Special Histology and Embryology: A Practical Course - V. K. Napkhanyuk 2001

Organs of Hematopoiesis and Immune Defense
Spleen

The Spleen (splen, lien) is a vital hemopoietic and immune defense organ involved in both the clearance of senescent or damaged erythrocytes and platelets, and the orchestration of immune responses against exogenous Antigens that evade the Lymph Nodes AND enter the bloodstream. The organ's mass and volume vary considerably depending on Blood pooling and The rate of hemopoiesis.

The human spleen is covered by a Connective Tissue capsule and the Peritoneum. Trabeculae extend inward from the capsule, anastomosing extensively in the deeper Regions of the organ. The splenic trabeculae contain a small number of smooth Muscle Cells. Elastic fibers are more abundant in the trabeculae than in the capsule.

The spleen is structurally divided into red and white pulp.

The red pulp constitutes the major portion of the splenic mass and consists of reticular tissue packed with erythrocytes—which impart its characteristic red color—along with sinusoidal Blood Vessels.

The white pulp is an aggregation of lymphoid tissue located in the adventitia of its Arteries, forming spherical clusters or lymphoid nodules, as well as periarterial lymphatic sheaths. The splenic lymphoid nodules exhibit four indistinctly demarcated zones: 1) periarterial; 2) germinal center; 3) mantle zone; 4) marginal zone. Typically, the nodule's central artery passes eccentrically through it, giving off radial capillaries. In histological preparations, the white pulp stains blue with hematoxylin.

Preparation 4. Cytology/practical/72.html">Cross section of a cat spleen (Figs. 33, 34).

Low magnification (Fig. 33). Make a drawing of the preparation. Label the following structures on the drawing: 1) capsule; 2) capsular trabeculae; 3) trabecular arteries and Veins; 4) red pulp; 5) white pulp.

High magnification (Fig. 34). Examine and draw the arterial sheaths (ellipsoids) and venous sinuses.

Arterial sheaths are terminal arterioles with narrow lumens and thickened walls. Typically, 2–3 endothelial Cell nuclei project into the arteriolar lumen. The endothelium is surrounded by a thick layer of tightly packed reticular fibers. These arterial sheaths function as sphincters regulating blood flow into the sinuses.

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Fig. 33. Cross section of a cat spleen. Hematoxylin-eosin staining. x 40:

1 — splenic capsule; 2 — trabecula; 3 — trabecular artery and vein; 4 — red pulp; 5 — white pulp (a — germinal center); 6 — central artery

Fig. 34. Cross section of a cat spleen. Detail of the previous preparation. Hematoxylin-eosin staining. x 600:

1 — splenic trabecula (a — smooth muscle cells; b — trabecular vein; c — trabecular artery); 2 — red pulp (d — pulp artery; e — splenic sinuses; f — reticular cells; g — blood formed elements)

Electron micrograph 21. Lymph node sinus. x 25,000:

1 — lumen of the lymph sinus; 2 — littoral cell; 3 — reticular fibers; 4 — reticular cells (after Sam and Clark)

The venous sinuses are numerous and permeate the entire red pulp. Their diameter ranges from 12 to 40 µm. The sinus walls are lined with rod-shaped endothelial cells that are continuous with the adjacent reticular tissue. Reticular fibers encircle the endothelial Cells of the sinus like hoops (Micrographs 21, 22).

Label the following on the drawing: 1) splenic trabecula; 2) smooth muscle cells; 3) trabecular vein and artery; 4) red pulp; 5) pulp artery; 6) splenic sinuses; 7) reticular cells.

Electron micrograph 22. Splenic sinus. x 8,000:

1 — erythrocytes within the sinus lumen; 2 — erythrocyte squeezing through a cleft in the sinus wall; 3 — reticuloendothelial cell; 4 — discontinuous basement membrane (after Yu. I. Afanasyev)

Review Questions

1. Which hemopoietic and immune defense Organs are considered primary (central)? Discuss their structural features and Functions.

2. Which organs of lymphocytopoiesis and immunopoiesis are referred to as peripheral? What role do they play in generating a response to antigenic stimulation?

3. Sources of development of Hematopoietic organs.

4. What tissue forms the structural framework of hematopoietic organs? Specific features of the Thymus as a hematopoietic organ.

5. What is myeloid tissue?

6. What tissue is termed lymphoid tissue?

7. Microscopic Structure and Main Functions of the Bone Marrow.

8. Features of the micro- and ultrastructure, as well as histophysiology, of the thymus.

9. Which structures in the lymph nodes constitute the B-zones and which constitute the T-zones? Features of their structure.

10. Functional features of lymph nodes associated with their Location along the course of Lymphatic vessels. Characteristics of lymph flow in lymph nodes.

11. White pulp of the spleen. Red pulp of the spleen.

12. Features of the Blood supply to the spleen.

13. Which structures serve as the T- and B-Zones of the spleen?

14. Morphological and functional features of the periarterial zone, mantle zone, and marginal zone of the lymphoid nodules of the spleen.

Situational Problems

1. A patient's blood test reveals a shift of the leukocyte formula to the left (an increased number of immature granulocyte forms). A change in the function of which organ does this indicate?

2. Epithelial cells are observed among the hematopoietic cells in a histological preparation. Which hematopoietic organ are we studying?

3. Two blood vessels are visible within a trabecula of the spleen on a histological slide. One of them has a well-defined tunica media that is clearly distinct from the adjacent trabecular tissue. The second vessel lacks a tunica media and appears as an endothelial-lined channel within the trabecula. What are these vessels?

4. A bacterial antigen has been administered into an animal's body. In which zones of the peripheral Organs of the lymphoid system will changes be observed?

Sample Examination Questions

1. Sources of development of hematopoietic organs and their histogenesis.

2. Structure and function of the bone marrow.

3. Morphofunctional Features of the thymus.

4. The spleen: its STRUCTURE AND FUNCTIONS.

5. Structure and functions of lymph nodes.

6. Functional features of lymph nodes associated with their location along the course of lymphatic vessels. Characteristics of lymph flow in lymph nodes.