The Ubiquitous Symmetry

Ever wondered why our bodies are so symmetrical? I mean, if you cut our (and also other animals') bodies in half (don't do a Jack the Ripper though!), through a line that passes through the nose, navel, the perineum (i.e., a sagittal section); you get two halves, left and right, which exhibit a near perfect mirror image symmetry. Not only are they found in the animal kingdom; in a molecular level (e.g. in crystal lattices), and subatomic level (electron spins) too they exist.

The question naturally arises why nature exhibits this mathematical expression. A complacent and plausible explanation would be that it was an act of the evolutionary selection process (in the case of animals). A symmetrical body would give us the advantage in standing upright, against gravity. Assymetricality would mean a 'couple' (the couple/torque of physics: and not husband and wife!) working on the body, thus making it fall.

We should not nevertheless, ignore the plants' own aptitude in mathematics too. They are expert in number theories, in that, they exhibit Fibonacci sequence ( 0, 1, 1, 2, 3, 5, 8, 13, 21, 34......every number is the sum of the previous two numbers), fractal geometry (image to the left) and many others. Another interesting thing that amazes me about plants is the way the tree trunks so faithfully point towards the center of the earth, even on a sloping hill. In other words, the tree trunks could be considered as an extension of the earth's radius.

Lets discuss the point why and how we are symmetrical. When a sperm meets and combines an ovum, a zygote is formed. The cells in this zygote multiplies resulting in a ball kind of stuff, called the blastocyst. In this sphere like stuff, only the central portion remains, in the form of a disk, while the rest of the sphere disappears. This central disk then differentiates (=evolves into different kinds of tissues) into three layers: an outer ectoderm, an inner endoderm and an intervening mesoderm sandwiched between them. Now this three layered disk then folds in such a way so that the edges of the disks appose and merge. This merging point is the navel or umbilicus (and other midline structures), via which the fetus gets nutrition from the placenta, via the umbilical cord. The limbs like arms or legs form as an offshoot from this folded structure. No wonder then, that the growing fetus will be symmetric, since the left and the right halves including the limbs are forming almost identically from a single entity.

Dysmorphisms do occur though, despite all this. In the case of other animals too, this same kind of embryogenesis is seen (and expected too), as ontogeny repeats phylogeny.

Can't Spam The Ovum

Do you have the courage to ask your friends to spam your email inbox? I guess not. Though I will not be talking about e-mails in this post, I'll only compare the challenges a tiny ovum throws to a spam (flock) of sperms and wins the challenge too.

During procreation, sperms from the males are ejected (ejaculated) through the external urethral meatus. They get vigor (energized, by the action of fructose, a simple carbohydrate found in the vagina; a process called capacitation) in the female genital tract. Spirited and intoxicated, as they become, they (actually, millions of them) now charge towards the lonesome ovum like a raging bull (of Spain!). But this microscopic ovum is quite fussy in choosing just one (among say those 200-300 million!) .

Whether the ovum chooses the sperm or any random sperm or the fittest sperm chooses the ovum is something I do not know. Nobody knows for certain. But the accepted theory says it is the random sperm that hits the bull's eye. As it takes two to tango, the sperm and the ovum dance a jig and they drop their "veils" (polar bodies). They kind of, have a one to one talk, before they finally unite to achieve their goal (zygote).

Meanwhile the 'other' sperms continue their ova-ward journey, but now that the ovum has activated it's own firewall, it dodges them very efficiently. This firewall called zona pellucida is active once one sperm has unloaded its DNA in the ovum.

May be we can improve our own computer's firewall if we learn the ova's dialect. We have learned a few things like AI (artificial intelligence) from our knowledge of the brain and the plasticity of its synapses.

What happens next is very interesting. The 'ball of cells' (blastocyst) metamorphoses into the animal that are us. This is what Embryogenesis is about.