lecture-3-physiology of female reproduction

Dr. HANAN AL-TAEE,Ph.D, IVF SPECIALIST.
IF THE OVA IS FERTILIZED:
Fertilization:
Is the fusion of male and female gametes and formation of the zygote.
First step is capacitation of the sperm which is the process by which the sperm acquire the ability to fertilize the ovum in female genital tract.


Figure demonstrates the steps of fertilization.
Second step is the Acrosome Reaction:
1. Hyaluronidase : dissolves the matrix of cumulus-oophorous. 2. A trypsin- like enzyme known as the acrosomal proteinase (acrosin) acts on the zona pellucida. The oocyte will complete meiosis II and second pollar body will be extruded.
Third step is the fusion of the sperm with the ovum and formation of the pronuclei. Thereafter a discharge of cortical granules leading to deposition of the granules contents in the perivitelline-space of the ovum. The secreted materials diffuse rapidly into the zona pellucida and provoke a change in its physiological and biochemical properties .The setting of a reduction in the membrane potential of the ovum may be responsible for prevention of polyspermy & polyploidy.
Implantation:
It takes 6–7 days for the embryo (after subsequent divisions) to travel down to the uterus. By then it is a blastocyst. Implantation involves an interactive process between the uterus and the embryo, with both responding mutually to one another. The process of implantation, usually on the dorsal wall of the uterus. Implantation may be separated into a series of developmental phases, starts with apposition, adhesion, penetration and invasion. Early embryo is capable of producing enzymes that degrade the basement membrane and, with prostaglandin- induced local edema; trophoblastic cells enter the endometrial stroma. The trophoblast and the decidua produce a large variety of grwth factors, cytokines and paracrine factors. Of them, are platelet- derived growth factor (PDGF), fibroblast growth factor (FGF), insulin-like growth factor (IGF) and transforming growth factor (TGF). The trophoblast and decidua also synthesize receptors for these and other implantation- promoting factors. The trophoblast actually digests its way into the endometrium, which then covers it up.
Gestation:
Gestation period refers to the pregnancy period. The average gestation period is about 280 days or 40 weeks from the date of last menstrual period (LMP). If the menstrual cycle is normal 28 day cycle, the fertilization of ovum by the sperm occurs on 14th day after LMP. Thus the actual duration of human pregnancy is 280 – 14 = 266 days. During gestation, the pregnancy is maintained by:
a) First by the corpus luteum.
b) Then by the blastocyst, which maintains the corpus luteum (hCG)
c) Then by the placenta.

Placentation:
The chorion of the trophoblast and the surrounding endometrium react to each other to form a placenta.
Placenta is a temporary membranous vascular organ that develops in females during pregnancy. It is expelled after childbirth. Placenta forms a link between the fetus and mother. It is the physical attachment and physiological connection between fetus and mother. Placenta is implanted in the wall of the uterus. It consists of two parts namely the fetal part and the mother’s part. It is connected to the fetus by umbilical cord, which contains blood vessels and connective tissue. The delivery of fetus is followed by the expulsion of placenta. After expulsion of the placenta, the umbilical cord is cut. The site of attachment of placenta in the center of anterior abdomen of fetus is called navel or umbilicus.
? Functions of placenta:
1. Nutritive function:
Nutritive substances, electrolytes and hormones necessary for the development of fetus diffuse from mother’s blood into fetal blood through placenta.
2. Excretory function:
Metabolic end products and other waste products from the fetal body are excreted into the mother’s blood through placenta.
3. Respiratory function:
Fetal lungs are nonfunctioning and placenta forms the respiratory organ for fetus. Oxygen necessary for fetus is received by diffusion from the maternal blood and carbon dioxide from fetal blood diffuses into the mother’s blood through placenta. Exchange of respiratory gases between fetal blood and maternal blood occurs mainly because of pressure gradient. Partial pressure of oxygen in the maternal blood is 50 mm Hg. In fetal blood, the partial pressure of oxygen is 30 mm Hg. This pressure gradient of 20 mm Hg causes the diffusion of oxygen into the fetal blood. This pressure gradient is very low, compared to the gradient existing between partial pressure of oxygen in arterial blood and alveoli in adults. Still, an adequate quantity of oxygen is available for fetus. This is because of two reasons:
1. The hemoglobin in fetal blood (Hb-F) has 20 times more affinity for oxygen than the adult hemoglobin. 2. The concentration of hemoglobin is about 50%more in fetal blood than in adult blood.
4. Endocrine function:
Hormones secreted by placenta are:
a.Human chorionic gonadotropin.
b. Estrogen.
c. Progesterone.
d. Human chorionic somatomammotropin.
e. Relaxin.
a. Human Chorionic Gonadotropin: (hCG):
IS a glycoprotein with chemical structure similar to that of LH. It comprised of two non-identical polypeptide chains (alpha and ?), with almost exactly the same properties as LH. hCG is secreted by growing chorion of the developing embryo and later by the placenta, it forms the basis of pregnancy tests, urine tests usually detect hCG 14 days after fertilization.
Actions of hCG:
i. On corpus luteum: hCG is responsible for the preservation and the secretory activity of corpus luteum. Progesterone and estrogen secreted by corpus luteum are essential for the maintenance of pregnancy. Deficiency or absence of hCG during the first 2 months of pregnancy leads to termination of pregnancy (abortion), because of involution of corpus luteum.
ii.Immune tolerance for the embryo.
iii.On fetal testes: Action of hCG on fetal testes is similar to that of LH in adults. It stimulates the interstitial cells of Leydig and causes secretion of testosterone. The testosterone is necessary for the development of sex organs in male fetus.
iv. it helps Invasion of the trophoblast into the endometrium.
v. it has an association to the severity of morning sickness of pregnancy.
b. Estrogen:
Placental estrogen is similar to ovarian estrogen in structure and function.
Actions of placental estrogen:
i. On uterus: Causes enlargement of the uterus so that, the growing fetus can be accommodated.
ii. On breasts: Responsible for the enlargement of the breasts and growth of the duct system in the breasts. Estrogen suppresses prolactin and thus inhibits milk production during pregnancy.
iii. On external genitalia: Causes enlargement of the female external genitalia.
iv. On pelvis: Relaxes pelvic ligaments. It facilitates the passage of the fetus through the birth canal at the time of labor.
d. Progesterone:
Placental progesterone is similar to ovarian progesterone in structure and function.
Actions of placental progesterone
i. On endometrium of uterus: Accelerates the proliferation and development of decidual cells in the endometrium of uterus. The decidual cells are responsible for the supply of nutrition to the embryo in the early stage.
ii. On the uterus: Inhibits the contraction of muscles in the pregnant uterus. It is an important function of progesterone as it prevents expulsion of fetus during pregnancy.
iii. On breasts: Causes enlargement of breasts and growth of duct system of the breasts.
Progesterone is responsible for further development and preparation of mammary glands for lactation.
d. Human Chorionic Somatomammotropin: human placental lactogen.
Human chorionic somatomammotropin (HCS) is a protein hormone secreted from placenta. It is often called placental lactogen. It acts like prolactin and growth hormone secreted from pituitary. Actions of HCS:
i. On breasts: In experimental animals, administration of HCS causes enlargement of mammary glands and induces lactation.
ii. On protein metabolism:
HCS acts like GH on protein metabolism. It causes anabolism of proteins and accumulation of proteins in the fetal tissues. Thus, the growth of fetus is enhanced.
iii. On carbohydrate metabolism: It reduces the peripheral utilization of glucose in the mother leading to availability of large quantity of glucose to the growing fetus.
iv. On lipid metabolism: It mobilizes fat from the adipose tissue of the mother. A large amount of free fatty acid is made available as the source of energy in the mother’s body. It compensates the loss of glucose from the mother’s blood to fetus.
e. Relaxin:
Relaxin is a polypeptide, which is secreted by corpus luteum. It is also secreted in large quantity by placenta at the time of labor.
Fetoplacental unit:
Fetoplacental unit refers to the interaction between fetus and placenta in the formation of steroid hormones. The interaction between fetus and placenta occurs because some of the enzymes involved in steroid synthesis present in fetus are absent in placenta and those enzymes, which are absent in fetus are present in placenta.

Figure demonstrates the fetoplacental unit of steroid synthesis.
Fetoplacental unit. DHEAS = Dehydroepiandrosterone sulfate, 16 OHDHEAS = 16hydoxydehydroepiandrosteronesulfate.