The Integument

BIO 408

Histology

Dr. D. L. Daley

The Skin or Integument

ⁿ Surface area - 1.2 to 2.2 M²

n 4-5 kg or 9-11 lbs.

n 7% total body weight

n 1.5- to 4.0 mm thick

n Layers

n Epidermis - stratified squamous keratinized epithelium cells

n Dermis - fibrous connective tissue

n Vascularized

n Hypodermis - mostly adipose & some areolar (loose) connective tissue

Cells of the Epidermis

n Keratinocytes - produce fibrous protein keratin

n Largest population of cells

n Joined by desomosomes

n From the stratum basale

n Eventually pushed to the surface as they fill with keratin filaments & die

Cells of the Epidermis

n Melanocytes - produce melanin

n In deepest layer of the epidermis

n Spider shaped

n The tryosinase is packaged into granules called melanosomes - The AA tyrosine is transported into the melanosomes and via tryosinase converted into melanin

n Melanosomes leave the cell body of the melanocytes and enters the cytoplasm of cells of the stratum spinosum

n Melanin accumulates on the “sunny”side of the nucleus - shielding it from UV radiation

Cells of the Epidermis

n Langerhans Cells

n Star shaped - from bone marrow

n Actually macrophages - immune function

n Merkel cells

n At the epidermal-dermal junction

n Associated with disc-like sensory ending

n Combination - Merkel’s disc - touch

Layers of the Epidermis

n Thick skin - palms, fingertips, and soles of the feet

n Five layers

n 400 - 600 mm thick

n Lacks hair follicles

n Thin skin - rest of the body

n 75 - 150 mm thick

n Strata lucidum missing

n Other layers reduced in thickness

Layers of the Epidermis

n Stratum Basale - basal layer

n Deepest layer

n Rapidly dividing cells

n Also called stratum germinativum

n 10-25% of the cells are melanocytes

n Occasionally Merkel cells are found

Layers of the Epidermis

n Stratum spinosum - prickly layer

n Keratinocytes - flatten and appear irregular or spiny in shape (prickle cells)

n Spines are an artifact of tissue preparation

n Scattered about are Langerhans’ cells

Layers of the Epidermis

n Stratum Granulosum

n Thin layer (3 to 5 cells thick)

n Keratinocytes drastically change appearance

n Cells flatten and nuclei & organelles begin to disintegrate

n Accumulate keratin hyaline (form keratin)and lamellated granules (contain a waterproofing glycolipid) spewed into the extracellular space

Layers of the Epidermis

n Stratum Lucidum - clear layer

n Only in thick skin

n Thin band above the stratum granulosum

n Contain densely packed keratin filaments oriented parallel tot he skin surface and eleidin ( from keratohyalin)

n Cytoplasmic aspect of these cells has a thickened appearance because of a nonkeratin protein called involucrin - function unknown

Layers of the Epidermis

n Stratum corneum

n Outermost layer

n 20 to 30 cells thick

n Keratin and thicken membranes protect skin against abrasion and penetration

n Glycolipid between cells waterproofs the layer

n All cells are dead

n Average person sheds 18 kg (40 lbs) of skin flakes in a lifetime

Thick Skin

Thin Skin

Dermis

n Dense irregular connective tissue

n Fibroblasts, macrophages, some mast cells, & white blood cells

n Matrix heavily embedded with collagen, elastin and reticular fibers

n Richly supplied with nerve fibers, blood vessels and lymphatic vessels

n Major portion of hair follicles, oil and sweat glands are derived from epidermis but reside in the dermis

Dermis

n Two layers - thin superficial papillary layer - areolar connective tissue

n Rich with collage and elastin fibers and blood vessels

n Superior surface has projections called dermal papillae

n Associated with sensory endings

n On the palms and soles the dermal papillae set atop large mounds - dermal ridges

n This elevates epidermis into epidermal ridges

n Sweat glands open on the ridges - leave trace of sweat - detected as fingerprints

Dermis

n Reticular layer - deeper dermal layer

n 80% of the thickness of the dermis

n Thick bundles of collagen that run in various planes - most parallel to surface

n Less dense regions - lines of cleavage or tension lines

n Incisions made parallel to these lines - skin gapes less and heals more readily

n Flexure lines - dermal folds found near joints

Skin Structure

Eccrine Sweat Glands

n Over entire body surface except nipples & parts of the external genitalia

n As many as 3 - 4 million per person

n Most numerous on palms, soles and forehead

n May form up to 10 liters of a sweat per day

n Simple coiled tubular gland - opens with a funnel shaped pore at the surface

n Contains 99% water & traces of salts, vitamin c, antibodies, urea, uric acid, ammonia, and lactic acid (attracts mosquitoes) - pH 4 to 6

Eccrine Sweat Glands

n Secretory unit

n Dark cells - line the lumen of the secretory unit - secrete a mucus rich substance

n Dense secretory granules

n Clear cells - - release watery substance - no secretory granules

n Myoepithelial cells - surrounding the secretory portion of the gland - actin and myosin impart contractile ability to these cells

n Duct - composed of basal and luminal cells and is high coiled - traverses the dermis and epidermis on the way to the surface

Apocrine Sweat Glands

n Axillary and anogenetial areas - larger

n Same components of sweat + fatty substances and proteins - more viscous

n Odorless but bacteria on the skin surface decompose the secretions - becomes musky unpleasant odor

n Activated by stress & pain - activity also increased by sexual foreplay

Apocrine Sweat Glands

n Ceruminous glands - modified apocrine sweat gland - lining of ear canal- secrete cerumen - earwax - may deter insects & block entry of foreign material

n Mammary glands - secrete milk - discussed later

Sebaceous Glands

n Simple alveolar glands - no visible lumen

n Generally small - except large on face neck and upper chest

n Secrete oily substance called sebum

n Generally secreted into a hair follicle or occasionally a o]pore of the skin

n Has bactericidal action

n Secretion stimulated by androgens - thus increase activity at puberty

Sebaceous Glands

Hair

n Composed of largely dead, keratinized cells

n Dominated by hard keratin - tougher & more durable - individual cells do not flake off

n Three concentric layers of keratinized cells

n Medulla of large cells& spaces

n Cortex of several layers of flattened cells

n Cuticle -single layer of overlapping cells - like shingles

n Hair pigment form melanocytes at base of the hair follicle - transferred to cortical cells

Hair Follicle

n From epidermal surface into the dermis

n Deep end - hair bulb

n Root hair plexus - network of sensory nerve endings surrounding the hair bulb - responds to hair movement

n Hair papilla - extends into the hair bulb - contains capillaries - supplies nutrients to the hair

n Arrector pilli - smooth muscle - pulls the follicle into an upright position - goose flesh

Hair Follicle Structure

Hair Follicle Structure

Types of Hair

n Vellus hair- fine pale hair of children and adult females

n Terminal hair - coarser, longer hair of the eyebrows, scalp

n At puberty terminal hair appear in the axillary & pubic regions of both sexes and on the face, chest and typically the arms and legs of males

n Stimulated by androgens

n Lanugo - very fine hair present on the fetus

Nails

n Scalelike modification of the epidermis

n Correspond to hooves & claws of other animals

n Hard kerratin

n Nail matrix of the nail bed is responsible for nail growth

Structure of a Nail

Burns

n Tissue damage inflicted by intense heat , electricity, radiation of certain chemicals

n All these denature cells proteins & cause cell death in the affected areas

n The major threat of burns is loss of body fluids

n As fluid seeps out - dehydration and electrolyte imbalance occurs

n This can result in renal shutdown, circulatory shock

n Fluids must be replace immediately

Rule of Nines

n Used to estimate the volume of fluid lost by computing the percentage of surface burned

n Body divided into 11 areas - each accounting for 9% of the body surface area - area around the genitals accounts for 1%.

Rule of Nines

Classification of Burns

n First-degree burns - only epidermis is damaged

n Local redness, swelling & pain

n Typically heal in 2 to 3 days without special attention

n Second-degree burns - injure epidermis and upper region of the dermis

n Symptoms mimic those of first-degree burns - but blisters may also occur

n Skin regeneration occurs usually in 3 to 4 weeks

n Both of the above are often called partial-thickness burns

Classification of Burns

n Third-degree burns (full-thickness burns) - involve the entire thickness of the skin

n Affected area maybe gray-white, cherry red or blackened

n Initially little or no edema

n Skin grafting is common treatment

n Burns are considered critical if

n 1. Over 25% second-degree burns

n 2. Over 10% third-degree

n 3. Third-degree burns on the face, hands or feet