Maternal-embryonic relationships in the goodeid teleost,Xenoophorus captivus

Summary The trophotaeniae and abdominal epidermis of Xenoophorus captivus embryos were studied by light, scanning and transmission electron microscopy, freeze-fracture replication, and histochemical techniques for unspecific phosphatases. The trophotaenial epithelium is continuous with both the intestinal mucosa and the epidermis, and contains structural elements similar to both. The predominant component is a simple brush-border epithelium consisting of cuboid cells showing signs of endocytotic activity at their apical surfaces. These are the absorptive elements of the trophotaeniae, and phosphatase ultracytochemistry demonstrates the presence of alkaline phosphatase on the external leaflet of their exposed plasma membranes. Enormously dilated intercellular spaces and large gaps occur in this epithelial covering.Beneath this absorptive epithelium lies an incomplete layer of dense squamous cells that appear to be derived from the stratified epithelium covering trophotaenial areas free of brush border epithelium and the abdominal wall. The exposed cell surfaces of this component are modified to form an elaborate pattern of microplicae which can be seen by scanning EM where gaps appear in the overlying absorptive epithelium. The stratified epithelium of the abdominal wall is underlain with collagen fibrils and an intricate network of capillaries, and is considered to be a site of cutaneous respiration. This cutaneous gas-exchange pathway averages 2–4 m in thickness. Chloride cells are constituents of the stratified epithelium of the trophotaenial base and abdominal wall.The involvement of the endodermal component of the trophotaenial epithelium in the transfer of nutrients and possibly antibodies, and the role of the abdominal epidermis and ectodermal trophotaenial epithelium in gas exchange and osmoregulation, are discussed.