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Structure Of The Skin
The skin is the largest organ of the human body. It is composed of tissue that grows, differentiates, and renews itself constantly. Because the skin is a barrier between the internal organs and the external environment, it is uniquely subjected to noxious external agents and is also a sensitive reflection of internal disease. An understanding of the cause and the effect of this complex interplay in the skin begins with a thorough understanding of the basic structure of this organ.
LAYERS OF THE SKIN
The skin is divided into three rather distinct layers. From the inside out, they are the subcutaneous tissue, the dermis, and the epidermis.
Subcutaneous Tissue
The subcutaneous tissue serves as a receptacle for the formation and the storage of fat, is a locus of high dynamic lipid metabolism, and supports the blood vessels and the nerves that pass from the tissues beneath to the dermis above. The thickness of the subcutaneous fat varies from one area of the body to another. The subcutaneous tissue constitutes the largest volume of adipose tissue in the body. Fat cells are derived from mesenchymal cells, as are fibroblasts. They are organized into lobules by fibrous septae, which contain most of the blood vessels, nerves, and lymphatics that nourish the skin.
Dermis
The dermis consists of connective tissue, cellular elements, and ground substance. It has a rich blood and nerve supply and contains pilosebaceous, apocrine, and eccrine structures. Anatomically, it is divided into two compartments. The first consists of thin collagen fibers, which are located beneath the epidermis (papillary dermis) and surrounding adnexal structures (periadnexal dermis). Together, the collagen fibers are regarded as a single unit called the adventitial dermis. This is an important unit because it is altered together with the adjacent epithelium in many inflammatory diseases. The second compartment, known as the reticular or deep dermis, is composed of thick collagen bundles and comprises the bulk of the dermis.
The connective tissue component of the dermis consists of collagen fibers, including reticulin fibers, and elastic fibers. These fibers contribute to the support and elasticity of the skin.
There are two different types of collagen that predominate in the dermis. Type I collagen forms the thick fibers in the reticular dermis. Type III collagen, also known as reticulin, forms the thin fibers within the papillary and periadnexal dermis. These reticulin fibers are not visible in hematoxylin and eosin-stained sections but can be identified with silver stains. They are abundant in certain pathologic conditions such as tuberculous granulomas, syphilis, sarcoidosis, and some mesodermal tumors. The proteins present in collagen fibers are responsible for nearly one fourth of a person's overall protein mass. If tannic acid or the salts of heavy metals, such as dichromates, are combined with collagen, the result is leather.
Elastic fibers are thinner than most collagen fibers and are entwined among them. They are composed of the protein elastin. Elastic fibers do not readily take up acidic or basic stains, such as hematoxylin and eosin, but they can be identified with Verhoeff–Van Gieson stain.
The cellular elements of the dermis include fibroblasts, endothelial cells, mast cells, and a variety of miscellaneous cells including muscle, nerve, and hematopoietic cells. The hematopoietic cells include histiocytes (macrophages), lymphocytes, and plasma cells that are present in the dermis under various pathologic conditions.
Fibroblasts form collagen and also produce ground substance. They are involved in immunologic and reparative processes and are increased in numerous skin disorders.
Mast cells arise from undifferentiated mesenchymal cells. They have intracytoplasmic basophilic metachromatic granules containing heparin and histamine. The normal skin contains relatively few mast cells, but their number is increased in many different skin conditions, particularly the itching dermatoses, such as atopic eczema, contact dermatitis, and lichen planus. In urticaria pigmentosa the mast cells may occur in tumor-like masses.
Histiocytes (macrophages) are present in only small numbers in the normal skin. However, in pathologic conditions they migrate to the dermis as tissue monocytes. They play a predominant role in the phagocytosis of the particulate matter and bacteria. Under special pathologic conditions they may form giant cells. They are also involved in the immune system by phagocytizing antigens.
Lymphocytes and plasma cells are found in only small numbers in normal skin but are significantly increased in pathologic conditions.
The ground substance of the dermis is a gel-like amorphous matrix not easily seen in routine sections, but it may be identified with colloidal iron and Alcian blue stains. It is of tremendous importance because it contains proteins, mucopolysaccharides, soluble collagens, enzymes, immune bodies, metabolites, and many other substances.
Epidermis
The epidermis is the most superficial of the three layers of the skin and averages in thickness about the width of the mark of a sharp pencil, or less than 1 mm. It contains several types of cells including keratinocytes, dendritic cells (melanocytes and Langerhans' cells), and Merkel cells.
The keratinocytes, or keratin-forming cells, are by far the most common and develop into four identifiable layers of the epidermis . From inside out, they are as follows:
VASCULATURE
A continuous arteriovenous meshwork perforates the subcutaneous tissues and extends into the dermis. Blood vessels of varying sizes are present in most levels and planes of the skin. In fact, the vascularization is so extensive that it has been postulated that its main function is to regulate heat and blood pressure of the body, with providing nutrition to the skin a secondary function. No blood vessels are present within the epidermis.
A special vascular body, the glomus, deserves mention. The glomus body is most commonly seen on the tips of the fingers and the toes, and under the nails. Each glomus body consists of a venous and arterial segment, called the Sucquet-Hoyer canal. This canal represents a short-circuit device that connects an arteriole with a venule directly, without intervening capillaries. The result is a marked increase in the blood flow through the skin. If this body grows abnormally, it forms an often painful, red, benign glomus tumor, commonly beneath the nail.
NERVE SUPPLY
The nerve supply of the skin consists of sensory nerves and motor nerves.
Sensory Nerves
The sensory nerves mediate the sensations of touch, temperature, and pain. The millions of terminal nerve endings, or Merkel's cell–neurite complexes, have more to do with the specificity of skin sensation than the better known, highly specialized nerve endings, such as the Vater-Pacini and Wagner-Meissner tactile corpuscles.
Itching is the most important presenting symptom of an unhappy patient. It may be defined simply as the desire to scratch. Itching apparently is a mild painful sensation that differs from pain in having a lower frequency of impulse stimuli. The release of proteinases (such as follows itch-powder application) may be responsible for the itch sensation. The pruritus may be of a pricking type or of a burning type and can vary greatly from one person to another. Sulzberger called those abnormally sensitive people “itchish,” analogous to “ticklish.” Itching can occur without any clinical signs of skin disease or from circulating allergens or local superficial contactants. The skin of atopic or eczema patients tends to be more itchy. Scratching makes the itching worse. This results in a perpetual itch-scratch cycle.
Motor Nerves
The involuntary sympathetic motor nerves control the sweat glands, arterioles, and smooth muscle of the skin. Adrenergic fibers carry impulses to the arrector pili muscles, which produce gooseflesh if they are stimulated. This is caused by traction of the muscle on the hair follicles to which it is attached. Cholinergic fibers if stimulated increase sweating and may cause a specific type of hives called cholinergic urticaria.
APPENDAGES
The appendages of the skin include both the cornified appendages (hairs and nails) and the glandular appendages.
Hairs
Hairs are produced by the hair follicles, which develop from germinative cells of the fetal epidermis. Because no new hair follicles are formed after birth, the different types of body hairs are manifestations of the effect of location, and of external and internal stimuli. Hormones are the most important internal stimuli influencing the various types of hair growth. This growth is cyclic, with a growing (anagen) phase and a resting (telogen) phase. The catagen cycle is the transition phase between the growing and resting stages and lasts only a few days. Ninety percent of the normal scalp hairs are in the growing (anagen) stage, and 10% are in the resting (falling out) stage, which lasts from 60 to 90 days. The average period of scalp hair growth ranges from 2 to 6 years. However, systemic stresses, such as childbirth, or systemic anesthesia may cause hairs to enter a resting stage prematurely. This postpartum effect or postanesthetic effect is noticed most commonly in the scalp when these resting hairs are depilated during combing or washing, and the thought of approaching baldness causes sudden alarm.
TYPES
The adult has two main types of hairs: (1) the vellus hairs (lanugo hairs of the fetus) and (2) the terminal hairs. The vellus hairs (“peach fuzz”) are the fine, short hairs of the body, whereas the terminal hairs are coarse, thick, and pigmented. The terminal hairs are developed most extensively on the scalp, brow, and extremities.
HAIR FOLLICLES
The hair follicle may be thought of as an invagination of the epidermis, with its different layers of cells. These cells make up the matrix of the hair follicle and produce the keratin of the mature hair. The protein synthesizing capacity of this tissue is enormous. At the rate of scalp hair growth of 0.35 mm per day, more than 100 linear feet of scalp hair is produced daily. The density of hairs in the scalp varies from 175 to 300 hairs per square centimeter. Up to 100 hairs may be normally lost daily.
Nails
The second cornified appendage, the nail, consists of a nail plate and the tissue that surrounds it. This plate lies in a nail groove, which like the hair follicle is an invagination of the epidermis. Unlike hair growth, which is periodic, nail growth is continuous. Nail growth proceeds at about one third of the rate of hair growth, or about 0.1 mm per day. It takes about 3 months to restore a removed fingernail and about three times that long for the regrowth of a new toenail. Nail growth can be inhibited during serious illnesses or in old age, increased through nail biting or occupational trauma, and altered because of hand dermatitis or systemic disease. Topical treatment of nail disturbances is very unsatisfactory, owing to the inaccessibility of the growth-producing areas.
Glandular Appendages
The three types of glandular appendages of the skin are the sebaceous glands, apocrine glands, and eccrine glands.
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