1. Stratum corneum
2. Stratum spinosum
3. Stratum Basale
4. Epidermis (stratified squamous keratinized epithelium)
5. Dermis (dense irregular C.T.)
6. Subcutaneous C.T. (Adipose Tissue)
Thick and thin skin are differentiated primarily on the basis of the relative thickness of the stratum corneum, the outermost of five layers.
1. Stratum corneum - Consists of several layers of dead squamous cells that contain large amounts of a fibrillar protein, alpha keratin, in an amorphous protein matrix. They are held together and to the cells of the next layer by desmosomes. Cell nuclei do not stain.
2. Stratum lucidum - Found only in thick skin. This relatively thin layer is characterized by its translucent appearance. Consists of dead cells without stainable nuclei.
3. Stratum granulosum - The living cells composing this layer are characterized by the presence of numerous basophilic keratohyalin granules that are the major source of the matrix proteins in keratin. "Membrane coating granules" release their glycolipid to the intercellular space in this layer. This material provides a hydrophobic seal that prevents nutrients from reaching more superficial layers. As those layers die lysosomal enzymes are released that digest the nuclei and organelles.
4. Stratum spinosum - Consists of several layers of large prickle cells. They tend to be shrunken away from each other except where they are attached by desmosomes. Bundles of tonofilaments anchor into these desmosomes. Near the stratum granulosum these cells contain, in addition to tonofilaments or keratin fibrils, "membrane coating granules" whose synthesis involves the Golgi apparatus. Some proliferating cells are found in the deeper layers of this layer so that no single layer can be truly called the stratum germinativum.
5. Stratum basale - A single layer of cuboidal to columnar cells next to the basement membrane, where hemidesmosomes are found. Proliferation usually takes place here, and the cells (keratinocytes) differentiate with many free polysomes to form the fibrillar protein, keratin, as they move to the surface. There are many hemidesmosomes along the basal membrane of these cells.
1. Papillary layer forms the dermal papillae that are capped over by the epidermis. The papillae have different shapes in different regions. They contain loops of nutrient blood vessels and specialized nerve endings. This layer is thin and consists of thin collagen fibers, largely Type III.
2. Reticular layer is the bulk of the dermis. It is thick and contains thick collagen fibers, Type I. Many elastic fibers are also found in this layer.
II. PHYSICAL AND NUTRITIONAL SUPPORT OF THE INTEGUMENT
A. Subcutaneous c.t.
Consists of adipose tissue and is the site of large blood vessels. It is subdivided into compartments by dense fibrous bands (the retinacula cutis) that attach the dermis to the deep fascia.
B. Blood Supply
Several arterial and venous plexuses are under and in the dermis. Control of blood flow in these plexuses and the capillary nets they supply is an important mechanism for controlling heat loss from the body.
1. Arterial plexuses
a. Rete cutaneum is located at the dermal - subcutaneous junction. Vessels from this network supply hair follicles and sweat glands on the subcutaneous side and dermal structures on the other side.
b. Rete subpapillare is located at the junction of the reticular and papillary layers. It receives blood from the rete cutaneum.
c. Papillary loops arise from the rete subpapillare and are large capillaries that supply each dermal papilla. Look for capillary loops in the dermal papillae and differentiate them from Meissner's corpuscles.
2. Venous plexuses
a. Subpapillary plexus is mingled with the rete subpapillare at the junction of the reticular and papillary layers.
b. Midreticular plexus, a distinctively venous plexus, is between the rete cutaneum and the rete subpapillare.
c. Deep plexus is at the same level as the rete cutaneum.
A specialized arterio-venous anastomosis found in palmar and plantar skin, including the fingers and toes. It consists of a coiled arterial segment, a middle segment surrounded by contractile epitheloid (glomus) cells, and a venous distal segment. They are very important in the thermoregulation of the extremities and of the body. Other arteriovenous anastomoses are found in the lips, nose, eyelids, and nail beds.
III. SPECIALIZED FEATURES
1. Melanocytes, derived from neural crest cells, are found among the basal cells of the epidermis of thin skin (fewer in thick skin). They contain tyrosinase-loaded vesicles that actively accumulate tyrosine. The tyrosinase converts tyrosine to DOPA and that to melanin. The vesicles lose their tyrosinase activity and be come melanin granules that are transported out through the processes of the melanocytes and transferred to the keratinocytes (keratin forming epithelial cells). Melanocytes typically contain few melanin granules. Melanin granules in phagosomes are concentrated on the sunny side of the nucleus of keratinocytes in the stratum basale. They are digested by lysosomal activity as the cell moves toward the surface.
2. Langerhan's cells - dendritic cells that are poorly stained with H&E and that are found throughout the epidermis but not attached to other cells by desmosomes. These cells migrate into the epidermis as well as the epithelia of mucosal surfaces from the bone marrow. They function to accumulate antigens, and then migrate to local lymphoid tissues to present those antigens to T-cells. They are therefore important in immune defense and allergy. The AIDS virus has been found in these cells. The dendritic cells of lymphoid tissues have the same origin and perform a similar function.
3. Merkel cells - are attached to other cells by desmosomes but contain granules similar to those in adrenal medulla. These cells are especially numerous in regions where nerve endings are abundant and may contribute to sensory transduction.
1. In epidermis, naked nerve endings are thought to be primarily for pain sensation.
2. In dermal papillae, several varieties of sensory endings have been described. The encapsulated are Meissner's, Kraus. Those of Ruffini are flattened expanded disks of axoplasm applied to the deep aspect of the epidermis.
3. Around hair follicles, a network of nerves can be demonstrated. They are activated when the hair is touched.
4. Pacinian corpuscles consist of a nerve ending surrounded by multiple layers of flat cells derived from the axolemma, Schwann cells. They are found in thick skin at the deep aspect of the dermis and in multiple sites through out the body. They serve as pressure receptors.
C. Pilosebaceous organ - found only in thin skin.
1. Hair follicle - develops from a down growth of epidermis, that forms the external root sheath. Connective tissue of the papillary dermis is pulled down with it to form a c.t. sheath. In the subcutaneous c.t. the external epithelial sheath turns inward to form the hair matrix, that caps over a vascular c.t. papilla. The matrix is the site where cellular proliferation gives rise to the cells that will become keratinized to form the hair shaft. In the lower third of the follicle the developing hair shaft is separated from the external root sheath by an internal root sheath that includes two cell layers: Henle's (outer) and Huxley's (inner). These cells degenerate releasing the hair shaft from the external epithelial root sheath and a space into which sebum is secreted. These can be studied on a section of scalp.
2. Sebaceous gland - an outgrowth from the mid portion of the hair follicle. Its secretion, sebum that is rich in a variety of lipids, is released into the hair follicle around the hair shaft. The gland secretes by the holocrine method, new cells being formed by proliferation at the periphery of each alveolus. In some sites, nose and eyelid, sebaceous glands have their own duct independent of hair follicles. In the eyelid the Meibomian glands have a long straight duct with multiple alveoli opening into it. The oily secretion creates a hydrophobic barrier at the margin of the eyelid.
3. Arrector pili muscle - a diagonally oriented fascicle of smooth muscle attaching to the c.t. sheath of hair follicle deep to the sebaceous gland and to the c.t. of the papillary dermis. Contraction forces secretion from the sebaceous gland and causes "goose bumps."
4. Hair growth cycle - The hair follicle has three stages of activity: 1) active growth, (anagen), 2) involution (catagen, hair falls out) and 3) resting (telogen). Follicles in different parts of the body and in different individuals differ in the timing of the cycle resulting in different hair lengths.
5. Hormone regulation - steroid stimulation, especially androgens, are responsible for the changes in hair growth during puberty. Vellus hair (fine and nonpigmented) in the child changes to terminal hair (coarse and pigmented). Changes in the levels of hormone or the expression of receptors and/or cellular response can lead to abnormal hair growth or hair loss.
D. Sweat glands - Simple coiled tubular glands with a duct portion and secretory portion. The coiled portion is at the dermis subcutaneous junction and contains the secretory tubule and part of the duct.
1. Merocrine (eccrine) gland - the most common type found in both thin and thick skin. Its duct is stratified (2 layers) cuboidal epithelium. These cells are small and darkly stained so that the duct has a smaller diameter and is darker than the secretory tubule. Myoepithelial cells are abundant and surround the secretory tubule that consists of 2 layers of secretory cells. The inner cells are dark (ribosomes) and contain glycoprotein secretion granules. The outer layer of cells that rest on the basal lamina communicate with the gland lumen by intercellular canaliculi. They are the source of the watery secretions. Secretion is stimulated by cholinergic nerves from the sympathetic system (this is an exception). Merocrine glands can be seen in sections of both thick and thin skin.
2. Apocrine- these are unusual sweat glands found in the axilla, groin, and perianal regions. They are rather large and their ducts empty into hair follicles. They only become active at puberty and are stimulated by adrenergic nerves. They probably secrete substances with pheromone activity. Their distinctive odor is partly the result of bacterial activity.
3. Ceruminous glands of the external auditory canal are a modification of the apocrine type.
Consist of hard keratin that forms in a manner analogous to hair. The nail bed is highly vascular - loops of capillaries extend into the longitudinally-arranged dermal ridges giving the pink color that can be seen through the translucent nail. Experienced clinicians can judge adequacy of oxygenation from that color. The epithelium of the nail bed consists only of the stratum basale and stratum spinosum. At the free border of the nail, cornification (keratinization) of this epithelium occurs, forming the hyponychium. The eponychium is the keratin of the epidermis of the nail wall as it extends onto the exposed surface of the nail.
IV. Specific regions
B. Ala of the nose - outside it is covered by thin skin with atrophic hair follicles but large sebaceous glands. It is supported by hyaline cartilage and the lining of the vesibule is like thin skin with large hair follicles.
C. Eyelid - has thin skin on the outside with fine hair follicles. At the margin there are large hair follicles (eye lashes). Two kinds of glands empty into these follicles; small sebaceous glands (glands of Zeis) and special spirally arranged sweat glands (glands of Moll). Just under the thin skin is a layer of skeletal muscle (orbicularis oculi) and deep to that and just under the conjunctiva is the tarsal plate made of dense irregular connective tissue containing the Meiobomian glands. The lipids secreted by these sebaceous glands create a hydrophobic barrier at the margin of the eyelid and spread over the lacrimal fluid to reduce its evaporation.
D. Scalp - this is thin skin with many large hair follicles, if it is not from a bald man.
E. Skin of the scrotum - is thin skin with a layer of smooth muscle (tunica dartos) in its dermis.