Skin and skin appendage

Overview

Structure of the skin

Microscopic skin structure

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

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):

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References:[1][2][3][4][5]

Cells of the skin

Cells of the epidermis

The epidermis is mainly composed of four different type of cells:

Sensory receptors of the skin

Receptor Type of fibers Function
Ruffini corpuscles
  • Dendritic endings
  • Adapt slowly to stimuli
  • Location: fingertips, joints
  • Pressure
  • Changes in joint angles
Meissner corpuscles
  • Large, myelinated fibers
  • Adapt quickly to stimuli
  • Location: glabrous (hairless) skin
  • Position sense
  • Dynamic, fine, and light touch
Pacinian corpuscles
  • Large, myelinated fibers
  • Adapt quickly to stimuli
  • Location: ligaments, joints, deep layers of the skin
  • Pressure
  • Vibration

Merkel discs

  • Large, myelinated fibers
  • Adapt slowly to stimuli
  • Location: fingertips, superficial skin
  • Position sense
  • Pressure
  • Static touch (e.g., edges, corners, shapes)
Free nerve endings
  • C fibers → slow, unmyelinated fibers
  • Aδ → fast, myelinated fibers
  • Location: all skin areas, epidermis, some viscera

Skin appendage

Overview

The skin appendages include:

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:

Glands

Sebaceous gland Sweat glands
Eccrine sweat gland Apocrine sweat gland
Description
  • Exocrine (holocrine) branched glands located in the dermis.
  • Controlled by circulating hormones
  • Exocrine glands with a spiral duct “acrosyringium” and secretory ducts that open into sweat pores
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

References:[6]

Functions of the skin

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

Clinical significance

Common skin disorders

Vascular tumors of the skin

Viral infections of the skin

Bacterial infections of the skin

Fungal infections of the skin

Bullous skin disorders

Malignant and premalignant skin lesions

Disorders of pigmentation

Other skin disorders

Nail disorders

last updated 11/18/2018
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