Stage 5 embryos have a postfertilization age of 7 to 12 days and are implanted in the uterine mucosa to varying degrees. The characteristic feature is implantation. The conceptuses are previllous, i.e., do not yet show definitive chorionic villi. The decidual reaction begins in the endometrial stroma. Two distinct layers are evident in the trophoblast; 1) a thicker outer layer without cell boundaries is called the syncytiotrophoblast and 2) a thinner inner layer with cell boundaries is called the cytotrophoblast. The chorion and chorionic cavity are defined with the formation of extraembryonic mesoblast. The definitive amniotic cavity appears between the embryonic disc and the chorion. The diameter of the embryonic disc measures approximately 0.1 to 0.2 mm. The thickness of the trophoblastic shell 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
Stage 5a specimens have a postfertilization age of 7 to 8 days and are characterized by a trophoblast that is mostly still solid. The greatest diameter of the trophoblastic shell is less than 0.5 mm. The endometrial stroma is edematous. The blastocystic cavity is flattened usually because of 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. Stage 5a is represented here by specimens #8020 (7 days) (stage 5a-1) and #8155 (8 days) (stage 5a-2).
The younger specimen has a postfertilization age of 7 days. The conceptus shows early superficial implantation, having eroded the endometrial epithelium but barely penetrated the endometrial stroma. A portion of the conceptus is still exposed to the uterine cavity. A small amniotic cavity is visible and amniogenesis is underway. The epiblast of the embryonic disc is composed of polyhedral cells that have no precise pattern of arrangement. The hypoblast is composed of relatively small, darkly stained, vesiculated cells with less distinct boundaries than those of the epiblast.
The older specimen has a postfertilization age of 8 days. The conceptus shows later superficial implantation being almost embedded within the endometrium and nearly flush with the endometrial epithelium. The abembryonic pole is barely exposed to the uterine cavity. The amniotic cavity is prominent and amnioblasts are present. The embryonic disc is dorsally concave. The epiblast is composed of pseudostratified columnar eipithelium. The hypoblast is composed of a cap like mass of smaller polyhedral cells with large nuclei.
The trophoblastic shell is composed largely of syncytiotrophoblast that varies from a thin indifferent type at the abembryonic pole to a thick, irregularly convoluted part at the embryonic pole. The embryonic pole is composed almost entirely of syncytiotrophoblast. The syncytiotrophoblast shows no definitive evidence of lacunae but intracytoplasmic vacuoles are seen occasionally. There are a few extraembryonic endoblast cells scattered around the periphery of the blastocystic cavity but too few to form the exocoelomic membrane.
Stage 5b is represented by Carnegie embryo #8004 that has an estimated postfertilization age of 9 days. The conceptus is superficially implanted and imperfectly covered by endometrial epithelium. The distinguishing characteristic at this substage is the presence of numerous irregular, slit like lacunae within the cytoplasm of the syncytiotrophoblast. Most of the 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 containing large, clear, epitheloid, vesiculated cells that have an oval or polyhedral shape. 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 membrane is considered to be the wall of the primary umbilical vesicle and the surrounding meshwork is thought to be extraembryonic endoblast rather than mesoblast. The amniotic cavity is smaller than the primary umbilical vesicle (yolk sac) and is almost enclosed by amniogenic cells.
The bilaminar embryonic disc is slightly oval and dorsally concave. The epiblast is composed of pseudostratified, columnar epithelium in which the cytoplasm is beginning to become vacuolated ventrally. The hypoblast is a single layer of cuboidal or polyhedral cells.
Stage 5c is represented by Carnegie embryo #7700 that has an estimated postfertilization age of 12 days. The conceptus is superficially implanted and imperfectly covered by endometrial epithelium. The distinguishing characteristic at this substage is the presence of large, irregular, intercommunicating lacunar spaces that contain enough blood to form a discontinuous red circle about one mm in diameter that is visible on the endometrial surface.
The bilaminar embryonic disc is composed of two, dorsally convex, cellular discs of approximately equal diameters; a thick epiblast and a thin hypoblast. The opposing surfaces of the two layers are flat without intervening mesoblast or mesoderm. The outline of the epiblast is oval in shape when viewed from above, revealing for the first time the embryonic disc axis. The epiblast is composed of large, vertically arranged cells whose nuclei are pseudostratified. Clear, oval, round or cylindrical vacuoles are mainly in the ventral part of the cells. The hypoblast is composed of medium-sized cuboidal or polyhedral cells that also contain cytoplasmic vacuoles. Extraembryonic mesoblasts are concentrated at the caudal end of the embryonic disc.
The conceptus lies just beneath an imperfectly eipthelialized surface. A fibrinous, leucocytic, hemorrhagic coagulum is present at the implantation site. The trophoblast just deep to the endometrial epithelium (abembryonic pole) is the least differentiated, consisting of only a single layer of cuboidal cells. Elsewhere the wall of the conceptus consists of an outer syncytiotrophoblast and an inner cytotrophoblast. The syncytiotrophoblast is composed of dark, eosinophilic cells with vacuolated cytoplasm containing dense nuclei of irregular size and shape. The cytotrophoblast is a single layer of large cuboidal or polyhedral cells of varying size and shape. Extraembryonic mesoblast and angioblast are forming deep to it and line the primordial chorionic cavity. Several villi appear as clumps of cytotrophoblast that project into the syncytiotrophoblast and have at their base a core of mesoblast and angioblast. The inner surface of the mesoblastic layer is composed of flattened, elongated mesothelial, spindle or stellate cells that form the exocoelomic membrane which together with the hypoblast forms the outer boundary of the primary umbilical vesicle (yolk sac). Above the dorsal surface of the embryonic disc is the nearly complete, dome-shaped, membranous amnion. Amnioblasts are delaminating from the adjacent cytotrophoblast.
All of the stage 5 specimens have been described by Hertig and Rock (1941, 1945 and 1949).