intoduction to skin

intoduction to skin epidermis part one

1. EPIDERMIS DR. SUBODH KUMAR SHAH

2. INTRODUCTION OF SKIN Skin is the largest organ in the body. In a 70-kg individual, the skin weighs over 5 kg and covers a surface area of approx. 2m 2 . Human skin consists of a stratified , cellular epidermis and an underlying dermis of connective tissue and beneath the dermis is a layer of subcutaneous fat . Mainly two kinds of human skin : - G labrous skin ( non-hairy skin ) and - H air-bearing skin

3. FUNCTIONS: The skin has very important vital functions for keeping the physiological and biochemical conditions of the body in its optimum state. The most important functions of the skin are:  Regulates body temperature. Prevents loss of essential body fluids, and penetration of toxic substances. Protection of the body from harmful effects of the sun and radiation. Excretes toxic substances with sweat. Mechanical support. Immunological function mediated by Langerhans cells. Sensory organ for touch, heat, cold, socio-sexual and emotional sensations. Vitamin D synthesis from its precursors under the effect of sunlight and introversion of steroids

4. INTRODUCTION OF EPIDERMIS Superficial layer of skin. Continually renewing, Stratified squamous epithelium that keratinizes and give rise to derivative appendages. Keratinocyte make 95% of cell population. Approximately 0.4 to 1 .5mm thick. Avascular . Project to dermis by rete ridges . Joined by complex basement membrane to dermis.

5. Epidermal Appendages are: 1) Hair 2) Nail 3) Sebaceous Gland 4) Sweat Gland(Apocrine & Eccrine ) Other cells of Epidermis are: 1) Melanocytes 2) Langerhan’s cell 3) Merkel cell

6. DEVELOPMENT Skin arises by juxtaposition of two major embryological components: Prospective epidermis- originates from surface area of early gastrula. Prospective mesoderm- brought into contact with inner surface of epidermis during gastrulation.  provides dermis.  induce differentiation of epidermal structures. Neural crests contribute: pigment cells.

7. DEVELOPMENT OF EPIDERMIS Epidermis develop from surface ectoderm of early gastrula. Development relies on Initiation signals. Notch and Wnt signalling pathways has opposing interplay with Beta Catenin, Lef1 & Notch peptide. Signals from sonic Hedgehog pathways Bone Morphogenetic Proteins(BMP’s ).

8. LAYERS OF EPIDERMIS From below up layers are- 1. Stratum Basale or Germinativum 2. Stratum Spinosum or Prickle layer 3. Stratum Granulosum 4. Stratum Corneum or Horny layer Stratum Lucidium is extra layer below horny layer found in thick skin of palms and soles. Malpighian layer include basal and spinous cell .

9. STRATUM BASALE Continuous layer generally one cell thick. -may be 2-3 cell thick in glabrous and hyperproliferative epidermis Cuboidal with large,dark staining nuclei and a dense cytoplasm. Site of mitotically active cell and give rise to other layer.

10. STRATUM SPINOSUM 8 to 10 cell layer. Polyhedral cell with a round nucleus. Become flatter and develop organelles. They look shrunken and contain spine like projection in histological section. Contain numerous desmosome and Calcium dependent cell surface modification. Function: adhesion of epidermal cells and resistance to mechanical stress . Limited mitosis occur.

11. STRATUM GRANULOSUM 2 to 5 cell layer Named for basophilic keratohyalin granules. Cytoplasm of granular layer and upper spinous layer also contain lamellar granule or membran e coating granule or Odland bodies . Lamellar granules appear in intercellular space between plasma membranes of adjacent cells. Cornified envelope begins to form. Keratinocyte of granular zone have keratin filament and keratohyaline granule.

12. STRATUM LUCIDUM On palms and soles there is stratum lucidum in between stratum granulosum and stratum corneum layers. These cells are still nucleated, and may be referred to as ‘ transitional ’ cells .

13. STRATUM CORNEUM Outer most layer of skin(epidermis). 20 to 25 cell layer thick. Cells ( corneocyte ) are flattened and have lost their nuclei and cytoplasmic organelles. Cells are stuck together by lipid in intercellular space. Keratin filament aggregate into disulphide cross link filament. Contain highly insoluble cornified envelope.

14. Stratum cornuem barrier is formed by a two-compartment system of lipid depleted, protein enriched corneocyte surrounded by a continuous extracellular lipid matrix. Desquamation : degradation of lamellated lipid + loss of residual intercellular demosomal interconnection.

15. OTHER CELL OF EPIDERMIS MELANOCYTE - Melanosome contain melan -Mostly basal melanocyte to keratinocyte ratio sun exposed part 1:1 sun protected part it is 1:10 - Dendritic - N o desmosome. Pale staining cytoplasm with ovoid nucleus and pigment containing melanosomes . Melanosome contain melanin.

16. Merkel cells(0.2-5%) -Mechanoreceptor in area of high tactile sensitivity -Basal cell -No Dendrites -Desmosomes Pale – staining cytoplasm with lobulated nucleus. Margin of cells project cytoplasmic ‘ spines’ towards keratinocytes. Contains neurosecretory granules. Fig: Merkel cell in human epidermis. The dermis (d) with collagen fibres is seen in the lower part of the picture ; b, basement membrane ; de , desmosomes making connections with adjacent basal keratinocyte; g, spherical granules (see inset ); n, nucleus of Merkel cell; t, tonofi laments.

17. Lagerhan’s cell -Antigen presenting cell. - Suprabasal . -Dendritic -No desmosome. -It recognizes, uptake, process and present antigen to sensitize lymphocyte and important in induction of delayed hypersensitivity. On electron microscopy clear cytoplasm with lobulated nucleus , well developed ER(endoplasmic reticulum) , golgi body and lysosome. Rod or racquet shaped granules – Birbeck granules. Fig: Langerhans ’ cell with its characteristically indented nucleus, situated between keratinocytes. Inset : Langerhans’ cell granules showing racquet-shaped profiles.