An Atlas of Opossum Organogenesis
The primary objective of this atlas is to gather under one cover a series of photographic illustrations and line drawings that summarize major events during organogenesis in the opossum (Didelphis virginiana). The opossum is of interest because of its short gestation period (about 12.5 days) and the fact that it is born in a near embryonic state. Because of the abbreviated period of intrauterine development, the opossum is an accessible biomedical model with which to study the major events that occur during organ development without the complications of doing intrauterine manipulations necessary if other mammalian species are used. The developing opossum can simply be examined and manipulated within the marsupium (pouch) during which time the majority of organogenesis takes place. The first nine days of gestation are concerned primarily with the establishment of the three germ layers. During this period of development, opossum blastocysts float freely within uterine secretions, each prevented from physically interacting with uterine tissues by a surrounding shell membrane. Initial organogenesis commences during the last three days of the 12.5 day gestation period and continues well into the postnatal period. It is during the last three days of gestation, when the yolk sac placenta is intimately associated with the uterine mucosa that development proceeds at an astonishing rate to form functional structures that will allow for the survival of the newborn of this species. Those structures essential for survival at and immediately after birth develop precociously but follow a typical mammalian pattern of development. Such structures are generally modified in some way to allow for the survival of the young following the abbreviated gestation period. Following their migration to the pouch, the lateral angles of the newborn opossum mouth seal around the teat due to a proliferation of cells within the epitrichium. As a result the mouth is sealed shut with the exception of a narrow orifice at the tip of snout that permits the entrance of the nipple. That portion of the nipple within the oral cavity swells thereby permanently anchoring the pouch young opossum to the same teat for about the next nine weeks of postnatal life.