Mode of uterine milk secretion in the white shark

Examination of the uterus of a dead female white shark (Carcharodon carcharias), which contained the earliest known white shark embryos, revealed that the uterine wall produces lipid-rich secretion (histotroph or "uterine milk") for embryonic nutrition. Uterine tissue was processed for light and electron microscopy, and immunohistochemical techniques to identify its secretory mechanism. Our results indicate that the white shark uterus secretes lipids via holocrine secretion. This type of secretion is characterized by the release of large lipid droplets accumulated in the epithelial cells into the uterine lumen through cell disintegration. The secretory epithelium of the uterus is stratified, and new surface epithelial cells are continuously supplied from deeper epithelial layers to replace the dead secretory cells at the surface. This vertical replacement possibly facilitates the active renewal of the surface epithelium, which is necessary for maintaining holocrine secretory mechanisms. These secretory mechanisms are different from those of myliobatiform stingrays, another elasmobranch taxon that exhibits lipid histotrophy. This may reflect the different origins of lipid histotrophy between these taxa.