Cell contact and surface coat alterations of limb-bud mesenchymal cells during differentiation.

Measurements of the cell perimeter, cell-cell contact number and length and of the thickness of the surface coat were performed in limb buds of mouse embryos during the process of cell condensation of the chondrogenic cell mass. This study starts with measurements of 'non-limb' mesenchyme of day-9 embryos and ends with the young cartilage of day 13. It is shown that until day 12 all cells in the limb bud are interconnected by cell contacts of the gap-junction type. Contact number and contact length increase during development, but increase about twice as much between central, prechondric cells as between peripheral or distal cells. Later in development, chondroblasts lose their contacts almost completely, whereas between peripheral cells, the amount of cell contacts drops to the initial value of mesenchymal cells. The cell size decreases during chondrogenesis. A decrease in the thickness of the surface coat of the cells during the whole differentiation period is shown. It may be assumed that this 'wave of cell contacts' is the last step for the initiation of the chondrogenic differentiation process.