FERTILIZATION OF THE OVUM

Ovum is released from graafian follicle of ovary into the abdominal cavity at the time of ovulation.

From abdominal cavity, ovum enters Fallopian tube through the fimbriated end. Entry of ovum is facilitated

by movement of cilia present in the inner surface of fimbriated end. Ovum of matured follicle in the ovary is in primary oocyte stage with diploid number (23 pairs) of chromosomes. Just before ovulation, meiotic division takes place in the ovum. Primary oocyte divides into a secondary oocyte and a first polar body. First polar body is expelled out. Secondary oocyte contains only 23 chromosomes (haploid). Remaining 23 chromosomes are lost in the expelled first polar body. Thus, when the ovum is released into abdominal cavity during ovulation, it is in the secondary oocyte stage with haploid number of chromosomes.

FERTILIZATION OF THE OVUM

 

FERTILIZATION OF THE OVUM

Fertilization refers to fusion (union) of male and female gamates (sperm and ovum) to form a new offspring. If sexual intercourse occurs at ovulation time and semen is ejaculated in the vagina, the sperms travel through the vagina and uterus to reach the fallopian tube. Sperms reach the ovarian end of fallopian tube within 30 to 60 minutes. Movement of the sperm through uterus is facilitated by the antiperistaltic contractions of uterine muscles. Uterine contractions are induced by oxytocin, whichis secreted from posterior pituitary by neuroendocrine reflex during sexual intercourse. Uterine contractions are also facilitated by prostaglandin (PGE2) present in male seminal fluid. Among 200 to 300 millions of sperms entering female genital tract, only a few thousand sperms reach the spot near the ovum. Among these few thousand sperms, only one succeeds in fertilizing the ovum. During fertilization, the sperm enters the ovum by penetrating the multiple layers of granulosa cells known as corona radiata present around the ovum. It is facilitated by hyaluronidase and proteolytic enzymes present in acrosome of sperm. Proteolytic enzymes from acrosome of the successful sperm diffuse through the structures of zona pellucida and inactivate the other sperms entering the ovum. Penetrating movement of sperm is enabled by a protein called CatSper present in the tail portion of the sperm. It is a tunnel-shaped protein and forms the ion channel for entry of calcium into sperm cell. Immediately after fertilization, ovum, which is in secondary oocyte stage, divides into a matured ovum and a second polar body. Second polar body is expelled. Nucleus of matured ovum becomes female pronucleus with 23 chromosomes, which include 22 autosomes and one sex chromosome called X chromosome. Simultaneously, head of sperm swells and becomes male pronucleus. Then 23 chromosomes of the sperm and 23 chromosomes of ovum arrange themselves to reform the 23 pairs of chromosomes in the fertilized ovum.

SEX CHROMOSOMES AND SEX DETERMINATION

SEX CHROMOSOMES

All the dividing cells in the body have 23 pairs of chromosomes. Among the 23 pairs, 22 pairs are called somatic chromosomes or autosomes. Remaining one pair of chromosomes is called sex chromosomes. Sex chromosomes are X and Y chromosomes.

SEX DETERMINATION

Sex chromosomes are responsible for sex determination. During fertilization of ovum, 23 chromosomes from ovum and 23 chromosomes from the sperm unite together to form the 23 pairs (46) of chromosomes in the fertilized ovum. Now, sex determination occurs. Ovum contains the X chromosome. Sperm has either X chromosome or Y chromosome. When the ovum is fertilized by a sperm with X chromosome, the child will be female with XX chromosome. And, if the ovum is fertilized by a sperm with Y chromosome, the sex of the child will be male with XY chromosome. So, the sex of the child depends upon the male partner.

IMPLANTATION

Implantation is the process by which the fertilized ovum called zygote implants (fixes itself or gets attached) in the endometrial lining of uterus. After the fertilization, the ovum is known as zygote. Zygote takes 3 to 5 days to reach the uterine cavity from fallopian tube. While travelling through the fallopian tube, the zygote receives its nutrition from the secretions of fallopian tube. After reaching the uterus, the developing zygote remains freely in the uterine cavity for 2 to 4 days before it is implanted. Thus, it takes about 1 week for implantation after the day of fertilization. During the stay in uterine cavity before implantation, the zygote receives its nutrition from the secretions of endometrium, which is known as uterine milk. Just before implantation, the zygote develops into morula and then the implantation starts. A layer of spherical cells called trophoblast cells is formed around morula. Trophoblast cells release proteolytic enzymes over the surface of endometrium. These enzymes digest the cells of the endometrium. Now, morula moves through the digested part of endometrium and implants itself.

DEVELOPMENT OF PLACENTA AND EMBRYO

Already uterus is prepared by progesterone secreted from the corpus luteum during secretory phase of menstrual cycle. After implantation, placenta develops between morula and endometrium. When implantation occurs, there is further increase in the thickness of endometrium because of continuous secretion of progesterone from corpus luteum. At this stage, the endometrial stromal cells are called decidual cells and the endometrium at the implanted area is called decidua. Now the trophoblastic cells of morula develop into cords, which are attached with decidual portion of endometrium. Blood capillaries grow into these cords from the blood vessels of the newly formed embryo. At about 16th day after fertilization, heart of embryo starts pumping the blood into the trophoblastic cords. At the same time, blood sinusoids develop around the trophoblastic cords. These sinusoids receive blood from the mother.

Trophoblastic cells form some vascular projections into which fetal capillaries grow. These vascular projections become placental villi. Thus, the final form of placenta has got the fetal part and the maternal part. Fetal part of placenta contains the two umbilical arteries, which carry fetal blood to the placental villi through the capillaries. The blood returns back to fetus through umbilical vein. Maternal part of placenta is formed by uterine arteries through which blood flows into sinusoids that surround the villi. The blood returns back to mother’s body through uterine vein.

MATERNAL CHANGES DURING PREGNANCY

During pregnancy, the changes are noticed in various organs, body weight, the metabolic activities and functional status of different physiological systems in the mother.

STRUCTURAL CHANGES

Various structural changes are noticed in the primary sex organs, accessory sex organs and in the mammary glands during pregnancy.

1. Ovaries

Follicular changes do not appear in ovary and ovulation does not occur because the secretion of FSH and

LH from anterior pituitary is inhibited. Corpus luteum enlarges and secretes a large quantity of progesterone and little estrogen, which are essential for maintaining the pregnancy. It continues for 3 months and then, corpus luteum degenerates. By this time placenta develops fully and takes over the function of secreting estrogen and progesterone. It continues throughout the period of pregnancy thus inhibiting the secretion of FSH and LH.

2. Uterus

When the fetus grows, uterus undergoes changes in volume, size, shape and weight.

 

i. Volume

Volume of uterus increases gradually due to fetal growth. From almost zero volume, uterus reaches about

5 to 7 liters at the end of pregnancy. Out of this, 50% of the volume is due to the fetus and rest is due to the placenta, amniotic fluid, etc.

ii. Size

Size of the uterus also increases due to: a. Hyperplasia (increase in number of cells) of myometrium

b. Hypertrophy (increase in size of the cells) of myometrium

c. Growth of fetus.

3. Vagina

Vagina increases in size and its color changes to violet due to increased blood supply. There is deposition of glycogen in the epithelial cells.

4. Cervix

In cervix, the number of glands, blood supply and mucus secretion increase. The tough cervix becomes soft and it is closed by mucus plug.

5. Fallopian Tube

The number of epithelial cells and blood supply increase in fallopian tubes.

6. Mammary Glands

Size of the mammary glands increases because of development of new ducts and alveoli, deposition of fat and increased vascularization. Pigmentation of nipple and areola occurs.

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