Stage 5 embryos are characterised by various degrees of implantation into the uterine mucosa. Two distinct layers are evident in the trophoblast; 1) a thicker outer layer without cell boundaries, called the syncytiotrophoblast and 2) a thinner inner layer with cell boundaries called the cytotrophoblast. The chorion and chorionic cavity are defined with the formation of extraembryonic mesoblast but the conceptuses are previllous, i.e., they do not yet show any definitive chorionic villi. The definitive amniotic cavity appears between the embryonic disc and the chorion. Stage 5 embryos have a postfertilization age of 7 to 12 days, the diameter of the embryonic disc measures approximately 0.1 to 0.2 mm and the greatest diameter of the chorion varies from 0.3 to 1 mm.
Stage 5 is divided into three substages based on the condition of the trophoblast and its relationship to the maternal vasculature.
Stage 5a specimens have a postfertilization age of 7 to 8 days and are characterized by a trophoblast that is still mostly solid. The greatest diameter of the trophoblastic shell is less than 0.5 mm. The endometrial stroma is edematous. The blastocystic cavity is usually flattened because of the collapse of the conceptus during implantation. Endoblast formation begins along the inner side of the trophoblast. The embryonic disc is approximately 0.1 mm in diameter and is composed of two layers, a thick layer called the epiblast and a thin layer called the hypoblast. The amniotic cavity is apparent for the first time and is formed by the curved epiblast of the embryonic disc.
The distinguishing characteristic of Stage 5b specimens is the presence of numerous irregular, slit like lacunae within the cytoplasm of the syncytiotrophoblast. Most of these lacunae communicate with each other and with the endometrial sinusoids but they contain relatively little maternal blood. The endometrial stroma shows an early decidual reaction and future villi begin as cytotrophoblastic clumps that project into the syncytiotrophoblast. There are few extraembryonic mesoblasts lining the inner surface of the trophoblastic shell. The umbilical vesicle (yolk sac) appears for the first time and becomes limited by a layer named the exocoelomic membrane. The bilaminar embryonic disc is slightly oval with an epiblast composed of pseudostratified, columnar epithelium and a hypoblast that is a single layer of cuboidal or polyhedral cells.
The distinguishing characteristic of Stage 5c embryos is the presence of large, irregular, intercommunicating lacunar spaces that contain enough blood to form a discontinuous red circle that is visible on the endometrial surface. The bilaminar embryonic disc is oval in shape when viewed from above. Extraembryonic mesoblasts are concentrated at the caudal end of the embryonic disc.