Skin and skin appendage

Microscopic skin structure

The skin is composed of several layers, which are categorized as follows (from superficial to deep):

• Cutis
  • Epidermis
    • Nonvascularized
    • Consists of five layers
    • Maintains the skin's barrier function
    • For more details, see layers of the epidermis.
  • Dermal-epidermal junction
    • Consists of a basement membrane that anchors the epidermis to the dermis
      • The basement membrane consists of two layers:
        • Lamina lucida (superficial)
        • Lamina densa (deeper)
    • Hemidesmosomes: connect epidermis to basal cell layer
    • Collagen fibrils: connect basal cell layer to papillary layer
  • Dermis: A layer derived from mesoderm that contains blood vessels and provides structural integrity.
    • Papillary dermis
      • Consists of fine, loosely arranged collagen fibers
      • Supplies the epidermis with nutrients
      • Important role in temperature regulation
      • Form dermal ridges, which connect dermal papillae with downgrowths of epidermis called epidermal ridges, or rete ridges.
      • Contains Meissner corpuscles (for fine-touch, and two-point discrimination receptors), and free nerve endings
      • Contains immune cells (mast cells and macrophages)
    • Reticular dermis
      • Consists of thick, densely packed fibers (e.g., reticular, elastic, and collagenous) that provide structure and support to the skin, and its components
      • Contains elastic fibers that provide elasticity
      • Contains Pacinian corpuscles (pressure receptors)
      • Contains roots of hair follicles, sweat glands, and sebaceous glands.
      • Langer's lines: Topographic lines that correlate with natural orientation of reticular fibers of the reticular dermis. Are of importance in wound healing, and to guide surgical incisions.
• Subcutaneous tissue
  • Also called hypodermis.
  • Derived from mesoderm.
  • Consists mainly of fat that protects from cold and trauma.
  • Contains superficial veins and free nerve endings
  • Lamellar corpuscles primarily located in this layer
    • Responsible for vibration and pressure sensation.
  • Contains Ruffini corpuscle (mechanoreceptors).
    • Responsible for mechanical pressure and distortion sensation
  • Collagenous and elastic fibers in this area function to anchor the skin (epidermis, dermis) to the deep fascia.
  • Allows for subcutaneous administration of medications.
• Skin appendage: hair, nails, glands (e.g., sweat glands, sebaceous glands)

Layers of the epidermis

The epidermis is the outer-most layer of the skin. The epidermis is derived from ectoderm; . It is primarily composed of keratinocytes, which constantly regenerate approximately every 30 days. The epidermis is composed of 4–5 layers, which are categorized as follows (from superficial to deep):

• Stratum corneum: outer layer of the epidermis
  • Consists of dead (anuclear), keratin-filled cells
  • This layer is constantly being sloughed off.
• Stratum lucidum: thin, translucent layer
  • Found only on thick skin (palms and soles).
  • Composed of a homogeneous layer of keratinocytes with no nuclei or organelles.
• Stratum granulosum: also called the granular layer
  • Contains keratohyalin.
  • This layer has waterproof properties.
• Stratum spinosum
  • Composed of actively dividing keratinocytes with spinous-like projections (prickle cells).
  • This layer produces keratin and induces keratinization.
• Stratum basale: also called the basal cell layer
  • Stem cells of the epidermis (daughter cells migrate upwards and differentiate into other cells).

Carolina Loves Guys that are Super Billionaires! (Corneum, Lucidum, Granulosum, Spinosum, Basalis)

References:^{[1][2][3][4][5]}

Overview

The skin appendages include:

• Nails
• Hair follicles
• Sweat gland
  • Apocrine sweat gland
  • Eccrine sweat gland
• Sebaceous gland

Nails

• Perionychium: epidermal tissue surrounding the root and base of the nail
• Eponychium: proximal layer of epidermis extending over the nail base
• Hyponychium: epidermal tissue immediately underlying the free distal edge of the nail
• Nail plate (nail body)
  • Covers the nail bed
  • Proximally: consists of the germinal matrix (responsible for new nail growth) and the lunula (white, crescent-shaped, and poorly vascularized portion of the nail)
  • Distally: sterile matrix (gives the nail bulk and strength)
• Nail fold: depression proximal to nail plate from which the nail grows
• Vascular supply
  • Arterial: two terminal branches of the volar digital artery
  • Venous: drains into a network in the proximal nail bed and the skin proximal to the nail fold
• Innervation: trifurcation of the dorsal volar digital nerve (supplying the nail fold, pulp, and distal tip of the finger)

Hair follicle

Invaginations of the epidermis and hair shaft into the deep dermis. It is composed of:

• Hair
  • A skin appendage that grows from follicles in the dermis
  • Divided into medulla, cortex, and cuticle
  • Functions include conservation of body heat, sensation, and protection of the skin.
  • Most prominent on the scalp, pubis, axilla, extremities, and face
  • See also phases of hair growth.
• Hair shaft
  • Extends above the epidermis
  • Connects to the hair root in the deep dermis
• Hair bulb
  • Root of the hair follicle, located deep in the dermal papilla
  • Receives blood and nutrients from capillary network of vessels to sustain hair growth
• Internal root sheath
  • Composed of the Henle layer, Huxley layer, and cuticle.
• External root sheath
  • Contains the stratum malpighii (composed of the stratum basale and stratum spinosum).
• Arrector pili muscle
  • Description: Obliquely directed smooth muscle fibers that attach to the dermal sheath surrounding hair follicles.
  • Function
    • Contraction is responsible for piloerection (goosebumps).
    • Conservation of body heat.
    • Innervated by the sympathetic autonomic nervous system.
• Hair disorders: see learning card on alopecia.

Glands

	Sebaceous gland	Sweat glands
		Eccrine sweat gland	Apocrine sweat gland
Description	Exocrine (holocrine) branched glands located in the dermis. Controlled by circulating hormones	Location: deep into the dermis and hypodermis. Composition: secretory unit called glomerulum or acrosyringium, surrounded by myoepithelial cells Controlled by circulating hormones and the autonomic nervous system The combination of sebaceous glands, arrector pili muscle, and hair follicle is known as the pilosebaceous unit.
		Exocrine glands with a spiral duct “acrosyringium” and secretory ducts that open into sweat pores	Exocrine glands with a secretory unit called “glomerulum”, and excretory ducts that open into hair follicles
Distribution	Predominant in the face and scalp Absent in palms and soles	Most areas of the body Absent in lips, ear canal, clitoris, labia minora, and glans penis	Mostly axilla, perineum, areola of the nipple, and external ear
Function	Secrete sebum (oily, waxy substance), a lubricant and a waterproof layer for the skin and hair. Has photoprotective, antimicrobial, and anti-oxidant activity.	Secrete sweat → thermoregulation	Modified apocrine cells produce ear wax (in the ear), or breast milk (in the breast). No significant role in thermoregulation

References:^[6]

The main functions of the skin include:

• Barrier and protection against ultraviolet (UV) radiation and the external environment.
• Thermoregulation (e.g., perspiration, regulation of blood circulation)
• Sensory function: Sensation of pressure, vibration, touch, pain, and temperature.
• Hormone synthesis of vitamin D
• Melanin synthesis
  • Melanin: Provides protection against ultraviolet radiation and determines the color of the skin and iris.
    • Tyrosine → DOPA (via the tyrosine hydroxylase; enzyme) → melanin (via tyrosinase, gamma-glutamyl transpeptidase, and tyrosinase-related protein-2).
  • Impaired melanin production: albinism

Common skin disorders

• Acne
• Allergic contact dermatitis
• Atopic dermatitis
• Psoriasis
• Rosacea
• Seborrheic keratosis
• Urticaria

Vascular tumors of the skin

• Angiosarcoma
• Bacillary angiomatosis
• Kaposi sarcoma
• Pyogenic granuloma
• Strawberry hemangioma

Viral infections of the skin

• Herpes
• Human papillomavirus
• Molluscum contagiosum
• Varicella-zoster virus
• Hairy leukoplakia

Bacterial infections of the skin

• Cellulitis
• Erysipelas
• Impetigo
• Necrotizing fasciitis
• Staphylococcal scalded skin syndrome

Fungal infections of the skin

• Tinea
• Cutaneous candidiasis
• Sporotrichosis

Bullous skin disorders

• Bullous pemphigoid
• Dermatitis herpetiformis
• Pemphigus vulgaris
• Stevens-Johnson syndrome
• Toxic epidermal necrolysis

Malignant and premalignant skin lesions

• Actinic keratosis
• Melanoma
• Squamous cell carcinoma
• Basal cell carcinoma

Disorders of pigmentation

• Albinism
• Melasma (Chloasma)
• Vitiligo
• Seborrheic dermatitis

Other skin disorders

• Acanthosis nigricans
• Erythema nodosum
• Lichen planus

Nail disorders

• Finger infections
• Psoriasis