Link to the Hippocrates Modules on LYMPHOID TISSUE and BLOOD
The development of erythrocytes (erythropoesis) and granulocytes (granulopoeisis) is completed in the bone marrow. However, not all blood cells complete development there. Platelets are formed from megakaryocytes in the bone marrow, but assume their mature shape after entering the circulation. Monocytes arise in the bone marrow and enter the circulation, but undergo further differentiation into macrophages after leaving the blood vascular system for other tissues.
The life cycle of lymphocytes is more complex. They are derived from precursors in bone marrow, but their differentiation and maturation occur elsewhere. Some precursors leave the bone marrow and settle in lymphoid tissue where they proliferate and differentiate into "bone marrow derived" (B) cells. Other precursors go to the thymus. There they acquire certain surface characteristics (e.g. expression of various lineage markers like theta antigen and the T cell receptor or TCR), as well as the development of functinoal capabilities, such as the ability to distinguish "self" from "non-self". After acquiring their specific characteristics, a process termed maturation, they migrate as "thymus derived" (T) cells to lymphoid tissues. There, upon contact with cognate antigen and with other support cells of the immune system, they differentiate further into 'effector' cells. T cells are more numerous in the circulation, amounting to approximately 85% of the circulating lymphocyte population while B cells comprise about 5-10%. T and B cells possess very different properties, but are indistinguishable under the microscope in routine histologic preparations.
Upon contact with cognate antigen and other support cells of the immune system, B lymphocytes develop into plasma cells which secrete antibody. B cells have surface immunoglobulins which serve as receptors for antigen. Each membrane-bound immunoglobulin is of a single specificity with regard to antigen recognition and all the antibodies on the surface of any one B cell have the same antigen specificity. B cells respond to antigen by proliferating into blast cells (large cells with much cytoplasm). Some antigens can stimulate B cells directly, but most require the help of T cells and antigen-presenting cells to convert B cells into blast cells. Some of the blast cells will become plasma cells which secrete large amounts of immunoglobulin of a single class and of the same specificity for the antigen. Plasma cells, which have the ultrastructure of a protein-secreting cell (abundant ER, large Golgi complex, several nucleoli), are short-lived (several days to a few weeks). Other blast cells will revert to the small lymphocyte morphology. Some of the long-lived lymphocytes are probably memory B-cells, which are clones of cells that are metabolically quiescent, but ready to divide and produce antibody-forming cells soon after re-exposure to the antigen they have previously encountered. They are antigen-specific memory cells.
T cells do not have surface immunoglobulins and receptors for immune complexes, but do bear a receptor for a specific antigen (TCR). Some of these receptors recognize foreign antigens ("non-self"). Others recognize antigens that originate from the body or, "self". T cells are involved in delayed hypersensitivity (cellular immunity), a mechanism important in some forms of transplant rejection, and some infectious diseases like tuberculosis. A subset of T-cells, those which express the differentiation antigen CD8 (cytotoxic CD8+ T lymphocytes) are able to lyse target cells in a process that does not require antibody or complement. Another subset of T cells, those that express the differentiation marker CD4 (helper CD4+ T lymphocytes), are involved in augmenting the B-cell immune response to antigen, a process termed 'T cell help'. Antigen is usually presented to CD4+ T cells by other cells collectively referred to as 'antigen presenting cells' (veil cells, dendritic cells, macrophages or B-cells). Recognition of the antigen by CD4+ T cells induces the formation of T cell clones, some cells of which will synthesize and release certain cytokines known as lymphokines. CD8+ T cells can directly recognize cognate antigen on the surface of target cells.
Most lymphocytes constantly recirculate: a given lymphocyte is thought to pass through each lymph node once per day. They leave lymphoid tissue through efferent lymphatics and enter the blood circulation at the level of the thoracic duct. Many of them migrate back to lymphoid tissue by going through the walls of small venules. In the lymph nodes these small venules characteristically possess a high endothelium.
The bone marrow and the thymus are central immune organs providing the rest of the immune system with cells. The spleen, the lymph nodes, GALT (gut associated lymphatic tissue) and BALT (bronchial associated lymphatic tissue) are peripheral components of the immune system and participate in filtration of blood and interstitial fluid. They concentrate the cells of the immune system and antigen from the blood or lymph, thereby facilitating induction of immune responses.
The function of the immune system is extensively studied in Immunology. Our objective is to provide you with information about the structure and function of these cells, tissues and organs and to enable you to correlate this with knowledge acquired through Immunology.
|Bone marrow - red and yellow|
cortex and medulla
Be sure to review the LYMPHOID TISSUE and BLOOD AND HEMATOPOESIS study units.
The purpose of this study is to examine bone marrow as a tissue, as well as to become acquainted with the development of individual blood cells. As in all of the hematopoetic tissues, the supporting framework of bone marrow is reticular connective tissue. Erythrocytes and granulocytes undergo maturation in the bone marrow prior to entering the circulating blood. When hematopoesis is occuring, these cells in different stages of development fill the intersticies of the reticular meshwork. In this active state, the marrow is referred to as red bone marrow. In the adult however, the marrow of long bones become infiltrated with fat and is then referred to as yellow bone marrow.
Slide 9, a section through developing bones of the finger, illustrates the location of red bone marrow within a bone. Note the heterogeneity of the cell population. Do not examine this slide using oil immersion as individual cells are difficult to recognize. Megakaryocytes, which are involved in formation of platelets, may be identified by teir larger size. What appear to be empty spaces are fat cells.
Slide 10, is also of a developing bone. Depending on the age, it may contain yellow bone marrow seen in most adult bone. This type of marrow is filled with fat, stored in reticular cells and the hematopoetic stem cells are quiescent.
Slide 12, is a bone marrow smear. The cells in this smear were probably aspirated from the marrow cavity of the sternum, which remains as red marrow in the adult. Identification of individual cells in a marrow smear is difficult even for a specialist. The object is rather to develop a concept of the progression of developmental changes in each of the cell lines, erythrocytic and granulocytic (see CT-14). Learn their patterns and then try to pick out cells of each line. First decide only if they are relatively immature or mature. The relatively mature cells are more numerous. Then try to be more specific. Use the photographs on HT-10 and its accompanying legend, in conjuction with the charts HT-8 and HT-9.
Study the cells of the erythroid series first (see HT-8), which comprise about 22% of the cells in marrow. Identify the more mature forms, the normoblasts and polychromatophilic erythroblasts. You may look for more immature forms, but you will not be held responsible for their identification, as they are difficult to distinguish with certainty. Study the cells of the granulocytic series (see HT-9), which total about 55% of the cells in bone marrow, in a similar manner. Identify myelocytes, metamyelocytes and mature granulocytes. Identify the members of the neutrophil series first because they are more numerous. Eosinophils could be considered next. It will be rather difficult to identify basophils or their precursors. Less mature cells, eg promyelocytes, are difficult to distinguish and you will not be held accountable for their identification. Megakaryocytes, on the other hand, can easily be identified at low magnification. Look for lymphocytes, reticular cells, monocytes and macrophages, as well as fat storing reticular cells.
These lymphoid organs are interposed along lymphatic vessels and serve as the site where antigen is processed and presented to helper T cells. Helper T cells in turn become activated, divide and secrete cytokines that amplify the immune response. Afferent lymphatics enter through the capsule of the organ, the lymph percolates through the reticular connective tissue and leaves through efferent lymphatics at the hilus. Lymph nodes such as those on slide 37 are encapsulated by connective tissue. Connective tissue trabeculae extend from the capsule into the organ. Lymphatic nodules, some of which show germinal centers of proliferating lymphocytes, form the cortex. Cords of denser lymphatic tissue extend into the medulla of the organ. These are the medullary cords. In the lymph node, B cells are located in the cortical nodules and T cells in the cortical regions between the B nodules or in areas bordering the medulla. Plasma cells are often seen in the medullary cords.
Arterial blood vessels enter the node at the hilus and form arching arcades in the deep cortex, which can extend toward the capsule, passing between the nodules. Capillaries drain into post-capillary venules (with an unusual high endothelium) which in turn enter veins that exit the node at the hilus. Lymphocytes can exit the blood vascular system to enter the lymph node by passing across the high endothelium of the post-capillary venules.
Slide 38, a silver stain of a lymph node, shows the distribution of reticular fibers in the node. If your slide is poorly stained, try sharing one with a neighbor. The reticular meshwork is more dense in the cortex and medullary cords than in the sinuses (subcapsular and medullary).
2. Spleen. Slides 39, 40, 99, 110, 111 and 117
The spleen is a filter of the blood interposed in the vascular system. It is the major site of removal of effete red blood cells. It is also site of lymphocyte accumulation and plasma cell formation. Slides 99, 117, 39 and 111 show the capsule that consists of connective tissue elements and smooth muscle, and surrounds the spleen. Trabeculae extend from it into the organ. Blood vessels entering and leaving through the hilus travel in the trabeculae for a part of their course. Lymphatics begin in the white pulp and leave via the trabeculae and hilus. Note that slide 111 has two pieces of tissue: spleen and liver.
Dense lymphatic tissue, called the white pulp, surrounds the major arterial vessels after they leave the trabeculae, as a periarterial sheath. The arteries thus surrounded are called central arteries. T cells are abundant in the periarterial sheaths. B cell-filled nodules occur as expansions of the and are surrounded by T cells. These splenic nodules (Malphighian corpuscles) may contain germinal centers. Lymphatics draining the white pulp course out beside the central artery to enter the trabeculae and exit with the blood vessels through the hilus.
The tissue between the white pulp is called the red pulp. The area of transition from red to white pulp is called the marginal zone. The marginal zone is quite apparent and well defined in slides 99 and 111. Some plasma cells may be found in the marginal zone.
In the red pulp, cords of dense lymphatic tissue, the splenic cords (Billroth's cords) are separated by sinuses filled with red blood cells, thus the term 'red pulp.' These are best seen in slide 99. In slide 39 the sinuses are collapsed and difficult to see. The cords contain numerous polymorphonuclear leukocytes. In slide 117, the sinuses are distended and the cords do not contain a dense population of lymphocytes. Spindle-shaped endothelial cells run parallel to the axis of the sinuses in a "barrel-stave" arrangement. They are frequently seen in cross-section in slides 99 and 117.
Macrophages are numerous in the marginal zone and cords. In slide 99 they can be identified by their content of brown pigment, hemosiderin, derived from phagocytized red cells. In slide 111 they are recognized by their content of ingested TiO2. In rodents (slides 110 and 111) the marginal zone is thicker than in the monkey (slide 99).
Reticular fibers in the spleen have a distinctive arrangement. On slide 40 notice that in the white pulp they are sparse in the germinal center and form a distinct rim around the nodules. In the marginal zone they form a delicate meshwork. In the red pulp they are arranged longitudinally in the cords and concentrically around the sinusoids. The reddish-brown masses are trabeculae cut in various planes of sections.
In slide 110 macrophages can be identified in the section of spleen by their PAS+ content. The reticular fiber meshwork is also PAS+. The megakaryocytes you may see in slide 110 are characteristic of rodent spleens.
3. Thymus. Slides 96 and 115
Slide 96 shows a thymus of an infant. Septation by connective tissue extending in from the capsule is incomplete in the thymus. It divides the cortex of dense lymphocytes into lobules, but the medulla, which has a less dense population of lymphocytes, is continuous between adjacent lobules. The proportion of cortex and medulla may vary significantly among individuals. Unlike other lymphoid organs, the supporting tissue of the thymus is not reticular connective tissue. It is an endodermally derived epithelium and lacks fibers. T lymphocytes enter the thymus and occupy the spaces between the epithelial cells forcing them apart until they come to form a meshwork very similar in appearance to the one formed by the mesenchymally derived reticular cells of the lymph nodes and spleen. Unlike the reticular cells derived from mesenchyme, however, the epithelial cells in the thymus are held together by multiple desmosomes. Hence, they are best described as epithelioreticular cells. These cells present 'self' antigen to the immature cells during T-cell development. They also secrete substances that promote maturation of the cells, such as thymosin and thymopoietin. Thymic corpuscles ( Hassal's corpuscles), concentrically arranged aggregates of epithelial-reticular cells, are characteristic of the medulla of the thymus, but have no known function. Lymphocytes do not usually form nodules in the thymus.
Slide 115 contains thymic tissue from a normal rat and one treated with corticosterone. Corticosterone treatment results in depletion of lymphocytes from the cortex, due to death of immature T lymphocytes and egress of mature T cells from the medulla. Therefore, the epithelial reticulum can be seen clearly as a pink band at the periphery of the shrunken thymus of the treated animal.
4. Palatine tonsil. Slide 41
Aggregations of lymphatic nodules form discrete organs, the tonsils, in specific locations of the upper digestive tract . These underlie the epithelium and are separated from adjacent structures by dense connective tissue. The overlying epithelium may be so heavily infiltrated by lymphocytes as to be unrecognizable in many places. This epithelium is stratified squamous in the palatine tonsil, differentiating it from the pharyngeal tonsil, which has ciliated pseudostratified epithelium.
5. Lymphoid infiltrates and nodules. Slide 47, 59 and 56
Diffuse lymphoid tissue is found scattered throughout the body. These lymphocytic infiltrations are particularly characteristic of loose connective tissue underlying epithelium exposed to the external environment, as for instance in the digestive tract, urogenital and respiratory systems. Diffuse infiltration may be focal as in the tongue, slide 47, or occupy a larger area, as at the gastroduodenal junction, slide 56.
In other sites the lymphocytes may aggregate into spherical nodules in the lamina propria. Several nodules may clump together as in the Peyer's patches characteristic of the ileum, slide 59, where both mucosa and submucosa are heavily infiltrated.
Upon antigenic stimulation lymphocytes in the nodules may begin to proliferate, forming a germinal center. Looking from the center to the periphery of such nodules, note the changes that occur in cell size and nuclear morphology during lymphocyte development.
Lymphoid Tissue POP-Quiz
Blood Development POP-Quiz