Biology Reference
In-Depth Information
although they adhere, the palate shelves never fuse.
20
The medial epithelium, like any other,
has a basement membrane at its basal surface and this, too, needs to be cleared away. Matrix
metalloproteinases are activated locally where cells are undergoing apoptosis, and presum-
ably account for the disappearance of basement membrane components such as collagen IV.
The production of metalloproteinases fails if apoptosis is blocked but is encouraged if
apoptosis is increased by the addition of retinoic acid, which is a powerful inducer of cell death
in this system.
19,20
If adhering palates are treatedwith inhibitors of matrix metalloproteinases,
the basement membrane remains and, although adhesion between shelves and apoptosis of
medial epithelial cells still takes place normally, fusion fails.
21,22
The main evidence for clearance of medial edge epithelial cells by epithelial-mesenchymal
transition comes from retroviral cell marking experiments in which cells in the palatal
epithelium are labelled permanently by retroviral transformation (
Figure 19.8
). After fusion,
FIGURE 19.8
Fusion of the secondary palate. As the palate shelves approach, filopodia rich in chondroitin
sulphate are produced, the cells meet and adhere. Periderm cells move away from the site of adhesion to form
a triangular collection of dying cells above and below the seam. Cells of the medial epithelium disappear by cell
death and by epithelium-mesenchymal transition.
