The skin is divided into three layers:
Epidermis
Dermis
Subcutaneous tissue.
The skin is thicker on the dorsal and extensor
surfaces than on the ventral and flexor surfaces.
Epidermis:
The #epidermis is the outermost part of the skin; it
is stratified
squamous epithelium. The thickness of the epidermis ranges from
0.05mm on the eyelids to 1.5mm on the palms and soles. The microscopic anatomy
of the epidermal-dermal junction is complex. The innermost layer of the
epidermis consists of a single row of columnar cells called basal
cells. Basal cells divide to form keratinocytes
(prickle cells), which comprise the spinous layer. The cells of the spinous
layer are connected to each other by intercellular bridges or spines, which
appear histologically as lines between cells. The keratinocytes synthesize
insoluble protein, which remains in the cell and eventually becomes a major
component of the outer layer (the stratum corneum). The cells continue to
flatten, and their cytoplasm appears granular (stratum
granulosum); they finally die as they reach the surface to form the stratum
corneum. There are three types of branched cells in the epidermis:
the melanocyte, which synthesizes pigment (melanin);
Langerhans'
cell, which serves as a frontline element in immune reactions of the
skin; and Merkel's
cell, the function of which is not clearly defined.
Dermis:
The dermis varies in thickness from 0.3 mm on the
eyelid to 3.0mm on the back; it is composed of three types of connective
tissue: collagen, elastic tissue, and reticular fibers. The dermis
is divided into two layers: the thin upper layer, called the papillary
layer, is composed of thin, haphazardly arranged collagen fibers;
the thicker lower layer, called the reticular layer,
extends from the base of the papillary layer to the subcutaneous tissue and is
composed of thick collagen
fibers that are arranged parallel to the surface of the skin.
Histiocytes are wandering macrophages that accumulate hemosiderin, melanin, and
debris created by inflammation.
Mast cells, located primarily about blood vessels, manufacture and release histamine
and heparin.
Dermal
nerves and vasculature:
The sensations of touch and pressure are received by Meissner's
and the Vater-Pacini corpuscles. The sensations of pain, itch, and temperature
are received by unmyelinated nerve endings in the papillary dermis.
A low intensity of stimulation created by inflammation causes itching, whereas
a high intensity of stimulation created by inflammation causes pain. Therefore,
scratching converts the intolerable sensation of itching to the more tolerable
sensation of pain and eliminates pruritus.
The autonomic system supplies the motor innervation of the skin. Adrenergic
fibers innervate the blood vessels (vasoconstriction),
hair erector muscles, and apocrine
glands. Autonomic fibers to eccrine sweat glands are cholinergic.
The sebaceous gland is regulated by the endocrine system and is not innervated
by autonomic
fibers.
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