Skin And Appendages PDF

Skin And Appendages Ms. Sharmili Ravindrarajan BSN (Hons) – UoP Lecturer (Probationary) Structure of Skin the skin is the largest organ in the body and it contains glands, hair and nails Between the skin and underlying structures is the subcutaneous layer composed of areolar tissue and...

Skin And Appendages Ms. Sharmili Ravindrarajan BSN (Hons) – UoP Lecturer (Probationary) Structure of Skin the skin is the largest organ in the body and it contains glands, hair and nails Between the skin and underlying structures is the subcutaneous layer composed of areolar tissue and adipose tissue There are two main layers: ✓The Epidermis ✓The Dermis In sections through the skin the line of junction of the two layers is not straight, but markedly wavy because of the presence of numerous finger like projections of dermis upward into the epidermis – Dermal papillae Anatomy of Skin Structure of Skin The surface of the epidermis is also often marked by elevations and depressions These are most prominent on the palms and ventral surfaces of the fingers, and on the corresponding surfaces of the feet The elevations forms a characteristic epidermal ridges that are responsible for the highly specific fingerprints of each individual The Epidermis Most superficial layer of the skin and it is composed of stratified keratinized squamous epithelium which varies in thickness in different parts of the body It is thickest on the palms of the hands and soles of the feet There are no blood vessels or nerve endings in the epidermis, but its deeper layers are bathed in interstitial fluid from the dermis, which provides oxygen and nutrients and drains away as lymph There are several layers of cells in the epidermis which extend from the deepest germinative layer to the most superficial stratum corneum Layers of Epidermis The Layers of Epidermis Basal Layer / Germinal Layer ✓The deepest layer is made up of a single layer of columnar cells that rest on a basal lamina ✓The basal layer contains stem cells that undergo mitosis to give off cells called keratinocytes ✓Keratinocytes form the more superficial layers of the epidermis The Layers of Epidermis Stratum Spinosum ✓The basal layer there are several layers of polygonal keratinocytes that constitute the Malpighian layer ✓Some mitoses may be seen in the deeper cells of the stratum spinosum ✓Stratum spinosum is included along with the basal cell layer in the germinative zone of the epidermis Stratum Granulosum ✓Few layers of flattened cells that are characterized by the presence of deeply staining granules in their cytoplasm ✓The granules consists of a protein called keratohyalin ✓The nuclei of cells in this layer are condensed and dark staining (Pyknotic) The Layers of Epidermis Stratum Lucidum ✓Superficial to the stratum granulosum ✓It appears homogeneous, the cell boundaries being extremely indistinct Stratum Corneum ✓This layer is acellular ✓It is made up of flattened scale like elements containing keratin filaments embedded in protein ✓The squamae's are held together by glue like material containing lipids and carbohydrates ✓The presence of lipid makes this layer highly resistant to water The Epidermis The maintenance of a healthy epidermis depends on three processes: ✓Desquamation (Shedding) of the keratinized cells from the surface ✓Effective keratinization of the cells approaching the surface ✓Continual cell division in the deepest layers with newly formed cells being pushed to the surface Hairs, secretion from glands and ducts of sweat glands pass through the epidermis to reach the surface The surface of the epidermis is ridged by projections of cells in the dermis – Papillae The pattern of ridges on the fingerprint is unique to each individual The Epidermis The downward projections of the germinal layer between the papillae are believed to aid the nutrition of epidermal cells and stabilize the two layers, preventing damage due to shearing force Blisters develop when trauma causes separation of the dermis and epidermis, and serous fluid collects between the two layers Pigmentation of the Skin The cells of the basal layer of the epidermis, and the adjoining cells of the stratum spinosum contain a brown pigment called melanin which is formed from the amino acid tyrosine Melanocytes are the cells responsible for the production of the pigment melanin Melanocytes are divided from Melano blasts. They may be present among the cells of the germinative zone or at the junction of the epidermis and dermis Each melanocyte has processes. Melanin granules formed in the melanocytes are transferred to surrounding non-melanin-producing cells through these processes – Dendritic cells Pigmentation of the Skin The number of melanocytes is constant, the differences in color depend on the amount of melanin secreted It protects the skin from the harmful sunlight. Exposure to sunlight promotes the synthesis of melanin The epidermis is sufficiently translucent for the color of blood, specially in light skinned individuals Structure of Melanocyte The Dermis The dermis is a tough and elastic connective tissue and the matrix contains collagen fibers interlaced with elastic fibers Rupture of the elastic fibers occurs when the skin is overstretched, resulting in permanent striae, or stretch marks that may be found in pregnancy (Linea gravidarum) and obesity Collagen fibers bind with water and give the skin its tensile strength, but this ability declines with age Just below the epidermis the connective tissue is dense and constitutes the papillary layer Deep to this there is a network of thick fiber bundles that constitute the reticular layer The Dermis The dermis is rest on the superficial fascia through which it is attached to deeper structures Fibroblast, macrophages and mast cells are the main cells found in the dermis Underlying its deepest layer there us areolar tissue and varying amounts of adipose (fat) tissue Two Layers of the Dermis Blood and Lymph Supply Blood vessels are not penetrating into the epidermis Arterioles form a fine network with capillary branches supplying sweat glands, sebaceous glands, hair follicle and the dermis Veins from the dermal papillae drain through plexus present in the dermis into a venous plexus lying on deep fascia A special feature is presence of numerous aretrio venous anastomoses Lymph vessels form a network throughout the dermis Nerve Supply Sensory receptors of touch, temperature, pressure and pain are widely distributed in the dermis The stimuli activates the different types of sensory receptors: ✓Meissner’s corpuscle: Light pressure ✓Pacinian corpuscle: Deep pressure ✓Free nerve endings: Pain Nerve impulses generated in the sensory receptors in the dermis are conveyed to the spinal cord by sensory nerves, then to the sensory area of the cerebrum where the sensations are perceived Appendages of the Skin Structure of Hair and Hair Follicle Each hair consists of a part that is seen on the surface of the body; and a part anchored in the thickness of the skin The visible part is known as shaft and the embedded part is known as root The root has an expanded lower end – Bulb The bulb is invaginated from below by part of the dermis that constitutes the hair papillae The root of each hair is surrounded by a tubular sheath, which is a downgrowth of epidermal cells into the dermis or subcutaneous tissue – Hair Follicle Appendages of the Skin Structure of Hair and Hair Follicle A hair may be regarded as a modified part of the stratum corneum An outer cortex and an inner medulla can be made out in large hair, but there is no medulla in thin hair The cortex is acellular and is made up of keratin The surface of the hair is covered by a thin membrane – Cuticle The cuticle is formed by flattened cornified cells which contain melanin Both in the medulla and cortex minute air bubbles are present which replace the melanin with the ageing results in white or grey color hair Structure of Keratin Structure of Hair and Hair Follicle The innermost layer, that immediately surrounds the hair root is continuous with the surface of the skin while the outermost layer of the follicle is continuous with the dermis The wall of the follicle consists of three layers: ✓Inner root sheath: Only in the lower part of the follicle ✓Outer root sheath: Continuous with the stratum spinosum ✓Connective tissue sheath: Derived from the dermis Layers of Hair Follicle Arrector Pili Muscles These are bands of smooth muscles attached at one end to the dermis, just below the dermal papillae; and the other end to the connective tissue sheath of hair follicle A sebaceous gland lies in the angle between the hair follicle and the arrector pili Contraction of the muscle has two effects: ✓The hair follicle becomes almost vertical relative to the skin. Simultaneously, the skin surface overlying the attachment of the muscle becomes depressed while surrounding areas become raised (During cold or excitement) ✓The sebaceous gland is pressed upon, and the its secretion are squeezed into the hair follicle Sebaceous Glands Sebaceous glands are seen most typically in the hair follicles Each gland consists of numerous alveoli that are connected to a broad duct that opens into a hair follicle Each alveolus is pear-shaped It consists of a solid mass of polyhedral cells and has hardly any lumen The outermost cells are small and rest on a basement membrane The inner cells are larger, more rounded, and filled with lipids. This lipid is discharged by the disintegration of the inner cells that are replaced by the proliferation of outer cells – Holocrine glands  Structure of Sebaceous Gland Sebaceous Glands The sebum is secreted by the sebaceous glands. Its oily nature helps to keep the skin and hair soft It helps to prevent dryness of the skin and makes it resistant to moisture Sebum contains various lipids including triglycerides, cholesterol, cholesterol esters, and fatty acids These are present numerously in the skin of the scalp, face, axillae, and groins except the palm and sole In regions of transition from one type of superficial epithelium to another, such as lips, eyelids, nipple, labia minora, and glans penis, there are sebaceous glands that are independent of hair follicles, directly secreting onto the surface Sweat Glands These are widely distributed throughout the skin and are most numerous in the palm, sole, axillae, and groin They are formed from epithelial cells. The entire gland consists of a single long tube The lower end of the tube is highly coiled on itself and forms the body or fundus. The body is made up of the secretory part of the gland. It lies in the reticular layer of the dermis or in subcutaneous tissue The part of the tube connecting the secretory element to the skin surface is the duct. It runs upwards through the dermis to reach the epidermis The orifice is funnel-shaped Structure of Sweat Gland Sweat Glands The wall of the tube making up the gland consists of an inner epithelial lining, basal lamina, and supporting layers of connective tissue In large seat glands flattened contractile, myoepithelial cells are present between the epithelial cells and their basal lamina Sweat glands are innervated by cholinergic nerves Nails Nails are present on fingers and toes The main part of the nail is known as the body. The body has a free distal edge The proximal part of the nail is implanted into a groove on the skin and is known as root or radix The tissue on which the nail rests is known as the nail bed. The nail bed is highly vascular Nail represents a modified part of the zone of keratinization of the epidermis. It usually regarded as a much-thickened continuation of the stratum lucidum The nail substance consists of several layers of dead, cornified cells filled with keratin Nails The nail rests on the germinative zone. The germinative zone is particularly thick near the root of the nail where it forms the germinal matrix The nail substance is formed mainly by the proliferation of cells in the germinal matrix. The superficial layers of the nail are derived from the proximal nail fold Distal to the nail fold a small semilunar white area called lunule is seen The germinal matrix is connected to the underlying bone by fibrous tissue The germinative zone beneath the body of the nail is thinner than the germinal matrix. It doesn’t involve in the growth of the nail – Sterile matrix Structure of Nail