STRUCTURE OF SKIN
Skin is not an inert wrapping but a dynamic membrane consisting of large number of structures and capable of performing numerous functions. It is the largest body organ contributing more than 15 percent of the body weight. It consists of three layers (Fig. 1.1):
- Epidermis
- Dermis
- Subcutaneous tissue
Epidermis
Layers of Epidermis
Epidermis is 0.5 mm to approximately 2 mm in thickness and is composed of various layers which are as follows (from outwards to inwards)
- Stratum corneum
- Stratum lucidum (only on palms and soles)
- Stratum granulosum
- Stratum spinosum
- Stratum germinativum
Stratum Germinativum
Stratum germinativum (Basal cell layer) is the innermost layer of the epidermis. The major cells of this layer are keratinocytes which are columnar in shape and wedged in between these cells are melanocytes and Langerhans’ cells.
The keratinocytes keep on dividing and move outwards undergoing change in shape, size and constituents. This process is known as keratinization. The skin is being shed off every day unseen to the naked eyes and the whole epidermis is replaced in about a month's time. Melanocytes synthesize and distribute melanin (the pigment) to keratinocytes and are responsible for the complexion of an individual. The Langerhans’ cells are the immunogenic cells.
Stratum Spinosum
Stratum spinosum (Prickle cell layer) is so named due to the spine like appearance of the cell margins in histologic sections. The spine like structures promote adhesion of epidermal cells and resist mechanical stresses. The cells in this layer contain bundles of keratin filaments.
Stratum Granulosum
Stratum granulosum (Granular cell layer) has 3–4 layers of elliptical cells which contain prominent basophilic keratohyalin granules.
Stratum Lucidum
Stratum lucidum is present only on palms and soles and is situated between stratum granulosum and stratum corneum.
Stratum Corneum
Stratum corneum is composed of multiple layers of flattened, dead horny cells which are the largest epidermal cells. These cells provide protection and act as barrier to the environment.
Epidermal Appendages
Sebaceous glands, sweat glands, hair and nails are the epidermal appendages.
Sebaceous Glands
Sebaceous glands are lipid producing structures which with the exception of the palms, soles and dorsa of the feet are distributed over the entire body surface. They are most numerous and most productive on the seborrheic areas, i.e. scalp, face and upper chest. The vast majority are connected to the hair follicles. Although sebaceous glands are fairly well developed at birth, they regress and remain small throughout childhood but mature around 10 years of age and become active under the influence of hormones at puberty. Their secretion (sebum) contains a complex mixture of lipids like triglycerides, wax esters, squalene, cholesterol and its esters.
Sweat Glands
Sweat glands are of two types, i.e. eccrine and apocrine. Eccrine sweat glands number 2 to 4 millions in humans and are distributed over the entire body surface. They are most abundant and active on the palms, soles, forehead and axillae. The eccrine sweat gland consists of a secretory coil and a duct. The duct opens directly on to the skin surface. These glands are stimulated by heat and emotional stimuli.
Apocrine Glands
Apocrine glands in humans are largely confined to the axillae and perineum and become functional at puberty. An apocrine gland consists of coiled secretory segment and straight excretory duct which opens into the follicular canal above the opening of sebaceous gland. These glands produce odoriferous secretion which is not known to serve any function in the human beings.
Hair
Hair in human beings, is of minimal functional benefit but is all important for personal adornment. Hair consists of cuticle, cortex and medulla. There are two types of hair, lanugo (vellus) hair and terminal hair. Terminal hair are thick and pigmented and are best developed on the face, scalp and extremities. Lanugo hair are fine and constitute the fuzz present all over the surface. There are about one million hair all over the body and about one lakh over the scalp.3
Growth of hair is not continuous but cyclic. There are two main phases, i.e. the growing phase (anagen) and resting phase (telogen). At the end of the growing phase, a club or resting hair is formed (catagen phase). Under normal conditions a hair count would reveal 85–95 percent in anagen phase, 1 percent in catagen and from 4–14 percent in the telogen phase. Scalp hair follicles normally rest for 3 months and are active for 2 to 6 years but are known to grow as long as 25 years. The hair usually grows at the rate of 0.4 mm per day.
Nail
Nail is an epidermal appendage consisting of hard keratinous nail plate and viable nail matrix under the proximal nail fold. The nail plate rests on the nail bed on distal phalanx and is covered by lateral nail folds. The nail bed under the free distal edge of the nail plate is known as hyponychium and the space between the nail plate and hyponychium is known as subunguium. The nails grow at the rate of 0.1 mm per day. The nails perform important functions of picking small objects, protect the distal phalanx, enhance tactile discrimination and add to the aesthetic appearance.
Dermo-epidermal Junction (Basement Membrane)
The dermo-epidermal junction forms an extensive interface between the epidermis and the dermis and plays a key role in epidermal dermal interactions. It is not visible on H and E staining but can be seen with PAS (Periodic acid schiff) staining as homogenous band. It consists of several planes, i.e. lamina lucida (basal portion of keratinocytes), lamina densa and the lamina fibroreticularis. It provides support for epidermis and serves as a semipermeable barrier.
Dermis
It varies in thickness from 1 mm on the eyelids to 5 mm on the back. It is a complex system consisting of a supporting matrix, several fibrous and filamentous tissues, epidermally derived appendages, cellular components like fibroblasts, mast cells, macrophages, blood borne cells (lymphocytes, plasma cells, other leukocytes) and nerve and vascular networks. The dermis consists of papillary dermis (adjacent to the epidermis) and reticular dermis which constitutes the lower nine-tenth and merges with the subcutaneous fat.
Ground substance or the supporting matrix consists of acid mucopolysaccharides and proteins. The main fibrous connective tissues of the dermis are collagen and elastic fibers. Collagen fibers are soft and flexible but strong and inelastic. The elastic fibers form a continuous network throughout the dermis and extend from dermo- epidermal junction to the subcutis and provide elasticity to the skin. Collagen fibers account for 75 percent and elastic fibers 3 percent of the dry weight of the skin.
The cellular components of the dermis are fibroblasts, mast cells, macrophages, monocytes and dendrocytes. Fibroblasts are the most common cells found in the dermis. They synthesize and degrade fibrous and non fibrous proteins of the connective tissue and the matrix. Mast cells are present in large numbers in the papillary dermis, around the appendages and blood vessels. They liberate histamine and other mediators in IgE mediated hypersensitivity reactions. The monocytes, macrophages and dendrocytes are immunogenic cells and participate in various immune mediated reactions.
The skin has a rich supply of vascular network. Blood vessels form two plexuses, i.e. the superficial plexus in the papillary dermis and deep plexus in the lower part of the reticular dermis adjoining the subcutaneous tissue and around the adnexa. Lymphatics roughly parallel the major vascular plexuses and clear the tissues of unwanted material.
The network of nerves in the skin consists of somatic sensory fibers and sympathetic autonomic fibers. Sensory fibers perceive the sensations of touch, pain, temperature, itching and mechanical stimuli while autonomic motor nerves control cutaneous vascular tone, pilomotor responses and sudoromotor activity.4
The smooth or involuntary muscles of the skin occur as arrector pili, tunica dartos of the external genitalia and in the areola of the nipples.
Subcutaneous Tissue (Hypodermis)
The subcutaneous tissue consists of lobules composed of adipocytes (fat cells) separated by thin fibrous septa through which small vessels pass. Adipocyte has a cytoplasm with membrane bound lipids that displace the nucleus to the periphery. Subcutaneous fat acts as shock absorber, insulator for heat, stores triglycerides which serve as a source of energy upon break down.
FUNCTIONS OF THE SKIN
Skin performs numerous functions and the important ones are:
- Protection: It protects the body from physical, chemical and biological injuries.
- Perception: It perceives various sensations like pain, touch, temperature, pressure and vibrations.
- Temperature regulation: Eccrine sweat glands and rich blood supply play a major role in thermoregulation.
- Barrier function: Skin acts as a permeability barrier that regulates the diffusion of substances particularly water and electrolytes.
- Secretory functions: Synthesis of vitamin D3 is an important secretory function which is achieved with the help of sunlight. Vitamin D3 is essential for bone metabolism.
- Storage functions: The dermis and subcutaneous fat act as a storage center of energy and other compounds.
- Excretory functions: Some of the harmful substances are excreted through the skin.
- Immunological functions: Recognition of exogenous substances (antigens) and elicitation of antigen specific cell responses are carried out in the skin.
- Cosmetic function: Color and texture of the skin along with the hair and nails play an important role in aesthetic appeal of an individual.