Chapter 2. Prehension

Chapter 6 - During Contact 207
digital skin, has an outer layer of anucleated, horny cells (the stratum
corneum), and 4-5 inner layers of viable cells (the stratum Malpighii).
Of the viable epidermal cells, the bottom, basal cells are oriented
perpendicular to the surface; as they migrate upwards, epithelial cells
become parallel to the skin surface. Ninety per cent of epithelial cells
are keratinocytes. Keratinocyte differentiation refers to the process
whereby dividing stem cells give rise to progeny cells destined to
migrate upward until they die and are lost from the surface (Eckert,
1989). The migration of newly formed mitotic cells from the basal
layer to the surface layer, and hence the epidermis replacing itself,
takes 12-30 days in unperturbed conditions (Eckert, 1989; Montagna
& Parakkal, 1974). In the transitional layer below the dead, cornified
surface cells, lipid5 filled granules fuse with the plasma membrane,
and release lipid contents into the extracellular space. These form
lipid-rich sheets that are believed to be extremely important in
"waterproofing" the epidermis (Eckert, 1989). Other functions of this
lipid sheet (e.g., as a lubricant) remain, to our knowledge,
uninvestigated. Adjacent epidermal cells are attached by intercellular
bridges (called desmosomes or maculae adherentes), maintaining the
integrity of the epidermis (Montagna & Parakkal, 1974).
The dermis, about 3 mm thick in digital skin, is a matrix of loose
connective tissue, composed of fibrous proteins, including collagen,
elastin and reticulin, permeated by a semigel matrix of
mucopolysaccharides. It is a tough and resilient tissue having
viscoelastic properties (Montagna & Parakkal, 1974; Wilkes, Brown
& Wildnauer, 1973). The elastin is insoluble, highly stable, and
returns to its original shape after deformations due to stress.
Microscopically, collagen has a hierarchical three dimensional
organization and is composed of fibers, fibrils and microfibrils. It
provides resistance to mechanical stress. Reticulin, a
lipoglycoprotein, is present in very small quantities. Least understood
of the dermal proteins, it may form a template for the extracellular
aggregation of collagen fibrils. The dermis has deeper reticular layers
composed of thick collagen fibers, and more superficial, papillary
dermal layers composed of thin collagen fibers, where fingers of
dermis project up into hollows of epidermis (Quilliam, 1978). It is
highly vascular, containing lymph vessels, nerve fibers, nerve
endings, and sweat glands.
In mammals, there are striking structural differences between
5A lipid is an organic compound consisting of fats and other similar substances;
they are insoluable in water.