Blackburn Daniel G, Vitt Laurie J
Department of Biology and Electron Microscopy Facility, Trinity College, Hartford, Connecticut 06106, USA.
J Morphol. 2002 Nov;254(2):121-31. doi: 10.1002/jmor.10005.
New World skinks of the genus Mabuya exhibit a unique form of viviparity that involves ovulation of tiny (1 mm) eggs and provision of virtually all of the nutrients for embryonic development by placental means. Studies of the Brazilian species M. heathi reveal that the chorioallantoic placenta is unlike those reported in any other squamate genus and exhibits striking specializations for maternal-fetal nutrient transfer. The uterine lining is intimately apposed to the chorioallantois, with no trace of an intervening shell membrane or of epithelial erosion; thus, the placenta is epitheliochorial. The uterus exhibits multicellular glands that secrete organic material into the uterine lumen. Opposite the openings of these glands, the chorion develops areolae, invaginated pits that are lined by absorptive, columnar epithelium. A single, mesometrial placentome develops, formed by radially oriented uterine folds that project into a deep invagination of the chorion. Uterine epithelium of the placentome appears to be syncytial and secretory and overlies a rich vascular supply. The apposed chorionic epithelium is absorptive in morphology and contains giant binucleated cells that bear microvilli. Several specializations of the placental membranes of M. heathi are found among eutherian mammals, signifying evolutionary convergence that extends to histological and cytological levels. The chorioallantoic placenta of M. heathi and its relatives warrants recognition as a new morphotype for reptiles, defined here as the "Type IV" placenta. This is the first new type of chorioallantoic placenta to be defined formally for reptiles in over half a century.
美洲石龙子属（Mabuya）的新大陆石龙子表现出一种独特的胎生形式，即排出微小（1毫米）的卵，并通过胎盘方式为胚胎发育提供几乎所有营养。对巴西物种赫氏石龙子（M. heathi）的研究表明，绒毛膜尿囊胎盘与其他任何有鳞目属所报道的胎盘不同，在母胎营养物质转运方面表现出显著的特化。子宫内膜与绒毛膜尿囊紧密贴合，没有中间壳膜或上皮侵蚀的痕迹；因此，该胎盘为上皮绒毛膜胎盘。子宫有分泌有机物质到子宫腔的多细胞腺体。在这些腺体开口的对面，绒毛膜形成乳突，即由吸收性柱状上皮衬里的内陷小窝。单个子宫系膜胎盘叶发育形成，由径向排列的子宫皱襞突入绒毛膜的深凹陷中形成。胎盘叶的子宫上皮似乎是合体且分泌性的，覆盖着丰富的血管供应。相对的绒毛膜上皮在形态上具有吸收性，含有带有微绒毛的巨大双核细胞。赫氏石龙子胎盘膜的几种特化在真兽类哺乳动物中也有发现，这表明进化趋同延伸到了组织学和细胞学水平。赫氏石龙子及其近缘种的绒毛膜尿囊胎盘值得被认定为爬行动物的一种新形态类型，在此定义为“IV型”胎盘。这是半个多世纪以来首次为爬行动物正式定义的新型绒毛膜尿囊胎盘。
