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Thời gian của khả năng sinh sản ngắn, từ thời điểm các trứng rụng cho đến khi nó không còn có thể được thụ tinh chỉ có khoảng 6-24 hours.As ngay như là một tinh trùng thâm nhập vào trứng, cơ thể cực thứ hai là ép đùn và trứng đã thụ tinh bắt đầu phân chia. | Fertilization and Implantation 427 indispensable way to fertility of the ovum or sperm or to their union as evidenced by modern techniques of in vitro fertilization that bypass it with no ill effects. The period of fertility is short from the time the ovum is shed until it can no longer be fertilized is only about 6 24 hours.As soon as a sperm penetrates the ovum the second polar body is extruded and the fertilized ovum begins to divide. By the time the fertilized egg enters the uterine cavity it has reached the blastocyst stage and consists of about 100 cells.Timing of the arrival of the blastocyst in the uterine cavity is determined by the balance between antagonistic effects of estrogen and progesterone on the contractility of the oviductal wall. Under the influence of estrogen circularly oriented smooth muscle of the isthmus is contracted and bars passage of the embryo to the uterus. As the corpus luteum organizes and increases its capacity to secrete progesterone P-adrenergic receptors gain ascendancy muscles of the isthmus relax and the embryonic mass is allowed to pass into the uterine cavity. Ovarian steroids can thus lock the ovum or embryo in the oviduct or cause its delivery prematurely into the uterine cavity. Implantation and the Formation of the Placenta The blastocyst floats freely in the uterine cavity for about a day before it implants normally on about the fifth day after ovulation. Experience with in vitro fertilization indicates that there is about a 3-day period of uterine receptivity in which implantation leads to full-term pregnancy. It should be recalled that this period of endometrial sensitivity coincides with the period of maximal progesterone output by the corpus luteum Figure 2 . In the late luteal phase of the menstrual cycle the outer layer of the endometrium differentiates to form the decidua. Decidualized stromal cells enlarge and transform from an elongated spindle shape to a rounded morphology and accumulate glycogen. .
