The foetal membranes and placenta of the African elephant ( Loxodonta Africana )

Abstract

The placenta of the African elephant sometimes extends completely around the equatorial zone of the chorionic sac, but it is more usually interrupted at one or more points on the circumference, in which case the villous areas are restricted to more or less elongated patches distributed meridionally. No chorionic villi were found over the polar areas of the sac. The chorion is distended, and completely separated from the amnion, by a voluminous quadrilocular allantois, the four sacs of which arise from a common antrum immediately below the point where the umbilical cord emerges from the amnion. These extend around the amnion, and fuse with it, until they meet over the back of the foetus. A yolk-sac was present in the youngest specimen examined (estimated gestation age 1 to 2 months, weight of foetus 2 g), but no trace of this structure could be found in another specimen in which the conceptus was still spherical (estimated gestation age 3 months, weight of foetus 10 g). The internal endodermal surface of the allantois bears a large number of protuberances, referred to as ‘allantoic pustules’. They are mainly concentrated in the vicinity of large allantoic vessels, but their function is obscure. The placental band comprises a central attachment area bordered by a distinct margin which contains extravasated blood and superficially resembles the marginal haematomata of the placenta of the dog. The extravasated blood, however, is not derived from vessels in the uterine wall, but from capillaries within the placental labyrinth. As seen with the light microscope, the placenta of the elephant is an endothelio-chorial or a vaso-chorial one, according to the respective classifications of Grosser and of Amoroso and Wislocki (see Amoroso 1955 a , b ). The trophoblast is almost entirely single-layered; it is cellular in the basal zones of the placenta but appears syncytial in the bulky distal part which constitutes the placental labyrinth. In the marginal regions of the band extravasation of maternal blood cells occurs, and the cytotrophoblast in this component of the placenta actively ingests some of these corpuscles. In the central portion of the placental band the major maternal vessels are invested by modified trophoblast which retains its cellular character. Beneath the labyrinth, in the central region of the placental band, an invasive zone of lobate villi progressively gives place to a zone where there is very active phagocytosis of the disorganized maternal decidua by the trophoblast. The latter is brought into close relation with the numerous foetal capillaries by the extreme attenuation of the foetal connective tissue. This zone is sharply demarcated from the overlying syncitiotrophoblastic labyrinth. Maternal blood enters and leaves the placenta by large vessels. The principal afferent vessels feed the labyrinth by arterial branches which pass more or less obliquely towards the foetal surface of the placenta. Small branches leave them at frequent intervals within the labyrinth, and enter the tortuous syncytial lamellae. At many points at the base of the placenta the efferent vessels become confluent with larger venous channels which return the blood to the uterine veins. In the marginal zone, even at our earliest stage, some blood escapes into small trophoblastic lacunae, where it becomes stagnant and undergoes phagocytosis (see above). By mid-pregnancy the marginal zones are clearly defined, and extravasation of blood also occurs within the junctional zone beneath the labyrinth. The essential features of the placenta and foetal membranes are already established in the earliest stages examined, and they undergo modifications of a relatively slight and secondary nature during the further course of gestation. The elephant, hyrax and manatee share a number of features in the arrangement of the foetal membranes and in the detailed structure of the placenta. These similarities, and accompanying differences, are briefly discussed.