The significance of vertical mitosis in epidermis.

In normal epidermis mitosis usually occurs in a plane horizontal to the dermo-epidermal junction, both daughter cells then remaining basal while an adjacent non-mitotic cell is detached from the baseline and forced distally. The evidence now indicates that only cells in a certain intermitotic state, called GIb, can be thus detached, and that when such cells are not locally available the expanding mitosis is forced into the vertical axis. The higher the mitotic rate the shorter is the intermitotic interval, and therefore the less probable it becomes that GIb cells will be locally available for extrusion and the more probable that mitosis will take place vertically. At the highest mitotic rates, as in hair bulbs, all mitoses are vertical. The relation of these findings to epidermal structure and to epidermal cellular homeostasis is discussed.