CHAPTER 11

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FERTILIZATION

INTRODUCTION

Fertilization is defined as the process of fusion of the sperm (haploid male gamete) with the oocyte (haploid female gamete) at the junction of the ampulla and isthmus of the fallopian tube. As a result of fertilization, a mononucleated diploid single cell is formed known as a zygote.

GAMETOGENESIS

The process of maturation of highly specialized spermatozoa of male and female before their fusion to form a zygote is known as gametogenesis.

Oogenesis

The process of maturation of ovum is called as oogenesis. Yolk sac releases the primitive germ cell after 3rd week and reaches the gonadal ridge at the end of the 4th week. The germ cells undergo rapid mitotic division and differentiate into oogonia.

Mature Ovum

A fully matured ovum is the largest cell in the body. It has a diameter of 130 microns. It has a cytoplasm, a nucleus and a nucleolus. During fertilization, the nucleus is converted into a female pronucleus.

A mature ovum has the following layer (Fig. 1).

• Outer layer: corona radiata
• Inner layer: zona pellucida

There is a space present between zona pellucida and the vitelline membrane known as perivitelline space. It accommodates the polar bodies.

A mature ovum

Spermatogenesis

The process of development of the spermatids from the primordial male germ cells and their differentiation into spermatozoa is called spermatogenesis.

Spermatozoa

A mature sperm is hypemotile cell. It is divided into different parts :

• Head with an acrosomal tip containing hyaluronidase enzyme to digest the zona pellucida.
• Mid piece containing mitochondria to provide nutrition.
• Tail to allow the motility of the sperm in a vibratory motion.

A male sperm

Schematic representation of gametogenesis

Capacitation

It is a series of functional changes that occur under the influence of the secretions of the fallopian tube resulting in the removal of glycoprotein coat, cholesterol, and protein around the head of the sperm cell.

Fertilization process

Sex of the Child Is Determined By The Genotype Of The Child

The male child has 44+XY chromosomes.

The female child have 44+XX chromosomes.

Monozygotic Twins

Monozygotic twins are also called the identical twins. They develop from the same fertilized egg, having the same genetic material with the same sex.

Conjoined Twins

Conjoined twins are the monozygotic twins in which the cells separated after the 8 days of fertilization. In this twins are joined together and share some body structures

Dizygotic Twins

Dizygotic twins are also called the fraternal twins. They develop due to the fusion of the two independent secondary oocytes, each by a different sperm. They are of the same age but they are genetically different.

Cleavage and formation of morula and blastocyst

A blastocyst

IMPLANTATION

DECIDUA

After conception, the endometrium of the pregnant uterus is known as decidua.

Decidual Reaction

It is the reaction that occurs due to the raised progesterone level after fertilization which will increase the structural and secretory activity of the endometrium.

These changes occur in all the endometrial layers but is significantly marked at the implantation site and around the maternal blood vessels. Endometrial stroma have fibrous connective tissue, which is transformed into epitheloid cells called decidual cells.

The decidua has 3 layers : the superficial layer, intermediate layer, and basal layer.

Structure of decidua

Blastocyst implants into the superficial layer of the deciduas , thus the different portions of the decidua are renamed as :

• Decidua basalis or serotina: The portion of the decidua which is in contact with the base of the blastocyst
• Decidua capsularis or reflexa: It is the thin layer that covers the blastocyst superficially.
• Decidua vera or parietalis: The remaining decidua that lines the uterine cavity except for the implantation site.

Location of the renamed decidua after implantation.

Functions

• It provides the site for the implantation of the blastocyst.
• It supplies nutrition to the developing ovum.
• The trophoblast penetrates to the deeper section of decidua and is been controlled by the local peptides, cytokines, and integrins.
• Decidua basalis contributes to the formation of the basal plate of the placenta