Fetal and perinatal ontogeny

Numerous natural killer (NK) cells appear in the endometrium of pregnant sow (day 9 of gestation), i.e. before attachment of the blastocyst (day 14). Pig conceptus is capable of antiviral activity (interferon a (IFNa) and IFNy) (day 14).

Hematopoiesis starts in the yolk sac (day 16) as in other mammals and lasts only for a short period

(day 24). The majority of cells are erythroid cells, stem cells and macrophage precursors. Erythropo-iesis continues (day 20) in the liver (rudiment day 17) where platelets are also observed.

Lymphoid cells appear in the liver after macrophages (day 25), natural IFNa-producing cells (day 26), and myelocytes (day 28). The CD2 molecule and Thy-1 mRNA are detected in embryonic liver (day 30). Lymphatic vessels appear (day 32). Erythropo-iesis is observed also (day 31) in the spleen (rudiment day 22); lymphocytes, however, occur later.

Thymic and cord blood cells are inducible to tumor necrosis factor a (TNFct) synthesis (day 34). The thymus (rudiment day 21 ) contains CD3 T cells (day 40).

Other sites of hematopoiesis (e.g. para-aortic area) cannot be excluded. Lymphocytes occur very early in the omentum (day 40). Few MAC320 lymphocytes appear early in the liver (day 40).

The sIgM+ B cells occur in the liver (day 44). The first serum immunoglobulins, specific antiflagellin antibody response, and CD4 h and CD8 thymocytes were found in the same developmental stage (day 44).

By 50 days of gestation, lymphocytes occur in bone marrow and gut, liver B cells secrete IgM, T cells are in the spleen and skin, thymus cortex and medulla are formed and lymphocyte mitogenic response is detectable.

By 55 days antiparvovirus antibodies can be detected, purified protein derivative (PPD) of Mycobacterium tuberculosis elicits a T cell response in vivo and slgM ' B cells are found in the spleen.

Hassal bodies appear (day 64), thymic B cells secreting IgM, IgG and IgA (only IgM in the spleen) arise (day 67), Peyer's patches are formed (day 77) and the first plasma cells appear in immunized fetuses (day 80).

Perinatally, lung alveolar and perivascular macrophages occur. Maternal immunoglobulins are transferred from the colostrum in the first 2 days after birth.

Macrophages gain functional maturity and germinal centers appear in the spleen at day 7. By day 10, a normal white blood cell picture with predominant lymphocytes is formed. By 2 weeks an influx of T cells into the lamina propria of the gut occurs, IgA and IgG+ B cells (mainly in the ileum) and new germinal centers appear (in lymph nodes and Pey-er's patches).

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