Anatomy & physiology of skin Skin Structure Skin is the single largest organ in the human body. It weighs an average of 4 kg and covers an area of 2 m2 Three distinct layers Epidermis: Composed of epithelial tissue Dermis: Composed of a combination of connective tissues Hypodermis: usually contains abundant fat. Epidermis It’s outermost layer of skin. It consists of many layers of closely packed cells. The most superficial of which areflattened and filled with keratins. It is a stratified squamous epithelium. Contains no blood vessels. It varies in thickness from less than 0.1 mm on the eyelids to nearly 1 mm on the palms and soles. Stratum Basale the deepest layer, rests on a basement membrane, which attaches it to the dermis. It is a single layer of columnar cells. In normal skin only 30% of basal cells are preparing for division. Once basal cell leaves basal layer in humans, normal transit time to stratum corneum is at least 14 days, and transit through stratum corneum to desquamation requires 14 days, 28 days total. Stratum Spinosum Consists of 8-10 layers of Keratinocytes. They are named for the spine-like appearance of the cell margins in histologic sections. As these cells differentiate and move upward through the epidermis, they become progressively flatter and develop organelles known as lamellar granules Composed of Keratinocytes attached to each other via desmosomes. Contains langerhans cells that aid in the immune system response. Stratum Granulosum Stratum Granulosum: The middle layer of 3-5 layers of cells that help form keratin. Contains keratohyline granules that produce a secretion These make up the thick and tough peripheral protein coating of the horny envelope. Stratum Lucidum Stratum Lucidum: Found only in the thick skin of the palms and soles of the feet Tends to be translucent. Stratum Corneum Stratum Corneum: The outermost layer of the epidermis. Contains up to 30 layers of flattened dead cells (corneocytes) separated by lipids in the intercellular space. Provides protection. Continuously shed and replaced. Cells of the epiderms Keratinocyte 80% Melanocyte 10% Langerhan’s cell 2-4% Merkle cell Melanocytes Melanocytes are the only cells that can synthesize melanin. They migrate from the neural crest into the basal layer of the ectoderm. They are also found in hair bulbs, the retina and pia arachnoid. Each dendritic melanocyte associates with a number of keratinocytes, forming an ‘epidermal melanin unit’ . Their cytoplasm contains discrete organelles, the melanosomes, containing varying amounts of the pigment melanin injected into surrounding keratinocytes to provide them with pigmentation to help protect the skin against damaging ultraviolet radiation. Difference between White & dark individual Melanocyte number Same Same Melansome number Few Many Melanosome size Small Large Degree of melanization Low High Grouped in Distribution in Individually membrane coated keratinocyte dispersed complex Langerhan’s cell Bone marrow –derived dendritic cell. Location in epidermis: St. sipnosum Langerhans cells constitute 2-4% of the total epidermal cell population. Characteristic finding on electron microscopy – Birbeck granules- tennis racket shaped bodies in the cell. Function: Antigen presenting cells, immune surveillance. Functionally impaired by ultraviolet radiation and topical steroid. Special stain: S100, CD1a Merckel’s cell Shape: Non dendritic cell Site: MM, lip, digit & near hair Location in epidermis: St. Basalis Origin: Unknown Function: Transducer of fine touch Cells of the epiderms Melanocyte Langerhans Merckle’s cell cell Prevalence Most common Next common Few Percentage 10% 2-4% - Location in epidermis St. basalis St. Spinosum St.Basalis Origin Neural crest Bone marrow Unknown Production of Immune Sensory Function melanin surveillance function MM, lip, All skin All skin Type of skin digit Basement Membrane Zone Site of interaction between epidermis and dermis Three layers 1.) Lamina lucida, 2.) Lamina densa, 3.) Sub- lamina densa Hemidesmosomal adhesion complex: 1.) hemidesmosomal plaque 2.) anchoring filaments 3.) anchoring fibrils Dermis Dermis: Composed primarily of connective tissue, mainly collagen, elastic fibers and ground substances. Divides into 2 Categories: Papillary Region: Superficial part that contains the dermal papillae and comprises the wavy boundary between the epidermis and dermis. Houses capillaries, nerve endings, and Meissner corpuscles (touch sensors). Reticular Region: Deeper region made up of dense irregular connective tissue. Contains many of the glands, nail roots, and root follicles. Hypodermis Hypodermis: Known as the subcutaneous tissue or superficial fascia. The boundary of the dermis is indistinct, but it is clear that there is more areolar and adipose tissue than in the dermis. Primarily serves as a storage site for large blood vessels and fat. Houses Lamellated or Pacinian corpuscles (nerve endings sensitive to pressure). Adnexal structures Sebaceous glands Sweat glands: Eccrine sweat glands Apocrine sweat glands Sebaceous glands Most sebaceous glands develop embryologically from hair germs, but a few free glands arise from the epidermis. The glands themselves are multilobed and contain cells full of lipid, which are shed whole (holocrine secretion) during secretion. Sebum contains a mixture of triglycerides, fatty acids, wax esters, squalene and cholesterol. Sebum is discharged into the upper partof the hair follicle. It lubricates and waterproofs the skin, and protects it from drying. It is also mildly bactericidal and fungistatic. Androgenic hormones stimulate sebaceous gland activity. The sebaceous glands react to maternal androgens for a short time after birth, and then lie dormant until puberty when a surge of androgens produces a sudden increase in sebum excretion and sets the stage for acne. Eccrine sweat glands There are 2–3 million sweat glands distributed all over the body surface but they are most numerous on the palms, soles and axillae. The tightly coiled glands lie deep in the dermis, and the emerging duct passes to the surface by penetrating the epidermis in a corkscrew fashion. Sweat is formed in the coiled gland by active secretion. Sweat glands have an important role in temperature control, the skin surface being cooled by evaporation. Three stimuli induce sweating: 1- Thermal sweating 2- Emotional sweating 3- Gustatory sweating The eccrine sweat glands are innervated by cholinergic fibres of the sympathetic nervous system. Apocrine sweat glands Apocrine glands are limited to the axillae, nipples, periumbilical area, perineum and genitalia. The coiled tubular glands (larger than eccrine glands) lie deep in the dermis, and during sweating the luminal part of their cells is lost (decapitation secretion). Apocrine sweat passes via the duct into the mid-portion of the hair follicle. The action of bacteria on apocrine sweat is responsible for body odour. The glands are innervated by adrenergic fibres of the sympathetic nervous system. Epidermal appendages Eccrine Appocrine Sebaceous Distribution All skin Axilla, areola, Hairy skin, free in Periumblical, eyelid, lip, nipple, genetalia genetalia Acinar part Coiled Coiled Multilobulated Duct shape Straight Straight Short & wide Duct opening Skin surface Hair follicle Hair follicle Excretion Merocrine Decapitation Holocrine Secretion Isotonic Milky & odorless Oily Stimulant Acetyl choline Adrenalin Androgen Unknown ,Sex Waterproof, lubricant Function Thermoregulation stimulant Anti-microbial. Blood supply of skin Although the skin consumes little oxygen, its abundant blood supply regulates body temperature. Blood vessels lie in two main horizontal layers. • Superficial H.P.: – Lies at papillary dermis – Arterioles form capillary loop in dermal papillae • Deep H.P.: – Lies just above subcutis – Arterioles supply hair follicle – & sweat glands Nerve supply of skin • It’s supplied by 1.000.000 fibers • Enervation are: – Autonomic N. – Sensory N. • Sensory fibers are: – Myelinated fiber – Non-myelinated fiber (itching) • Nerve ending are: – Free (Nocecepters) – Specialized end organ (mechanoceptors) • Specialized end organ are: Meisseners & paccianian’s corpuscles 6 Main Functions of the Skin 1. Protection: Physical and solar barrier. 1. Keratin protects tissues from heat, chemicals, injury, & infection. 2. Melanin protects against solar radiation. 2. Vitamin D Synthesis: Vitamin D regulates blood calcium levels – production is stimulated by ultraviolet radiation (UVB) 3. Extensive Sensory Functions: Detects heat, cold, touch, texture, pleasure, vibration, and tissue injury. 6 Main Functions of the Skin 4. Thermoregulation: Via sweat production & blood flow; preserves homeostasis. 5. Excretion & Absorption: Excretes & absorbs lipid-soluble materials; sweat allows secretion of small amounts of waste materials including salt, carbon dioxide, ammonia, and urea. 6. Reservoir: For 8-10% of adult blood flow .