A nice, worm surprise
an old dogma in developmental biology is that there are two classes of developmental systems. Each class embodies principles that ensure the orderly progression of a relatively unstructured fertilised egg to the highly structured adult. The classes are called mosaic and regulative. A mosaic embryo is believed to come pre-equipped with instructions that enable different parts to perform different functions as the embryo develops. On the other hand, in a regulative embryo, functional differentiation is caused by spontaneous self-organisation involving a significant extent of intercellular signalling or mutual interactions between the cells that comprise the embryo.
One might compare a mosaic embryo to a society in which professions are pre-determined by birth and a regulative embryo to one in which what a person does for a living depends on what others are doing.The text book examples of mosaic development are provided by insects and worms. In contrast, mammalian development is believed to be highly regulative.
This clear dichotomy has been recently thrown into question by experi-mental work carried out on the roundworm Caenorhabditis elegans. Early reassert suggested that in this worm, all the embryos had same patterns of cell division and subsequent development, thereby implying a mosaic form of development. Later research showed that the situation was not this simple at all, because when some cells were rearranged in the embryo, it went on to develop normally - implying the cells displayed a fair degree of flexibility. The latest finding by T Kaletta, H Schnabel and R Schnabel of the Max Planck Institute in Martiasried, Germany, (Nature , Vol 390, No 6657) shows that a defect in a single gene, lit-1, results in a roundworm with defective development all over the body. More specifically, there seems to be a strange tendency in a cell that develops a posterior body part, to develop an anterior body part instead. That a single gene can give rise to such "global" aberrations implies that intracellular signalling must play an important role in the development of this supposedly mosaic organism. The dichotomy between mosaic and regulative development may be much weaker than had been thought so far.