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 2nd 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