Examination of early embryos of the Senegal galago (Galago senegalensis senegalensis) show that the centrally implanted bilaminar blastocyst is temporarily attached to the surface of the endometrium which has been denuded of its covering epithelium. This attachment is mediated by an area of giant polyploid trophoblast cells which are situated at its abembryonic pole. At no stage do these cells penetrate into the depths of the stroma which does not undergo any decidual reaction. At the height of its development in the blastocyst of 4.0 to 5.0 mm diameter the giant cell trophoblast forms one-third of the blastocyst surface. The remainder of the trophoblast shell consists of small cells which do not exhibit any invasive ability. By the time the primitive streak appears nearly all the giant cell trophoblast has degenerated and lost contact with the surface of the endometrium. As this takes place the endometrium becomes re-covered by uterine epithelium. From this stage onwards the trophoblastic shell consists entirely of small, non-invasive cells lying in contact with the uterine epithelium. Simultaneously chorionic invaginations appear over the mouth of each uterine gland. Shortly after the 18 somite stage the chorion is vascularized by the allantoic mesoderm to form the definitive placenta which is diffuse and epithelio-chorial. Re-examination of the twin blastocysts of Loris lydekkerius (Loris 5) from the J. P. Hill Embryological Collection indicates that the abembryonically situated “absorptive area” is the remnant of a similar temporary trophoblastic attachment. Circumstantial evidence suggests that the early blastocyst of Nycticebus coucnng is also temporarily attached to the uterine wall. Tarsius, Tupaiu and some insectivores (Sorex, Crociduvn and Talpa) also have primary attachment of their bilaminar blastocyst by invasive trophoblast situated on the abembryonic pole. In Tarsius and Tupniu the definitive haemochorial allantoic placenta is formed at the primary attachment site. In the others the primary attachment site is temporary and the definitive allantoic placenta is formed at the embryonic pole after amnion formation by folding. Thus it appears that invasive trophoblast occurs on the mammalian blastocyst at different times and places and that some trophoblast never exhibits invasive ability. Apart from the initial invasion by the temporary attaching trophoblast, the blastocysts of Loriscids and Lemuroids retain a covering of non-invasive trophoblast hence theirepithelio-chorial placenta is regarded as being of primary, and not secondary origin. Despite the very marked difference in the structure of the definitive allantoic placentas of Lorisoids, Lemuroids, Tarsius and Insectivora all stem from essentially the same type of blastocyst. Hence it is concluded that structural differences in the definitive allantoic placenta are not necessarily of phylogenetic significance.
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