Okay,so when the little swimmer touches the egg, it isn't a person, when the egg is surrounded by swimmers, it isn't a person, if one gets in, it isn't a person, but when the nuclei touch it becomes a person? Here is a link to the biochemistry of fertilization:
matweb.hcuge.ch
You know, I read it several times, and I still don't see that part about where the gametes bring their haploid souls together to generate a conjugate soul? Could you explain this process in scientific terms? I don't want to be vague about the instant "life" begins to become human.
Just askin'. I liked this quote (inside the little ""s, in case anyone is confused - emphasis added). "Eventually, there is an intermixing of the maternally and paternally derived chromosomes to establish the genome of the embryo and hence the process of fertilization can be considered as concluded."
So, E, just how long do you suppose "eventually" is? God should just send out a burst of light or something to let us all know, don't you think! That would have made it easy.
Gamete interaction
In comparison to the large number of spermatozoa laid down in the vagina at coitus, only very few sperm cells reach the ampulla and are found in the proximity of the egg. Although sperm attraction to follicular factor(s) has been claimed, sperm chemotaxis in mammalian fertilization has not been demonstrated. The leading role in the sperm-egg encounter is played by the molecular organization of their surfaces, and abundant evidence suggests that the species-specific gamete recognition and binding is mediated by receptor molecules at the gamete surface.
Initial contact between gametes occurs when the sperm attach to the unfertilized extracellular coat or zona pellucida. Capacitated, acrosome-intact sperm are capable of binding to the zona pellucida via the plasma membrane of the sperm head. Binding is an important prerequisite step for zona penetration because it initiates events that culminate in induction of the acrosome reaction.
One of the components of the zona pellucida (ZP3) representing the primary sperm receptor, is responsible for both the sperm-binding activity and the ability to induce a complete acrosome reaction. Acrosome-intact sperm bind to ZP3 in a relatively species specific manner, this gamete recognition and binding is mediated by carbohydrates and not by the polypeptide chain. Many sperm are released from the zona pellucida after undergoing the acrosome reaction, yet maintenance of sperm binding is achieved by interaction of acrosome-reacted sperm with ZP2; thus, ZP2 serves as a secondary receptor.
Putative ZP-binding-glycoproteins of spermatozoa have been recognized in various species. Several egg-binding proteins are envisaged on the sperm membrane that impart species specificity. The postulated candidates are the following: a 95kDa protein (p95SPERM) showing a tyrosine kinase activity that is stimulated on binding and whose activation requires aggregation, a 56kDa protein (p56) of unknown function, an antigen designated p200/220 (whose monoclonal antibody is named M42) necessary for zona-induced acrosomal reaction, another related antigen the SAA-1 antigen detected on all mammalian sperm acrosomes, a ß-1,4-galactosyl-transferase mediating fertilization by binding oligosaccharide residues on zona pellucida glycoprotein. Many evidence suggest also the possible involvement of protease inhibitor sites and mannosidase sites or of other molecules called spermadhesins showing features of serine proteases having lectin-like activity.
Proteolytic enzymes appear to participate in multiple phases of mammalian fertilization, including acrosome reaction, sperm binding to zona pellucida (ZP), ZP penetration and zona reaction, however, the enzymes involved have not been completely identified. A role for sperm proacrosin and acrosin, the best characterized sperm protease, in ZP binding and penetration has been postulated. Several observations suggest that the plasminogen activator/plasmin system might play a role in mammalian fertilization. First, both mouse gametes express plasminogen-dependent proteolytic activities: ovulated eggs contain and secrete tissue-type PA (t-PA) and ejaculated spermatozoa exhibit urokinase-type PA (uPA). Second, t-PA is significantly higher in follicular fluids and granulosa cells from follicles containing oocytes that can be fertilized in vitro compared to follicles containing oocytes that fail to fertilize. Third, the addition of plasminogen to the fertilization medium increases the frequency of eggs fertilized in vitro.
Sperm cells must undergo the acrosome reaction before they can penetrate the zona pellucida and fuse with the egg plasma membrane. Acrosome reaction progresses from multiple fusion-points between the plasma and outer acrosomal membranes, which expose the inner acrosomal membrane and the acrosomal contents (enzymes), to complete vesiculation and loss of the integrity of the acrosome. The acrosome reaction bears a strong resemblance to ligand-mediated exocytotic reactions in somatic cells proceeding through an intracellular signal transduction system, it involves the participation of a 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. Acrosome reaction can be induced by biological agents such as follicular fluid (progesterone), cumulus cells or zonae pellucidae or by physiochemical agents such as calcium ionophores, lysophosphatidylcholine and electropermeabilization or by the aggregation of zona binding sites on the sperm heads.
Sperm-egg fusion
After sperm entry into the perivitelline space, the final stages of sperm-egg interaction include the binding and fusion of the sperm and egg plasma membranes, and entry of the sperm into the egg. Sperm binding to the egg surface occurs on the lateral face of the head, with the firm point of attachment between the sperm and egg plasma membranes occurring at the equatorial segment. Little is known concerning the sperm and egg surface complementary molecules (binding sites) that participate in gamete plasma membranes fusion in mammals. It has been recently shown that a 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 contains an integrin ligand domain. These results suggest that an integrin-mediated adhesion event takes place and leads to fusion.
Fusion of a single sperm sets in motion a series of egg reactions to prevent additional sperm entry, thus avoiding the lethal consequences of polyspermy. Egg cortical reaction takes place soon after fusion, causing the zona pellucida to become " hardened " and refractory to both binding and penetration of supernumerary sperm. Zona binding is prevented by the inactivation of the sperm primary receptor (and acrosome inducer) ZP3 and zona penetration is stopped through modification of the sperm secondary receptor ZP2. The cortical reaction involves the exocytosis of cortical granules and the release of their enzymatic content into the perivitelline space. The oligosaccharides of ZP3 responsible for gamete recognition and adhesion are modified by cortical granule glycosidase(s) and the glycoprotein ZP2 undergoes limited proteolysis making the zona pellucida more insoluble and " hardened ", preventing the maintenance of binding of acrosome-reacted sperm to the zona pellucida. It has been suggested that the oocyte plasminogen activator may participate in this proteolytic process although the evidence is poor.
Egg activation and pronuclei formation
Gamete fusion triggers responses within the egg that culminate in the activation of the embryonic developmental program. Activation may also be induced parthenogenetically under various physical or chemical stimuli, in all cases, calcium is an obligatory mediator. In mammals, sperm may cause both a persistent production of inositol trisphosphate (InsP3) and an increase in calcium permeability of the plasma membrane to maintain internal calcium oscillations. The early calcium increase induces cortical granule exocytosis (cortical reaction), which involves a signal transduction system that is similar to that of somatic cells, and that leads to the hardening of the zona pellucida. Activation leads to the resumption of the cell cycle: the second meiotic division is achieved, by the extrusion of the second polar body and the egg enters into interphase with formation of pronuclei. Pronuclear formation takes place a few hours after fertilization, and requires a calcium increase and a cytoplasmic alcalinization of the zygote. Following anaphase II, the egg chromosomes remaining in the cytoplasm disperse and the female pronucleus forms. Similarly, after cell fusion, the sperm nucleus is decondensed and transformed into a male pronucleus. The biochemical transitions responsible for the remodelling of the sperm nucleus consist of changes in the majority of sperm specific chromatin proteins and the acquisition of chromosomal proteins which induce a chromatin conformation compatible with fusion of male and female pronuclei. Maternal chromatin and sperm pronuclear development are regulated by common egg cytoplasmic factors involved in the regulation of the cell cycle and dependent on oocyte maturation. The pronuclear development in fertilized eggs is known to proceed through a series of transformations, which restore the transcriptional competence of the inactive gamete chromatin and re-establish the functional diploid genome of the embryo. Two stages of decondensation are observed: i) a very rapid chromatin expansion dependent on egg nucleoplasmin, and ii) a slow membrane-dependent decondensation involving protein migration into the nucleus reliant on nuclear envelope formation recruited from maternal pool.
The two pronuclei move towards the egg center, and spermaster increases in size during their migration. The end result of the migration of the pronuclei is their juxtaposition, following pronuclear envelope breakdown, giving rise to a group of chromosomes for the ensuing division. The spatial organization of microtubule arrays in a cell is largely dependent on organizing centers, the centrosomes. The proximal centriole of the sperm and its centrosomal material between apposed pronuclei are involved in the fertilization events. Human centrioles as those of other animals except the mouse are paternally derived. Eventually, there is an intermixing of the maternally and paternally derived chromosomes to establish the genome of the embryo and hence the process of fertilization can be considered as concluded.
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