Skin Structure and Function

• The skin is a protective covering of the whole body
• The skin is the largest and heaviest organ

Functions of the skin

• Protection against infection, trauma, dehydration, and UV rays
• Excretion of water and some waste products
• Sensation
• Thermoregulation (sweating)
• Metabolic function: synthesis of vitamin D

Types of skin

• Thick (non-hairy) skin has a thick epidermis and is present in palms and soles
• Thin (hairy) skin covers all body except the palms & soles.
• Thin skin has basic skin structure as thick skin but with some differences.

Structure of thick skin

• Consists of the epidermis, dermis, and hypodermis.
• The epidermis is the outer epithelial layer
  • Composed of keratinized stratified squamous epithelium
  • Derived from ectoderm.
• The dermis is a thick, deep connective tissue layer
  • Derived from mesoderm
• The hypodermis consists of subcutaneous tissues

Epidermis

• It is regenerated continuously every 2-4 weeks.
• It is formed of keratinocytes and non-keratinocytes
• According to the degree of maturation of keratinocytes, the epidermis consists of 5 layers.

Layers of Epidermis

• Stratum germinativum (basal layer)
  • Single layer of columnar cells
  • Basophilic cytoplasm
  • Junctional complex
  • Undergoes mitosis
  • Gives rise to new cells
• Stratum spinosum (prickle cell layer)
  • Several layers (4-8)
  • Polyhedral cells, less basophilic
  • Contains spines
  • Connected together by desmosomes
• Stratum granulosum (granular layer)
  • Few layers of 3-5 cells
  • Flat diamond cells
  • Deeply basophilic, granular
  • Contains keratohyaline granules
• Stratum lucidum (clear layer)
  • Few layers
  • Flat non-nucleated cells
  • Contains immature keratin
• Stratum corneum (horny layer)
  • Thick esinophilic
  • Several layers of flat cornfield cells
  • Contains mature keratin

Non-keratinocytes

• Langerhans cells = macrophage of skin
  • Stellate shape
  • Present in the spiny layer
  • Derived from monocyte
  • Is an antigen presenting cell to t lymphocyte
  • Has a role in the skin's immunological reaction
• Melanocytes are the coloring cells
  • Branched cells
  • Present in the basal layer
  • Forms melanin pigments
  • Protects from ultraviolet rays
• Merkel's cells are sensation cells.
  • Consists of modified sensory receptors cells
  • In the basal cell layer
  • Acts a receptor cells for touch sensation

Skin Color

• Depends on carotene content (yellow color), hemoglobin in the blood capillaries of the dermis (reddish color), and melanin content (brown to black)

Dermis

• It consists of Loose CT
• Supports the epidermis
• Binds dermis to hypodermis
• Has projections called papillae

Hypodermis

• It consists of Loose CT
• Binds the skin loosely with the underling tissues
• Helps the skin to slide easily over them

Thin (Hairy) Skin

• It covers all body except palms, soles, tips and sides of fingers and toes.
• The eyelid has the thinnest skin in the body.
• Has the basic skin structure as thick skin but with some differences.

Differences between thick and thin skin

Skin Appendages

• During development, the cells of the epidermis grow down the dermis to form sweat glands, sebaceous glands, hair follicles & hairs, and nail beds and nails

Sweat Glands

• Eccrine sweat gland (merocrine)
  • All over the body except lips and nail bed
  • Simple tubular
  • Secretory part is formed of cubical epithelium
  • Deep in dermis
  • Surrounded with myoepithelium
  • Secrete sweat (watery secretion)
  • Sweat evaporation plays a role in thermoregulation
• Apocrine sweat glands
  • Modified sweat glands
  • In the axilla, pubic region, areola of the breast
  • Only secrete after puberty
  • Viscid cloudy secretion
  • Become odorous after bacteria action
  • Ducts open into hair follicles

Sebaceous Glands

• Simple alveolar
• Sites are the dermis of thin skin
  • Usually associated with hairs
• Mode of Secretion: Holocrine
  • (The whole cell in the center degenerates during secretion)
• Secretes sebum (oily secretion)
• It keeps thin skin and its hairs soft, and it is antifungal and antibacterial.

Hair

• Hair shaft projects above the skin.
• Hair root part is embedded inside hair.
• Arises from the hair follicle.
• Expansion of the follicle creates the bulb.
• The bulb is present over a vascular part called the papilla.
• Papilla is rich in nerve fibres and blood capillary
• Both the shaft and root of hair consist of:
  • Medulla: Keratinizing cells, Keratin is soft.
  • Cortex: Keratinizing pig. Cells, Keratin is hard
  • Cuticle: Keratinizing cells, Keratin is hard.

Arrector Pili Muscle

• A bundle of smooth muscle fibers arises from the dermis and inserts into the hair follicle.
• In case of fear and cold → sympathetic stimulation → contraction of arrector pili muscle → erection of hair with depression of skin over the muscle and elevation of skin around the hair (goose skin).