• It is a flattened bilobed organ lying behind the sternum.
• It is very large in infants. At puberty it starts to invalute and decrease in size.
Histological structure
The thymus gland consists of:-
1- Stroma.
2-Parenchyma
I- Stroma:
a- Capsule: thin loose C. T.
b- Trabeculae: Are thin and arise from the capsule dividing the gland into two lobes, and each lobe is divided into incomplete lobules of variable size and shapes.
II- Parenchyma:
Outer part (cortex).
Each thymus lobule is composed of
Central part (medulla).
A- The cortex:
• Is the outer dark part of the lobule (due to the high content of lymphocytes).
• It contains the following cells:
1- Lymphoblasts:
- Present in the outer part of the cortex.
- They have dark nuclei and are derived from the CFUs (Colony forming units) in the bone marrow.
2- T- lymphocytes:
- Present in the deeper part of the cortex
- Numerous and derived from the lymphoblasts.
- They migrate to the medulla and pass to the blood via the post-capillary venule.
3- Reticular epithelial cells:
- They are branched with a pale nucleus.
- Their processes are joined together to form a reticulum (so their name).
- The interspaces between the reticular cells are filled with proliferating lymphocytes.
- Functions:
• They are secretory cells (responsible for secretion of thymic hormones).
• Their processes form the supporting element of thymic lobule.
4- Few number of macrophages: (phagocytic cells).
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B- The medulla:
• It is the central paler pale part of the thymus lobule (due to low content of lymphocytes).
• The medullae of some adjacent lobules are continuous with each other.
• The medulla consists of the following:
1- T- lymphocytes:
- They are less in number than in the cortex.
- They have completed their development in the cortex and become pushed into the medulla.
- They enter the circulation through the post-capillary venules which found in the medulla. Then they go to the thymus dependent zone.
2- Reticular epithelial cells (described above).
3- Hassall’s corpuscles:
- It is a central hyaline acidophilic mass surrounded by concentric layers of reticular epithelial cells.
- The acidophilic mass is degenerated cells.
4- Few macrophages: (phagocytic cells).
Негізгі бет 1 lymphatic system: thymus
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