Page 470 - Atlas of Histology with Functional Correlations

characterized by dense irregular connective tissue fibers (mainly type I

collagen), and is less cellular than the papillary layer. Also, this layer can
withstand more mechanical stresses and supports nerves, blood vessels, hair
follicles, and all the sweat glands. There is no distinct boundary between the two
dermal layers; the papillary layer blends with the reticular layer. Also, the dermis
blends inferiorly with the hypodermis, or the subcutaneous layer, which

contains the superficial fascia and adipose tissue.

The connective tissue of the dermis is highly vascular and contains blood
vessels, lymph vessels, and nerves. Certain regions of the skin exhibit
arteriovenous anastomoses used for temperature regulation. Here, blood passes

directly from the arteries into the veins. In addition, the dermis contains such
sensory receptors as Meissner corpuscles that are located in dermal papillae and
Pacinian corpuscles deeper in the connective tissue of the dermis.

FUNCTIONAL CORRELATIONS 12.1 ■

Epidermal Cells and Cell Layers

There are four cell types in the epidermis of the skin, with the keratinocytes
being the most dominant cells. Keratinocytes divide, grow, migrate up,
undergo keratinization, or cornification, and form the protective epidermal
and surface layer for the skin. The epidermis is composed of stratified

keratinized squamous epithelium. There are other less abundant cell types in
the epidermis. These are the melanocytes, Langerhans cells, and Merkel
cells, which are interspersed among the keratinocytes in the epidermis. In

thick skin, five distinct and recognizable cell layers can be identified.

STRATUM BASALE (GERMINATIVUM)—

THE DEEPEST LAYER

The stratum basale is the deepest or basal layer in the epidermis. It consists
of a single layer of columnar to cuboidal cells that rest on a basement

membrane separating the dermis from the epidermis. The cells are attached
to one another by cell junctions, the desmosomes, and to the underlying
basement membrane by hemidesmosomes. Cells in the stratum basale serve
as stem cells for the epidermis; thus, increased mitotic activity is seen in this

layer. The cells in this layer continually divide and mature as they migrate up
toward the superficial layers. All cells in the stratum basale produce

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