Ovulation General Embryology Fertilization Notes

General Embryology And Development Of Face Oral And Paraoral Structures

‘Embryology’ deals with different stages of development of human body in the womb of the mother (intrauterine life) starting from the fertilization of the ovum till the birth of the baby.

General Embryology

General embryology is a branch of embryology which involves the events of human development during first eight weeks of intrauterine life.

Mature graafian follicle:

Graafian follicle is formed in the ovary of the mother under the influence of folliclestimulating hormone (FSH) secreted by the pituitary gland.

Read And Learn More: General Embryology Important Notes

A mature graafian follicle consists of:

• An eccentrically placed oocyte.
• A well-defined zona pellucida (layer of glycoproteins on the surface of oocyte secreted by granulosa cells and oocyte) surrounding the oocyte.
• A large fluid-filled antrum that pushes the oocyte to a side.
• Stratified layer of granulosa cells around the oocyte which can be differentiated into cumulus oophorus (ovaricus) which are intact granulosa cells surrounding the oocyte and discus proligerus cells that attach the oocyte to the wall of the follicle.
• Two layers of stromal cells surrounding the granulosa cells are called theca folliculi. Out of the two layers:
  1. The theca (tunica) interna is a cellular layer which is rich in vascularity and it secretes oestrogen.
  2. Theca (tunica) externa is a fibrous layer which merges with ovarian Stroma.

At puberty, each ovary contains about 2 lakh oocytes (at prophase i) every month about 15–20 reinitiate the development under the influence of fsh. Only one will develop into mature graafian follicle in each cycle.

Embryology Ovulation:

• A mature graafian follicle gradually reaches the ovarian surface and forms a bulge.
• At the apex of the bulge, an avascular spot, the stigma appears.
• An increase in the prostaglandin levels causes local muscular contractions in the ovarian wall to release the oocyte from the ovary.
• Secondary oocyte surrounded by the cells of cumulus oophorus is shed off from the ovary. It floats freely to reach the uterine tube.
• The corona radiata is formed as cumulus oophorus cells gradually reorganize themselves around the secondary oocyte.
• It carries the oocyte into the ampulla of the uterine tube by sweeping movements of the fimbriae and by the motion of cilia of the epithelial cells lining uterine tube.
• Once the secondary oocyte reaches the ampulla of the uterine tube, it is ready for fertilization.

Embryology Fertilization:

Fertilization is a process by which a mature male gamete (spermatozoon) fuses with a mature female gamete (ovum) to form a zygote with diploid number of chromosomes.

• It occurs in the ampulla of uterine tube.

Preparation for fertilization:

Once the spermatozoa enter the female genital tract, they undergo series of changes to get ready for fertilization.

These changes are:

• Capacitation
• Acrosome reaction

Capacitation of sperms:

Capacitation is one of the final steps of maturation of spermatozoa in the uterine tube.

• The process of capacitation may take about 7 hours.
• It helps the sperms to get conditioned to the female reproductive tract.
• The glycoprotein coat and seminal plasma proteins are removed from the Plasma membrane overlying acrosomal region of spermatozoa.
• Only sperms that are capacitated can undergo acrosome reaction after passing through the corona radiata cells

Acrosome reaction:

Once the sperm comes in contact with zona pellucida, zona proteins induce acrosome reaction. Acrosin and trypsin enzymes which are needed for the penetration of the zona pellucida are released.

Fertilization takes place in three stages:

1. Penetration of corona radiata by spermatozoon
2. Penetration of zona pellucida by spermatozoon
3. Fusion of cell membranes of the oocyte and spermatozoon

These three stages of fertilization have been explained.

It can be noted that the sex is determined by the end of fertilization.

• If the fertilizing sperm carries y chromosome it results in a male embryo.
• If the fertilizing sperm carries x chromosome it results in a female embryo.