Haemopoietic development in the foetal lamb from 85-135 days measured by (3H)-thymidine incorporation in vitro.

Abstract

Cell suspensions from the bone marrow (BM), spleen, thymus, liver and lymph nodes of 14 foetal lambs from 85-135 days gestation were incubated in vitro with 3H-thymidine (TdR). Cells from the long bones, the humerus, tibia and femur, yielded similar results, viz., the total leucocyte population of the bone increased from a mean of 8.8 X 10(6) at 85-90 days (N = 4) to a mean of 487 X 10(6) at 130-135 days (N = 3); the rate of incorporation of TdR varied from 22,460-45,960 c.p.m./h/10(6) cells with no clear trend related to foetal age; this rate is consistent with active mitosis occurring in the expanding BM cell population; bone weight and cell number/bone increased in parallel, and the total mitotic capacity of the bone showed a similar pattern. In the liver, spleen and thymus the incorporation of TdR per 10(6) cells decreased during the latter part of gestation. Despite growth of the liver (from 18 g to 71 g), the number of white blood cells (WBC) extracted from this tissue was lower at 135 days than at 85 days, suggesting that the haemopoietic capacity of the liver declined during this period. The marked increase in total WBC numbers in the thymus suggests this tissue may be supplying circulating lymphocytes to the growing foetus.