BLASTOCYST, GASTRULATION AND FERTILIZATION

Fertilization

This is the fusion of the male and female gametes. It is a complex process that requires contact between the sperm and the oocyte. It occurs in the ampulla of the uterine tube.

What is the viability of gametes?

Human oocytes are usually fertilized within 24 hours after ovulation. The oocyte cannot be fertilized after 24 hours as it degenerates after. The human sperm do not survive more than 48 hours.

What are the five stages of fertilization ?

1. The passage of sperm through the coronal radiata

2. Penetration of the zona pellucida

3. Fusion of the plasma membrane of the oocyte and sperm

4. Completion of the second meiotic of division of the oocyte and formation of the female pro-nucleus

5. Formation of the male pro-nucleus

Describe the first stage of fertilization

Only capacitated sperm pass freely through the corona radiata. Capacitated sperm are sperm that have undergone a series of changes that takes about 7-8 hours that makes them high motile and hyperactive. The action of the enzyme hylarudinase released from the acromosome of the sperm helps to dispel the corona radiata cells and the propulsion of the sperm’s tail.

Describe the second stage of fertilization

The zona is a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces acromosome reaction. The intact acromosome binds with the zona glycoprotein 3 on the pellucida. Release of acromosomal enzymes allows the sperm to penetrate the zona pellucida thereby coming in contact with the membrane of the oocyte. As soon as the sperm comes in contact with the oocyte, the permeability of the ZP changes. Lysosomal enzymes are released from the cortical granules lining the plasma membrane of the oocyte. These enzymes alter the properties of the ZP to prevent sperm penetration and inactivate the binding sites for sperm on the ZP surface.

Explain stage 3 of fertilization

The plasma membranes of the oocyte and the sperm fuse and break down at the site of fusion. The head and tell of the sperm enter the cytoplasm of the oocyte but the plasma membrane of the sperm is left behind.

Explain stage 4 of fertilization

Penetration of oocyte by the sperm activates the oocyte into completing the second meiotic division and forming a mature oocyte and the second polar body.

Explain the final stage of fertilization

Within the cytoplasm of the oocyte, the nucleus of sperm enlarges to form male pronucleus and the sperm tail degenerates.

Cleavage

This is a series of mitotic divisions of a zygote resulting in a rapid increase in the number of embryonic cells. Cleavage normally occurs as the zygote passes along the uterine tube to the uterus. Division of the zygote into classmates begins 30 hours agree fertilization. The first cleavage divides the cells into two.

What is compaction ?

After the eight cell stage , the blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells. This is probably mediated by the cell surface adhesion glycoproteins. Approximately, three days after fertilization, cells of the compacted embryo divide to form morula.

Explain the process of blastocyst formation

Shortly after the morulla enters the uterus (approximately four days later), a fluid filled spaces called blastocystic cavity appears inside the morulla. As the fluid increases, it separates the blastomere into two parts; the inner mass of cells called the embryoblast covered by an outer layer of cells called the trophoblast. By the late blastocyst stage, the zona pellucida has degenerated. Although cleavage increase the number of blastomeres, the daughter cells are smaller than the parent so there is no increase in size until the ZP degenerates.

Implantation

Approximately, six days after fertilization, the blastocyst attaches to the endometrial epithelium . As soon as this happens, it begins to proliferate rapidly and differentiate into two layers: the inner layer of cytotrophoblast and the outer layer synctiotrophoblast.

What are the three layers of the uterus?

1. The endometrium or mucusa lining the inside wall

2. Myometrium a thick layer of smooth muscle

3. Perimetrium the peritoneal covering of the outside wall

What events occur during the second week of development?

1. Completion of implantation of the blastocyst

2. Formation of the bi-laminar embryonic disk

3. Formation of extra-embryonic structures

What happens on the 8th day of development

At the 8th day of developments, the blastocyst is partially embedded in the endometrium. The synctiotrophoblast continues to invade the endometrium while more cells in the cytotrophoblast migrate into the synctiotrophoblast where they fuse and lose that individual cell membranes . Cells of the embryo last also differentiate into two layers; the hypoblast which are small cuboidal cells adjacent to the blastocystic cavity and the epiblast which are columnar cells adjacent to the amniotic cavity. These layers combine to form a flat ovoid disk called the bi-laminar disk. At the same time, a cavity forms within the epiblast that enlarges to become the amniotic cavity.

What happens on day 9

The blastocyst is deeply embedded in the endometrium and the penetration defect on the surface of the epithelium is closed by fibrin . Vacuoles appear at the region of the trophoblast and they fuse to form a larger lacunae, this is called the lacunar stage. The hypoblast cells adjacent to cytotrophoblast form a thin layer called the exo-coelemic membrane. This membrane together with the hypoblast form the lining of the exo-coelemic cavity, or the primitive yolk sac or the primary umbilical vesicle.