Dermis, the true skin

The dermis, or true skin (derma, cutis, corium) is located between the epidermis and the subcutaneous fat. Two layers are distinguished in it, the papillary (stratum papillare), or the subepithelial layer and the reticular layer (stratum reticulare). The papillary layer is that part of the dermis which is found between the epidermis and the superficial network of blood vessels. The reticular layer merges with the subcutaneous fat and is not demarcated from it sharply. The epidermis is molded into the dermis in the form of rounded strands between which the papillae penetrate, which lends the boundary between the epidermis and the dermis the appearance of an uneven wavy line; this ensures close joining of the epidermis and the true skin. The height and shape of the papillae in the same layer but in different areas of the skin differ. On the palms and soles, for instance, the papillae may be very high, whereas on skin areas with a thin epidermis they form only a wavy line which can hardly be distinguished. The presence of the papillae probably improves the conditions for the nutrition of the epidermis because in this way the area of contact between it and the dermis is greatly increased as a result of which the supply of nutrients to the epidermis from the papillary capillaries is easier. The dermoepidermal junction is an undulating basement membrane that adheres the epidermis to the dermis. It is composed of 2 layers, the lamina lucida and lamina densa. The lamina lucida is thinner and lies directly beneath the basal layer of epidermal keratinocytes. The thicker lamina densa is in direct contact with the underlying dermis. These structures are the target of immunologic injury in bullous pemphigoid and epidermolysis bullosa.

The true skin is composed of a fibrous substance of collagen, elastic, and argyrophil (precollagenous) fibres, and an astructural amorphous interstitial substance found between the connective-tissue fibres. The dermal papillary layer consists of thin bundles of collagen fibres and many fine elastic and argyrophil fibres. In the reticular layer the collagen bundles are more compact and thick and intertwine into a thick network of loops.

Here also numerous elastic fibres, thicker than those in the papillary layer, lie between the collagen bundles and form a thick reticuluna. The structure of the reticular layer determines on the main the strength of the skin which differs with the skin area. The reticular and particularly the papillary layer of normal skin have a small number of various cell elements: fibroblasts, histiocytes, lymphocytes, mast and plasma cells, and peculiar pigment cells (melanophages).

Hairs, glands (epithelial appendages of the skin), muscles, vessels, nerves and nerve endings are located in the dermis.

The deep part of the skin (subcutaneous fatty tissue), the hypoderm

The subcutaneous tissue, or hypoderm, consists of thick bundles of collagen and elastic fibres stretching from the reticular dermal layer and forming a wide-loop reticulum in which accumulations of large fat cells, lobules of fatty tissue, are lodged. The fat cells are almost completely formed of a large drop of fat which displaces the cell nucleus to the periphery, and a very small amount of protoplasm. The skin fascia which is a thick connective-tissue plate is also part of the hypoderm; it often fuses with the underlying periosteum or the aponeurosis of the muscles to form a single structure. Bundles of connective-tissue fibres stretch from the superior surface of the fascia to the dermis and form a sort of a lattice called retinaculum cutis. The thickness and length of the elastic and collagen bundles determine the degree of mobility of the skin over the underlying tissues and organs.

The thickness of the hypoderm is marked by considerable variability both in different persons and on different skin areas of the same person. It is usually about 2 mm thick over the skull, 5 to 10 mm thick on the back, while its thickness in the region of the abdomen and buttocks may reach several centimetres. There is no subcutaneous fatty tissue on the eyelids, under the nails, on the prepuce, small pudendal lips, and scrotum, and there is very little of it on the nose, ears, and lips (the vermilion border).

The subcutaneous fat protects the body from mechanical injuries and cooling. Vessels and nerves are found in its thickness and hair bulbs and coils of sweat glands may grow into its upper part.

Muscles of the skin. Muscles stretching obliquely in relation to the hairs and raising them on contraction (mm. arrectores pilorum) and muscle fibres in the walls of vessels and sweat glands are the smooth voluntary muscles of the skin. When the arrectores pilorum muscle contracts, it raises the hair and squeezes out the secretion from the sebaceous gland. Smooth muscle fibres which are not connected to the hair follicles are present in the skin of the scalp, forehead, cheeks, and dorsal surfaces of the hands and feet. Considerable layers of these fibres are found in the papillary layer of the skin on the scrotum, the axillae, the nipple, and in the region of the anus and the prepuce. The muscles which lend expression and mobility to the face are the striated muscles of the skin.