Chapter 3 Conception and placental development
In contrast to other body cells, the oocyte has only 23 chromosomes. At some stage of its growth it undergoes a meiotic division of its nucleus and an unequal division of its cytoplasm, to become a secondary oocyte. The smaller cell is expelled into the perivitelline space and is termed a polar body (Fig. 3.1B). The oocyte and its polar body both contain 23 chromosomes.
One of the 20 follicles (occasionally two or more, particularly if the ovaries are hyperstimulated in an in-vitro fertilization programme) outstrips the others, developing a large fluid-filled antrum and migrating to bulge through the thickened surface of the ovary (Fig. 3.2). With the release of the luteinizing hormone (LH) surge by the pituitary at midcycle the follicle bursts, expelling the ovum, which is gathered into the Fallopian tube by the fimbriae that project from its proximal end. The ovum, surrounded by the perivitelline, is contained in a condensed opaque substance 5–10 μm thick called the zona pellucida. Adherent to the zona pellucida are theca granulosa cells derived from the mature follicle.
Once the ovum has been expelled the follicle collapses and turns yellow, forming the corpus luteum (Fig. 3.2). The ovum is now ready to be fertilized should it be reached by a sperm.
Of the 60–100 million sperm ejaculated into the vagina at the time of ovulation, several million will negotiate the helical channels in the cervical mucus to reach the uterine cavity. Several hundred will pass through the narrow entrance to the fallopian tubes, and a few will survive to reach the ovum in the fimbrial end of the Fallopian tube. One sperm may penetrate the zona pellucida of the ovum, its head entering the substance of the ovum. When this occurs a chemical reaction prevents the entry of any other sperm. At the same time the oocyte undergoes another division of its chromosomes and a second polar body is formed (see Fig. 3.1D).
Once inside the cytoplasm of the ovum the sperm’s nuclear membrane dissolves, leaving a naked male pronucleus. The ovum, having divided to produce a second polar body, also loses its nuclear membrane. The two naked nuclei approach each other and fuse (see Fig. 3.1E). Fertilization and conception have occurred.
Within a few hours of fertilization, the fused nuclei divide to form two cells (see Fig. 3.2E). Once this has occurred, further cell division proceeds rapidly until, within 3–4 days, a solid mass of cells (the morula) has formed (see Fig. 3.2G, H).
IMPLANTATION
Implantation occurs 8–10 days after ovulation in most healthy pregnancies.
The morula is rapidly propelled along the Fallopian tube to enter the uterine cavity. During its passage, fluid passes through canaliculae in the zona pellucida to create a central fluid-filled cavity in the morula, forming a blastocyst (see Fig. 3.2I). On reaching the uterine cavity the zona pellucida becomes distended and thin. It soon disappears, leaving the surface cells of the blastocyst in contact with the endometrial stroma. About 50% of blastocysts adhere to the endometrium. The surface trophoblastic cells of the adhering blastocyst differentiate into an inner cellular layer, the cytotrophoblast, and an outer syncytiotrophoblast.
Knobs of trophoblast rapidly form and invade the endometrial stroma in a controlled manner (Fig. 3.3). By the 10th day after fertilization the knobs of trophoblastic tissue have developed a mesodermal core and have pushed deep into the endometrial stroma (Fig. 3.4). The stromal cells react to the invasion by becoming polyhedral in shape and filled with glycogen and lipid, converting into a decidua, which supplies the energy needed by the invading trophoblast. At the same time a number of deep cells at one pole of the blastocyst differentiate to become an inner cell mass, from which the embryo will develop.
By the 9th to 10th day after fertilization the inner cell mass has differentiated into an ectodermal layer, a mesodermal layer and an endodermal layer, in which a small fluid-filled cavity, the amniotic sac, has formed (Fig. 3.5). The further development of the amniotic sac, in which the fetus will float relatively weightless until it is born, is shown in Figure 3.6.
The inner cell mass projects into the original blastocystic cavity, the walls of which are formed from cytotrophoblast, and the cavity is filled with mesoderm (Fig. 3.7). Quickly, the surface layer of ectoderm divides to surround a fluid-filled cavity in the mesoderm – the yolk sac (see Fig. 3.6A–C).
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