Stingray placental analogues: structure of trophonemata in Rhinoptera bonasus.

The cownose ray, Rhinoptera bonasus, displays a non-placental form of viviparity since direct maternal-embryonic connections are lacking. Early stage embryos depend on yolk reserves for growth to 215 mm disc width; growth to term, 405 mm disc width, is effected by ingestion of uterine histotrophe. During late gestation, the maternal uterine epithelium possesses 2-3 cm long spatulate, villiform appendages, termed trophonemata. These secrete histotrophe, which is a viscous, nutrient fluid. Scanning electron microscopy of the trophonematal surface reveals branching ridges, each of which is supplied by a capillary. The secretory unit is composed of 8-10 cells joined by extensive junctional complexes. Characteristically, secretory cells have a rough endoplasmic reticulum whose irregular cisternae are grossly distended and filled with low density flocculent material. Uncoated vesicles are given off by an extensive juxtanuclear Golgi complex. Coated vesicles are also present, but are not directly associated with the Golgi complex. Electron dense granules, larger lysosome-like vesicles, and multivesicular bodies are in the vicinity of the Golgi.