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Skin and skin appendage

Last updated: March 4, 2021

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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 and epidermis), 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.

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

Cutis

Epidermis

  • Derived from ectoderm
  • Primarily composed of keratinocytes, which constantly regenerate approximately every 30 days
  • The outermost and nonvascularized layer of the skin that maintains the skin's barrier function
  • There are 5 layers of the epidermis, 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
      • Located 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
    • Stratum basale (also called the basal cell layer of the epidermis)

Come on, Let's Get Some Beers”: Corneum, Lucidum, Granulosum, Spinosum, and Basalis are the 5 layers of the epidermis.

Dermal-epidermal junction

Dermis

Derived from mesoderm; contains blood vessels and provides structural integrity to the skin

Subcutis (also called hypodermis)

Skin appendages

References:[1][2][3]

Cells of the epidermis [1]

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

Sensory receptors of the skin [1]

Sensory receptors of the skin
Receptor Types of fiber Function
Ruffini corpuscles
  • Encapsulated dendritic endings
  • Large myelinated fibers that are coiled together with bundles of collagen fiber
  • Adapt slowly to stimuli
  • Location: fingertips, joints
  • Changes in joint angles
  • Pressure
  • Slippage detection
  • Stretch
Meissner corpuscles
  • Large myelinated fibers
  • Adapt quickly to stimuli
  • Location: glabrous skin (an area of skin that does not contain hair follicles, e.g., over the palms and soles)
  • Dynamic, fine, and light touch (e.g., stretching, tugging, folding, twisting, striking, squeezing)
  • Vibration (low-frequency)
  • Position sense
Pacinian corpuscles
  • Large myelinated fibers
  • Adapt quickly to stimuli
  • Location: ligaments, joints, deep layers of the skin
  • Vibration (high-frequency)
  • Deep touch and 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

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.

A Delta plane is fast, but a Cab is slow”: A-delta fibers have fast conduction velocity and C fibers have slow conduction velocity.

Overview [1]

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 matrix unguis or onychostroma (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
  • 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
    • An oily, waxy substance which is a lubricant and a waterproof layer for the skin and hair
    • Has photoprotective, antimicrobial, and antioxidant properties
Regulation of secretion

The main functions of the skin include: [1]

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

Hair disorders

  1. Marks JG Jr, Miller JJ . Lookingbill and Marks' Principles of Dermatology. Saunders Elsevier ; 2013
  2. Layers of the Skin. https://training.seer.cancer.gov/melanoma/anatomy/layers.html. Updated: May 15, 2017. Accessed: May 15, 2017.
  3. Zhang S-X. An Atlas of Histology. Springer Science & Business Media ; 2013