Histology Of Skin

Histology of Skin Integument system

Objectives: The students should be able to: 3. list the functions of the skin. 4. describe the structure of the thick skin. 5. list the differences between thick skin and thin skin. 6. describe the structure of the dermis. 7. describe the blood supply and nerve supply of the skin.

6.describe briefly the structure and the functions of the following appendages of the skin a) sebaceous glands b) sweat glands c) hair follicles d) nail

General Concepts & Considerations • heaviest organ: ? 16% total body weight & 1.2 to 2.3 m2 surface area • composed of epidermis and dermis • functions include: 1. protection: physical, biological, against UV light, from dehydration 2. regulation of body temperature 3. synthesis of vitamin D with UV absorption 4. sensory • classified as thick or thin skin depending on thickness of epidermis

Skin: layers Epidermis surface layers that are keratinized Dermis dense fibro-elastic connective tissue containing glands and hair Hypodermis loose connective tissue consisting largely of adipose tissue

Characteristics: •keratinized stratified squamous epithelium •regenerated by the keratinocyte stem cells in the basal layer differentiate as they move outwards •consists of 5 layers or strata 1.stratum basale deepest 2.stratum spinosum | 3.stratum granulosum | 4.stratum lucidum ? 5.stratum corneum surface • turnover from basal to superficial ranges from 25 to 50 days •specialized structures include sweat glands and hair follicles

Stratum Basale: •single layer of columnar or cuboidal keratinocyte stem cells, which are mitotically active. deepest layer •attached to basement membrane by hemidesmosomes; •attached to each other with desmosomes. •melanocytes and Merkel cells present.

Stratum Spinosum: •several layers of cuboidal, polygonal and slightly flattened cells, with a central euchromatic nucleus, mitotically active •concentrated tonofilaments in the cytoplasm •Langerhans cells •projections of melanocytes Psoriasis= skin disorder where excessive cell division leads to increased thickening of strata basale and spinosum

Stratum Spinosum: cytoplasm is rich in tonofilaments that terminate with desmosomes in spiny projections, hence “spinosum” - they hold the cells together and help to protect the skin from abrasions •shrinkage of the keratinocytes reveals the spines

Stratum Granulosum: •3 to 5 layers of flattened polygonal cells •cells accumulate keratohyalin granules with phosphorylated proteins, “granulosum” •contain lamellar granules which are lipid and protein rich • are discharged extracellularly to produce a “cement” that seals the skin to foreign objects & water •most superficial layer in which nuclei are present, but no cell division occurs

Stratum Granulosum:

Stratum Lucidum: •a translucent thin layer of extremely flattened eosinophilic cells •nuclei and organelles not present •filaments and desmosomes retained •cells contain eleidin, a transformation product of keratohyalin

Stratum Corneum: •outermost layer •composed of 15 to 20 layers of cells •flattened, nonnucleated, keratinized cells: •filled with filamants of keratin •surface cells continuously desquamated

Epidermis - specialized cells Langerhans cells: •bone marrow derived monocyte / macrophage cell that is antigen-presenting •present in all layers, but predominantly in stratum spinosum •pale nuclei, granular cytoplasm, processed •increase in number in chronic inflammatory skin diseases

• Merkel’s cells: – rare in thin skin – in the stratum basale – contain small dense granules – may function as sensory mechanoreceptors or as neuroendocrine cells

Melanocytes •in the stratum basale pale “halo” of cytoplasm •neural crest produce melanin and pass it on to nearby keratinocytes •melanin covers nuclei of keratinocytes •skin color depends on activity of cells, rather than number

Dermis dense irregular connective tissue •type I collagen •networks of elastic fibers •blood vessels nerves & nerve endings •in old age cross linking of fibers increase and number of elastic fibers decreases •blood vessels in skin important in blood temperature and pressure regulation

Dermal Papillae: •interdigitations of the dermis and the epidermis which counteract shearing force between the two layers – • prominent in areas that grip or experience friction e.g. fingertips, palms, soles of feet

Layers papillary layer - loose FECT that forms the dermal papillae, loops of small blood vessels and capillaries, nerve endings reticular layer - dense irregular FECT that forms bulk of dermis, with blood vessels and a-v shunts, lymphatics and nerves Blisters an accumulation of fluid at dermo-epidermal junction due to: excessive shearing force or heat Dermatitis - rash in response to viral infections or allergies epidermis thickens and disrupted by infiltration of leukocytes and accumulation of extracellular fluid blood vessels in upper dermis are dilated

Dermis - sweat gland Eccrine sweat glands (aka merocrine) •distributed in skin throughout the body, •particularly abundant on forehead, scalp, axillae, palms and soles •simple coiled tubular •sweat is hypotonic, watery, neutral or slightly acidic

sweat gland ducts: do not divide are narrow and lined by stratified cuboidal epithelium, and pass through dermis and epidermis secretory portion: in dermis surrounded by myoepithelial cells which help discharge secretion dark cells line the lumen and contain secretory granules clear cells underlie the dark cells and pass intercellular canaliculi to the lumen

Sweat Glands Apocrine sweat glands •large specialized sweat glands localized in axilla, areola, circumanal region •begin to function in puberty and respond to hormones •large coiled secretory portion: inner cuboidal cells, outer flat cells on basement membrane, wide lumen, myoepithelial cells present but not prominent •empty into hair follicles •innervated by adrenergic fibers •produce viscid milky secretions in response to external stimuli such as fear or sexual excitement

Meissner’s corpuscle •a specialized structured nerve ending •touch receptor •confined to dermal papillae • most numerous on hands and feet

Pacinian corpuscle •an encapsulated nerve ending •pressure receptor •found in deep dermis or hypodermis •General Skin Nerve supply: - free nerve endings detect pain and temperature - innervation by sympathetic nervous system controls blood flow and hair

Thick vs Thin skin All before has been about thick skin: •on the palms, fingertips or the soles of the feet •lacks follicles, sebaceous glands, arrector pili muscles in contrast thin skin: •is over most of the body, contains hair follicles, sebaceous glands and arrector pili muscles •thinner epidermis •less well developed strata granulosa and lucida, and the stratum corneum may be quite thin

Hair - thin skin only •made of keratin •follicle derived from epidermal epithelium •begins deep in dermis •connective tissue sheath •sebaceous glands •medulla, cortex and cuticle •arrector pili muscle -bundles of smooth muscle attached to hair follicles in dermis and papillary layer of dermis •contraction elevates hairs goose bumps

Sebaceous glands distributed over most of the body (at 100 / cm2), •with (400 to 900 / cm2) on face, forehead and scalp •acinar glands with several sacs •most have short ducts that empty into neck of hair follicle, or onto the skin directly (eyelids, lips, glans penis and glans clitoridis )

•acini consist of basal layer of undifferentiated flattened epithelium •these cells divide, differentiate, and then break down to release their fat droplets into the lumen of the gland forming sebum, which is then released by the gland •sebum is a complex mix of triglycerides, waxes, cholesterol and esters, with mild antibacterial and anti-fungal activity •activity controlled by sex-hormones

Nail – • located on dorsal distal phalanx of each finger and toe • nail plate composed of hard keratin lying on nail bed • the stratum corneum of the epidermis that overlies the nail root forms the eponychium (cuticle) • hyponychium or nail plate consists of the stratum corneum of the underlying nail bed, and so is a keratinized epithelial layer • nail bed epidermis has only strata basale and spinosum • growth due to cells in nail matrix at nail root