Science: Under the microscope: The right heart

SYMMETRY AND HANDEDNESS

Did Christ have situs inversus? That is, were all his organs, his heart in particular, on the opposite side to normal? I have been told by an art historian that almost all drawings and paintings put the wound he received from the centurion on the right side, instead of the left, and that when the heart is visible it, too, is on the right. If this were true then Christ could have had reversed symmetry of the body organs, a rare condition that occurs in about one in 10,000 people without any ill effect.

However, an absence of full reversal of handedness of the organs, particularly the heart, is associated with a number of congenital abnormalities. The heart, an asymmetrical structure, has well-defined left and right chambers. During development the embryo has to know which is the left and which is the right side. Left and right have no meaning on their own and can only be identified once both the head-to-tail axis and the front-to-back axis have been set up. This can be clearly seen by looking in a mirror: the left hand becomes the right because looking in the mirror reverses the front-to-back axis.

How does the embryo do it? We do not know, but recent work is beginning to provide clues. We have a mode which suggests that the only way for the embryo to know its left from right is to make use of an asymmetrical molecule and to somehow convert this molecular asymmetry to asymmetry of cellular behaviour, so making the two sides of the embryo different. There is a mutation in a gene which can make the left/right asymmetry of mice random - about half of all mice have their hearts on the left and the rest on the right. This gene codes for a motor protein involved in moving cellular components along rod-like structures in the cell. One idea is that if these structures are properly oriented, then this motor can move material from one side of the embryo, across the midline, to the other and so define left and right. It is when this and the further cell activities go wrong that left/right-handedness is incorrectly specified and so leads to heart abnormalities.

There is more to handedness than organs. There is handed localisation of certain brain functions, the centre for speech, for example, being usually on the left side of the brain. And then there is handedness itself, left- and right-handedness, with we left-handers being a minority, about 10 per cent. There is a clear genetic component to left-handedness and even evidence that left-handers have on average a very slightly lower IQ, and that we are also more asymmetrical in our normally symmetrical structures like limbs and faces.

Symmetry matters and there is abundant evidence from bird studies that the preferred mate is the most symmetrical one. The reason is that the symmetry of structures that should be symmetrical is an indication of the reliability of embryonic development and the absence of developmental errors. There is also evidence that human facial beauty is linked to symmetry.

Left-handers may live in a right-handed world but there is one major invention that gave them the advantage - the Singer sewing machine. The machine was driven by the right hand but the tricky manipulations were done with the left. The persistence of left-handedness may be related to the small advantage it can give in certain circumstances, like sword- fighting. But handedness is not confined to humans - about two thirds of apes are right-handed and most parrots are left-footed.