Viviparity and the maternal-embryonic relationship in the coelacanth Latimeria chalumnae

Synopsis Embryos of Latimeria chalumnae develop in well-vascularized compartments in the uterine region of the right oviduct. Compartments conform to the shape of their embryos and yolksacs; they represent a stable, gestation-induced oviductal modification. Late-term pups possess large, flaccid, vascular yolksacs almost devoid of yolk. The sac is in close contact with, but does not adhere to, the lumenal uterine surface. A massive vascular plexus occurs in the wall of the compartment at the site of contact with the yolksac; together they constitute a non-adherent, transposable placenta. The exterior surface of the yolksac is bounded by an attenuated, single-layered, squamous epithelium that surrounds an intercommunicating bed of cortical sinuses. The cortex of the sac is composed mostly of connective tissue stroma. The inner surface is bounded by a layer of yolk-digesting merocytes. Residual yolk occurs as yolk platelets that include yolk crystals. The interior surface of the sac is invested by an uniquely specialized vitelline circulation; no connection seems to exist between the interior of the yolksac and gut. The uterine wall consists of: (1) a lumenal surface composed of an anastomosing network of capillaries with a layer of attenuated, very thin, squamous epithelium, (2) a well-vascularized connective tissue stroma, (3) alternating transverse and longitudinal layers of smooth muscle, also well-vascularized, and (4) an external epithelial layer. Comparison of egg dry weight (184 g) with the estimated dry weights of a late-term pup (171 to 239 g) and a neonate (200 to 280 g) reveals a weight change of – 7 to + 30% and + 9 to + 52%, respectively. This is indicative of matrotrophy. In one female specimen, 19 remarkably large ovulated eggs were found and in another about 30 somewhat smaller ovarian ones. These are many more than ever could be accommodated in the uterine space. During the early and middle phases of development, embryos must be lecithotrophic, using their yolk reserves, with oophagy of fragmented supernumerary eggs as the most probable source of additional nutrients. The well-developed embryonic gut contains brown, amorphous yolk-like material. The limited amount of metachromatic secretory product of the uterine glands can play little or no role in embryonic nutrition.