1. Skin

2. Introduction About 15-20% of total body weight (adult).
m2 of surface area to the external environment.
composed of :
1- Epidermis, an epithelial layer of ectodermal origin
2- Dermis, a layer of mesodermal connective tissue
Beneath the dermis lies hypodermis, a loose connective tissue that may contain pads of adipocytes.

3. Introduction
Beneath the dermis lies hypodermis, a loose connective tissue that may contain pads of adipocytes.
The hypodermis binds skin loosely to the underlying tissues and corresponds to the superficial fascia of gross anatomy.
Projections of the dermis called papillae interdigitate with evaginations of the epidermis known as epidermal ridges.

4. Introduction Epidermal derivatives include: Hairs Nails Sebaceous
Sweat glands

5. Functions of the skin
Protection: physical, biological, against UV light, from dehydration.
Sensation: Diff. receptors for touch, pressure, pain, temp.
Thermoregulation: Thick hair (in most mammals) – for heat conservation, Adipose and sweat glands (humans).

6. Functions of the skin Metabolic function:
Synthesis of Vit.D3 with UV absorption – for Ca metabolism.
Excess electrolytes can be removed in sweat
Adipose (subcutaneous) – major storage of triglycerides for energy.

7. Epidermis
Primarily made up of keratinized stratified squamous epithelium(keratinocytes).
Gives strength to the skin.
Varies in thickness from thick skin to thin skin.
It does not have any vascularization, so it relies on the connective tissues deep to it.
Also contain melanocytes, merkel’s cells and Langerhans cell.

8. Types of skin
Thick skin (found on the palms and soles) and thin skin (found elsewhere on the body) refer to the thickness of the epidermal layer.
Total skin thickness (epidermis + dermis) also varies according to site.
Skin on the back is about 4 mm thick
Skin of the scalp is about 1.5 mm thick.

9. Layers of epidermis Stratum basale (the deepest layer)
Stratum spinosum
Stratum granulosum
Stratum lucidum (only in thick skin)
Stratum corneum (most superficial layer of epidermis)

10. Stratum Basale Single layer of basophilic columnar or cuboidal cells.
Cells are bound to each other by desmosomes and to basal lamina by hemidesmosomes.
Characterized by intense mitotic activity.
is responsible with the initial portion of the next layer, for constant production of epidermal cells.
keratinocytes in the stratum basale contain intermediate filaments composed of keratins.

11. Stratum Spinosum The thickest epidermal layer.
polyhedral or slightly flattened.
Also contain the dividing cells as in basale.
The keratin filaments form visible bundles called (tonofibrils) converge and terminate at the numerous desmosomes .
Tonofibrils provide resistant to the abrasion ( thick skin)

12. Stratum Granulosum 3–5 layers of polygonal cells.
cytoplasm is filled with intensely basophilic masses , keratohyaline granules.
The membrane-coated lamellar granules ovoid structures containing many lamellae composed of various lipids. undergo exocytosis, discharging their contents into the intercellular spaces of the stratum granulosum, produces sheets that envelop the cells.
Form a barrier against the loss of water from skin.

13. Stratum Lucidum Is only seen in thick skin.
A translucent thin layer of extremely flattened eosinophilic cells.
Nuclei and organelles not present
Desmosomes retained
The cytoplasm consists almost only of densely packed keratin filaments embedded in an electron-dense matrix.

14. Stratum Corneum
The main difference between thick skin and thin skin relates to the thickness of the Stratum corneum.
15–20 layers of flattened, nonnucleated keratinized cells.
Cytoplasm is filled with filamentous keratins.
After keratinization, the cells contain only fibrillar and amorphous proteins with thickened plasma membranes and are called horny, cornified cells.
These cells are continuously shed at the surface of the stratum corneum.

15. Epidermal cell types Keratinocytes (structure)‏.
Melanocytes (pigment)‏.
Merkel cells (sensation). ‏
Langerhans’ cells (immune defence)‏.

16. Keratinocytes
Most numerous epidermal cell found in all layers of the epidermis.
Chief function is the production of keratin: a tough fibrous protein that gives strength and confers a lot of protective ability.
Tightly connected to one another by desmosomes.
Provides continuity, strength, and protection.
Is the reason skin flakes off in sheets rather than as individual

17. Melanocytes- neural crest cells
specialized cell of the epidermis found beneath or between the cells of the stratum basale & in the hair follicles.
Derived from neural crest cells.
rounded cell body.

18. Melanin
Melanin granules migrates & transferred to the keratinocytes of the germinativum layer.
Accumulate as a supranuclear cap which absorbs and scatters sunlight, protecting nuclear DNA from the deleterious effects of UV radiation.

19. Langerhans cells
These are star shaped bone marrow derived cells. forming a fairly dense network in the epidermis Migrated to the skin via blood.
Present in the S. spinosum of the skin, represent 2–8% of the epidermal cells
These cells are antigen presenting cells so participate in immune related reaction.

20. Merkel’s cell
Mechanoreceptors resemble pale-staining keratinocytes with keratin filaments in their cytoplasm but few if any melanosomes.
Neural crest in origin.
Found in the basal layer of the thick skin & bases of hair follicles.
Has sensory function.
have functions related to the diffuse neuroendocrine system.

21. Dermis Dermis is connective tissue –Support epidermis
– Binds epidermis to hypodermis Its surface
The thickness : varies (maximum of 4 mm on the back)
Projections (dermal papillae) interdigitate with projections (epidermal pegs or ridges) of the epidermis
Contains 2 layers :
– Papillary layer
– Reticular layer

22. Dermis
A basement membrane found between the stratum basale and papillary layer.
The basement membrane consisting of the basal lamina and the reticular lamina and can usually be seen with the light microscope.
Nutrients diffuse into the avascular epidermis from the dermis vasculature through this basement membrane.

23. Papillary layer The major part of the dermal papillae.
1/5 of the dermis.
Is composed of loose connective tissue, with fibroblasts and other cells such as mast cells and macrophages.
Anchoring fibrils of type VII collagen insert into the basal lamina and bind the dermis to the epidermis

24. Reticular layer Irregular dense connective tissue (type I collagen).
Reticular dermis is the lower 4/5.
More fibers and fewer cells than the papillary layer.
A network of elastic fibers is also present providing elasticity to the skin.
Blue arrow indicates the reticular dermis

25. Dermis In addition, dermis contains epidermal derivatives :
– Hair follicle
– Sweat & sebaceous glands
Rich network of blood & lymph vessels and nerves supply.

26. Subcutaneous Tissue
Loose connective tissue which helps in sliding the skin over the deep structure.
Also called the hypodermis or superficial fascia
Often contains fat cells that vary in number in different regions of the body and vary in size according to nutritional state.
An extensive vascular supply in the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.

27. Appendages of the skin
Hairs
Sebaceous gland
Sweat gland
Nails

28. Hair Elongated keratinized structure.
• Develop from epidermis & invade the dermis & hypodermis.
• Everywhere except palms, sole, some part in genitalia.
• Hairs grow discontinuously, have periods of growth ollowed by period of rest .

29. Arrector pili muscle
The surrounding dermis forms a connective tissue sheath. Running from a midpoint on this sheath and to the dermal papillary layer is a small bundle of smooth muscle cells, the arrector pili muscle.
Contraction of these muscles pulls the hair shafts to a more erect position

30. Keratinization of the hair
Keratin of hair has a harder and more compact nature than that of stratum corneum, maintaining its structure much longer.
Although keratinization in the epidermis occurs continuously and over the entire surface, it is intermittent in the hair and occurs only in the hair root.

31. Nail
Hard, flexible plates of keratin on the dorsal surface of each distal phalanx
The nail root is the most proximal region of the nail plate , is formed by a matrix of proliferating, differentiating keratinocytes.
These cells make up the nail matrix which contribute keratinized cells to the nail root.

32. Sebaceous gland
Embedded in the dermis over most of the body surface, except the thick, hairless skin of the palms and soles.
Branched acinar glands with several acini converging at a short duct which usually empties into the upper portion of a hair follicle.
In certain hairless regions, such as the genital glands, eyelids, and nipples, sebaceous ducts open directly onto the epidermal surface.

33. Sebaceous gland
The acini consist of a basal layer of undifferentiated flattened epithelial cells on the basal lamina.
Their nuclei shrink and undergo autophagy along with other organelles and near the duct the cells disintegrate and release the lipids via holocrine secretion.
The product of this process is sebum

34. Eccrine sweat gland Are widely distributed in the skin.
Most numerous on the soles of the feet.
Sweating is the physiological response to increased body temperature and in humans the most effective means of temperature regulation.
The secretory portions and ducts are coiled and have small lumens.
Secretory part is generally more pale-staining than the ducts and has stratified cuboidal epithelium

35. Apocrine gland
Largely confined to skin of the axillary and perineal regions
much larger lumen than Eccrine gland.
The secretory portions of apocrine sweat glands consist of simple cuboidal, eosinophilic cells with numerous apical secretory granules that undergo exocytosis.
Their cells show merocrine, not apocrine, secretion

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