Structure and Function of the Skin

The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue. The skin is composed of surface epithelium Surface epithelium Epithelium is classified according to the cells (squamous, cuboidal, columnar), the number of layers, and other unique characteristics either due to function (transitional epithelium allowing distention) or appearance (pseudostratified epithelium giving a false impression of multiple layers). Surface epithelium has multiple functions, which include protection, secretion, filtration, and sensory reception. Surface Epithelium, exocrine components, connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue, muscles, and nerves. The primary role of the skin is to serve as a protective barrier between the internal body and the external environment; it also protects the body from excessive fluid loss.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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Structure of the Skin

Cells of the skin

  • Keratinocytes:
    • Primary cell type of the epidermis
    • Undergo continuous mitosis (stimulated by epidermal growth factor)
    • Produce keratin
  • Melanocytes:
    • Spider-shaped epithelial cells
    • Produce melanin (UV protection)
    • Melanin granules accumulate on the apical surface of keratinocytes.
  • Dendritic cells (Langerhans cells):
    • Resident macrophages of the skin
    • Antigen-presenting cells
    • Activate the immune system
  • Mechanoreceptors:
    • Merkel cells:
      • Slowly adapting light-touch receptors
      • Present at the dermal–epidermal junction
      • In contact with a sensory ending (Merkel or tactile disk)
    • Ruffini corpuscles:
      • Slowly adapting stretch receptors
      • Located in deep dermis
    • Meissner corpuscles:
      • Rapidly adapting light-touch receptors
      • Located in superficial dermis
    • Pacinian corpuscles:
      • Detect deep pressure and vibration
      • Located in deep dermis

Epidermis

  • Derived from ectoderm
  • Keratinized stratified squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium
  • 4 or 5 main layers:
    • Stratum basale (stratum germinativum):
      • Deepest layer
      • Germinating layer
      • Single row of epithelial cells bound to the dermis by hemidesmosomes (macula adherens)
      • Divide constantly while moving toward the surface
      • 10%–15% of the cells are melanocytes.
    • Stratum spinosum (prickly layer):
      • Consists of several layers (thickest layer of epidermis)
      • Web-like system of filaments
      • Cells are linked together by desmosomes.
      • As cells move up through the spinous layer, they accumulate keratohyalin granules.
      • Other elements present are dendritic cells and melanin granules.
    • Stratum granulosum:
      • Thin layer of 4–6 cells
      • Location of keratinization
      • Cells flatten and nuclei and organelles Organelles A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. The Cell: Organelles begin to disintegrate.
      • Cells acquire 2 types of granules, keratohyalin granules and lamellar granules (protect against water loss).
    • Stratum lucidum (clear layer):
      • Seen ony in thick skin
      • Thin translucent band
      • Made up of 2–3 layers of keratinocytes 
    • Stratum corneum (horny layer):
      • 20–30 layers of flattened anucleate cells
      • Thinner on lens of the eye, thicker on calluses 
  • 4 types of cells: 
    • Keratinocytes:
      • Deepest, produce keratin (tough fibrous protein)
      • Cycle through the epidermis every 30 days
    • Melanocytes:
      • Produce melanin
      • Live within the stratum basale
    • Merkel cells:
      • Linked with sensory nerve endings
      • Only mechanoreceptor with connection to epidermis
    • Langerhans cells:
      • Macrophage-like dendritic cells
      • Activate immune system
Layers of the epidermis

There are 5 layers of epidermis:
Beginning from the cells located in the stratum basale, they differentiate and proliferate toward the skin surface and integrate themselves into the other layers, the last layer being the stratum corneum, which is composed of dead keratinized cells.

Image: “The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.” by OpenStax College. License: CC BY 4.0

Dermis

  • Derived from mesoderm
  • Components:
    • Strong flexible connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue (primarily collagen)
    • Contains fibroblasts, macrophages, occasional mast cells, and WBCs
    • Elastic fibers provide flexibility to the dermis
    • Sebaceous glands maintain elasticity and strength.
  • Supplies the epidermis with nutrients + moisture
  • Contains rich supply of blood vessels, lymphatic vessels, and nerve endings 
  • 3 layers:
    • Papillary layer:
      • Thin superficial layer
      • Made up of elastin and collagen fibers
      • Contains phagocytes, fibroblasts, and adipocytes
      • Projections of this layer into the epidermis are called dermal papillae.
      • Dermal papillae form epidermal ridges at the surface of the epidermis (fingerprints).
      • Capillary loops and nerve endings (free nerve endings and Meissner corpuscles) are located in the dermal papillae.
    • Reticular layer:
      • Accounts for 80% of the thickness of the dermis
      • Irregular dense fibrous connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue
      • Well-vascularized cutaneous plexus
      • Gives the skin its elasticity and helps to toughen the skin
      • Contains sweat glands and hair follicles
Layers of the dermis

The dermis is made up of 2 layers:
The papillary layer is made up of collagen and elastin fibers and contains phagocytes, fibroblasts, and adipocytes. The reticular layer gives the skin its elasticity and helps to toughen the skin. Sweat glands and hair follicles are found at this layer. This layer is also well vascularized.

Image: “This stained slide shows the two components of the dermis—the papillary layer and the reticular layer.” by OpenStax College. License: CC BY 4.0

Function of the Skin

Each layer of the skin has a unique function.

Epidermis and dermis:

  • Mechanical protection
  • Protection against external pathogens
  • Maintenance of cutaneous water loss
  • Temperature regulation:
    • ↑ Temperature → Blood vessels in papillary layer of the dermis dilate.
    • Body also cools by releasing sweat.
  • Protection against UV radiation is provided by melanocytes.
  • Sebaceous glands:
    • Associate with hair follicles
    • Secrete sebum to moisturize skin

Hypodermis:

  • Located deep to the dermis
  • Contains adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or "brite" adipose tissue, which is a transitional form. Adipose Tissue, which serves as an energy reserve

Skin Appendages

Hair

  • Growth begins in the hair papilla of the dermis.
  • Composed of keratinized cells that emerge and migrate upward inside the hair follicle
  • Each hair follicle has an associated sebaceous gland.
  • Arrector pili muscle causes piloerection of hair shaft (goosebumps).
  • Terminal hairs: coarse hair on scalp, eyebrows, etc.
  • Vellus hair: very fine hair covering a newborn Newborn A neonate, or newborn, is defined as a child less than 28 days old. A thorough physical examination should be performed within the first 24 hours of life to identify abnormalities and improve outcomes by offering timely treatment. Physical Examination of the Newborn or the cheeks of an adult (peach fuzz)
  • Puberty Puberty Puberty is a complex series of physical, psychosocial, and cognitive transitions usually experienced by adolescents (11-19 years of age). Puberty is marked by a growth in stature and the development of secondary sexual characteristics, achievement of fertility, and changes in most body systems. Puberty:
    • Growth of terminal hairs
    • Accumulates in certain areas (pubic/facial hair)
  • Glabrous skin: Areas of the skin not covered by hair (4%)
  • Growth cycle:
    • Phase 1:
      • Growth/anagen phase
      • Lasts for 2–10 years
    • Phase 2:
      • Transitional/catagen phase
      • Lasts for 2 weeks
    • Phase 3:
      • Resting/telogen phase
      • Lasts for 3–8 months

Nails

  • Made up of dense keratinized cells of the epidermis
  • Protect fingertips and toes against injuries
  • Lunula: whitish crescent-shaped area at proximal end of the nail
  • Cuticle: 
    • Between lunula and skin
    • Keeps germs from entering below the skin
  • Nail root: 
    • Behind the cuticle
    • Forms keratinized cells that push the nail forward
  • Structure of nail:
    • Nail plate: actual fingernail, composed of keratin
    • Nail bed: 
      • Below nail plate
      • Continuous with strata basale and spinosum
    • Nail matrix (nail root): 
      • Proximal end of nail, deep to the skin
      • Responsible for new nail formation
    • Eponychium: cuticle
    • Hyponychium: distal extension of nail
Structure of the nail

Structure of the nail:
The whitish crescent-shaped end of the nail (toward the body) is referred to as the lunula (“small moon”). Between the lunula and skin, there is a protective layer, called the cuticle, that keeps pathogens from entering beneath the skin. Behind the cuticle (toward the body), is the nail root, which forms keratinized cells that push the nail forward.
Fingernails grow faster than toenails. The typical growth rate of a fingernail is 1 mm per week; a toenail grows 0.5 mm per week.

Image: “Nails” by OpenStax College. License: CC BY 4.0

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Clinical Relevance

  • Erysipelas Erysipelas Erysipelas is a bacterial infection of the superficial layer of the skin extending to the skin's superficial lymphatic vessels. This infection presents as a raised, well-defined, tender, and bright red rash. Typically on the legs or face, but erysipelas can occur anywhere on the skin. Erysipelas: superficial skin infection that affects the dermis in addition to the superficial cutaneous lymphatics. The condition is caused by bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology: Overview, most commonly Streptococcus Streptococcus Streptococcus is one of the two medically important genera of gram-positive cocci, the other being Staphylococcus. Streptococci are identified as different species on blood agar on the basis of their hemolytic pattern and sensitivity to optochin and bacitracin. There are many pathogenic species of streptococci, including S. pyogenes, S. agalactiae, S. pneumoniae, and the viridans streptococci. Streptococcus pyogenes. The incidence of erysipelas has been decreasing since the introduction of antibiotics. Possible risk factors include poor hygiene, poor sanitation, and a history of lymphedema. 
  • Molluscum contagiosum Molluscum contagiosum Molluscum contagiosum is a viral infection limited to the epidermis and is common in children below 5 years of age. Lesions appear as grouped, flesh-colored, dome-shaped papules with central umbilication. Molluscum Contagiosum: viral infection of the skin that only affects humans. The infection is generally mild and should not be a reason for concern. Molluscum contagiosum Molluscum contagiosum Molluscum contagiosum is a viral infection limited to the epidermis and is common in children below 5 years of age. Lesions appear as grouped, flesh-colored, dome-shaped papules with central umbilication. Molluscum Contagiosum causes multiple raised dome-shaped skin-colored papules with central umbilication. The condition is self-limited and thus does not require treatment. The virus Virus Viruses are infectious, obligate intracellular parasites composed of a nucleic acid core surrounded by a protein capsid. Viruses can be either naked (non-enveloped) or enveloped. The classification of viruses is complex and based on many factors, including type and structure of the nucleoid and capsid, the presence of an envelope, the replication cycle, and the host range. Virology: Overview belongs to the Poxviridae family.
  • Basal cell carcinoma Basal cell carcinoma Basal cell carcinoma is the most common skin malignancy. This cancer arises from the basal layer of the epidermis. The lesions most commonly appear on the face as pearly nodules, often with telangiectatic blood vessels and ulceration in elderly individuals. Basal Cell Carcinoma: the most common malignant neoplasm of the skin. Basal cell carcinoma Basal cell carcinoma Basal cell carcinoma is the most common skin malignancy. This cancer arises from the basal layer of the epidermis. The lesions most commonly appear on the face as pearly nodules, often with telangiectatic blood vessels and ulceration in elderly individuals. Basal Cell Carcinoma is a nonmelanoma skin cancer (NMSC) that forms de novo from cells within the stratum basale. Basal cell carcinoma Basal cell carcinoma Basal cell carcinoma is the most common skin malignancy. This cancer arises from the basal layer of the epidermis. The lesions most commonly appear on the face as pearly nodules, often with telangiectatic blood vessels and ulceration in elderly individuals. Basal Cell Carcinoma solely affects the skin with hair and usually appears in areas exposed to UV radiation from the sun. The most common locations are the face, head, and neck. 
  • Melanoma Melanoma Melanoma is a malignant tumor arising from melanocytes, the melanin-producing cells of the epidermis. These tumors are most common in fair-skinned individuals with a history of excessive sun exposure and sunburns. Melanoma: Malignant melanoma involves the malignant transformation of melanocytes. Melanoma Melanoma Melanoma is a malignant tumor arising from melanocytes, the melanin-producing cells of the epidermis. These tumors are most common in fair-skinned individuals with a history of excessive sun exposure and sunburns. Melanoma accounts for more than 90% of the deaths caused by skin cancer. Chronic UV light exposure, damage caused by sunlight (including sunburns, especially during childhood and adolescence), fair skin type, and the cumulative appearance of melanocytic nevi Nevi Nevi (singular nevus), also known as "moles," are benign neoplasms of the skin. Nevus is a non-specific medical term because it encompasses both congenital and acquired lesions, hyper- and hypopigmented lesions, and raised or flat lesions. Nevus/Nevi (> 100) or the presence of dysplastic nevi Nevi Nevi (singular nevus), also known as "moles," are benign neoplasms of the skin. Nevus is a non-specific medical term because it encompasses both congenital and acquired lesions, hyper- and hypopigmented lesions, and raised or flat lesions. Nevus/Nevi represent significant risk factors. The most important risk factor is exposure to sunlight, particularly UVB radiation.
  • Squamous cell carcinoma Squamous cell carcinoma Cutaneous squamous cell carcinoma (cSCC) is caused by malignant proliferation of atypical keratinocytes. This condition is the 2nd most common skin malignancy and usually affects sun-exposed areas of fair-skinned patients. The cancer presents as a firm, erythematous, keratotic plaque or papule. Squamous Cell Carcinoma: nonmelanoma skin cancer arising from the suprabasal epidermal keratinocytes. Squamous cell carcinoma Squamous cell carcinoma Cutaneous squamous cell carcinoma (cSCC) is caused by malignant proliferation of atypical keratinocytes. This condition is the 2nd most common skin malignancy and usually affects sun-exposed areas of fair-skinned patients. The cancer presents as a firm, erythematous, keratotic plaque or papule. Squamous Cell Carcinoma is the 2nd most common nonmelanoma skin cancer, after basal cell carcinoma. This carcinoma is seen most frequently in sun-exposed areas in people > 40 years of age. White males are more likely to be affected by this cancer; however, SCC is the most common skin cancer in patients with darker skin types.

References

  1. Kierszenbaum, A. L., Tres, L. L. (2019). Histology and Cell Biology: An Introduction to Pathology, 5th ed. Elsevier.
  2. Navarette-Dechent, C., et al. (2020). Human surface anatomy terminology for dermatology: a Delphi consensus from the International Skin Imaging Collaboration. Journal of the European Academy of Dermatology and Venereology 34:2659–2663. https://doi.org/10.1111/jdv.16855
  3. Yousef, H., Alhajj, M., Sharma, S. (2020). Anatomy, skin (integument), Epidermis. StatPearls. Retrived October 15, 2021, from https://europepmc.org/article/nbk/nbk470464

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