Skin and skin appendage

Summary

The skin is the largest organ of the body, covering an area of approximately 2 m2. The skin is composed of the cutis (including the dermis, epidermis, and subcutaneous tissue) and skin appendages. The epidermis, which is derived from ectoderm, is the outermost layer of the skin and is mainly composed of keratinocytes. The dermis, which is derived from mesoderm, is located underneath the epidermis and is mainly composed of elastic fibers, type I collagen, and connective tissue. It is formed by the papillary dermis and the reticular dermis. The subcutaneous tissue, which is derived from the mesoderm, is the innermost layer of the skin and is mainly composed of fat and connective tissue. Skin appendages are derived from the skin and include hair, nails, and glands. The main functions of the skin are protection (barrier against ultraviolet radiation, microorganisms, and water loss), the synthesis of vitamin D, detection of sensation (e.g., touch, temperature, pain), and the regulation of body temperature.

Structure of the skin

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

Cutis

Subcutis (also called hypodermis)

  • Derived from mesoderm
  • Consists ; mainly of fat that protects from cold and trauma
  • Contains superficial veins and free nerve endings
  • Contains Pacinian corpuscles (mechanoreceptors)
    • Responsible for the sensations of vibration and pressure
  • Collagenous and elastic fibers in this area anchor the skin (epidermis, dermis) to the deep fascia.
  • Allows for the subcutaneous administration of medication

Skin appendages

Hair, nails, glands (e.g., sweat glands, sebaceous glands)

Layers of the epidermis

  • The epidermis is the outermost layer of the skin and 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):

To remember the layers of the epidermis, think of the phrase “Come Let's Get Some Beers”: Corneum, Lucidum, Granulosum, Spinosum, and Basalis.

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

Cells of the skin

Cells of the epidermis

The epidermis is mainly composed of four different types of cells.

Keratinocytes

Melanocytes

Vitiligo is an acquired condition characterized by loss of melanocytes. Albinism is an inherited condition characterized by impaired melanin production.

Langerhans cells

Merkel cells

  • Definition: mechanoreceptors for deep static touch features (e.g., shapes, edges)
  • Location
  • Characteristics
    • Rich in keratin filaments, desmosomes, and small, dense blue granules
    • Merkel cells are connected to enlarged axon terminals via synapse-like junctions.
    • The Merkel cell-neurite complex is known as a Merkel disc receptor.

Sensory receptors of the skin

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

Merkel discs

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

MeiSsner corpuscles are located on Smooth, hairleSs skin and detect Smooth (fine) touch. MerKel discs are located on folliKles (hairy skin) and detect Krude touch.

References:[6][7]

Skin appendage

Overview

Skin appendage includes:

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 (the white, crescent-shaped, poorly vascularized portion of the nail)
    • Distally: sterile matrix (provides the nail with bulk and strength)
  • Nail fold: depression proximal to the 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 follicles

Invaginations of the epidermis into the deep dermis, forming a cavity where the hair grows and develops. Hair follicles are composed of the following:

Glands

Sebaceous glands Sweat glands
Eccrine sweat glands Apocrine sweat glands
Description
  • Exocrine (holocrine) branched glands
  • Exocrine glands with a spiral duct (acrosyringium) and secretory ducts that open into sweat pores
Location and distribution
  • Dermis
  • Predominantly located on the face and scalp
  • Absent on palms and soles
Function
  • Secretion of sebum (oily, waxy substance), which is a lubricant and a waterproof layer for the skin and hair
    • Has photoprotective, antimicrobial, and antioxidant properties
  • Secretion of sweat (thermoregulation)
  • Modified apocrine cells produce ear wax or breast milk.
  • No significant role in thermoregulation
Regulation of secretion
  • Stimulated by androgens, especially dihydrotestosterone
  • Inhibited by estrogens

References:[8]

Functions of the skin

The main functions of the skin include:

  • Barrier and protection against the external environment
  • Thermoregulation (e.g., perspiration, regulation of blood circulation)
  • Sensory function: sensations of pressure, vibration, touch, pain, and temperature
  • Hormone synthesis of vitamin D
  • Melanin synthesis: Provides protection against ultraviolet (UV) radiation and determines the color of the skin and iris.

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 09/09/2019
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