In the embryo itself, the formation of the gut tube and the apposition of two sides of the intraembryonic coelom yields a tube surrounded by visceral mesoderm and suspended between two mesenteries, dorsal and ventral. The ventral mesentery will degenerate except for part of the foregut where the liver will form. This leaves the rest of the gut hanging suspended only by a dorsal mesentery in the peritoneal cavity.
Lateral folding also serves to bring the paired primitive heart tubes adjacent in the midline where they will undergo fusion (Fig. 11.1/12.1, 11.3/12.3). The portion of the intraembryonic coelom ventral to the heart tubes enlarges as the heart grows, such that the heart tube is suspended within is by connections at the cranial and caudal ends (Fig. 11.5/12.5). As this portion of the coelom, now known as the pericardial cavity, expands, it is soon separated from the pleural portions of the coelom on either side by the growth of septae from the body wall (Fig. 10.4/11.6, 10.5/11.7). These pleuropericardial folds will form the fibrous pericardium and carry the cardinal veins together to the midline where they can participate in the formation of the vena cava. The lining of the dorsal body wall also contributes septae (pleuroperitoneal folds/membranes) that close off the canals that separate the pleural cavities from the peritoneal cavity below it. In doing so, they meet the septum transversum to form the diaphragm (Fig. 10.6/11.8). Subsequent growth of the body wall will complete this process and the diaphragm as well. To recapitulate, the diaphragm is formed from the septum transversum, the somatic lining of the intraembryonic coelom (pleuroperitoneal folds) and the body wall (see Chapter on Development of the Gastrointestinal Tract: Foregut).
As alluded to above, the umbilicus is formed by longitudinal folding, as the connecting stalk, containing the umbilical blood vessels and the allantois, moves from a caudal to a ventral position. There it meets and fuses with the narrowing yolk stalk containing the vitelline duct and surround tissue (Fig. 5.16/6.16). The umbilicus will elongate and narrow further as the fetus grows, acquiring the name umbilical cord (Fig. 6.9/7.9).
Folding of the embryo also alters the topographical relationship of the amniotic and chorionic cavities. The amniotic cavity grows much larger and folds together with ectoblast around the embryo to completely surround it (Fig. 6.1/7.1). The chorionic cavity is then outside the amniotic cavity. As the embryo grows, the chorionic cavity is obliterated as the amnion and chorion fuse (Fig. 6.4/7.4).
In order for the embryo to grow substantially, it needs a better supply of nutrients than diffusion can provide. The fetal circulatory system is one of the first organ systems to develop, beginning at the end of the second week, and consists of three portions (Fig. 11.32/12.31). The vitelline circulation develops in the extraembryonic mesoderm surrounding the yolk sac, while the umbilical circulation forms in the connecting stalk near the allantois and includes the chorionic circulation in the extraembryonic mesoderm of the chorion surrounding the embryo. At the start of the third week the fetal (cardinal) circulation itself forms from the splanchnic portion of the lateral plate mesoderm. It establishes connections with the umbilical and vitelline circulations shortly thereafter.
In general, the formation of the circulatory system involves the aggregation of cells known as angioblasts into masses or cords called blood islands that develop spaces within them (Fig. 5.14/6.13). Fusion of these cords accounts for the initial formation of blood vessels that can then grow and expand (Fig. 5.15/6.14). Vascular endothelial growth factor, a protein secreted from hypoblast and endodermal cells, controls the formation of the blood vessels themselves ). At first, the angioblasts not only form the endothelial lining of the blood vessels but those in the yolk sac and around the allantois form the primitive blood cells. Only in the fifth week does blood cell production occur in the embryo proper and even then in the liver and not until later in the spleen and bone marrow. Areas lacking mesoderm, such as the buccopharyngeal and cloacal membranes do not form blood vessels.
The development of blood vessels from mesodermal precursors is under the control of vascular endothelial growth factors (VEGF) (Fig. 5.14/6.13). In the embryo, VEGF is produced by cells of the hypoblast and endoderm. VEGF has generated a lot of excitement recently since agents that block its function can inhibit the growth of tumors. Tumors also produce VEGF to induce blood vessels that they require for continued proliferation. Because it is also responsible for blood vessel growth and regeneration in adults, VEGF is also potentially useful in many clinical situations where collateral blood vessel formation would be beneficial.
Cells in a given tissue or organ express a unique set of genes and proteins. The challenge of developmental biology is to understand how cells undergo appropriate differentiation during embryogenesis. In early development, many cells are pluripotent, that is, they can have a number of different potential cell fates. As development proceeds, cells become progressively restricted in their developmental pathways until a point where they become committed to a very defined set of developmental choices. For example, the ectoderm initially can differentiate into many structures, including neuronal structures, the lens or cornea of the eye, epidermis of skin and glands such as the mammary gland. Once neurulation starts, those ectodermal cells that become part of the neural plate are committed to forming part of the nervous system and can no longer differentiate into non-neuronal cell types.
The process of restriction of cell fate is greatly influenced by signals from other tissues within the organism. A complex set of signals is received by a particular cell from several different sources. These signals impart to the cell positional information that instructs it in its developmental choices. The ability of one tissue or agent to influence the restriction/commitment process of another in order to alter its developmental fate is known as induction. Several examples of induction are discussed in the textbook including the induction of somite differentiation in response to signals from the notochord, neural tube, ectoderm and lateral mesoderm (Fig. 5.12/6.11). In early embryogenesis, there are many examples of induction. Some inductive processes can result in the formation of an entire body axis. Experiments performed over 50 years ago by Spemann and Mangold showed that a region of amphibian embryos known as the dorsal blastopore lip was essential for the initiation of gastrulation. When an additional blastopore lip was grafted into embryos, a second notochord formed along with a second neural tube, a second set of somites, even a second head. More recently, the same effect has been achieved by injecting mRNA encoding goosecoid, a regulator of axis development produced in the blastopore lip (Fig. 4.7/5.6). This region of the amphibian embryo, known as an organizer, is the equivalent in mammalian embryos to the primitive node. In chick embryos, transplantation of an ectopic node also initiates a new gastrulation event and the formation of a second neural tube.
On a molecular level, growth factors mediate most inductive processes. Growth factors resemble hormones. They are secreted molecules, usually proteins but also other smaller organic compounds that can diffuse away from their cell of synthesis. Receptors on the taget cells then bind these growth factors and initiate an appropriate response. A good example is the establishment of ventral/dorsal polarity of the neural tube. Growth factors produced by the notochord and by the ectoderm adjacent to the neural tube are key players (Fig. 19.14/17.14). The notochord produces the growth factor sonic hedgehog (shh) which induces the cells in the ventral portion of the neural tube to form what is known as the floorplate (Fig.1). The floorplate cells themselves produce more sonic hedgehog and other growth factors to induce nearby cells to produce the motor neurons of the spinal cord. The function of hedgehog depends critically on its concentration -- cells further away from the notochord are exposed to much lower levels than those close by and hence fail to become motor neurons. Cells of the ectoderm produce growth factors known as bone morphogenetic proteins (BMP's) which act antagonistically to shh and have a dorsalizing effect on the neural tube, inducing the roof plate and the adjacent alar plate. It is also important to appreciate from this example that the same growth factor can be involved in several different inductive processes, often at different times and with different tissues. As we will see later in the course, for example, sonic hedgehog plays an important role in development of the face as well.
Figure 1. Molecular regulation
of neural tube differentiation. Top) Sonic hedgehog secreted by the notochord
(N) ventralizes the neural tube and induces formation of the floor plate (F),
which also produces SHH. Bone morphogenetic proteins are secreted from the
alar plates. Bottom) Initally, PAX 3 and 7 are expressed uniformly in the
neural plate. SHH represses their expression in the ventral half of the neural
tube. BMP’s upregulate PAX 3 and 7 expression in the dorsal portion of the
tube. PAX6 begins to be expressed as the neural folds elevate and close. Other
growth factors and transcription factors also play a role in spinal cord development.
Figure 1. Molecular regulation of neural tube differentiation. Top) Sonic hedgehog secreted by the notochord (N) ventralizes the neural tube and induces formation of the floor plate (F), which also produces SHH. Bone morphogenetic proteins are secreted from the alar plates. Bottom) Initally, PAX 3 and 7 are expressed uniformly in the neural plate. SHH represses their expression in the ventral half of the neural tube. BMP’s upregulate PAX 3 and 7 expression in the dorsal portion of the tube. PAX6 begins to be expressed as the neural folds elevate and close. Other growth factors and transcription factors also play a role in spinal cord development.
Growth factors control the fate of cells by activating the transcription of specific sets of genes whose protein products will in turn initiate developmental programs. For example, BMP's and shh activate different members of the Pax family of transcription factors, which are required for the dorsal/ventral patterning of the neural tube (Fig. 19.14/17.14). Some of the genes activated by transcription factors are capable of regulating the development of not just a specific differentiation pathway of a particular tissue, but whole segments of the body plan. Such regional segmentation is seen most clearly in the formation of the somites, which presages the overall segmented organization of the spinal cord, its associated musculature and patterns of innervation. A family of transcription factors, encoded by the HOX genes, serves to control segmentation in the mammalian embryo, as they do in organisms as distantly related as insects (Fig. 5.22/6.22). In mammals, expression of specific combinations of HOX genes (and their protein products), rather than expression of a single HOX gene at a given site, impart the positional information. One example is in the head, which is also segmented and has regionally specific HOX gene expression (Fig. 15.12/16.12).
Cell migration, programmed cell death, cell-cell adhesion, and cell interaction with the extracellular matrix are also important regulators of embryonic morphogenesis. These processes occur in early development as well. For example, cells migrate through the primitive streak to form the endoderm and mesoderm. At the beginning of their migration they also lose the cell surface molecules that mediate tight cell-cell adhesion in the epiblast, thereby allowing for unrestricted cell movement. And programmed cell death (apoptosis) is the eventual fate of the cells forming the oropharyngeal and cloacal membranes, among many other embryonic structures. Examples of these types of interactions as well as other inductive processes and the role of growth factors and hormones in development will be discussed when the embryology of the different organ systems is presented during the remainder of the course. Some of the best understood events occur in the formation of the genital system and the face. While the names of the players may change, the general principles governing developmental events, as enumerated briefly above, will nevertheless apply.
Further Reading. For those who are interested in learning more about this subject, the textbook by Gilbert is highly recommended and has been placed on reserve in the library. It is a wonderfully comprehensive and reasonably up to date compendium that integrates principles of modern molecular biology with those of classical embryology and developmental biology:
Gilbert, Scott F. (2002) Developmental Biology. 7th ed. Sinauer Associates, Sunderland, MA.
(Note: These will serve as the basis for the group discussion sessions Thursday/Friday, Sept. 8-9. Students should be prepared to answer questions in italics in class.)
What layer of the bilaminar embryo gives rise to embryonic tissue (as opposed to extraembryonic)?
What is an embryonic stem cell?
What is the primitive streak and what does it give rise to?
What specific structures are derived from the primitive node?
What does a sacrococcygeal teratoma represent? What tissues may be present?
How does the neural tube form?
Where does the neural crest come from and what does it give rise to?
What causes neural tube defects such as anencephaly and spina bifida?
What are the divisions of early mesoderm and what, in general, does each give rise to?
Where do the somites arise and how do they develop in the early embryo?
Miscarriage is a fairly common phenomenon. Why do you think it may occur?
What are the derivatives, in general, of each of the three definitive germ layers? What structures derive from splanchnic/visceral lateral plate mesoderm as opposed to somatic/parietal?
What does the intraembryonic coelom give rise to?
How does embryonic folding affect the formation of the gut tube? The position of the heart in the embryo?
What parts of the embryo eventually contribute to the umbilicus?
What is embryonic induction?
What is an organizer and what region of the embryo serves this function?
What tissues participate in neural induction? Induction of the floorplate?
What kinds of molecules have inducing signal activity? What is sonic hedgehog? BMP?
What kinds of genes participate in establishing positional information in the body axis? What are the characteristics of HOX genes?