36.1 Physiology of pregnancy
36.2 Functions of the placenta
36.3 Amniotic fluid and its physiological role
36.4 Physiological changes in female body functions in pregnancy
36.5 Physiology of childbirth
36.6 Physiology of lactogenesis and lactation Control questions
Situational tasks
36.1. Physiology of pregnancy
Pregnancy lasts about 280 days (40 weeks, 9 months) from the beginning of the last period; it is divided into three trimesters.
Fertilization and implantation.
After ovulation the oocyte leaves the ovary and enters the fallopian tube, being in the metaphase of the second division of meiosis. It has a transparent membrane and follicular cells attached to it (radiate crown). The ovum remains viable in the female reproductive tract for 6-24 hours, while sperm cells are viable for 24-48 hours.
Spermatozoa easily pass through the cervical canal into the uterine cavity in the middle of the cycle, when, under the influence of high concentration of estradiol and low progesterone levels, the cervix becomes softer, the cervical canal expands, and the secret of its plug becomes more watery. In order for the spermatozoon to pass through the membrane of the oocyte and fertilize it, it must carry out a process of capacitation and acro-somal reaction.
In the process of capacitation, the spermatozoon acquires the ability to pass through the oocyte membranes: it loses the glycoprotein layer from the cell membrane, the wavelike movements of the flagellum are replaced by whip-like movements, and the concentration of cAMP and Са2+ in spermatozoa, necessary for implementation of the acrosome reaction, increases. Capacitation takes several hours.
Acrosomal reaction begins with interaction of the sperm cell with the Zp3 receptor of the transparent membrane of the oocyte, which further activates its ad-enylate cyclase system. The acrosome (apical body) of the sperm cell is enlarged and fuses with its cell membrane. At the same time, the protein Zp2 of the acro-some membrane binds to the transparent membrane of the oocyte, thereby fixing the spermatozoon to it. The acrosome enzymes (trypsin, hyaluronidase, etc.) lyse a site of the oocyte transparent membrane within 15 minutes. As a result of acrosomal reaction, the sperm nucleus enters the cytoplasm of the oocyte. At this time, the oocyte cell carries out the second division of meiosis; as a result, four daughter ovocytes with a haploid set of chromosomes that have unique genetic material different from the mother oocyte are produced from one maternal oocyte. DNA synthesis begins in the haploid nuclei of the sperm and oocyte cells, their nuclear membranes dissolve. The parental chromosomes are intermixed, a diploid set of chromosomes is formed resulting in a zygote, a single-cell embryo.