The evolution and involution of Peyer's patches in fetal and postnatal sheep.

The Peyer's patches (PP) of sheep have a number of important anatomical features and functional characteristics which are similar to tissues that have been classified as primary lymphoid organs. The prenatal maturation of PP occurs in the absence of any antigenic stimulus as immunogenic molecules are not normally encountered by the sheep fetus. Primordial PP were first detected in the small intestine of fetal sheep at about 60-days gestation; lymphoid follicles were present by 75-days gestation and vigorous lymphopoiesis was occurring in these follicles by 100 days. From 120-days gestation until birth, at about 150 days, the PP follicles were histologically mature and they had the greatest density of proliferating lymphoid cells found anywhere in the body. The total number of PP and their constituent follicles had developed before birth when there were 25-40 discrete PP in the jejunum and proximal ileum and one single continuous PP in the terminal ileum. There was no evidence of any change in the rate of growth of the PP follicles at birth which could be related to the advent of the first antigens in the gut. The total weight of PP tissue was greater than any other single lymphoid tissue by about 6 weeks after birth weighing around 120 g or about 1.2% of the body weight; about 50-60 g of the PP tissue was calculated to be lymphoid tissue. At this time the ileocecal PP (IPP) extended 2.5 m along the terminal ileum and accounted for about 90% of the total mass of PP. From about 12 weeks after birth the IPP began to involute and only a few PP follicles remained in this region of the intestine by 18 months of age. Follicles in PP in other parts of the small intestine remained and continued to produce lymphocytes throughout the life of the animal. PP contain a number of anatomically and functionally distinct lymphoid compartments that could play different roles in the body's immune defense. Explicit in most theories on the function of PP is the notion that antigenic stimulation is the cause of the lymphopoiesis in the follicles; our results do not support this view. Instead they suggest that the follicles in the PP of sheep may play a role similar to that played by the bursa of Fabricius in birds.