Prenatal hematopoiesis and blood characteristics of the cat

Summary The blood, yolk sac, liver, spleen, and bone marrow from 65 timed embryos of 22 cats were examined using light microscopic means in six different age categories. Nucleated primitive erythroblasts derived from the yolk sac are mature by the 19th day and represent 98% of the circulating blood cells, some of them found circulating even at birth. Definitive yolk sac erythropoiesis comprises a span of up to 30 days. On the 36th day, hematopoietic contribution drops to 15%. Neutrophils and the first thrombocytes are present on the 17th day, eosinophils and lymphocytes by the 25th day. Hepatic hematopoiesis most likely begins with definitive erythropoiesis on abouth the 20th day; granulopoiesis occurs in the liver on the 25th day. Blood forming tissue in the liver amounts to 28% which drops to 4% at birth. Splenic hematopoiesis begins on about the 36th day but contributes little to the blood. Bone marrow activity begins at mid-term and supplies about 50% of the blood cells on the 45th day. Hematocrit values increase from 22% on the 36th day to 47% at birth, thus exceeding the normal value of adult cats. The red blood cell number increases from 0.8 million/mm³ on the 25th day to 3.8 million on the 45th day and 6.3 million at birth. The total leukocyte count (880 on the 45th day and 6.480 at birth) must be calculated from the differential count of nucleated cells. Primitive erythroblasts represent the most common nucleated cells on the 25th day; on the 36th and 45th day, definitive erythroblasts predominate, but are outnumbered by leukocytes at birth. On the 36th and 45th day, lymphocytes are the predominating cell type in the white blood picture. The contribution of the hematopoietic organs to the feline prenatal blood formation is shown graphically.