Embryonic erythropoiesis in vertebrates is characterized by development of pluripotent hematopoietic stem cells from embryonic mesoderm and formation of blood islands in the yolk sac. The production of nucleated erythroid precursors in the yolk sac (primitive erythropoiesis) continues until “seeding” of the newly forming liver by circulating erythroid cells (around Day 10 of gestation in mice, Week 6 in humans). Thereafter, the liver is the major site for definitive erythropoiesis (production of mature non-nu-cleated red blood cells) until the spleen and bone marrow become major hematopoietic sites in neonatal and postnatal animals. The transition from primitive to definitive erythropoiesis occurs on approximately Day 20 of gestation in swine after the formation of the hepatic anlage on Day 18. ventolin inhalers
Litter size in swine is controlled by several factors, including ovulation rate, uterine capacity, and fetal survivability. Despite years of research into these mechanisms, increase in litter size in the domestic pig has been slow (0.037 pigs/litter/yr from 1966-1996). Previous studies attempting to increase litter size in pigs by superovulation, genetic selection for ovulation rate, or embryo transfer have resulted in dramatically increased fetal numbers by Day 30 of gestation, but litter size at term was increased by only approximately 1.0 pig because of fetal loss later in gestation. These studies and others employing experimental models for uterine crowding indicate that “extra” fetuses are lost beginning between Day 25 and Day 50 of gestation. Hallmarks of fetal development in swine at this time include organogenesis and sexual differentiation. Failure of sexual differentiation is unlikely to be lethal, and few organ systems other than circulatory and excretory systems are likely to be functioning on Day 25 of gestation. However, fetal erythropoiesis is being initiated at this time in the liver.