Jerez Adriana, Ramírez-Pinilla Martha Patricia
Laboratorio de Biología Reproductiva de Vertebrados, Escuela de Biología, Universidad Industrial de Santander, Bucaramanga, Colombia.
J Morphol. 2003 Nov;258(2):158-78. doi: 10.1002/jmor.10138.
Topological and histological analyses of Mabuya mabouya embryos at different developmental stages showed an extraembryonic membrane sequence as follows: a bilaminar omphalopleure and progressive mesodermal expansion around the whole yolk sac at gastrula stages; mesodermal split and formation of an exocoelom in the entire embryonic chamber at neurula stages; beginning of the expansion of the allantois into the exocoelom to form a chorioallantoic membrane at pharyngula stages; complete extension of the allantois into the exocoelom between limb-bud to preparturition stages. Thus, a placental sequence could be enumerated: bilaminar yolk sac placenta; chorioplacenta; allantoplacenta. All placentas are highly specialized for nutrient absorption from early developmental stages. The bistratified extraembryonic ectoderm possesses an external layer with cuboidal cells and a microvillar surface around the whole yolk sac, which absorbs uterine secretions during development of the bilaminar yolk sac placenta and chorioplacenta. During gastrulation, with mesodermal expansion a dorsal absorptive plaque forms above the embryo and several smaller absorptive plaques develop antimesometrially. Both structures are similar histologically and are active in histotrophic transfer from gastrula stages until the end of development. The dorsal absorptive plaque will constitute the placentome and paraplacentome during allantoplacental development. At late gastrula-early neurula stages some absorptive plaques form chorionic concavities or chorionic bags that are penetrated by a long uterine fold and seem to have a specialized histotrophic and/or metabolic role. The extraembryonic mesoderm does not ingress into the yolk sac and neither an isolated yolk mass nor a yolk cleft are formed. This derived pattern of development may be related to the drastic reduction of the egg size and obligatory placentotrophy from early developmental stages. Our results show new specialized placentotrophic structures and a novel arrangement of extraembryonic membrane morphogenesis for Squamata.
对不同发育阶段的绿鬣蜥胚胎进行的拓扑学和组织学分析显示，胚外膜序列如下：原肠胚阶段为双胚层卵黄囊脏壁和围绕整个卵黄囊的中胚层逐渐扩展；神经胚阶段为中胚层分裂并在整个胚胎腔形成外体腔；咽胚阶段尿囊开始扩展到外体腔形成绒毛尿囊膜；从肢芽期到分娩前期尿囊完全延伸到外体腔。因此，可以列举出胎盘序列：双胚层卵黄囊胎盘；绒毛胎盘；尿囊胎盘。所有胎盘从发育早期就高度特化以吸收营养。双分层胚外外胚层在整个卵黄囊周围具有一层立方体细胞外层和微绒毛表面，在双胚层卵黄囊胎盘和绒毛胎盘发育过程中吸收子宫分泌物。在原肠胚形成过程中，随着中胚层扩展，胚胎上方形成一个背侧吸收斑，反中膜侧发育出几个较小的吸收斑。这两种结构在组织学上相似，并且从原肠胚阶段到发育结束都活跃于组织营养传递。背侧吸收斑在尿囊胎盘发育过程中将构成胎盘叶和副胎盘叶。在原肠胚晚期 - 神经胚早期阶段，一些吸收斑形成绒毛膜凹陷或绒毛膜袋，被一条长的子宫皱襞穿透，似乎具有特殊的组织营养和/或代谢作用。胚外中胚层不侵入卵黄囊，也不形成孤立的卵黄块或卵黄裂。这种衍生的发育模式可能与卵大小的急剧减小以及从发育早期就必须的胎盘营养有关。我们的结果显示了有鳞目新的特化胎盘营养结构和胚外膜形态发生的新排列。
