Thymus

I. THYMIC CORTEX (Figure 10-1) consists of:

A. Immature T cells (also called thymocytes)

B. Thymic epitheliocytes

1. Thymic epitheliocytes form a cellular meshwork into which immature T cells are tightly packed.

2. They secrete thymosin, serum thymic factor, and thymopoietin, which transform immature T cells into mature T cells.

3. They secrete thymotaxin, which attracts progenitor T cells (stem cells) from the hone marrow to populate the thymus.

4. Thymic epitheliocytes are derived embryologically from endoderm.

5. They are joined by desmosomes and contain cytokeratin intermediate filaments.

6. They are sometimes called thymic nurse cells.

II. THYMIC MEDULLA consists of the following:

A. Mature T cells

B. Thymic epitheliocytes

C. Thymic (Hassall's) corpuscles, which are whorl-like structures composed of keratinized thymic epitheliocytes (see Figure 10-1)

III. MATURATION OF T CELLS (see Chapter 9 V E). As T cells mature, they express:

A. T-cell receptors (TcR) that are antigen-specific

B. Cluster of differentiation (CD) surface markers

1. Helper T cells are CD4+.

2. Cytotoxic T cells are CD8+.

3. Suppressor T cells are either CD4+ or CD8+.

IV. BLOOD-THYMUS BARRIER

A. The blood-thymus barrier is found only in the thymic cortex.

B. This barrier assures that immature T cells undergo maturation in an antigen-free environment.

Figure 10-1. (A) Thymic cortex shows a large number of densely packed immature T cells of various sizes. In addition , thymic epitheliocytes (arrows) are apparent. (B) Thymic medulla showing the whorl-like Hassall's corpuscles, which arc keratinized thymic epitheliocytes. (C) Thymoma is a tumor of thymic epitheliocytes. A huge proliferation of thymic epitheliocytes occurs (compare with normal thymus in A). Inset shows high magnification of thymic epitheliocytes. Note that this figure shows thymic epitheliocytes in three different states: normal (in A), keratinized (in B), and tumorous (in C). (C: Reprinted with permission from the East Carolina University, School of Medicine, Department of Pathology slide collection.)

Figure 10-1. (A) Thymic cortex shows a large number of densely packed immature T cells of various sizes. In addition , thymic epitheliocytes (arrows) are apparent. (B) Thymic medulla showing the whorl-like Hassall's corpuscles, which arc keratinized thymic epitheliocytes. (C) Thymoma is a tumor of thymic epitheliocytes. A huge proliferation of thymic epitheliocytes occurs (compare with normal thymus in A). Inset shows high magnification of thymic epitheliocytes. Note that this figure shows thymic epitheliocytes in three different states: normal (in A), keratinized (in B), and tumorous (in C). (C: Reprinted with permission from the East Carolina University, School of Medicine, Department of Pathology slide collection.)

C. It consists of tight junctions between nonfenestrated endothelial cells, basal lamina, and thymic epitheliocytes.

V. INVOLUTION OF THE THYMUS

A. At birth, the thymus is fully developed.

B. In early childhood, involution begins.

C. At puberty, involution greatly accelerates.

D. By early adulthood, the thymus is transformed primarily into adipose tissue with isolated areas of lymphatic tissue.

1. Despite the apparent involution, the rhymus remains a source of T cells throughout life.

2. Involution can be accelerated by stress, adrenocorticotrophic hormone (ACTH), or steroids.

VI. THYMECTOMY

A. Neonatal thymectomy severely impairs cell-mediated immunity and also somewhat diminishes humoral immunity because helper T-cell function is compromised. The lymph nodes and spleen are reduced in size because the thymic-dependent zone of the lymph nodes and periarterial lymphatic sheath of the spleen, respectively, do not become populated with T cells.

B. Adult thymectomy causes less severe impairment of cell-mediated immunity and humoral immunity because the lymph nodes and spleen are already well populated with long-lived T cells.

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