Histomorphological development of lymphoid organs in chicken: II. Spleen and Caecum

Histomorphological observations were made on the spleen and caecum of 130 chicks aged day 1 to day 60 at 5 day interval. The splenic capsule was thin and fibrous on the day 1. The thickness gradually increased up to 30 days of age and remained unchanged thereafter. It was significantly thickened at the point of transaction with blood vessel. Connective tissue fibers i.e. collagen, reticular and elastic, in the decreasing order of their abundance were associated with only the splenic trabecular artery. As the trabecular artery branched in the parenchyma into central arteries, penicilliform arteries (sheathed arterioles) and terminal arteriole, the collagen fiber content gradually reduced and was absent from the wall of terminal arterioles. On the day of hatching the white pulp appeared as small irregularly ovoid or polyhedral masses carrying mostly reticular cells and few lymphocytes with heterochromatic nuclei and scanty cytoplasm. The hemapoietic cells were seen to congregate in the form of diffused foci that were scattered in between the primitive venous channels and lymphoid aggregations. A constant reorganization of the red and white pulp was evident up to 10^th day of development. The concentration of lymphoid tissue was highest at the open end of caecum followed by its blind end and least at the body. The lymphoid tissue appeared as caecal tonsil at open end of caecum about 6–12 mm away from the ileo-caecal junction. It was diffuse lymphoid tissue at day 1 which was distributed either in the lamina propria or in the sub-mucosa. It included mostly small lymphocytes, reticular cells, macrophages, plasma cells and isolated RBCs. By 25^th day, distinct lymphatic follicles appeared both at deeper and superficial locations with a higher frequency at the basal zone of caecal wall.