Friday, November 27, 2009

Salute the SPERM

                     a group of sperms with a distinct Head and Tail

How are we made? How do we end up having millions of cells when we have only started with 2 of them (actually 1, after they have fused)?

Imagine this: we are totally non-existent until one among the father's millions-of sperms would have located and fused with the mother's egg (oocyte as it is called). Imagine also that the sperm and the egg are totally powerless to make us, all alone by themselves - i.e to make us, it's inevitable that they beat all odds and meet each other!

And what's even more incredible is that once they have met successfully, the destiny is sealed- they must forgo their individual entities and must now function as a whole, a new entity, which is both similar and different to each other. In other words, they must die as individuals, to make us!

The sperm is the only motile cell of the body. The only cell, which can, by virtue of its huge reserves of energy and a peculiar structure (it has a 'head' and a huge tail stcuk to its behind), undergoes a swift biochemical change (Capacitation, which I'll explain later), beats its tail frantically to generate a forward propulsive force and moves extremely rapidly in order to meet its paramour, the oocyte. The sperm, has a haploid (x) number of chromosomes (so does the oocyte), in order to give rise to a normal (2x) number upon fusion.

During Spermatogenesis, the spermatogonia (2x) undergo meiosis to form mature spermatids (x), which in the female tract, undergo this fantastic biochemical-switch called Capacitiation (which is actually a cumulative term for 2 separate events - Hyperactivation and Acrosome reaction) and is characterised with a distinct, almost-violent, whiplash movement of the tail (resembling the letter '8').

Interestingly, a huge-Calcium ion influx into the sperm, aided by the sudden high-fluidity of the it's cell-membrane is necessary and sufficient to bringing about capacitation. It has been shown experiments with Ca-channel blockers, ionophores etc, that sperm-capacitation could be blocked.

It is also evidenced by various groups, that the Capacitation-event is a result of distinct tyrosine-residue phosphorylation of several important proteins (as it turns out ser-thr phosp. play a relatively-smaller role in it). Obviously, protein kinases, mainly of the EGF-R family and also RTK and TK's of the NR type are the major culprits for bringing our this phenomenon!  Inhibition of tyr. residue phosphorylation-events by suitable inhibitors have shown the dependancy of the event on crucial catalytic-activity of the kinases. Interestingly, only some Phosphatases have been shown to play a critical role in the event. As it turns out, the sperm has a super-compact DNA (Histones are replaced by Protamines, which are more basic-proteins and therefore package the DNA even more tightly). Hence, only proteomic and not genomic actions, decide the fate of this motile-cell!!

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