Morphological and functional changes to the uterus of lizards with different placental complexities

We compared the uterine component of the placentae of each placental region of two species of lizards, Eulamprus tympanum and Pseudemoia entrecasteauxii, to identify potential routes and mechanisms of nutrient transport. We focused on the uterine epithelium because nutrients must pass though it to reach the embryo. At this point, we have only been able to study two species in detail, but each has a placenta of different complexity and quantitatively different placentotrophy. The uterine epithelium of the chorioallantoic placenta has a different morphology from the omphaloplacenta, and the chorioallantoic placenta of P. entrecasteauxii, the most placentotrophic of the two species, is further differentiated in placentomal and paraplacentomal regions. The chorioallantoic region of E. tympanum, and the paraplacentomal region of P. entrecasteauxii have morphologies indicative of gas exchange epithelia. The blood-uterine lumen distance in P. entrecasteauxii, however, is about one tenth that in E. tympanum, suggesting a greater diffusive capacity for gases in P. entrecasteauxii. The omphaloplacenta of both species has a similar structure, with hypertrophied cells, electron-dense granules in the apical cytoplasm and vesicles budding off into the uterine lumen. This result is particularly interesting because E. tympanum exhibits little net nutrient transfer to the embryo during pregnancy. Nutrient provision across the omphaloplacenta in both species occurs by histotrophy. The placentome of P. entrecasteauxii also has a morphology suggestive of nutrient provision, but by some non-histotrophic mechanism and may be very important in the evolution of substantial placentotrophy. © 2006 by The Herpetologists' League, Inc.