Fine-structural identification and organization of the epidermal proliferative unit.

Mouse dorsal interfollicular epidermis has a vertical organization consisting of approximately 1400 units/mm*. Each unit is identified by its uppermost 4 to 6 layers of roughly hexagonal flattened cornified cells or squames, which are stacked in a precise vertical column. Beneath each column of squames there are 3 differentiating cells and a group of 10 or 11 basal cells, which provide cells to replace those lost by desquamation at the top of the squame column. The whole unit is termed an epidermal proliferative unit or EPU. In the central region of the basal layer of each EPU is a single dendritic Langerhans cell, devoid of desmosomes, but held in position by the surrounding keratinocytes. The dendrites radiate outwards between the keratinocytes to the periphery of the unit. Keratinocyte migration into the spinous layer takes place from the edge of the unit. In the squame column itself, a modified cell-to-cell attachment (squamosome) runs around the edge of each cornified cell, and attaches it to the overlapping region of cells from adjacent squame columns.