Fertilisation In Human

Fertilization in humans is the process by which a male gamete (sperm) and a female gamete (egg or ovum) fuse to form a single-celled structure called a zygote, marking the beginning of human development. This process occurs in the female reproductive system, typically in the ampulla of the fallopian tube.

1.0Introduction

• The process in which union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, is called fertilisation.
• The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube.
• The ovum (secondary oocyte) released by the ovary is also transported to the ampullary region where fertilisation takes place. 
• Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. 
• This is the reason why not all copulations lead to fertilisation and pregnancy.
• The union of male and female gametes is called Syngamy.
• Intermixing of their cytoplasm is called plasmogamy.
• The fusion of the pronuclei of sperm and ovum is called karyogamy.
• The intermingling of their chromosomes is called amphimixis.

2.0Mechanism of Fertilization

• Sperm Movement: Millions of sperms move towards the secondary oocyte in the fallopian tube.
• Penetration of Corona Radiata: Enzyme hyaluronidase helps sperm penetrate the outer layer of the ovum.
• Acrosomal Reaction: Adherence of sperm to the ZP3 receptors/region on zona pellucida, this leads to initiation of acrosomal reaction. Various enzymes are released. E.g. Acrosin (Zona lysin).Acrosin facilitates the penetration of sperm through zona pellucida.
• Fusion: Sperm fuses with the secondary oocyte membrane. This is syngamy.
• Phagocytosis: The ovum engulfs the sperm completely.
• Completion of Meiosis-II: Ovum completes its second meiotic division, releasing a second polar body.
• Formation of Fertilisation Membrane: Structural changes in zona pellucida through cortical reaction and discharge of cortical granules in perivitelline space forms a fertilization membrane, which prevents polyspermy.

It is followed by plasmogamy in which all the structures of sperm dissolve in egg cytoplasm except sperm nucleus and proximal centriole (this centriole will help in cleavage after formation of zygote).

• Formation of Male Pronucleus: Sperm’s nucleus swells and becomes the male pronucleus.
• Karyogamy and Amphimixis: Male and female pronuclei fuse, completing fertilisation.

3.0Significance of Fertilisation 

• Secondary oocyte completes its second maturation division before completion of fertilization on coming in contact with the sperm.
• Amphimixis process leads to the formation of a diploid zygote to restore the normal diploid number of the chromosomes.
• The centriole of sperm after entering into the egg induces the egg to undergo cleavage.
• The paternal and maternal characters are transmitted to the offspring through the process of fertilisation.
• The peripheral changes occurring in the egg membrane prevent the further entry of sperm into the ovum, thus checking polyspermy.
• The male gametes (sperms) the sex chromosome could be either X or Y, hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y. 
• After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male.

4.0Implantation, Pregnancy & General Stages of Embryonic Development

Morulation 

• As a result of segmentation or cleavage activities, unicellular zygote changes into a solid ball-like multicellular structure. In the later stage of cleavage, clusters of sticky, cohering, protruding (outside) blastomeres are produced, which look like mulberry. 
• This stage is termed as morula stage. The embryo with 8 to 16 blastomeres is called a morula. 
• The cleavage starts as the zygote moves through the isthmus of the oviduct towards the uterus.

Cleavage vs Normal Mitosis

Blastulation 

• The morula continues to divide and transforms into blastocyst. Blastula of eutherian and metatherian mammals is called blastocyst, because blastula is in the form of a cyst. 
• The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. 
• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. 
• In human beings, the blastocyst gets attached to the uterine endometrium in about four days after entering the uterus. 
• After attachment, cells of trophoblast secrete some enzymes which helps the embryo enter in the endometrium. The uterine cells divide rapidly and cover the blastocyst. 
• By the 12th day after fertilization, blastocyst is completely buried in the wall of the uterus. This is called implantation and it leads to pregnancy. 
• In humans, the site of implantation is generally the mid-dorsal/fundus part of the uterus.

Gastrulation  

• In the gastrula stage the rate of cleavage division is slow and ultimately stops at the end of gastrula. 
• Gastrula stage is the most important stage in embryonic development because formation of three germinal layers i.e. ectoderm, mesoderm and endoderm takes place. 
• During gastrulation, Inner Cell Mass differentiates into an outer layer called ectoderm and an inner layer called endoderm. 
• Mesoderm soon appears between the ectoderm and the endoderm. 
• These three layers give rise to all tissues (organs) in adults.
• It needs to be mentioned here that the inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.

5.0Summary of Developmental Stages in Human