Lymphoid organ

Organs dominated by lymphoid tissue

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Lymphoid organ ）Yes lymphoid tissue Main organ, realized in the body immunity , hence the name immune organ , including thymus, spleen, tonsil They are composed of lymphoid tissue, and their functions are lymph gland Similarly, they can produce lymphocytes. The thymus is located in the upper part of the chest, above the heart. The spleen is located in the left upper part of the abdominal cavity, which is the largest lymphoid organ of the human body. The tonsil is located in the root of the tongue And around the pharynx.^[1]

Chinese name: Lymphoid organ
Foreign name: lymphoid organ
Also called: immune organ

Composition: lymphoid tissue
Features: immunity
Classification: Central and peripheral lymphoid organs

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Introduction to Organs

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Lymphoid organ

According to the difference of genesis and function, it can be divided into central lymphoid organs and peripheral lymphoid organs. Mainly by lymphoid tissue Composition, including lymph gland , spleen and thymus 。

Central lymphoid organ

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Central lymphoid organ Embryogenesis It occurs earlier, such as thymus and bone marrow, whose occurrence and function are not affected by antigen stimulation. Due to the effects of hormones and microenvironment Before birth It is basically well developed. It is the proliferation and differentiation of hematopoietic stem cells into virgin T B lymphocytes And deliver lymphocytes to peripheral lymphoid organs to promote the development of peripheral lymphoid organs. T lymphocytes It is produced by the thymus, and B lymphocytes are from the upper Supraluminal bursa (Bursaof Fabricius), or bladder like organ (Bursa equivalent ）。 The supraluminal bursa is birds Unique, Mammals And no one. It is suggested that the cystic organs may be embryonic liver and bone marrow in humans. From the 7th week of human embryo, the liver has Hematopoietic stem cells The proliferation and differentiation of B lymphocytes. After 6 months, B lymphocytes gradually transferred from the liver to the bone marrow. Therefore, the liver and bone marrow have the same function as a cystic organ, but the liver is generally not called a central lymphoid organ.

Peripheral lymphoid organ

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Peripheral lymphoid organ Embryogenesis Occurs later, such as lymph gland , spleen tonsil 、 Cephalic kidney And intramucosal lymphoid tissue 。 Accepts and accommodates lymphocytes from central lymphoid organs. Under antigen stimulation. lymph cell proliferation Differentiation and participation immune response T of Effector cell Or plasma cells. T effector cells produce and release various Lymphokine （lymphokin）， plasma cells Secretory antibody 。

So, the peripheral lymphoid organs are Immunocompetent cell The site of settlement and proliferation is also an important part of immune response. Peripheral lymphoid organs are widely distributed in various important parts of the body, forming the second line of immune defense.

thymus

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The thymus is a central lymphoid organ. The size and structure of thymus changed significantly with age. The thymus develops fastest from embryonic stage to two years old (10-15g). From two years old to adolescence, the thymus continued to grow (30-40g), but the speed slowed down. After adolescence, the thymus degenerated and shrunk (about 10g), adipose tissue Increase. Although the adult thymus degenerates, it still remains Immune potential Required.

Microstructure of thymus 　

The thymus is a substantial organ bread yes connective tissue The model is connected with interlobular septa formed by connective tissue in the thymus. The interlobular septum divides the thymus into many incompletely separated lobules, whose diameter is about 1-2 mm. The lymphocytes around the lobule are dense and deeply stained, called cortex; Central Department Light staining, called medulla 。 The septa between the stem lobules are incomplete, and the medulla of adjacent lobules are interconnected.

(1) Cortex : Mainly by Thymic epithelial cell (Epithelial cell), dense lymphocytes and some macrophage form. There are two types of thymic epithelial cells:

① The ones distributed under the capsule and on the surface of interlobular septum are single-layer and flat epithelial cells , also known as subcapsular epithelial cells（ subcapsular epithlial cell）， Put the Microenvironment Isolation from the outside world;

② The rest epithelial cells They are all stars, called stellate epithelial cells, and are usually called superior epithelia Reticular cell （epithelial reticular cell）； nucleus Large, round, light colored, Cytoplasm It contains many Tension wire ； The processes of adjacent astrocytes Desmosome The pores are filled with lymphocytes and macrophages. In the thymus, there is a special cell with the characteristics of epithelial cells, called thymus Nurse cell (thymic nurse cell), with a diameter of about 30~50 μ m, is a large round or oval cell, and the cytoplasm contains several Thymocyte This kind of cells is generally difficult to distinguish from the epithelial cells under the capsule.

Lymphocytes in the thymus, also known as thymocytes, are derived from the division of stem cells entering the thymus. Lymphocytes in the superficial cortex near the capsule and the interlobular septum are large, immature and common Schizoid image ； The middle layer of cortex is medium sized lymphocytes; Lymphocytes in the deep cortex are small and mature, and degenerated lymphocytes are common. Medullary Hematopoietic stem cells After entering the thymus through blood flow, it is cultured and differentiated into T lymphocytes , but most of them (about 95%) were apoptotic macrophage Phagocytosis, only a small part of which mature into T lymphocytes and penetrate into the Posterior capillary venule And migrate to specific areas of peripheral lymphatic organs through blood flow.

（2） medulla （medulla） : There are many epithelial reticular cells, Thymocyte Less, so the staining is shallow, and the medulla is usually oval or irregular Thymic corpuscle (thymus corpuscle) or Hassal corpuscle, about 20~50 μ m in diameter, is surrounded by several layers of flat epithelial reticular cells in concentric circles. The peripheral cells of thymic corpuscles are immature, with clear nuclei and cytoplasm Acidophilic ； The nucleus of the cell in the center of the corpuscle disappeared and had degenerated and disintegrated. It is also common in corpuscles macrophage and Eosinophils 。 The function of thymic corpuscles is not clear.

Cephalic kidney

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some Circularia And a few bony fish Adult Remaining Pronephrine , called head kidney, its position is equivalent to Whole kidney Front of with immunity 。（ carp The kidney of the fish is close to the back of the chest and abdominal cavity and is dark red. The two kidneys are partially connected. The front end of each kidney is a head kidney.

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