Skin Cells: Types, Structure, Functions and Disorders

Introduction to Skin Cells

Skin cells are the building blocks of our skin, which is the largest organ of the body.

The skin is outer covering of body that acts as protective barrier against external environment and performs various other vital functions.

The skin contains different layers composed of different types of cells. Together, layers and cells create complex organ that provides protection and helps in regulation of different body functions.

Anatomy and Structure of Skin

Skin contains three layers of tissue: Epidermis, Dermis, and Hypodermis.

Epidermis

Epidermis is outermost layer of skin that contains different types of cells:

• Keratinocytes
• Melanocytes
• Langerhans cells
• Merkel cells

There are four main layers of skin cells in epidermis. Starting from surface, layers are:

• Stratum corneum

• Stratum granulosum

• Stratum spinosum

• Stratum basale

• In areas with thick skin, like palms and soles, there is additional layer called stratum lucidum found beneath stratum corneum.

Dermis

Dermis is layer that lies under epidermis.

• Contains connective tissue, blood vessels, nerves, hair follicles, and various specialized cells.
• Specialized cells include fibroblast, macrophages, mast cells, adipocytes, and stem cells.
• Further divided into two layers:
  • Papillary layer
  • Reticular layer

Hypodermis or Subcutaneous Layer

Hypodermis lies under dermis and forms deepest layer of skin.

• Composed mainly of adipose cells and connective tissue.
• Provides insulation and energy storage.
• Connects skin to muscles and bones.

Types of Skin Cells

Main Types in Epidermis

Four main types of skin cells found in epidermis are keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

Keratinocytes

• Most common type of cell found in epidermis.
• Produce protein keratin, which provides strength to skin.
• Produced from basal cells in innermost layer of epidermis and move to surface in process called keratinization.
• As cells move toward surface, they eventually die and form outer protective layer of dead skin cells.

Melanocytes

• Cells found in lower layer of epidermis that produce melanin, which is pigment responsible for skin color.
• Protects skin against harmful UV radiation.
• Originate from neural crest cells.
• Transfer pigment granules to neighboring keratinocytes through structures called dendrites.

Langerhans Cells

• Immune cells found in stratum spinosum layer of epidermis.
• These cells are dendritic cells involved in immune response.
• Detect and alert immune system to potential threats.
• Originate from mesenchymal cells.
• Carry antigens from skin to lymph nodes to alert immune system.

Merkel Cells

• Oval-shaped skin cells found in stratum basale layer of epidermis.
• Function as touch receptors and involved in sensory perception.
• Allow us to sense different stimuli such as touch and pressure.
• Attached to keratinocytes by structures known as desmosomes.

Other Specialized Cells in Dermis and Hypodermis

Inner dermis and hypodermis layer also contain other specialized cells:

• Dermis contains:

  • Fibroblasts - produce fibers like collagen and elastin that provide skin its strength and elasticity.
  • Macrophages - immune cells that engulf pathogens and other foreign substances.
  • Mast cells - immune cells involved in immune and inflammatory response of body.

• Hypodermis layer mainly consists of:

  • Adipose cells or adipocytes - store energy as fat and provide insulation for body.

Functions of Skin Cells

• Keratinocytes produce protein keratin, which forms tough barrier on surface of skin.

  • Barrier protects internal organs from pathogens and injury.
  • Keratin provides strength and structure to skin.
  • Also play role in immune response and wound healing.

• Melanocytes produce melanin pigment that provides pigmentation or skin color.

  • Also protect against UV radiation.

• Langerhans cells take up foreign antigens and transport them to lymph node to induce immune response to antigens.

  • This immune response defends against foreign invaders.

• Merkel cells function as mechanoreceptors, helping sense stimuli such as touch, pain, and temperature.

  • Convert mechanical stimuli into electrical signals and transmit these signals to sensory nerve fibers.

• Macrophages found in dermis engulf pathogens, regulate inflammation, promote tissue repair, and maintain tissue homeostasis.

• Mast cells are involved in immune responses and inflammation.

  • Contain several receptors that detect pathogens and tissue damage in skin.
  • Communicate using mediators such as histamine and cytokines.

Diseases and Disorders of Skin Cells

Basal Cell Carcinoma

Common type of skin cancer that arises in basal cells of epidermis layer.

• Mainly occurs on sun-exposed skin.
• Slow-growing tumor that is less likely to metastasize.
• Usually occurs as small, shiny bump on skin.
• Primary cause is change or mutation in DNA of skin cells that occurs due to exposure to ultraviolet (UV) rays.
• Treatment methods involve:
  • Excision
  • Cryosurgery
  • Photodynamic therapy
  • Laser therapy
• Protective measures to reduce risk include:
  • Minimizing sun exposure
  • Avoiding tanning beds
  • Using sunscreen
  • Wearing protective clothing
  • Quitting smoking

Squamous Cell Carcinoma

Another most common type of skin cancer following basal cell carcinoma. It starts in squamous cells which are flat keratinocytes.

• Also mainly develops in sun-exposed parts of body.
• Main cause is mutation in p53 gene due to long-term exposure to UV radiation.
• Other factors include:
  • Weak immune system
  • Old age
  • Exposure to certain chemicals
• Primary treatment is surgery.
• Other treatment options include use of oral or topical retinoids to reduce risk of development.

Melanomas

Rare type of skin cancer that arises in melanocytes. They have high potential to metastasize.

• May occur as moles, unusual bumps, scaly patches, sores, or rashes.
• Main cause is exposure to UV rays.
• Other risk factors include:
  • History of melanoma
  • Fair skin
  • Use of tanning beds
  • Weak immune system
• Treatment includes:
  • Surgery
  • Targeted cancer therapy
  • Immunotherapy

Langerhans Cell Histiocytosis (LCH)

Rare cancer where Langerhans cells build up in body. It primarily affects children.

• Can cause tissue damage and lesion formation in various parts of body.
• Genetic mutations in genes like BRAF, along with environmental factors and infections can cause LCH.
• Risk factors include:
  • Family history
  • Smoking
  • Exposure to chemicals
  • Incomplete vaccinations
• Treatment approach includes:
  • Steroid therapy
  • Surgery
  • Targeted therapy
  • Chemotherapy
  • Immunotherapy
  • Stem cell transplants

Merkel Cell Carcinoma

Rare type of cancer that arises in Merkel cells. It is neuroendocrine tumor of skin.

• Main cause is exposure to UV radiation.
• Other risk factors include:
  • History of skin cancer
  • Weak immune system
  • Using tanning beds
• Presence of Merkel cell polyomavirus (MCP) infection is also associated with development.
• Treatment involves:
  • Excision
  • Chemotherapy
  • Adjuvant radiotherapy
  • Immune-based treatments

Psoriasis

Autoimmune condition that involves rapid skin cell growth.

• Causes proliferation of keratinocytes that results in formation of white plaques of dead skin commonly found on areas such as elbows, knees, and trunk.
• Various factors that increase risk include:
  • Infections
  • Skin injuries
  • Sun exposure
  • Certain medications
  • Stress
  • Alcohol, smoking, and obesity
• Treatment strategies may include:
  • Topical agents
  • Phototherapy
  • Systemic medications
  • Biologic therapies

Vitiligo

Skin disorder where melanocytes are absent from epidermis layer. It is autoimmune condition where melanocytes are attacked by immune system.

• Appears as symmetrical white spots on skin in areas such as hands, arms, feet, and face.
• More common in people with:
  • Family history of disorder
  • Certain autoimmune conditions such as psoriasis, rheumatoid arthritis, thyroid disease, or type 1 diabetes
• Treatment options include:
  • Topical treatments
  • Phototherapy
  • Laser therapy
  • Surgery

Albinism

Genetic condition characterized by reduction or absence of melanin in skin.

• Occurs due to mutations that disrupt function of melanocytes.
• Individuals have very pale skin and patches of white skin.
• Can also cause:
  • Crossed eyes
  • Rapid eye movements
  • Vision problems such as nearsightedness or light sensitivity

Summary

Skin cells are building blocks of largest organ skin providing protective barrier. Skin has three layers: epidermis with four main layers stratum corneum, granulosum, spinosum, basale plus stratum lucidum in thick skin containing keratinocytes most common producing keratin and keratinization, melanocytes from neural crest producing melanin and protecting from UV via dendrites, Langerhans cells dendritic immune cells in spinosum carrying antigens to lymph nodes, and Merkel cells in basale attached by desmosomes acting as mechanoreceptors; dermis with papillary and reticular layers containing fibroblasts producing collagen and elastin, macrophages, mast cells, adipocytes and stem cells; and hypodermis of adipose cells providing insulation and connecting to muscle and bone. Functions include barrier formation, pigmentation, immune defense, touch sensation, inflammation regulation via histamine and cytokines, and wound healing. Disorders include basal cell carcinoma from UV mutation, squamous cell carcinoma from p53 mutation, melanomas with high metastasis, Langerhans cell histiocytosis with BRAF mutation in children, Merkel cell carcinoma associated with MCPyV, psoriasis with rapid keratinocyte proliferation forming white plaques, vitiligo with autoimmune melanocyte loss causing white spots, and albinism from melanocyte dysfunction causing pale skin and eye problems.

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