Mitotic rate maturation in the Peyer's patches of fetal sheep and in the bursa of Fabricius of the chick embryo.

The metaphase arrest technique was used to quantitate the mitotic rate in sheep Peyer's patches (PP) and in the chicken bursa of Fabricius. It was shown that the mitotic rate in these tissues was the same whether or not the animal had been exposed to extrinsic antigen. The mitotic rate was calculated from the rate of accumulation of metaphase cells in tissue sections taken up to 4 h after i.v. vincristine. The follicular B cells in PP of fetal sheep at 3-5 days before birth, and in the bursas of chick embryos 2-3 days before hatching, none of which had experienced extrinsic antigen, entered mitosis at a rate of 5.2%/h and 7.5%/h, respectively. A similar analysis was done on animals at approximately 6 weeks of age, at which stage the PP and bursa had received chronic exposure to antigen from the gut. The mitotic rates were not significantly different from those in fetuses, i.e., 5.4%/h for PP and 6.0%/h for the bursa. In the follicles of both young lambs and juvenile chickens, the mitotic rate was 2-3 times greater in the outer, or cortical zone, than in the inner, or medullary zone. In a similar study of the thymus, no significant differences were found between the mitotic rates in fetal sheep and young lambs. The mitotic rate for the thymus was 15-20-fold less than for the PP. It is proposed that sheep PP, the chicken bursa and the thymus have in common the important characteristic of establishing a maximal rate of lymphocyte division prior to being exposed to antigen.