Haematology of foetal sheep

Objective To describe the ontogeny of blood cells throughout foetal develop ment in sheep. Design A haematological study on blood and bone marrow from 42 sheep foetus es aged between 19 days gestation and full term. Procedure Virgin Merino ewes were mated and the developing foetuses removed surgically at different stages of gestation. Blood and bone marrow samples were collected, stained for cytological examination or processed for elect ron microscopy. Blood samples were also examined haematologically. Foetuses were incubated with H-3-thymidine and autoradiographed. Results During the first 4 weeks of development primitive erythroblast cons tituted the majority of the circulating blood cells. Definitive erythroid c ells, originating in the liver, first appeared in the blood at around 27 da ys gestation and entirely replaced the primitive erythroblasts by 50 days g estation. Leukocyte numbers, especially lymphocyte count, increased rapidly after 49 days gestation. Erythropoiesis predominated in the marrow of all foetuses older than 70 days. In bone marrow, myelopoiesis was the major act ivity and lymphopoiesis was not significant. Conclusions Red blood cell numbers and haemoglobin content progressively in creases during foetal development. Primitive erythroblasts are not the prec ursors of the definitive erythroblasts. There are no significant difference s in morphological features or maturation sequence between hepatic and bone marrow erythroblasts. Myelopoiesis is a major activity of bone marrow rath er than of foetal liver.