New world lizards of the genus Mabuya have the most specialized level of placentotrophy among reptiles known to date, and related to that, they have the most complex allantoplacenta characterized by a series of morphological specializations that converge with those known for eutherian mammals. One of these specializations is the placentome that is found in the embryonic pole of the incubation chamber. In the mature allantoplacenta, this structure is morphologically the most complex, which could support an important amount of nutrient exchange between mother and fetus. According to the relationship between the chorioallantois and the syncytial uterine epithelia, the placenta of Mabuya populations shows some interesting similarities to the synepitheliochorial type. Recently, cells of chorionic origin have been found invading the syncytial uterine epithelium, and in very close proximity with uterine blood vessels. In this study, we describe the relationship between these invasive chorionic cells, the uterine syncytium, and the subjacent blood vessels of several populations of this genus, by means of high resolution optical microscopy and transmission electron microscopy. Cell groups originating from the chorion, of variable size and shape, penetrate the uterine syncytial epithelium extending complex cytoplasmic projections that come in contact with uterine capillaries and form an extensive and complex double-membrane system that surrounds the capillary. The close relationship between the chorion and the maternal circulation suggests that the Mabuya placentome shows some characteristics of an endotheliochorial placenta. This finding constitutes so far the only documented example of an endotheliochorial placentation in Reptilia. Anat Rec, 290:1508Rec, 290: -1518Rec, 290: , 2007 2007 Wiley-Liss, Inc.

We studied the ultrastructural organization of the ovarian follicles in a placentotrophic Andean lizard of the genus Mabuya. The oocyte of the primary follicle is surrounded by a single layer of follicle cells. During the previtellogenic stages, these cells become stratified and differentiated in three cell types: small, intermediate, and large globoid, non pyriform cells. Fluid-filled spaces arise among follicular cells in late previtellogenic follicles and provide evidence of cell lysis. In vitellogenic follicles, the follicular cells constitute a monolayered granulosa with large lacunar spaces; the content of their cytoplasm is released to the perivitelline space where the zona pellucida is formed. The oolemma of younger oocytes presents incipient short projections; as the oocyte grows, these projections become organized in a microvillar surface. During vitellogenesis, cannaliculi develop from the base of the microvilli and internalize materials by endocytosis. In the juxtanuclear ooplasm of early previtellogenic follicles, the Balbiani's vitelline body is found as an aggregate of organelles and lipid droplets; this complex of organelles disperses in the ooplasm during oocyte growth. In late previtellogenesis, membranous organelles are especially abundant in the peripheral ooplasm, whereas abundant vesicles and granular material occur in the medullar ooplasm. The ooplasm of vitellogenic follicles shows a peripheral band constituted by abundant membranous organelles and numerous vesicular bodies, some of them with a small lipoprotein core. No organized yolk platelets, like in lecithotrophic reptiles, were observed. Toward the medullary ooplasm, electron-lucent vesicles become larger in size containing remains of cytoplasmic material in dissolution. The results of this study demonstrate structural similarities between the follicles of this species and other Squamata; however, the ooplasm of the mature oocyte of Mabuya is morphologically similar to the ooplasm of mature oocytes of marsupials, suggesting an interesting evolutionary convergence related to the evolution of placentotrophy and of microlecithal eggs.