ABSTRACT
Species that exhibit histotrophy show considerable variation in the quality and quantity of uterine secretions. There is a general trend, however, from mucous to lipid histotrophy that is correlated with increased dry weight gain. There is a fine, indistinct line between lecithotrophic yolk sac viviparity and matrotrophic histotrophy. Structural modifications of the uterus, however, occur in all viviparous species and it is likely that all species, regardless of reproductive mode, have some varying degree of histotrophy. Reliable dry weight determinations between uterine eggs and term fetuses are lacking for most Chondrichthyes. Very few studies (Ranzi 1934; Guallart and Vicent 2001) have appropriately dried samples and subsequently incinerated them so that only water, organic matter and minerals can be accounted for. Few relatively isolated biochemical analyses of uterine secretions exist and ultrastructural analyses of the uterus are fragmentary. These data are critical to defining reproductive modes. Definitive lipid histotrophy with a large dry weight gain is characteristic of the stingrays in which the uterus is festooned with secretory villi termed trophonemata. Uterine secretions or "uterine milk" in stingrays is particularly rich in lipid. Other uterine specializations associated with viviparity are: 1) formation of abundantly vascularized uterine folds, flaps or villi, most of which are non-secretory, 2) uterine compartments, 3) thinning and reduction of epithelial layers separating maternal and fetal vessels thereby reducing gas diffusion distance and 4) presence of dilated intercellular spaces which are involved in water and mineral transfer.
