In a description of the earliest stages of the human blastocyst, the wall of the primitive blastodermic vesicle was characterized as consisting of a single layer of ectoderm. As early as 72 hours after ovum fertilization, the 58-cell blastula had differentiated into 5 embryo-producing cells and 53 cells destined to form trophoblasts (Hertig, 1962). Although trophoblasts have not been distinguished before blastocyst implantation, both cytotrophoblasts and syncy-tiotrophoblast are present in the earliest implanted blastocyst of the monkey. Indeed, evidence has been presented that chorionic gonadotropin (hCG) is secreted by cells of the human blastocyst at the time of implantation.
[...] Hamilton and Boyd (1966) give credit to Friedlander (1870) for the first description of the striking structural changes of the spiral arteries of the decidua basalis during human placentation. During implantation, the spiral arteries acquire a lining of cells within the endothelium that is derived from the invading cytotrophoblasts. During this vascular invasion, degenerative changes occur in the arterial wall, affecting all layers of these vessels. The most striking change involves the vascular smooth muscle, which becomes nonrecognizable. The cytotrophoblasts that invade the spiral arteries can pass several centimeters along the vessel lumen; indeed, Hamilton and Boyd observed cytotrophoblasts in myometrial portions of these vessels. [...]
[...] EMBRYONIC AND PLACENTAL DEVELOPMENT EARLY BLASTOCYSTS In a description of the earliest stages of the human blastocyst, the wall of the primitive blastodermic vesicle was characterized as consisting of a single layer of ectoderm. As early as 72 hours after ovum fertilization, the 58-cell blastula had differentiated into 5 embryo-producing cells and 53 cells destined to form trophoblasts (Hertig, 1962). Although trophoblasts have not been distinguished before blastocyst implantation, both cytotrophoblasts and syncy-tiotrophoblast are present in the earliest implanted blastocyst of the monkey. [...]
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