Thymus-Explained

Table of Contents

1. Introduction:
2. Location;
3. Structure of the thymus
   1. Cortex:
   2. Medulla
   3. Secretion and Functions
4. Clinical

Introduction:

The thymus is a small (5 mm x 5mm) gland, a central or primary lymphoid organ. It has two lobes that are connected by a fibrous isthmus.

Location;

It is situated in the superior mediastinum and extends upward to the root of the neck and downward to the anterior mediastinum.

     Location of the thymus

Structure of the thymus

A capsule covers the thymus and sends septa into the organ and divides it into lobules. Each lobule consists of an outer cortex and an inner medulla.

Cortex:

1. A densely populated area where actively multiplying lymphocytes are present, with immature T-lymphocytes or thymocytes. The cells undergo differentiation.

2. Epithelial reticular cells (TECs- Types I, II, III) are present that give structural support.

3. Macrophages phagocytose apoptotic T-cells.

Medulla

It is pale in color and contains

1. A few more mature T-lymphocytes

2. Specialized epithelial cells (Types IV, V, VI)

3. Hassall’s corpuscles or thymic corpuscles are characteristic of the thymus. A central calcified core is surrounded by concentrically arranged keratinized epithelial reticular cells.

Secretion and Functions

Epithelial reticular cells produce endocrine and paracrine effects vital for T-cell differentiation.

Thymosin and Thymulin promote T-cell proliferation and differentiation.

Thymopoietin regulates T-cell differentiation.

Thymic Humoral Factor enhances cell-mediated immunity.

Hassall’s corpuscles secrete TLSP to induce regulatory T-cells for central tolerance.

Clinical

Thymoma or thymic carcinoma usually causes pressure syndrome.

Ectopic thymic tissue.

Associated with Myasthenia Gravis and DiGeorge syndrome.