Organizing the Embryo: Segmentation

Insects, like all arthropods, are segmented. The body of Drosophila melanogaster is built from 14 segments:

What signals guide segment formation?

The process begins with the gradients of messenger RNA (mRNA) that the mother deposited in her egg before it was fertilized.

For the details, see Embryonic Development: Getting Started

Shortly after fertilization, these are translated into their proteins with

An Example: eve stripe 2

The gene even-skipped (eve) is expressed in 7 bands or stripes corresponding to 7 of Drosophila's 14 segments (skipping the even-numbered ones). The photo (courtesy of Peter A. Lawrence and Blackwell Scientific Publications) shows the 7 stripes of eve activation.

At first the gene is expressed in fairly broad zones, but in time its expression becomes restricted to ever-narrower stripes. The mechanism by which this occurs is known for the second stripe.

The eve promoter has binding sites for the proteins encoded by
bicoid (bcd)
hunchback (hb)
giant (gt)
Krüppel (Kr)

  • Trapped in a valley between high levels of the giant and Krüppel proteins, expression of eve in the second stripe finally becomes limited to a band of cells only one cell thick. (A different set of promoter sites is used in the third eve stripe so expression is not repressed there.)
In principle, then, such a system of interacting gradients of transcription factors could act as on-off switches, which in time partition the embryo into its future segments.

Drosophila development (and probably that of animals in general) passes through three rather different (although often overlapping) phases:
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25 April 2014