ABSTRACT
After repeated mitotic divisions, the zygote, composed of the fused male and female pronuclei, transforms into a 32-cell ball of blastomeres or a morula (from Latin word morum, mulberry) by day 4 post-fertilization. The morula then begins to take on fluid, blastomeric segregation and compaction occur, and a fluid-filled cavity develops within the morula, changing it to a blastocyst. The blastomeres of the blastocyst form an outer shell of cells, now called trophoblast (from the Greek, trophe, meaning nutrition), and a localized, inner cell mass, the embryoblast. The side of the blastocyst with the inner cell mass is called the embryonic pole. The blastocyst emerges from its covering of zona pellucida, and is thereby enabled to attach tightly to the endometrium on about day 6. The site of attachment is at the embryonic pole, and, upon implantation, the trophoblast cells at the embryonic pole rapidly proliferate and differentiate into an outer, leading layer of syncytiotrophoblast and an inner, proliferating mass of cytotrophoblasts. These broad solid trophoblast columns invade the endometrium, which, due to progesterone from the corpus luteum, has become decidualized, and additional cytotrophoblast differentiation into an intermediate type occurs. Invasion (implantation) enables the conceptus to derive nourishment from the endometrium. Before implantation is completed, the embryoblast differentiates into a bilaminar embryo, composed of epiblast and hypoblast. On about day 6-7, epiblasts nearest to the site of implantation (i.e. the dorsal epiblast) differentiate into amnioblasts. The amnioblasts proliferate, become decohesive, and fluid collects between them; the amnioblast is split and forms a small space, the amniotic cavity, by about the end of day 7. Thus, amnioblasts cover the epiblast, line the newly formed amniotic cavity, and separate the embryo from the trophoblast. Simultaneously, the trophoblasts invade the decidual interstitium and its blood vessels (capillaries and spiral arterial vessels); this vascular invasion results in blood extravasation and the formation of decidual blood lakes. At about day 9, the invading trophoblast also develops internal sites of cellular decohesion that progress to form lacunae; these become filled by maternal blood of the lakes. These lacunae then progressively enlarge and coalesce to form a network of blood-filled channels, the early intervillous spaces. These, the lacunar-lake formations, represent the beginning of the uteroplacental circulation as the maternal blood drains back into the maternal circulation via maternal veins. In addition, during the second week of development, the bilaminar embryonic disk exhibits an extraembryonic extension at its lateral aspects, the extraembryonic mesenchyme. Also, the invading fingers of syncytiotrophoblast have formed a radiating but spherical shell around the embryo and the amnion. The invasive syncytiotrophoblast projections gain a central, conical component of cytotrophoblast between days 11 and 13 and become the primary stem villi. The primary villi, which lie
within blood-filled lacunae, in turn become infiltrated by protrusions of the extraembryonic mesenchyme, which has come to line the shell of the radiating trophoblast villous projections, and become secondary villi. The innermost core of extraembryonic mesenchyme within each secondary villus then gives rise to villous blood vessels, changing them to tertiary villi. By the end of the third week, the vessels of the tertiary villi become connected through channels in the extraembryonic chorionic plate
and root of a connecting stalk to those vessels of the intmembryonic mesenchyme of the embryo proper. Thus, the extraembryonic mesenchyme develops into the vascular chorion plate of the placenta and the vascular cores of the chorionic villous tree. Due to the folding of the embryo during the 4th week, the chorion forms a sac whose projections vascularize the villous cytotrophoblast and its syncytiotrophoblast covering. The embryo and its amniotic sac thereby lie suspended within this chorionic sac via the connecting stalk of vessels; this connecting stalk develops into the umbilical cord and contains the allantois (a ventral, tubular extension of the developing cloaca in the embryo), the yolk sac (the ventral extension of the primitive endodermal canal) and its tiny vascular supply, and two umbilical arteries and two veins. Later, the right umbilical vein disappears and the umbilical cord begins to lengthen. Up to about 8 weeks, chorionic villi cover the entire chorionic sac, but with growth of the sac, there is compression atrophy of the villi along the decidua capsularis. This compressed and atrophic chorion is called the chorion leve, or smooth chorion, and constitutes the free membranes. The villi along the decidua basalis rapidly proliferate, forming the chorionic plate and villous chorion (chorion frondosum), which constitute the placental disk. The amniotic sac enlarges faster than the chorionic sac, resulting in fusion of the amnion with the chorion leve, by about 12 weeks of gestation. The chorioamnion, in turn, fuses with the decidua capsularis (Figure 1). By about 18-20 weeks of gestation, progressive enlargement of the fetal sac results in obliteration of the uterine cavity as the decidua capsularis fuses with the decidua parietalis of the opposite uterine wall. At 20 weeks of gestation, the dome of the uterus is typically palpable at the level of the maternal umbilicus; thereafter, uterine size, as measured by fundal height above the umbilicus, increases by about 1 cm/gestational week.
