Question

Somites.

What is the relationship between the determination front and cycling levels of Notch activity in the formation of somites?

Answer 1

Formation of somites

The periodic formation of somites during vertebrate segmentation has been suggested to involve a molecular ‘segmentation clock.The details of somitogenesis, and the terminology, are slightly different for the different classes of vertebrates . The recent revelations have come mainly from mouse and chick embryos, on which we concentrate here. Somites are formed from presomitic mesoderm — a continuous slab of loosely packed cells on each side of the body axis, generated in the primitive streak and deposited by it as it moves tailwards, like a trail of vapour from a travelling jet engine. As the newly-generated mesoderm cells mature, they change their adhesive properties and, at a certain critical distance from the primitive streak, abruptly aggregate, in such a way that the trail of pre-somitic mesoderm becomes broken up into discrete epithelial balls of cells — the somites. These form a precise, regular series, with perfect symmetry between the two sides of the body.

The outcome of somite formation is an alternating pattern in space, but to an observer watching the region where cells are becoming mature for segmentation, the process appears as an oscillation in time, between two styles of cell behaviour — cohesion to form a somite at one moment, and detachment to form a cleft at the next. This prompted long ago the speculation that somite formation might be controlled by some kind of oscillator in the cells of the presomitic mesoderm: the cells would cohere or detach from one another according to which phase of their oscillation they were in at the moment when they reached maturity. This is the ‘clock-and-wave-front’ model where ‘clock’ refers to the oscillator, and ‘wavefront’ refers to the travelling interface between immature (presomitic, oscillating) and mature (somitic, oscillation-arrested) mesoderm.

We now know, from studies in the chick , that there is indeed an oscillation of just the type hypothesized . Newly-generated mesoderm cells emerging from the primitive streak oscillate, in synchrony with one another, in their level of expression of hairy1, which encodes a basic-helix–loop–helix transcription factor and is a homologue of one of the Drosophila ‘pair-rule’ segmentation genes. The time for one oscillation cycle equals the time for formation of one somite — 90 minutes in the chick. As the cells mature, the rate at which they go round the gene expression cycle slows down, until finally their cycling comes to a halt as they embark on somite formation: cells arrested in a state of high hairy1 expression can be seen to form posterior portions of somites, whereas those arrested in a state of low hairy1 expression can be seen to form anterior portions.