FERTILIZATION By Dr Mujahid Khan Fertilization Is a

FERTILIZATION By: Dr. Mujahid Khan

Fertilization • Is a complex sequence of coordinated molecular events • Begins with a contact between sperm and ovum • Ends up with intermingling of maternal and paternal chromosomes

Site • Usual site is the ampulla of uterine tubes • Ampulla is the longest and widest part • If the oocyte is not fertilized here, it slowly passes to the uterus, where it degenerates and absorbed • Fertilization may occur in other parts of uterine tubes

Site • Does not occur in the uterus • Chemical signals from oocyte attract the gamete specific proteins on the surface of the sperms • The fertilization process takes about 24 hours

Phases of Fertilization • • • Passage of sperm through corona radiata Penetration of zona pellucida Fusion of the plasma membrane of the oocyte and sperm Completion of second meiotic division & formation of female pronucleus Formation of male pronucleus

Passage of Sperm Through Corona Radiata • Dispersal of the follicular cells of the corona radiata surrounding the oocyte and zona pellucida • This results mainly from the action of hyaluronidase enzyme released from the acrosome • Tubal mucosal enzymes also appear to assist in dispersal

Penetration of Zona Pellucida • Passage of a sperm through the zona pellucida is the important phase in the initiation of fertilization • The enzymes estrases, acrosin and neuraminidase appear to cause lysis of zona pellucida • Forming a path for the sperm to follow the oocyte

Penetration of Zona Pellucida • Zona reaction occurs once the sperm penetrates the zona pellucida • It makes zona pellucida impermeable to other sperms • The composition of this extracellular glycoprotein coat changes after fertilization

Fusion of the Plasma Membrane of the Oocyte and Sperm • The plasma membranes of the oocyte and sperm fuse and break down at the area of fusion • The head and tail of the sperm enter the cytoplasm of oocyte • Sperm’s plasma membrane remains behind

Completion of Second Meiotic Division • Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division • Mature oocyte and a second polar body are formed • Nucleus of the mature oocyte becomes a female pronucleus

Formation of Male Pronucleus • Cytoplasm of oocyte and nucleus of the sperm enlarges to form the male pronucleus • Tail of the sperm degenerates • Morphologically the male and female pronuclei are indistinguishable • The oocyte containing two haploid pronuclei is called an ootid

Formation of a Zygote • As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote

Early Pregnancy Factor • Is an immunosuppressant protein • Secreted by trophoblast • Appears in maternal serum within 24 -48 hrs • It forms the basis of a pregnancy test during the first 10 days of development

Chromosome • Zygote is genetically unique • Half of chromosomes from father and half from mother • New combination is formed which is different from either of parents • This mechanism forms biparental inheritance and variation of human species

Sex of the Embryo • Embryo's chromosomal sex is determined at fertilization • Sex is determined by the type of sperm (X or Y) fertilizes the oocyte • It is father whose gamete decides the sex

Results of Fertilization • Stimulates the penetrated oocyte to complete 2 nd meiotic division • Restores the normal diploid number of chromosomes • Determines chromosomal sex of embryo • Initiates cleavage (cell division) of zygote

Cleavage of Zygote • Consists of repeated mitotic divisions of zygote • Rapid increase in the number of cells • These smaller embryonic cells are called Blastomeres • Normally occurs in the uterine tube

Cleavage of Zygote • Zygote divides first into 2 then 4 and 8 • Zygote is within the thick zona pellucida during cleavage • Zona pellucida is translucent under light microscope • Begins about 30 hrs after fertilization

Compaction • Blastomere change their shape after 9 cell stage • Tightly align themselves against each other • Form a compact ball of cells • Probably mediated by cell surface adhesion glycoprotein • Permits greater cell to cell interaction

Morula • When there are 12 -32 blastomeres the developing human is called MORULA • Enters the uterus at this stage • Spherical morula forms about 3 days after fertilization • Has outer and inner cell layer • Resembles mulberry or blackberry

Formation of Blastocyst • Morula enters the uterus 4 days after fertilization • A fluid-filled space called the blastocystic cavity appears inside the morula • The fluid passes from the uterine cavity through the zona pellucida to form this space

Formation of Blastocyst • Blastomere is separated into two parts as the fluid increases in the blastocystic cavity • A thin outer cell layer called trophoblast • Inner cell mass that gives rise to embryo called embryoblast