Introduction• Gametogenesis is the general process of gamete formation in both males and females.• Oogenesis is the process of meiosis in female organisms from an oogonium to a primary oocyte, to a secondary oocyte, and then to an ovum.
• Spermatogenesis is a similar process of forming sperm by meiosis in males.
Process of Oogenesis• Oogenesis begins soon after fertilization, as primordial germ cells travel from the yolk sac to the gonads, where they begin to proliferate mitotically.• The germ cells multiply from only a few thousand to almost 7 million.
• They become oocytes once they enter the stages of meiosis several months after birth.• Now called primordial follicles, they are made up of oogenic cells from the primordial germ cells surrounded by follicle cells from the somatic line.• The oocyte is then arrested in the first meiotic prophase until puberty.
• At puberty, between 4 to 10 follicles begin to develop, although only 1-2 are actually released. Surrounding each oocyte is a zona pellucida, membrana granulosa, and theca cell layer.• Each oocyte finishes its first meiotic division, creating a secondary oocyte and polar body, which serves no further function.• It begins the next meiosis cycle and is arrested in its second metaphase, at which point it is released from the ovary in ovulation.
• It will not finish the meiosis cycle until it encounters the stimuli of a sperm.
Spermatogenesis• Spermtozoa are formed in the wall of semineferous tubules of the testes. The various cell-stage in spermatogenesis are as follows:• The spermatogonia (type A) or germ cells (44 +X+Y) divide mitotically, to give rise to more spermatogonia of type A, and also to spermatogonia of type A, and also to spermatogonia of type B.
• The spermatogonia (typeB) (44+X+Y) enlarge, or undergo mitotis, to form primary spermatocytes.• The primary spermatocytes (44+X+Y) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division : it reduces the number of chromosomes to half.• Each secondary spermatocyte has 22+X or 22+Y chromosomes. It divides to form spermatids.
• This is the second meiotic division and this time there is no reduction in chromosome number.• Each spermatid 22+X or 22+Y gradually changes its shape to become a spermatozoon . This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis.
FertilizationDefinition:• Fertilization means the fusion of two mature cells an ovum and a spermatozoan to form a single cell the zygote.• Fertilization is the process of fusion of spermatozoan with the mature ovum.
Stages1. Approximation of the gametes.2. Contact and fusion of the gametes.3. Effects of fertilization.
1. Approximation of gametes. It involves the transport of the sperms and ova in the female genital tract to reach the uterine tube. The ovum is picked up after ovulation by tubal fimbrae and is by muscular or kind of suction or ciliary action or chemotaxis by tubal secretions. Then transported to ampullary part.
2. Contact and fusion of the gametes• A direct contact between the spermatozoa and the zona pellucida of the secondary oocyte is necessary before actual fusion of the gametes.• The cells of corona radiata provide obstacle to the penetration of the sperms.• 200 to 300 million sperms emitted at single ejaculation about 300 to 500 sperms reach the ovum, only 1 unites with oocyte & rest engaged in disintegration of corona radiata by secreting enzyme hyalerinidase
• Prior to penetration spermatozoa undergo process of ;Capacitation.Acrosome reaction.
3.Effects of fertilization Completion of second meiotic division of female gamete: -It contains haploid number of chromosomes (22+ X). The bigger one is female pronucleus & smaller polar body pushed to perivitteline space. -Head and tail of sperm enter the cytoplasm. Head and neck of spermtaozoan nos of chromosome ie 22+X or 22+Y.
•Restoration of diploid nos of chromosomes in the zygote • Both male & female pronuclie meet near centre of ovum, nuclear membrane disappear. This results in nuclear fusion with restoration of diploid nos of chromosome 44+2X or 44+xy
• Determination of the chromosomal sex Out of the total population in that half of them contain x bearing chromosome and remaining half contain y bearing . If x bearing spermatozoon fertilizes an ovum the zygote contain 2x chromosome +44autosome & female child is formed & Zygote containing 2y chromosome +44autosome male child is formed.
Initiation of cleavage division ofzygote• Two cell formed undergo a series of division. One cell divides to form three cell stage of embryo followed by 4cell, 8cell, 16cell. This process of division of ovum into smaller cells called CLEAVAGE
• Cleavage reaches 16 cell stage a cluster/ mass of cell like mulberry called ‘MORULA’ surrounded by zonapelucida. Consists of inner cell mass i.e. completely surrounded by outer layer of cell which gives rise to structure called ‘TROPHOBLAST’. Inner cell mass cell gives rise to embryo called ‘EMBROYOBLAST’.
• Some fluid passes into the morula from uterine cavity and partially separate the cells of inner cell mass from those of trophoblast & fluid increases morula acquires shape of cyst. Trophoblast becomes flattened & inner cell mass gets attached to inner of trophoblast on one side and morula becomes BLASTOCYST.