Anatomy & Physiology: The Skin Anatomy

Largest Organ: The skin accounts for 12-15% of body weight.
Layers of Skin:
- Epidermis:
- Type: Keratinized stratified squamous epithelium.
- Dermis:
- Type: Connective tissue layer.
- Hypodermis:
- Description: Another connective tissue layer but not part of the skin; it consists mainly of subcutaneous fat.

Boundary: Acts as a boundary between external and internal environments.
Vitamin D Synthesis: Crucial for calcium metabolism.
Other Functions:
- Temperature regulation
- Sensory perception
- Excretion of wastes and salts
- Protection from trauma and bacteria

Fat
Accessory Structures
- Hair shaft
- Sweat gland duct opening (pore)
- Tactile (Meissner's) corpuscle
- Sebaceous gland
- Arrector pili muscle
- Lamellated (Pacinian) corpuscle
- Nerve fibers
- Sweat gland
- Artery
- Vein
- Cutaneous plexus

Main Cells:
- Keratinocytes
Layers of the Epidermis:
- Stratum corneum
- Stratum lucidum
- Stratum granulosum
- Stratum spinosum
- Stratum basale

Thin Skin (4 layers) vs. Thick Skin (5 layers):
- Thick skin covers palms and soles of the feet, thin skin covers most other body surfaces.

Micrographs:
- Thin skin at 154x magnification and thick skin at similar magnification.

Stratum Basale:
- Contains hemidesmosomes, ridges, and dermal papillae.
- Basal Cells: Germ cells that replace keratinocytes above.
- Merkel Cells: Sensory touch receptors.
- Melanocytes: Responsible for pigment production.
Stratum Spinosum:
- Dividing keratinocytes migrate into this layer.
- Contains Dendritic Cells: Immune respondents to microorganisms and cancer.
Stratum Granulosum:
- Non-dividing, grainy layer of keratinocytes.
- Contains keratin and keratohyalin.
Stratum Lucidum:
- Only present in thick skin; features flattened non-dividing keratinocytes.
Stratum Corneum:
- Composes 15-30 layers of keratinocyte fibers.
- Keratinization: Formation of protective cellular layer filled with keratin.
- Features:
  - Strong connections
  - Dry surface
  - Reduces water loss

Types of Fibers:
- Collagen Fibers: Provide strength and firmness.
- Elastin Fibers: Essential for elasticity and suppleness.
Fibroblasts: Cells that produce collagen and elastin.

Papillary Layer:
- Made up of areolar tissue; allows for immune cell migration; contains numerous blood vessels.
Reticular Layer:
- Dense layer composed of collagen and elastic fibers.

Composition: Not considered “skin”, composed mainly of areolar and adipose tissue.
Functions:
- Stabilizes skin in relation to underlying organs.
- Provides thermal insulation and energy supply.
- Contains abundant blood supply, facilitating subcutaneous injections.

Key Components:
- Cutaneous Plexus
- Papillary Plexus
Functions:
- Temperature regulation
- Low temperature can lead to cyanosis (bluish color due to lack of oxygen).

Sensory Receptors: Found in the skin detecting various stimuli such as touch, vibration, temperature, and moisture.
Types of Receptors:
- Merkel's Disks: Located at the epidermal-dermal border.
- Free Nerve Endings: Detect pain and temperature changes.
- Meissner's Corpuscles: Sensitive to light touch.
- Hair Follicle Receptors: Respond to hair movement.
- Pacinian Corpuscles: Detect deep pressure and vibration.
- Ruffini's Endings: Detect skin stretch.
Skin Types:
- Hairy Skin: Contains hair follicles.
- Glabrous (Hairless) Skin: No hair follicles, found on palms and soles.

Skin Pigments:
- Carotene:
- An orange-yellow pigment accumulating in the stratum corneum and subcutaneous layers.
- Thickened areas like corns and callouses can appear more prominent due to carotene.
- Melanin:
- Brown/black pigment produced by melanocytes in the stratum basale.
- Packaged into melanosomes, which are then taken up by keratinocytes.
- In darker skin types, melanosome transfer occurs in upper epidermal layers.
- UV exposure can concentrate melanosomes around the nucleus to protect DNA, additionally stimulates melanocyte activity.

Sweat Glands (Sudoriferous):
- Types:
- Apocrine Glands:
  - Located in armpits, nipples, and pubic region.
  - Extend deep into the dermis and produce odorous, milky, sticky sweat.
- Merocrine Glands:
  - More prevalent, found on palms and soles of the feet.
  - Not as deep in the dermis; primarily secrete water and salts through vaporization.
- Myoepithelial Cells: Actively participate in secretion under nervous system stimulation.

Function: Secrete oily lipid (sebum) into hair follicles.
Composition of Sebum:
- Contains triglycerides, proteins, electrolytes, and cholesterol.
Role of Sebum:
- Inhibits bacterial growth, lubricates, and conditions the skin.
Issues: Clogged glands can lead to inflammation and acne.

Accessory Organs: Composed of hard keratin and non-living.
Structure:
- Slender filament produced in hair follicles.
Functions:
- Protection and cushion (primarily on scalp).
- Prevention of foreign body entry (in nostrils and ears).
- Sensory reception.
Anatomical Features:
- Shaft:
- Medulla: Soft keratin
- Cortex: Keratinized cells
- Cuticle: Thin outer layer
- Bulb: Bundle of epithelial cells at the base of the hair.
- Root Hair Plexus: Sensory nerves surrounding the hair follicle.
- Arrector Pili Muscle: Located in the papillary layer of the dermis; causes hair to stand on end (goosebumps).