Menkhorst Ellen, Nation Angela, Cui Shuliang, Selwood Lynne
Department of Zoology, University of Melbourne, Victoria, Australia.
J Exp Zool B Mol Dev Evol. 2009 Sep 15;312(6):625-38. doi: 10.1002/jez.b.21235.
Two characters distinguish oogenesis and early development in marsupials and monotremes: (1) the shell coat that persists from the zygote to somite stages in marsupials or until hatching in monotremes; and (2) the numerous, apparently almost empty vesicles that appear in primary oocytes, increase during oogenesis in marsupials and monotremes before being shed into the cleavage cavity and are preferentially distributed to the trophoblast lineage in marsupials, but comprise the latebra in monotremes. Analysis of these unusual characters used Southern analysis of genomic DNA dot blots and histology and electron microscopy. The evidence suggests that the marsupial shell coat protein, CP4, was probably characteristic of the egg of the mammalian ancestor. Further, the vesicles, present in marsupials during oogensis and cleavage and in eutherian mammals during blastocyst formation are the residual elements of white yolk present in the larger yolky eggs of monotemes and sauropsids. By comparison with the function of the vesicle components in marsupials, it is suggested that one role for the white yolk in monotremes and the sauropsids is to provide extracellular matrix (ECM), especially hyaluronan containing stabilizing proteins, for epithelial construction. Thus, as oviparity was replaced by viviparity, egg size was reduced, the germinal cytoplasm was retained, and yellow yolk was markedly reduced or lost in marsupials and eutherians. The white yolk was retained in monotremes and marsupials where blastocyst epithelial construction requires ECM support, and its appearance is heterochronously shifted to after compaction, when blastocyst formation and expansion occurs, in eutherian mammals.
（1）有袋类动物中从合子到体节阶段持续存在的卵壳，或单孔类动物中直到孵化时一直存在的卵壳；（2）在初级卵母细胞中出现的大量、看似几乎空的囊泡，在有袋类动物和单孔类动物的卵子发生过程中数量增加，然后被排入卵裂腔，在有袋类动物中优先分布到滋养层谱系，但在单孔类动物中构成卵黄心。对这些异常特征的分析采用了基因组DNA斑点杂交的Southern分析、组织学以及电子显微镜技术。证据表明，有袋类动物的卵壳蛋白CP4可能是哺乳动物祖先卵子的特征。此外，在有袋类动物卵子发生和卵裂期间以及有胎盘类哺乳动物囊胚形成期间出现的囊泡，是单孔类动物和蜥形纲动物较大的多黄卵中存在的白色卵黄的残余成分。通过与有袋类动物中囊泡成分的功能进行比较，有人提出单孔类动物和蜥形纲动物中白色卵黄的一个作用是为上皮构建提供细胞外基质（ECM），尤其是含有稳定蛋白的透明质酸。因此，随着卵生被胎生取代，有袋类动物和有胎盘类动物的卵大小减小，生殖细胞质得以保留，黄色卵黄明显减少或消失。白色卵黄在单孔类动物和有袋类动物中得以保留，在这些动物中囊胚上皮构建需要ECM支持，而在有胎盘类哺乳动物中，其出现时间异时性地转移到致密化之后，即囊胚形成和扩张发生的时候。
