skin

The skin (integument) is the largest sensory organ in our body.It makes up to 16% of body weight with a surface area of 1.8m². Its normal temperature is 37°C.

Skin has several functions,the important ones are, Provides a protective barrier against mechanical, thermal and physical injury and hazardous substances. Prevents loss of moisture. Reduces harmful effects of UV radiation. Acts as a sensory organ (touch, detects temperature). Helps regulate temperature. An immune organ to detect infections Production of vitamin D.

1-Epidermis :Stratified squamous keratinized epithelium- originated from ectoderm 2-Dermis :Dense connective T.- originated from mesoderm 3- hypodermis(subcutaneous) deepest layer of the skin.It consists of loose connective tissue and fat.It may reach up to 3cm thick in the abdomen.

The junction between dermis & epidermis is irregular there are dermal projections called dermal papillae &epidermal evaginations called epidermal ridges

-These interdigitations have different patterns In thin skin—they are pegs & sockets In thick skin—they are ridges & grooves

-outer surface of skin & in certain regions of body showed ridges & grooves arranged in special patterns These patterns are unique to each individual & appear as loops-arches-whorls

Which are more prominent on palmar & plantar surfaces of hand & foot as a fingerprint & footprint These finger&foot print are determined genetically

"Dermatoglyphics" (from ancient Greek derma=skin, glyph=carving) is the scientific study of fingerprints, lines, mounts, and shapes of hands.ancient Greek glyph fingerprints

Dermatoglyphics refers to the formation of naturally occurring ridges on certain body parts, namely palms, fingers, soles and toes. These are areas where hair usually doesn’t grow and these ridge allow for increased leverage when picking up objects or walking on barefoot.

The finger prints of both hands are not the same. They do not change size or shape throughout a person's life, except in cases of serious injuries that scar the dermis.

The uniqueness of a person’s finger prints have been thought by some to be tied to a person's personality and future prediction as a part of analyzing dermatoglyphics. This belief is the basis for the study of palmistry. analyzing dermatoglyphics palmistry In addition it plays an important role in anthropology & criminology

Thickness of skin Histologically Anatomically depend on thickness of epidermis depend on total skin thickness Thick Thin Thick Thin μm μm 4mm 1.5mm Palm of hand other regions Back eyelid Sole of foot of skin Hairless contains hairs

These slides show the contrast between thick and thin skin. Note the dermis is thick in both, but the epidermis is much thicker in the thick skin on the left.

Epidermis Stratified squamous keratinized epithelium (keratinocytes) + Melanocytes + Langerhans cells + Merkel's cells Keratinocytes are arranged in 5 layers

1-Stratum basale(germinativum) *Single layer of basophilic columnar or cuboidal cells lie on basement Membrane * This layer contains stem cells which are involve in mitotic activity *Epidermis is renewed every 15—30 days-depend on age& body region *Cells of stratum basale contain intermediate keratin filaments

2-Stratum spinosum Consists of many layers of cuboidal or flattened cells with central nucleus *Each cell has processes or extensions(spiny projections) * Which are filled with bundles of keratin filaments (tonofilaments)

These tonofilaments are inserted into desmosomes to maintain adhesion between cells Tonofilaments & desmosomes are more numerous where skin is subjected to friction & pressure mitoses occurs in malpighian layer(Str.basale+str.spi nosum)

3-Stratum granulosum * Consists of 3—5 layers of flattened polygonal cells where * Cytoplasm is filled with basophlic keratohyalin granules

there are 2 types of granules a-Non membrane coated granules called keratohyline granules- which contain phosphorylated histidine rich protein &protein containing cystine

b-Membrane coated lamellar granules— they are rod like structures consists of bilayer lipid. * These granules fuse with cell membrane & discharge into intercellular spaces & deposited in form of sheets of lipid * Which acts as a barrier to prevent entry of foreign materials

4-Stratum lucidum *Translucent thin layer— consists of flattened eosinophilic epidermal cells *Nuclei & organells are not clear *Cytoplasm contains densely packed keratin filaments Which are embedded in electron-dense matrix

*Desmosomes are still present between cells *Which are more obvious in thick skin

5-Stratum corneum *Consists of 15—20 layers of flattened nonnucleated cytoplasm filled with filamentous protein scleroprotein(keratin) (which is polypeptides of high molecular weight)

*After keratinization—the cells consist of fibrillar and amorphous proteins +thickened plasma membranes *they are called horny cells

During keratinization—lysosomal hydrolytic enzymes play role in disappearance of organelles These cells are continuously shed out from surface of skin which occurs as a result of lack of cells cohesion—this process is called desquamation Turnover time of stratum corneum is normally 2—4 weeks Desquamation occurs to maintain normal thickness of epidermis

In certain diseases like psoriasis & ichthyosis, there is excessive production of corneocytes (which occurs every 3—4 days)—causes thickening of skin and scaling

Intercellular junction There are 4 types responsible about adhesion & interactions between cells

1-Desmosomes –major type—presents between keratinocytes * by anchoring intermediate filaments to cell membrane & bridging adjacent keratinocytes

2-Adhesion junction— membrane proteins of adjacent cells hooked together—responsible about cell motility,cell shape &cell interactions

3-Gap junction— there are cluster of intercellular channels (connexons)—to form connections between cytoplasm of adjacent keratinocytes

4-Tight junction — consists of transmembrane proteins that fuse together on outer surfaces of adjacent cells it regulates the permeability of skin

Other cells of epidermis

1-Melanocytes : are dendritic cells consists of rounded cell bodies with irregular extensions running between stratum basale & stratum spinosum *Melanocyte contains mitochondria—well developed Golgi apparatus –rough endoplasmic reticulum

*They are attached to basal lamina by hemidesmosomes *the ratio of melanocyte to keratinocyte ranges from 1: 4 to 1: 10 in different parts of body *it is constant in all races

*The number of melanocytes per unit area of skin varies from region to another—it is In thigh /mm² In scrotum /mm² * This is called epidermal- melanocyte unit

synthesis of melanin Melanin pigment is produced by oxidation of tyrosine by tyrosinase enzyme(from ribosomes) tyrosine dopa dopaquinone melanin

transfer of tyrosinase (transmembrane protein) in ribosomes of rER to vesicles of Golgi apparatus

Process of formation

Stage I :Vesicle shows tyrosinase activity (premelanosome) Stage II : vesicle is ovoid & shows the beginning of melanin formation (early melanosome)

Stage III: Vesicle shows increase in melanin formation (nearly mature melanosome) Stage IV: Melanin completely fills the vesicle –ellipsoid in shape (mature melanosome)

Melanin pigments are transferred to keratinocyte by pigment donation as follow -Phagocytosis of tip of melanocyte processes by keratinocyte -This type of secretion is called cytocrine secretion

-within keratinocyte—melanin granules accumulate in the supranuclear region to protect the nucleus from the hazard of U.V. radiation

Color of skin depend on 1-Rate of synthesis of melanin granules 2-Rate of transfer of melanin granules into keratinocyte 3-Content of carotene(result in yellow discoloration)comes from overconsumption of carotene containing food 4-Number of blood vessels in dermis

The tanning of skin after exposure to sun light is due to 1-physicochemical reaction darkens the preexisting melanin 2-increase in rate of synthesis of melanin

There are 2 types of melanin pigment 1-Eumelanin--dark brown pigment (found in skin) 2-Pheomelanin--reddish yellow pigment (found in red hair)

melanocyte stimulating hormone(MSH)is produced by pituitary gland This hormone controls the melanin synthesis (by activation of tyrosinase)

2-Langerhans cells *They are dendritic cells—present mainly in stratum spinosum *They form about 2—8% of epidermal cells *They are antigen presenting cells

On E.M * Nucleus is indented— cytoplasm has Tennis racket- shaped granules called Birbeck granules *They are derived from bone marrow

3-Merkel´s cells *Present in stratum basale *They are found where sensory perception is needed like finger tips *They are connected to keratinocytes by desmosomes

*Nucleus is lobed&cytoplasm contains many melanosomes &dense-cored neurosecretary granules *Free nerve ending forms an expanded disk at base of merkel´s cell to form a merkel´s corpuscle which is mechanoreceptor