Anatomy and Physiology of Adult Friction Ridge Skin
CHAPTER 2
FIGURE 2–9 Electron micrograph of a focal tight junction between adjacent keratinocytes. Magnification = 12500 X. (Reprinted with permission from Cavoto and Flaxman (1972), p 372.)
FIGURE 2–10 Basal cells of the primary ridges. Scale bar is 10 µm. (Reprinted with permission from Lavker and Sun (1983), p 123.)
primary ridges with the basement membrane is slightly undulated (Figure 2–10), whereas basal cells of the secondary ridges contain long projections that extend deep into the dermis (Figure 2–11) (Lavker and Sun, 1982, p 1240). The differences in the structure of the basal cells in the primary and secondary ridges explain their differences in function. The basal cells of secondary ridges, with long projections into the dermis, serve an anchoring function (Lavker and Sun, 1982, p 1239). The basal cells of the primary ridges have a morphology similar to stem cells and can be induced to multiply by tissue demand or injury (Lavker and Sun, 1982, p 1239). The basal cells also differ in the rate at which they multiply. The basal cells of the secondary ridges divide more frequently than the primary ridges because the basal cells of the primary ridges give rise to cells that divide in the suprabasal layer. Suprabasal Layer. The basal keratinocytes of the secondary ridges continuously divide—each basal cell dividing to push one cell at a time into the stratum spinosum. The basal cells of the primary ridges behave a little differently.
The basal keratinocyte of the primary ridge divides to create a new cell. This new cell does not immediately enter the stratum spinosum and commit to differentiation. The newly generated cell, termed a transient amplifying cell, undergoes a couple of cell divisions while it sits in the suprabasal layer (Lavker and Sun, 1983, p 121). After cell divisions are complete, the transient amplifying cells are pushed upward into the stratum spinosum and begin differentiation. More cells are produced in the primary ridges than in the secondary ridges because of the transient amplifying cells. The cells of the primary ridges maintain the surface ridges, where more cells are needed because of greater abrasion. 2.2.6.2 Stratum Spinosum. As the keratinocytes are pushed toward the surface, they begin to undergo differentiation. The cells become polyhedral in shape and desmosomes (cell junctions) are reinforced. Keratin production is increased, and the keratin filaments are organized concentrically around the nucleus and extend into the desmosomes (Freinkel and Woodley, 2001, p 23). New
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