Placental development Directory UMM :Data Elmu:jurnal:A:Animal Reproduction Science:Vol60-61.Issue1-4.Jul2000:

2. Placental development

Following fertilization, the embryo goes through stages of division, modeling and Ž . cellular compaction morula stage . Cells that make up the early embryo are called ‘‘blastomeres’’. The ‘‘blastocyst stage’’ occurs when the embryo develops a fluid-filled central cavity, the ‘‘blastocoel’’, that is surrounded by a single layer of cells, the Ž . ‘‘trophectoderm’’ Fig. 1 . These cells, forming this outer wall, have assumed special- ized functions and are known as trophoblast cells. They will remain as the outermost Fig. 2. Bovine Fetuses — Early Placentation. These bovine fetuses are contained in a clear membrane the ‘‘amnion’’ forming a round fluid-filled structure Ž . surrounding them. The chorioallantois has been torn to better demonstrate the amniotic sacs. These three fetuses are approximately 30, 40 and 50 days of gestation. Placental vessels are evident extending from the fetus along the umbilical cord to the chorioallantois, which initially attaches to the endometrium at about 33 days of gestation. Cotyledons are not yet visible. layer of cells essentially forming a single cell layer covering the outside of the placenta of all domestic animal species. This outer layer of trophoblast cells, together with the somaticrparietal mesoderm, form the ‘‘chorion’’. With time, more fluid is produced within the blastocoel resulting in an expansion of the size of the embryo, which is then called the ‘‘expanded blastocyst’’. The fluid-filled Ž . blastocoel will become the cavity of the yolk sac Fig. 1 . During the blastocyst stage, another specialized population of cells, those that will become the embryo proper, replicate and appear as a flat cluster of cells at one pole of Fig. 3. Bovine fetus and placenta with cotyledons evident. The amnion has expanded and discrete cotyledons are now readily visible. Specialized areas called ‘‘cotyle- dons’’ begin to develop on the surface of the chorioallantois at about 30 days gestation and are readily visible in this photograph of an approximately 70-day fetus. Cotyledons on that part of chorioallantois filling the uterine horn are larger than those lining the placental membranes in the other horn. Cotyledons closer to the tip of the uterine horn will also be smaller. Total numbers vary greatly from 70 to 120. Ž . the hollow blastocyst. This cluster of cells is the embryonic disc inner cell mass that Ž . will become the embryo. Another cell layer endoderm grows from the inner cell mass Ž to line the cavity of the yolk sac, and a third layer of cells, the mesoderm meso s . middle , will then extend from the embyonic disc and form a layer between the trophectoderm and the endoderm. The expanded blastocyst maintains its round structure, with essentially three concen- Ž . tric layers of cells trophectoderm, mesoderm and endoderm surrounding the early yolk sac cavity with the mass of cells that will differentiate into the embryo located along one pole. Following this, the embryo will elongate. The amnion will form as folds of the trophectoblast cells, together with somatic mesoderm, rise up from around the embryo Ž . proper to ultimately join and create a fluid-filled space around the embryo see Fig. 2 . As the embryo begins to form into a fetus, but before the ventral abdominal wall is closed, an outpocketing of the hindgut extends from the fetus into the loose tissues of the splanchnic mesoderm. This sac-like stucture is the ‘‘allantois’’ and with the splanchnic mesoderm, it carries with it the vessels that will ultimately vascularize the Ž . chorion and amnion Fig. 1 . Only one additional major remodeling event remains. The expanding allantois becomes directly apposed to the chorion . The allantois and the chorion fuse to form the chorioallantois, which, with the amnion, are the ‘‘extra-embryonic fetal membranes’’. Depending on the species, the chorioallantois will assume different shapes and micro- scopic forms as it apposes and, in some cases, invades the endometrium of the gravid uterus. Fig. 4. Placentome from a term pregnancy with umbilical arteries injected with colored latex. Note the intricate vascular pattern of branches of the umbilical arteries and veins as they penetrate or leave the placentome. D.H. Schlafer et al. r Animal Reproduction Science 60 – 61 2000 145 – 160 150 Fig. 5. Drawing of two bovine placentome and fetal membranes attached to the endometrium. The relationship between the chorioallantois forming cotyledonary villi and the endometrium is demonstrated in this drawing. Branching cotyledonary villi penetrate Ž Ž . the crypts of the endometrium at specialized sites called caruncles. Together, they form the placentome. Reprinted from Davies et al. 2000 by permission of the . publisher W-B Saunders .

3. Gross anatomy