ABSTRACT
Shortly after the blastocyst enters the uterus, implantation is achieved by attachment of the trophoblast to the endometrium, usually before the end of the second week of gestation in domestic mammals. In large domestic mammals, the functional placenta is formed from separable trophoblastic and maternal tissues, each with distinct, albeit intimately associated, vasculature. After initial attachment, the closely apposed chorionic and endometrial epithelia progressively fold and interdigitate to form microvilli. Simultaneously, rapid development of blood vessels on both sides of the newly formed placenta allows completion of the transition from histotrophic to hemotrophic nutrition of the implanted embryo.[4]
Postembryonic Growth
After implantation, the placenta undergoes a period of explosive mitotic growth characterized by rapid synthesis and accumulation of nucleic acids and protein in trophoblastic and uterine tissues. In sheep, this period begins at around 40 days of gestation and ceases abruptly by midgestation at approximately 75 days, resulting in 50-fold and 30-fold increases in wet and dry placental mass, respectively (Fig. 1).[6] Cessation of absolute placental growth is preceded by sharp peaks in rates of DNA and protein synthesis at 50-55 days of gestation, associated with a maximal rate of dry matter accretion (Fig. 1). In the cow, hyperplasia of the cotyledonary placenta continues into late gestation, although at a
Om ega-
Poultry
diminishing rate, whereas in the pig, macroscopic growth of the diffuse placenta is complete by about day 60 or slightly after midpregnancy.[7]
Notwithstanding some species variation in pattern and timing, in all domestic mammals, the phase of major proliferative growth of the placenta occurs in early to midpregnancy, and precedes rapid growth of the fetus in late gestation. This is significant because the trajectory of early placental growth can determine ultimate size and func-
tional capacity of the placenta and fetal growth during late gestation.
