Gametogenesis is a procedure of formation of gametes from germ cells in testes and ovaries. Diploid or haploid precursor cells experience cell division and differentiation to create mature haploid gametes. Depending on biological life cycle of organism, gametogenesis takes place by meiotic division of diploid gametocytes in different gametes or by mitotic division of haploid gametogenous cells. At the phase of spermatogonia and oogonia, germ cells multiply by mitosis, consequently; they experience meiosis to become the completely matured gametes. Meiosis comprises two consecutive divisions with only one DNA replication cycle and results in production of haploid gametes. Pairing of homologous chromosomes is unique to meiosis. First meiotic division improves genetic variability by independent assortment (random distribution) of different maternal and paternal homologs and by crossing-over between homologous non sister chromatids. Second meiotic division signifies normal mitosis without DNA replication. Meiosis is dominated by prophase of first meiotic division, which occupies the long period and is separated in 5 sequential phases- zygotene, leptotene, diplotene, pachytene and diakinesis-stated by morphological criteria.
Development of Sperm:
Spermatogonia develop from primordial germ cells which migrate in undifferentiated gonad early in embryogenesis. In wall of forming seminiferous tubules two different types of cells are already clearly noticeable at this phase: supporting Sertoli cells, thought to derive from surface epithelium of genital ridge, and spermatogonia, derived from primordial germ cells. During fetal period, spermatogonia enter the dormant or arrested stage of development, and Sertoli cells comprise most of seminiferous epithelium. At sexual maturity, spermatogonia start to increase in number. It is at this time that spermatogenesis actually begins as this term generally refers to sequence of events by which spermatogonia are transformed in spermatozoa. Spermatogenesis comprises three main stages: spermatogonial multiplication, meiosis, and spermiogenesis. Cells at these different phases are known as spermatogonia, spermatocytes and spermatids, respectively. In men spermatogonial multiplication takes place through regular intervals of 16 days. Spermatogonia can be separated in two major kinds, noncycling ones (Ao), and those which will differentiate in spermatocytes after 6 mitotic divisions. Type (Ao) spermatogonia are able to repopulate seminiferous epithelium when cycling spermatogonia reduce in number. Cycling spermatogonia give the stem cell population for meiosis that starts when preleptotene spermatocytes begin DNA replication. Each primary spermatocyte, really the largest germ cell in the tubules, experiences the first meiotic division, forming two secondary spermatocytes which are about half the size of primary spermatocyte. Spermatids are progressively transformed in mature sperm by the extensive procedure of differentiation called as spermiogenesis; lastly differentiated sperm is released from seminiferous epithelium and becomes free spermatozoon, a procedure known as spermiation. In human procedure of spermatogenesis extends over the period of approx 60 days.
The sperm cell comprises of two morphologically and functionally distinct regions. Head comprising strangely highly condensed haploid nucleus and the tail propelling sperm to egg assisting to enter through egg coat. DNA in the nucleus is inactive and very tightly packed due to its association with extremely positively charged proteins, protamines, instead of histones, that have been displaced during spermiogenesis. Head also has a membrane-limited organelle, acrosome, whose contents are believed to have function in penetration of spermatozoon in ovum. The variety of enzymes, comprising glycosidases, proteinases, arylsulfatases, phosphatases and phospholipases are present in acrosome and in the preacrosomal membrane.
Sperm released from seminiferous epithelium are not proficient of fertilization. Long series of changes that spermatozoa undergo between casting off from the Sertoli cells, and fusing with egg, i.e. till the completely functional state of spermatozoa, is referred to as sperm maturation. All through their journey from testis to the closeness of the ovum, sperm cells are suspended in transudations and secretions of male and female genital tracts. Chemical and physical nature of this medium increasingly changes and spermatozoa also change structurally, chemically and behaviorally. Many proteins from testicular and epididymal environment have been illustrated to bind to particular regions of sperm surface which are involved in sperm maturation and in part of gamete recognition procedure. Biochemical modifications of some sperm surface components are also involved, and an increase in interchromatin disulfide bonds for chromatin condensation during this travel that lasts numerous days. Sperm cells develop slow motility and ability to bind and penetrate eggs as they progress from caput to the cauda epididymidis. Last step of sperm cell maturation is known as capacitation that is a functional term utilized to point to changes in mammalian spermatozoa which should take place in female genital tract, or during in vitro incubations, as preparation for acrosome reaction. Capacitation comprises lowering of cholesterol/phospholipid ratio in sperm membrane, a loss of sperm surface coating components (loss of antifertility factor from human seminal plasma) maybe involved with acquisition of zona pellucida binding activity, and phosphorylation of some plasma membrane proteins.
Development of the egg:
Unfertilized egg is the outcome of the discontinuous course known as oogenesis, which starts during fetal development and ends in sexually mature adult. Oogonia develop from primordial germ cells in ovary, and multiply by mitosis simply during the fetal life. By the 5th month of gestation in women, all germ cells discontinue proliferation and enter meiosis but break at prophase of the first meiotic division; arrest may last from 12 to 50 years. Spherical dictyate oocytes become enclosed inside a few squamous somatic cells to form what is known as primordial follicles; oocytes are then known as primordial oocytes. It is in this period of life that ovary has highest number of oocytes-about one to two millions-as several of them will degenerate before puberty and through reproductive life of the woman. At puberty only approx 300'000 primary oocytes remain. They symbolize stockpile from which the few are selected at any given time for development towards preovulatory follicles having fully grown oocytes. Oocyte and its surrounding follicle grow coordinately, rather than concurrently. Certainly, the oocyte completes its development before formation of follicular antrum, i.e., the main part of follicular growth takes place after oocyte has stopped growing. Completion of growth takes about 2.5-3 months. Nucleus of growing oocyte, known as germinal vesicle, is mainly apparent and has a very refractile nucleolus. During oocyte growth extracellular coat develops around plasma membrane. This acellular layer, known as the zona pellucid (ZP), is comprised by 3 main glycoproteins (ZP1, ZP2 and ZP3) which are assembled in long, interconnected filaments to create relatively porous coat approx 5 μm thick. From time of puberty, one developing follicle is stimulated every month to mature to finish development and to ovulate. This signifies that during about 40 years of a woman's reproductive life, merely 400 to 500 eggs will have been released. All the rest will have deteriorated. LH surge released by pituitary will, each month, activate one antral follicle to mature. Completely grown primary oocytes surrounded in Graafian follicles resume meiosis just prior to ovulation. The oocyte then progresses through metaphase, anaphase, and telophase of first division, emits first polar body, and, without stopping, enters second division up to metaphase. It is around this time that ovulation takes place, by rupture of follicle wall at the surface of ovary. In the oviduct, oocyte remains at metaphase II until it is triggered by fertilization to finish second meiotic division.
In comparison to large number of spermatozoa laid down in vagina at coitus, just very few sperm cells reach ampulla and are found in proximity of egg. Though sperm attraction to follicular factors has been claimed, sperm chemotaxis in mammalian fertilization hasn't been shown. Leading role in sperm-egg encounter is played by molecular organization of their surfaces, and abundant proof recommends that species-specific gamete recognition and binding is mediated by receptor molecules at gamete surface.
Initial contact between gametes takes place when sperm attach to unfertilized extracellular coat or zona pellucida. Capacitated, acrosome-intact sperm are capable of binding to zona pellucida using plasma membrane of sperm head. Binding is the significant requirement step for zona penetration as it starts events which culminate in induction of acrosome reaction.
One of the components of zona pellucida (ZP3) signifying primary sperm receptor is liable for both sperm-binding activity and ability to induce the complete acrosome reaction. Acrosome-intact sperm bind to ZP3 in the relatively species particular manner, this gamete identification and binding is mediated by carbohydrates and not by polypeptide chain. Several sperm are released from zona pellucida after undergoing acrosome reaction, thus far maintenance of sperm binding is attained by interaction of acrosome-reacted sperm with ZP2; hence, ZP2 serves as secondary receptor.
Sperm cells should experience acrosome reaction before they can go in zona pellucida and fuse with egg plasma membrane. Acrosome reaction progresses from multiple fusion-points between plasma and outer acrosomal membranes that expose inner acrosomal membrane and acrosomal contents (enzymes), to complete vesiculation and loss of integrity of the acrosome. Acrosome reaction bears the strong resemblance to ligand-mediated exocytotic reactions in somatic cells proceeding through the intracellular signal transduction system it comprises the participation of Gi protein, of phospholipase C and of protein kinase C. In addition, an increase in intracellular calcium is concomitant with the induction of acrosomal loss.
After sperm entry in perivitelline space, final phases of sperm-egg interaction comprise binding and fusion of sperm and egg plasma membranes, and entry of sperm in the egg. Sperm binding to egg surface takes place on lateral face of the head, with firm point of attachment between sperm and egg plasma membranes occurring at equatorial segment. Little is known concerning sperm and egg surface complementary molecules (binding sites) which participate in gamete plasma membranes fusion in mammals. It has been lately shown that sperm surface protein (PH-30, a guinea-pig sperm antigen), known to be involved in sperm-egg fusion, shares biochemical characteristics with viral fusion proteins and has integrin ligand domain. These results recommend that integrin-mediated adhesion event occurs and leads to fusion.
Egg activation and pronuclei formation:
Gamete fusion triggers replies within egg which culminate in activation of embryonic developmental program. Activation may also be induced partheno genetically under different physical or chemical stimuli, in all cases, calcium is the obligatory mediator. In mammals, sperm may cause both persistent production of inositol trisphosphate (InsP3) and the increase in calcium permeability of plasma membrane to maintain internal calcium oscillations. Early calcium increase makes cortical granule exocytosis (cortical reaction) that engages signal transduction system which is similar to that of somatic cells, and that leads to solidifying of zona pellucida. Activation leads to resumption of cell cycle: the second meiotic division is attained, by extrusion of second polar body and the egg enters in interphase with formation of pronuclei. The biochemical transitions liable for remodeling of sperm nucleus comprise of changes in majority of sperm specific chromatin proteins and attainment of chromosomal proteins that tempt chromatin conformation compatible with fusion of male and female pronuclei. Two phases of decondensation are seen: i) very rapid chromatin expansion dependent on egg nucleoplasmin, and ii) slow membrane-dependent decondensation involving protein migration in nucleus dependent on nuclear envelope formation recruited from maternal pool.
Two pronuclei move towards egg center, and spermaster increases in size during migration. Outcome of the migration of pronuclei is their combination, following pronuclear envelope breakdown, giving rise to the group of chromosomes for resulting division. Spatial organization of microtubule arrays in the cell is mainly dependent on organizing centers, centrosomes. Proximal centriole of the sperm and its centrosomal material between apposed pronuclei are involved in fertilization events. Human centrioles as those of other animals except mouse are paternally derived. Finally, there is the intermixing of maternally and paternally derived chromosomes to set up genome of embryo and therefore procedure of fertilization can be regarded as accomplished.
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