Chapter 2: First Week Of Development

Question 1

What is fertilization?

Answer 1

The union between the male gametes (sperms) and female gametes (ovum) to form the zygote.

Question 2

Where is the site of fertilization?

Answer 2

Ampulla of the uterine tube.

Question 3

What are the steps of fertilization?

Answer 3

1. Capacitation of the sperms.
2. Phase I: penetration of the corona radiata.
3. Phase II: penetration of zona pellucida.
4. Phase III: Penetration of the cell membrane of the oocyte.
5. Events that occur during and after the entrance of the sperm to the cytoplasm of the oocyte.

Question 4

Capacitation of the sperms

Answer 4

The process of removal of glycoprotein coat that covers the acrosomal regions of the sperms in the female genital system. Takes about 7 hours.

Question 5

Penetration of corona radiata

Answer 5

Only 300 - 500 sperms out of 200 - 300 million sperms. Ejaculation will reach the ovum and start dispersion of corona radiata by the release of Hyaluronidase enzyme that dissolves the Hyaluronic acid that fixes the cell of corona radiata together.

Question 6

Penetration of Zona pellucida

Answer 6

Only one sperm will pass through the Zona pellucida by the following steps:
1. Sperms bind to Zona to specific binding sites (sperm receptors).
2. Spearmint secrete acrosomal enzymes (acrosin and trypsin like substances) that dissolve a passage through Zona.
(Acrosomal reaction) helped by the movement of the tail.
3. The head of the fertilizing sperm comes in contact with the plasma membrane of the secondary oocyte.

Question 7

Penetration of cell membrane of the oocyte

Answer 7

1. Plasma membranes of sperm and oocyte fuse together.
2. Fused plasma membranes open to allow a passage of sperm contents (nucleus, neck, middle piece, and axial filament) to cytoplasm of the ovum, leaving the sperm cell membrane on the outer surface of the oocyte.

Question 8

Events that occur during and after the entrance of the sperm to cytoplasm of the oocyte

Answer 8

1. Cortical and Zona reactions: during the entrance of fertilizing sperm, the oocyte releases lysosomal enzymes from the cortical granules. These enzymes prevent polyspermy by:
   - Changing sperm binding sites of the Zona pellucida to prevent binding and passage of more sperms.
   - Making the plasma membrane of the fertilized oocyte impregnable to their sperms.
2. The secondary oocyte completes the 2nd meiotic division to form mature ovum (23Ch) and 2nd polar body (23Ch).
3. Male nucleus becomes larger (male pronucleus) and come to be in contact with nucleus ootid (female pronucleus).
4. Nuclear membranes of both the male and female pronuclei fuse together to form the nucleus of the zygote.

Question 9

What are the results of fertilization?

Answer 9

1. Formation of zygote.
2. Determination of sex of the zygote either male (XY) or female (XX).
3. Restoration of diploid number (46 Ch).
4. Start of cleavage and migration of the zygote from the fertilization to site of implantation in the uterine cavity.

Question 10

Types of artificial fertilization

Question 11

Types of artificial fertilization

Answer 11

1. In-vitro fertilization (IVF)
2. Intracytoplasmic sperm injection (ICSI)
3. Gamete intrafallopian (GIFT)

Question 12

In-vitro fertilization (IVF)

Answer 12

1. In case a female suffering from obstructed uterine tubes.
2. Giving the mother gonadotropins to stimulate FSH secretion, which induces multiple Graafian follicles formation in the ovary.
3. Withdrawal of secondary oocyte from the Graafian follicles just before ovulation using laparoscope.
4. Transfer of the secondary oocyte to a special culture medium and then addition of the sperms.
5. When fertilization occurs and the embryo reaches the 8 cell stage, it is taken from the culture medium and is implanted in the endometrium.

Question 13

Intracytoplasmic sperm injection (ICSI)

Answer 13

1. In case of low count of sperms or failure of its penetration to walls of the oocyte.
2. In-vitro injection of a single sperm into the cytoplasm of the oocytes to cause the fertilization.
3. After the formation of morula, it is implanted into the uterine endometrium using a catheter.

Question 14

Gamete infrafallopian transfer (GIFT)

Answer 14

1. Oocytes and sperms are introduced into the ampulla of the fallopian ( uterin) tube, where fertilization takes place. Development then proceeds in a normal manner.

Question 15

Cleavage

Answer 15

The zygote divides mitotic ally inside the Zona pellucida giving rise to smaller blastomeres (46Ch) in the following manner:
1. 2 cell stage: 1st day.
2. 4 cell stage: 2nd day.
3. 16 cell stage (morula) followed by 32 and 64: 3rd day.

Question 16

Migration

Answer 16

During cleavage, the morula migrates inside the uterine tube to reach the uterine cavity at the 4th day by the following mechanisms:
1. Muscular peristalsis of the uterine tube.
2. Motion of the cilia of the mucosal lining of the tube.
3. Mucus secreted by the mucus membrane of the uterine tube provides a fluid vehicle that facilitates migration and provides nutrition for dividing zygote.

Question 17

Formation of the blastocyst

Answer 17

1. Zona starts to degenerate at the end of the 5th day.
2. Fluid pass through degenerating Zona to form multiple spaces between the cells of the morula.
3. Gradually, those spaces fuse together to form a single cavity called blastocele. This stage is called blastocyst.

Question 18

Blastocyst is composed of?

Answer 18

1. Outer cell mass (Trophoblast): a single layer of cells.
2. Inner cell mass (Embryoblast): a mass of cells on one side of the inner aspect of trophoblast.
3. Blastocele: a cavity of the blastocyst.
4. Embryonic pole: part of the trophoblast that lies outer to the embryoblast.
5. Abembryonic pole: opposite side of the embryonic pole of the blastocyst.