Thymus : brief and easy handout

Type of organ: lympho-epithelial organ

Location:  in the superior mediastinum.  

Parts: It has two lobes

Cells of thymus:

i) Developing T lymphocytes (which come from bone marrow lymphoid stem cells) 

 ii) epithelioreticular cells

Development:

    i) Lymphocyte: mesodermal in origin

    ii) Epithelium primordium : developed from the endoderm of 3rd and 4th pharyngeal pouches.

Thymus function:

Maturation and differentiation of T lymphocytes so they can distinguish between self antigen and non-self antigen.

Histological features of thymus :

a) Capsule is present which penetrate and divides the thymus into incomplete lobules.

b) Each lobulus is divided into outer cortex and inner medulla

c) Outer cortex of each lobules contain large number of developing T lymphocytes

    but contain no lymphatic nodule

d) Medulla  contain Hassall’s corpuscles which are formed by epithelio-reticular cells
 Thymic or Hassall’s corpuscles

•          Location: thymic medulla.

•          Formation: by epithelioreticular cells. The epithelioreticular cells are concentrically arranged and cytoplasm of the cells contain keratohyalin granules, lipid droplet and intermediate filaments

Blood thymic barrier

1.Lining endothelium of the capillary  wall

2.Basal lamina of endothelial cells

3.Macrophages in the surrounding  perivascular connective tissue

4. Basal lamina of epithelioreticular    cells

5.Epithelioreticular cells

Important of blood thymic barrier:

Prevent entering of antigen into the thymic cortex

Two important negative points regarding Thymus:

It contain

i)  No lymphatic nodules and

ii) No B lymphocytes

 Involution of the thymus:  After puberty much of the parenchyma of the thymus, in particular cortical lymphoid tissue, is replaced by adipose tissue.