Anatomy and Physiology of the Skin Dr. M. Arif Abid Dermatologist www.doctorabid.com Abid@afghan-doctors.com

Dermatological disorder = 10% -15% of primary care consultations Skin is the largest organ in human body Dermatological diseases can cause social and psychological problems, also it may affect ability to work (e.g. Chronic hand dermatitis.) Skin is the gate of the body(might reflect systemic disease). 2

Structure of skin Epidermis Basement membrane (dermoepidermal junction) Dermis Subcutaneous fat Epidermis: Four layers (from outside – inside) Cornified layer Granular layer Spinous layer Basal layer Dermis contains: Collagen fibers Elastic fibers Ground substances Blood vessels Nerves. 3

Skin appendages: Hair follicle Sebaceous gland Arrector pili muscle Eccrine sweat gland Apocrine sweat glands Nail 4

Basement membrane (dermoepidermal junction) Epidermis Basement membrane (dermoepidermal junction) Dermis Subcutaneous fat 5

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Epidermis: Consist of several zones Cornified layer (Stratum corneum ) Granular layer (Stratum granulosum) Spinous layer (Stratum spinosum) Basal layer (Stratum basale) 7

Cornification (keratinization) Occur in the cytoplasm of epidermal keratinocytes during their terminal differantiation into dead horny cell(corneocyte)

Cornification (keratinization) The total process takes approximately 2 months. It involves the formation of keratin polypeptides. Abnormalities in this process leads to roughness and scaling of the skin e g psoriasis.

Epidermis (thinner outer layer of skin) Dermatology deals with disorders of skin, hair, nails, and mucous membranes. Structure Epidermis (thinner outer layer of skin) Dermis (thicker connective tissue layer) Hypodermis (subcutaneous layer) 10

Stratum lucidum : Found in thick skin of palms and soles above granular layer

Basal layer Rest on the basement membrane they divide continuously and move upwards. Melanocytes are dendritic cells lying between basal cells in a ratio of 1:10 . Melanocytes synthesize melanin stored in melanosomes.

Basal Cell layer Melanosomes are transferred to adjacent cells forming the Epidermal Melanin unit. Small, unmelanized, clumped melanosomes are found in white skin Large melanized, dispersed melanosomes in dark skin

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The Spinous cell layer: Usually 5 -10 layers thick . Adhere to each other by desmosomes (complex modification of the cell membrane ). Desmosomes appear like spines.

Granular Cell layer : Diamond shaped cells. Cytoplasm is filled with keratohyaline granules. In thin skin it is 1 -3- cell layers and 10 cell layers in thick skin like palms and soles.

Stratum Corneum layer: The cells in this layer have no nucleous . Cells have thick envelope that resist chemicals. Cells are held together by desmosomes. It is 25 cell layer .

Basement Membrane (BM) Pink homogenous area between epidermis and dermis . Consists of number of proteins like BP antigens, collagen 4 The site of attack injury in blistering diseases.

Skin Structure Components of BM Lamina lucida: thin clear amorphous space Lamina densa: an electron dense area containig type4 collagen Anchoring fibrils : anchors epidermis to dermis ,type 7 collagen

Skin Structure Dermis is divided into Papillary dermis . Reticular dermis Consists of : Collagen fibers Provides strength . Elastic Fibers: Provides elasticity, protection against shearing forces.

Dermis cells Histocyte Fibroblast Mast cell

Skin Structure Ground substance : Binds water and maintains the skin turgor. Blood vessels: To nourish the overlying epidermis.

Skin Structure The function of dermis: It provides nourishment to the epidermis and interact with it during wound repair. It gives the skin its strength elasticity, and softness.

Skin structure Subcutaneous Fat: Composed of lipocytes

Skin structure Subcutaneous Fat: Composed of lipocytes

Skin Structure Skin Appendages include: Eccrine/ apocrine sweat glands. Sebaceous glands. Hair Follicles. Nails

Skin appendages Eccrine sweat glands Tubular structures open freely on the skin ;not attached to hair follicles. Under the influence of cholinergic stimuli.

Skin appendages Eccrine sweat glands Present everywhere except the vermilion border of the lips, nail Beds, labia minora, glans Abundant in palms ; soles.

Skin appendages Present in the axillae ; anogenital area modified Apocrine glands: Present in the axillae ; anogenital area modified glands in the external ear canal the eye lids(moll’s glands ) and areolae.

Attached to hair follicles or open freely. Sebaceous glands: Attached to hair follicles or open freely. Present in the scalp, forehead face upper chest except palms and soles. Under the control of androgens. Ectopic glands in the mucous membrane are called fordyce spots.

Sebaceous glands:. Present in the areola as Motgomery’s tubercles, in the eye lids as Meibomian glands.

Skin appendages Hair follicles: The hair follicle with it’s attached sebaceous gland form the Pilosebaceous Unit. Structure : Infundibulum : Region between skin surface and sebaceous gland.

Skin Appendages Isthmus begin at sebaceous glands and end at bulge (site of insertion of arrector pilli muscle) Hair bulb envelopes the dermal papilla

Skin Appendages Arrector pili is smooth muscle located in the dermis and is attached to the side of the hair shaft. - Cold and emotions will contract muscle and pull hair in vertical position.

Skin Appendages Hair shaft has: Cuticle Cortex contain pigment cells Medulla found only in terminal hair

Type of hair Terminal hair lanugo hair Bristle hair Villus hair

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Skin appendages: Nails Anatomy Nail plate made up of dead keratinized cells of matrix. Nail bed is visible through nail plate. Nail matrix lies above the midportion of distal phalanx

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Skin appendages: The lunula is the visible part of the matrix Proximal nail fold can be altered in connective tissue Fingernails grow 3 mm/m Toenails grow 1 mm/m

Skin immune system Langerhans cells interact with keratinocytes, which secrete a number of immunoregulating cytokines, and T-cells forming the skin immune system. 47

Physiology (function) 1- Protection - physical barrier that protects underlying tissues from injury, UV light and bacterial invasion. - mechanical barrier and immunity 2- Excretion - sweat removes water and small amounts of salt, uric acid and ammonia from the body surface 3- Synthesis of Vitamin D

4- Regulation of body temperature - high temperature or strenuous exercise; sweat is evaporated from the skin surface to cool it down. - vasodilation (increases blood flow) and vasoconstriction (decrease in blood flow) regulates body temp. 5-Sensation - nerve endings and receptor cells that detect stimuli to temp., pain, pressure and touch.

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