In addition to water and small inorganic ions, macromolecules traverse the uterine epithelium in viviparous skinks to be absorbed by the developing fetus. In some species of lizards with complex placenta, the paracellular pathway across the uterine epithelium becomes tighter and more highly regulated as gestation progresses, suggesting that the transcellular pathway may be an alternative route for molecules to travel across the epithelium. In this study, we identified an extensive formation of a lysosomal system in the apical region of uterine epithelial cells in the highly secretory omphaloplacental region of the skink placenta in two species from the Pseudemoia genus. We suggest that this lysosomal system assists apocrine secretion by digesting large macromolecules into smaller particles, allowing more effective transport across the plasma membrane of uterine epithelial cells. We also demonstrate alkaline phosphatase (AP) activity along the apical plasma membrane of uterine epithelial cells in the omphaloplacental region of skink uterus, an enzyme usually associated with active transport in secretory cells. Apocrine secretion, an extensive lysosomal system and AP activity, offer strong evidence that macromolecules are transported across uterine epithelium of the omphaloplacenta. Our study is the first to provide histochemical evidence of macromolecular transport across this region of the placenta in two species of skinks from the genus with the most complex placenta described in Australia.
(c) 2009 Wiley-Liss, Inc.