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GAMETOGENESIS AND FERTILIZATION By DR.ARCHANAVIKRAM YMC GAMETOGENESIS :The process involved in the maturation of the two highly specialised cells, spermatozoon in male and ovum in female, before they unite to form zygote, is called gametogenesis. OOGENESIS :•Definition – The process involved in the development of a mature ovum is called oogenesis. •The primitive germ cells take their origin from the yolk sac at about the end of 3rd week . •In the female gonads, the germ cells undergo a number of rapid mitotic division and differentiate into oogonia. •The number of oogonia reaches it’s maximum at 20th week, numbering about 7 million. •While the majority of the oogonia continue to divide, some enter into the prophase of the first meiotic division and are called primary oocytes. •These are surrounded by flat cells and are called primordial follicles and are present in the cortex of the ovary. •At birth, all the oogonia are replaced by primary oocytes which have finished the prophase of the first meiotic division and remain in the resting phase (dictyotene stage) between the prophase and the metaphase. •Out of these, some 400 are likely to ovulate during the entire reproductive period. Maturation of the oocytes – The essence of maturation is reduction of the number of chromosomes to half. -Before the onset of first meiotic division, the primary oocytes double its DNA by replication, so they contain double the amount of normal protein content. -There are twenty two pairs of autosomes which determine the body characteristics and one pair of sex chromosomes, named “XX”. -The first stage of maturation occurs with full maturation of the ovarian follicle just prior to ovulation but the final maturation occurs only after fertilization. •The secondary oocyte completes the second meiotic division only after fertilization by the sperm in the fallopian tube and results in the formation of two unequal daughter cells, each possessing 23 chromosomes (23x), the larger one is called the mature ovum and the smaller one is called the second polar body containing the same number of chromosomes. •The first polar body may also undergo the second meiotic division. •In the absence of fertilization, the secondary oocyte does not complete the second meiotic division and degenerates as such. Structure of a mature ovum:•A fully mature ovum is the largest cell in the body and is about 130 microns in diameter. It consists of cytoplasm and a nucleus with it’s nucleolus which is eccentric in position. •It contains 23 chromosomes (23x). •During fertilization, the nucleus is converted into a female pronucleus. SPERMATOGENESIS:•Definition – The process involved in the development of spermatids, from the primordial male germ cells and their differentiation into spermatozoa is called spermatogenesis. •Shortly before puberty, the primordial germ cells develop into spermatogonia and remain in the wall of seminiferous tubules. •The spermatogonia differentiate into primary spermatocytes which remain in the stage of prophase of the first meiotic division for a long time (about 16 days) •Each spermatocyte contains 22 pairs of autosomes and 1 pair of sex chromosomes named “XY”. •With the completion of the first meiotic division, 2 secondary spermatocytes are formed having equal share of cytoplasm and haploid number of chromosomes either 23X or 23Y. •Immediately follows the second meiotic division with the formation of four spermatids, each containing haploid number of chromosomes, two with 23X and two with 23Y. •Extensive morphological differentiation of the spermatids occurs without further cell division to convert them into spermatozoa. This process is called spermiogenesis, after which maturation & capacitation of spermatozoa occurs. •The time required for a spermatogonium to develop into a mature spermatozoon is about 61 days. Structure of a mature spermatozoon: • The spermatozoon has a head, a neck, a middle piece and a principal piece or tail. • An axial filament passes through the middle piece and extends into the tail. 1. The head : - The head of the human spermatozoon is derived from the nucleus . It consists of chromatin (mostly DNA). - The head is covered by a cap like structure called the acrosome (also called the acrosomic cap or galea capitis). 2. The neck : -The neck is narrow -It contains a funnel shaped basal body and a spherical centriole. -The basal body is also called the connecting piece because it helps to establish an intimate union between the head and the remainder of the spermatozoon. 3. The axial filament : • The axial filament begins just behind the centriole. • It passes through the middle piece and most of the tail. • At the point where the middle piece joins the tail, the axial filament passes through a ring like structure called the annulus. • The part of the axial filament which lies in the middle piece, is surrounded by a spiral sheath made up of mitochondria. -The axial filament is composed of several fibrils. -There is a pair of central fibrils, surrounded by 9 pairs (doublets) arranged in a circle around the central pair. -Outside the fibrils, there is a fibrous sheath. -In the region of the middle piece, the fibrous sheath is surrounded by spirally arranged mitochondria. -Finally, the entire sperm is enclosed in a plasma membrane. OVULATION : •Definition –. it is a process whereby a secondary oocyte is released from the ovary following rupture of a mature graffian follicle and becomes available for conception •If gonadatrophin stimulation is adequate, one of the several follicle units propelled to varying degrees of maturity will advance to ovulation. FERTILIZATION FERTILIZATION : •Definition – It is the process of fusion of the spermatozoon with the mature ovum. • it occurs in the ampullary part of the uterine tube. •The fertilizable life of the human oocyte is between 12 and 24 hrs. •Out of hundreds of millions of sperms deposited in the vagina, only thousands of sperms enter the uterine tube while only 300 to 500 reach the ovum. •The tubal transport is facilitiated by muscular contraction & aspiration action of the uterine tube. Morula:- After the zygote formation, typical mitotic division of the segmentation nucleus occurs producing 2 blastomeres. •The 2 cell stage is reached approximately 30hrs after feritilization. •The blastomeres continue to divide by binary division through 4,8,16 cells stage until a cluster of cells is formed called morula. •The morula after spending 3 days in the uterine tube enters the uterine cavity through the narrow uterine ostium on the 4th day. •The central cell of the morula is known as inner cell mass -the embryo proper and the peripheral cells are called outer cell mass which forms the protective & the nutritive membranes of the embryo. Blastocyst:- While the morula remains free in the uterine cavity on the 4th & 5th day, it is covered by a film of mucus. •The fluid passes through the canaliculi of the zona pellucida which seperates the cells of the morula & is now called the blastocyst. •Lysis of the zona and escape of embryo is called as zona hatching. •The cells on the outer side of the morula become the trophectoderm and the inner cells become the inner cell mass. •Trophectoderm differentiates into the placenta and inner cell mass into the embryo. IMPLANTATION : Implantation is defined as the process by which an embryo attaches to the uterine wall and penetrates first the epithelium and then the circulatory system of the mother to form the placenta. Implantation begins 2 – 3 days after the fertilized egg enters the uterus; entry is on day 18 or 19 of the cycle. Thus, implantation occurs 6 – 7 days after fertilization. Implantation consists of 3 stages: 1. Apposition 2. Adhesion 3. Invasion (also called migration to denote its benign nature). INVASION AND PLACENTATION: In the second week after ovulation, the placenta is formed. By this time, the trophoblasts at the implantation site have formed masses of cytotrophoblasts and syncytiotrophoblasts, and invasion of maternal blood vessels has begun.