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

Organs of Hematopoiesis and Immune Defense
Thymus

The Thymus gland, or simply the thymus, is a central organ of lymphocytopoiesis and immunogenesis.

Within the thymus, Bone Marrow precursors of T-lymphocytes undergo antigen-independent differentiation into T-lymphocytes, various subtypes of which mediate cellular Immunity responses.

Externally, the thymus is enclosed by a Connective Tissue capsule that extends inward as septa, dividing the organ into lobules. A porous basal lamina lies between the capsule and the thymic parenchyma.

The Structural and functional unit of the thymus is the lobule. The framework of each lobule is formed by epithelio-reticulocytes—stellate Cells whose processes interconnect to form a reticular syncytium. Thus, epithelio-reticulocytes create a unique scaffold whose interstices are populated by T-lymphocytes and macrophages.

A lobule consists of the cortex and the medulla.

The cortex contains T-lymphocytes that densely pack the spaces between the epithelio-reticulocytes. Macrophages and their variant, dendritic cells, are encountered somewhat less frequently.

The subcapsular zone of the cortex contains larger lymphoid cells known as lymphoblasts. These are T-lymphocyte precursors that migrated from the Cytology/practical/86.html">Red bone marrow. They proliferate under METABOLISM/18.html">The Influence of thymosin, which is secreted by the stromal epithelio-reticulocytes and macrophages.

The epithelial reticular cells, macrophages, and dendritic Cells of the subcapsular zone are referred to as thymic "nurse cells" because they provide the necessary microenvironment for T-lymphocyte maturation. The cortical cells are separated from the bloodstream by the Blood-thymus barrier, which shields the differentiating cortical lymphocytes from an excess of Antigens.

The blood-thymus barrier is composed of the endothelial cells of blood capillaries with their basal lamina; perivascular spaces containing isolated lymphocytes, macrophages, and Extracellular matrix; and epithelio-reticulocytes along with their basal lamina.

The medulla of the thymic lobule is populated by small, medium, and large T-lymphocytes, T-lymphoblasts, epithelio-reticulocytes, and macrophages.

In histological preparations, the medulla appears lighter in color because it contains a lower density of lymphocytes. The lymphocytes in this zone constitute a recirculating pool of cells capable of entering and leaving the bloodstream via post-capillary venules and Lymphatic vessels.

A characteristic feature of the medulla is the presence of Hassall's corpuscles (thymic corpuscles). These are formed by concentric layers of stellate epithelio-reticulocytes, whose Cytoplasm contains large vacuoles, keratin granules, and coarse bundles of fibrils. The thymus synthesizes biologically active compounds such as thymosin, thymulin, and thymopoietin.

These substances promote the proliferation and maturation of T-lymphocytes, as well as the synthesis of certain compounds, such as Insulin-like growth factor, Calcitonin-like factor, and growth factor.

Slide 2. Section of the thymus gland (Fig. 29).

Low magnification. Examine the slide. Locate the capsule and the connective tissue septa extending from it into the interior of the organ, dividing the gland into lobules. The lobules consist of a cortex and a medulla. The delicate stroma (framework) of the lobule is formed by epithelial reticulocyte-like cells, known as epithelio-reticulocytes. These are epithelial cells whose contacting processes form a network. The bulk of the lobule is made up of T-lymphocytes, which are concentrated mainly in the cortex (appearing darker on the slide). The medulla is considerably lighter than the cortex because it contains fewer lymphocytes. In the medulla, layered epithelial cells of varying sizes (Hassall's corpuscles) are visible. They are formed by concentric layers of epithelio-reticulocytes.

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Fig. 29. Thymus gland. Hematoxylin and eosin staining. x 56:

1 — capsule; 2 — trabeculae (a — Blood Vessels); 3 — lobules of the gland (b — cortex; c — medulla; d — Hassall's corpuscle)

Label on the figure: 1) capsule of the thymus gland; 2) trabeculae: a) blood vessels; 3) lobules of the thymus gland: b) cortex; c) medulla; d) Hassall's corpuscle.

High magnification (Fig. 30). In the medulla, identify epithelial cells that differ from lymphocytes by their larger size, stellate shape, transparent cytoplasm, and light-staining Nucleus; sketch them.

Fig. 30. Medulla of the thymic lobule. Fragment of the previous preparation. Hematoxylin and eosin staining. x 600:

1 — lymphocyte; 2 — stellate Cell; 3 — Hassall's corpuscle; 4 — blood vessel

Label on the figure: 1) lymphocytes; 2) stellate cells; 3) Hassall's corpuscle; 4) blood vessel.