With each ovarian cycle, a number of primary follicles begin to grow, but usually only one reaches full maturity, and only one oocyte is discharged at ovulation. At ovulation, the oocyte is in metaphase of the second meiotic division and is surrounded by the zona pellucida and some granulosa cells (Fig. 2.4). Sweeping action of tubal fimbriae carries the oocyte into the uterine tube.
Before spermatozoa can fertilize the oocyte, they must undergo (a) capac-itation, during which time a glycoprotein coat and seminal plasma proteins are removed from the spermatozoon head, and (b) the acrosome reaction, during which acrosin and trypsin-like substances are released to penetrate the zona pellucida. During fertilization, the spermatozoon must penetrate (a) the corona radiata, (b) the zona pellucida, and (c) the oocyte cell membrane (Fig. 2.5). As soon as the spermatocyte has entered the oocyte, (a) the oocyte finishes its second meiotic division and forms the female pronucleus; (b) the zona pellucida becomes impenetrable to other spermatozoa; and (c) the head of the sperm separates from the tail, swells, and forms the male pronucleus (Figs. 2.6 and 2.7). After both pronuclei have replicated their DNA, paternal and maternal chromosomes intermingle, split longitudinally, and go through a mitotic division, giving rise to the two-cell stage. The results of fertilization are (a) restoration of the diploid number of chromosomes, (b) determination of chromosomal sex, and (c) initiation of cleavage.
Cleavage is a series of mitotic divisions that results in an increase in cells, blastomeres, which become smaller with each division. After three divisions, blastomeres undergo compaction to become a tightly grouped ball of cells with inner and outer layers. Compacted blastomeres divide to form a 16-cell morula. As the morula enters the uterus on the third or fourth day after fertilization, a cavity begins to appear, and the blastocyst forms. The inner cell mass, which is formed at the time of compaction and will develop into the embryo proper, is at one pole of the blastocyst. The outer cell mass, which surrounds the inner cells and the blastocyst cavity, will form the trophoblast.
The uterus at the time of implantation is in the secretory phase, and the blastocyst implants in the endometrium along the anterior or posterior wall. If fertilization does not occur, then the menstrual phase begins and the spongy and compact endometrial layers are shed. The basal layer remains to regenerate the other layers during the next cycle.
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