Integumentary System: Skin, Hair, Nails, and Glands

The integumentary system is the largest organ system, comprising 15-20% of body weight. It provides barrier protection, thermoregulation, sensation, and immune surveillance.

Skin

Layers

Layer	Thickness	Composition	Function
Epidermis	0.05-0.1 mm (thin) to 1.5 mm (thick)	Stratified squamous epithelium	Barrier, pigmentation, immune
Dermis	0.5-2 mm	Connective tissue (collagen, elastin)	Strength, elasticity, blood supply
Hypodermis (subcutaneous)	Variable	Adipose tissue, connective tissue	Fat storage, insulation, shock absorption

Epidermis

Cell types:

Cell	Function
Keratinocyte	Produce keratin (major cell, 90%)
Melanocyte	Produce melanin (pigment, UV protection)
Langerhans cell	Antigen-presenting (immune)
Merkel cell	Sensory (touch)

Layers (strata):

Layer	Description	Thick Skin	Thin Skin
Stratum basale	Deepest (germinativum), single layer of dividing keratinocytes	Yes	Yes
Stratum spinosum	Several layers, desmosomes (prickle cells)	Yes	Yes
Stratum granulosum	Keratohyalin granules	Yes	Yes
Stratum lucidum	Clear layer, only in thick skin	Yes	No
Stratum corneum	Dead keratinocytes (squames), 15-30 layers	Yes	Yes

Keratinization (cornification):

• Basale → spinosum: 14 days
• Spinosum → corneum: 14 days
• Total turnover: ~28 days (increases with age)

Dermis

Layer	Composition	Contents
Papillary (superficial)	Loose connective tissue	Dermal papillae (capillary loops, Meissner corpuscles)
Reticular (deep)	Dense irregular connective tissue	Sweat glands, hair follicles, sebaceous glands, Pacinian corpuscles

Lines of cleavage (Langer lines): Direction of collagen bundles in the dermis. Surgical incisions parallel to these lines heal best.

Skin Thickness

Region	Thickness	Type
Eyelids	0.5 mm	Thin
Palm, sole	1.5 mm	Thick
Back	4 mm	Thick (dermis)
Scalp	3 mm	Thick

Skin Appendages

Hair

Feature	Description
Types	Vellus (fine, body), Terminal (thick, scalp/eyebrows/genital)
Structure	Follicle (in dermis/hypodermis), shaft (cuticle, cortex, medulla)
Hair bulb	Matrix (dividing cells), dermal papilla
Muscle	Arrector pili (smooth muscle, sympathetic)
Cycle	Anagen (growth, 2-6 years), Catagen (regression, 2-3 weeks), Telogen (resting, 3-4 months)
Loss	~100 hairs/day (normal)

Nails

Feature	Description
Plate	Hard keratin
Matrix	Germinal cells (lunula = visible matrix)
Bed	Underlying epidermis (attached)
Hyponychium	Under free edge
Eponychium (cuticle)	Proximal fold
Growth	3 mm/month (fingernails), 1 mm/month (toenails)

Sebaceous Glands

Feature	Description
Location	Everywhere except palms, soles
Secretion	Sebum (lipids, triglycerides, wax esters, squalene)
Duct	→ Hair follicle (or directly to surface)
Function	Lubrication, waterproofing, antimicrobial
Activity	Androgen-stimulated, highest on face/scalp/chest

Sweat Glands

Type	Eccrine	Apocrine
Distribution	Everywhere (2-4 million)	Axilla, areola, perineum, external ear
Secretion	Hypotonic saline (99% water)	Organic + proteins (odorless until bacterial breakdown)
Duct	Direct to surface	→ Hair follicle
Function	Thermoregulation (cooling)	Pheromone? (vestigial in humans)
Activation	Cholinergic (sympathetic)	Adrenergic (sympathetic)

Skin Functions

Function	Mechanism
Barrier	Physical (keratin), chemical (acid pH, antimicrobial peptides), immune (Langerhans cells)
Thermoregulation	Sweat evaporation, vasodilation/constriction, insulation (fat, hair)
Sensation	Touch (Meissner), pressure (Pacini), pain (free nerve endings), temperature (Krause/Ruffini)
Vitamin D synthesis	UVB → 7-dehydrocholesterol → cholecalciferol
Pigmentation	Melanin (Eumelanin: brown/black, Pheomelanin: red/yellow)
Immune surveillance	Langerhans cells, dendritic cells
Wound healing	Inflammation → proliferation → remodeling
Water homeostasis	Prevents excessive water loss (insensible perspiration: ~500 mL/day)