Integumentary System (Lecture 1/2)

Integumentary System Functions

1. Protection 2. Sensation 3. Thermoregulation 4. Metabolism 5. Attraction

Dermis Layers

2 layers: Papillary layer & reticular layer

Epidermal-melanin unit

= 1 melanocyte + keratinocytes (where melanocytes transfer melanosomes) -density of units in skin is similar in all individuals Though melanocytes produce melanosomes (make the melanin), keratinocytes are melanin depot (contain more than the cells that make it) Melanocytes of ppl w/ ancestral origins near equator produce melanin granules more rapidly & accumulate them more abudantly (& superficially) in keratinocyte -need greater sun protection

Vitiligo

= acquired condition involving skin depigmentation, often only in affected patches, due to loss/decreased activity of melanocytes Causes of meloncyte loss aren't clear, but may involve environment, genetics or autoimmune conditions

Albinism

= congenital disorder producing skin hypopigmentation due to defect in tyrosinase or some other component of melanin-producing pathway

Dermis

= layer of connective tissue that supports the epidermis & binds it to the subcutaneous tissue (hypodermis) -thickness of the dermis varies with region of body & reaches max of 4 mm on back. Surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate w/ projections (epidermal pegs or ridges) of the epidermis, especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction.

Friction blisters

= lymph filled spaces created between epidermis & dermis of thick skin by excessive rubbing If continued, such activity produces protective thickening and hardening of the outer cornified epidermal layers (corns and calluses) In adults, 1/3 of all cancers originate in the skin -Most derive from cells of the basal or spinous layers, producing (basal cell carcinomas & squamous cell carcinomas)

Subcutaneous tissue

= subcutaneous layer consists of loose CT that binds the skin to the subjacent organs, making it possible for the skin to slide over them -AKA the hypodermis or superficial fascia Contains adipocytes that vary in number in different body regions and vary in size according to nutritional state. Extensive vascular supply at the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.

"ABCDE" rule

A: Asymmetry B: Border is irregular, notched, or blurred C: Color is variable or irregularly distributed D: Diameter is greater than 6 mm E: Evolving or changing lesion or exceptional nevus that appears different from other nevi

Langerhans cells

APCs derived from monocytes Represent 2-8% of cells in epidermis & usually most clearly seen in spinous layer Cytoplasmic processes extend from these dendritic cells between keratinocytes of all layers, forming fairly dense network in epiderm.

Epidermis meets Dermis

Basement membrane always between s. basale & dermis -follows contour of interdigitations between layers Avascular epidermis receives nutrients for keratinocytes via diffusion from dermal vasculature (through basement membrane)

Bullous Pemphigold

Blistering disorder caused by abnormality of dermal-epidermal junction

Pemphigus

Blistering disorder caused by autoimmune damage to intercellular jxns between keratinocytes (desmosomes)

What is the integumentary system?

Comprised of skin, hair & nails Skin is largest single organ of the body (~15-20% of total body weight) -approx. 1.5-2 square meters of surface to external environment AKA integument or cutaneous later Composed of.. - epidermis (epi layer of ectodermal origin) - dermis (layer of mesodermal CT)

Epidermis

Consists of stratified squamous keratinized epithelium -composed of keratinocytes (most abundant) 3 less abundant epidermal cells: -melanocytes, Langerhans cells & Merkel cells Epidermis forms major distinction between thick (palms & soles) & thin skin -thick/thin refer to thickness of epidermal layer -thin (75-150 um) -thick (400-1400 um) *dermis of thin skin is more cellular & well vascularized than thick skin

Integ Sys (Thermoregulation)

Constant body temp is normally easily maintained due to skin's insulating components (fatty layer & hair on head) Mechanisms for accelerating heat loss (sweat production & dense superficial microvasculature)

Integumentary System Organization

Dermal papillae interlock with invaginating epidermal ridges at the irreg. jxn of dermis and epidermis -ridges strengthen adhesion of two layers Epidermal derivatives include hairs, nails, sebaceous & sweat glands Below the dermis is the subcut. tissue hypodermis = a loose CT layer usually containing pads of adipocytes - the subcut. tissue binds the skin loosely to underlying tissues & corresponds to superficial fascia of gross anatomy

Melanin Formation

Granules containing melanin mature via 4 stages 1. Vesicle w/ tyrosinase & other enzymes form melanin 2. Melanin production begins on organized matrix in vesicle 3. Melanin pihment accumulates on vesicular matrix 4. Melanin fills mature pigment granule Melanin granules transported to tips of processes of melanocyte & then transferred to neighboring keratinocytes of basal & spinous layers Melanin granules are transported to region near nucleus in keratinocytes where they accumulate as a supranuclear cap -shades DNA from harmful UV radiation effects

Filaggrin

Intermediate filament binding protein which bundles keratin filaments in skin cells (in keratohyalin granules)

Integ Sys. (Attraction)

Many features of skin are visual indicators of health involved in attraction between sexes (hair, pigment) Effects of sex pheromones produced by apocrine sweat glands & other skin glands may also be important for attraction

Integ Sys (Sensation)

Many skin sensory receptors allow constant monitoring of environment Various skin mechanoreceptors help regulate body's interactions w/ physical objects

Moles

Melanocytes can normally proliferate in skin to produce moles or benign melanocytic nevi (varying) Changes in size or appearance of moles are sometimes indicative of dysplasia (abnormal cells, pre-C) that can progress to further malignant melanoma -dividing rapidly, malignantly transformed melanocytes often penetrate basal lamina, enter dermis & metastasize by invading blood & lymph vessels

Reticular Layer

Much thicker, consists of dense irregular connective tissue (mainly bundles of type I collagen) -more fibers and fewer cells than the papillary layer. Network of elastic fibers is also present, providing elasticity to the skin. -Between the collagen and elastic fibers are abundant proteoglycans rich in dermatan sulfate Dermatan sulfate = key role as a constituent of tissues and organs; thought to be involved in wound repair, regulation of blood coagulation and the response to infection

Stratum Spinosum

Normally the thickest layer, especially in the epiderm ridges; consists of generally polyhedral cells w/ central nuclei -nucleoli and cytoplasm actively synthesizing keratins Some cells may still divide; combined zone sometimes called stratum germinativum Keratin filaments (tonofilaments) assemble here into microscopically visible bundles called tonofibrils that converge and terminate at the numerous desmosomes holding the cell layers together. Cells extend slightly around the tonofibrils on both sides of each desmosome leading to the appearance of many short "spines" or prickles at the cell surfaces. Epiderm. of thick skin subject to continuous friction and pressure (foot soles) has a thicker stratum spinosum with more abundant tonofibrils and desmosomes

Integ Sys. (Protection)

Provides physical barrier against thermal & mechanical insults (friction) & against most potential pathogens/other materials Microorgs. that penetrate alert resident lymphocytes & antigen-presenting cells (APCs) in skin & immune response mounts Melanin in epidermis protects cell nuclei from UV radiation Permeability barrier against excessive water loss/uptake (allows terrestrial life) Selective permeability allows some lipophilic drugs (steroid hormones & meds) to be administered via skin patches

Stratum Basale

Single layer of basophilic cuboidal or columnar cells on basement membrane at derm-epiderm jxn -Hemidesmosomes in basal cell membrane join these cells to basal lamina -Desmosomes bind the cells of this layer together in their latreal & upper surfaces Characterized by intense mitotic activity & contains progenitor cells for all epiderm. layers (progenitor cells also in deepest part of stratum spinosum) Renews in humans every 15-30 days, depending on age, region of body, etc *An important feature in all keratinocytes in the S.B. is the cytoskeletal keratins, intermediate filaments Keratins move UP superficially

Integ Sys. (Metabolism)

Skin cells synth. vitamin D3 through local action of UV light on vitamin's precursor -needed in Ca2+ metabolism & proper bone formation Excess electrolytes can be removed in sweat & subcut. layer stores a sig. amt of energy in fat

Melanocytes

Specialized cells of the epidermis found among the cells of the basal layer and in hair follicles The color of the skin is the result of several factors -most important of which are the keratinocytes' content of melanin & carotene & the # of blood vessels in dermis. -Eumelanins are brown or black pigments produced by the melanocyte. •The similar pigment found in red hair is called pheomelanin •1:6 melanocyte:keratinocyte (in S. basale)

Epidermal Layers (top --> bottom)

Stratum Corneum Stratum Lucidum Stratum Granulosum Stratum Spinosum Stratum Basale Come Let's Get Sun Burnt

Keratinocytes

The most abundant epidermal cells, they function mainly to produce keratin.

Papillary Layer

Thin layer, including the dermal papillae, consists of loose connective tissue -types I and III collagen fibers, fibroblasts and scattered mast cells, dendritic cells, and leukocytes From here, anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis.

tonofibrils

a bundle of protein filaments that extend to one side of the cell to the other in the stratum spinosum. The tonofibrils act as cross braces, strengthening and supporting the cell junctions.

Melanocytes

a mature melanin-forming cell, typically in the skin.

Stratum Corneum

•15-20 layers of squamous, keratinized cells filled with filamentous keratins. •Keratin filaments contain at least six different polypeptides, synthesized during cell differentiation in the immature layers •As they form, keratin tonofibrils become heavily massed with filaggrin and other proteins in keratohyaline granules •By the end of keratinization, the cells contain only amorphous, fibrillar proteins with plasma membranes surrounded by the lipid-rich layer. •These fully keratinized (cornified) cells (squames) are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes break down. -lost nuclei & cytoplasm -worn away at surface

Stratum Granulosum

•3-5 layers of flattened cells, undergoing terminal differentiation process of keratinization. -Their cytoplasm is filled with intensely basophilic masses called keratohyaline granules. -These are dense, non-membrane-bound masses of filaggrin and other proteins associated with the keratins of tonofibrils, linking them further into large cytoplasmic structures. •Cells also contain lamellar granules which produce lipids that form an impermeable layer around the cells (major barrier against water loss in the skin)

Merkel cells

•AKA epithelial tactile cells; sensitive mechanoreceptors essential for light touch sensation. •Joined by desmosomes to keratinocytes of the basal epidermal layer -resemble the surrounding cells but with few (if any) melanosomes. Abundant in highly sensitive skin (fingertips, bases of some hair follicles) *Originate from the same stem cells as keratinocytes Characterized by small, Golgi-derived dense-core neurosecretory granules containing peptides. -basolateral surfaces of the cells contact expanded terminal discs of unmyelinated sensory fibers penetrating the basal lamina.

Stratum Lucidum

•Found only in thick skin; consists of a thin, translucent layer of flattened keratinocytes held together by desmosomes. •Nuclei and organelles have been lost; cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix.