Study Notes: Structure and Function of Human Skin (Histology)

The Skin: Structure and Function

Overview of Skin Anatomy

The skin is the largest organ of the human body, serving as a protective barrier and performing multiple physiological functions. It consists of three main layers: the epidermis, dermis, and hypodermis (subcutis).

• Epidermis: Outermost layer, composed of stratified squamous epithelium.

• Dermis: Middle layer, made of connective tissue, provides strength and elasticity.

• Hypodermis (Subcutis): Deepest layer, mainly adipose tissue, functions in insulation and energy storage.

Functions of the Skin

The skin performs several essential functions for maintaining homeostasis and protecting the body.

• Protection: Acts as a barrier against microorganisms, chemicals, UV radiation, mechanical stress, and water loss.

• Thermoregulation: Regulates body temperature through sweat production and blood flow.

• Sensory Organ: Contains receptors for touch, pain, temperature, and pressure.

• Production of Vitamin D: Synthesizes vitamin D when exposed to UV light.

Types of Skin

Thick Skin vs. Thin Skin

Human skin is classified into two types based on location and structure: thick skin and thin skin.

• Thick Skin: Found on the palms of the hands and soles of the feet. Characterized by a thick epidermis and absence of hair follicles.

• Thin Skin: Covers the rest of the body. Contains hair follicles and has a thinner epidermis.

Layers of the Epidermis

Stratified Squamous Epithelium

The epidermis is composed of 4 or 5 distinct layers, depending on the skin type.

• Stratum basale: Deepest layer; contains stem cells and melanocytes. Responsible for cell proliferation. Anchored to the basement membrane by hemidesmosomes.

• Stratum spinosum: Several layers of polygonal keratinocytes connected by desmosomes. Known as the prickle cell layer.

• Stratum granulosum: 1-3 layers of squamous keratinocytes containing keratohyalin granules and lamellar bodies (lipids).

• Stratum lucidum: Present only in thick skin; a thin, clear layer of dead cells.

• Stratum corneum: Outermost layer; many layers of dead, keratinized cells. Main barrier against water loss and environmental damage.

Keratinization: The process by which keratinocytes mature, fill with keratin, lose their nuclei and organelles, and form the protective stratum corneum.

Thickness Variation

The thickness of the stratum corneum varies by body region:

• Sole of foot: Very thick stratum corneum.

• Abdomen: Medium to thin stratum corneum.

Regeneration of the Epidermis

• Keratinocyte Lifecycle: Keratinocytes remain in the epidermis for approximately 4 weeks.

• Proliferation: Occurs in the stratum basale.

• Differentiation: Takes about 2 weeks to become cornified cells.

• Stratum corneum: Keratinocytes spend about 2 weeks before being shed.

Cell Types of the Epidermis

Keratinocytes

Main cell type; produce keratin and form the protective barrier.

Melanocytes

• Located in the stratum basale.

• Synthesize melanin in granules (melanosomes).

• Transfer melanin to keratinocytes, protecting their DNA from UV radiation.

• Number of melanocytes is constant across ethnicities, but melanin production varies.

Impairments: Genetic defects can cause birthmarks (nevus) or melanoma (malignant tumor derived from melanocytes).

Langerhans Cells

• Mainly in the stratum spinosum.

• Dendritic, antigen-presenting cells; no desmosomes.

• Phagocytose antigens and migrate to lymph nodes to activate T lymphocytes.

Merkel Cells

• Located in the basal layer.

• Function as mechanoreceptors for pressure.

The Dermis

Layers of the Dermis

The dermis is a connective tissue layer beneath the epidermis, providing strength and elasticity.

• Stratum papillare: Indented with the epidermis; loose connective tissue with thin collagen fibers, capillaries, and Meissner's corpuscles (touch receptors).

• Stratum reticulare: Dense connective tissue with thick collagen (type I) and elastic fibers; responsible for mechanical resistance.

The Hypodermis (Subcutis)

The hypodermis is the deepest layer, composed mainly of adipose tissue.

• Cushioning: Protects underlying structures (e.g., sole of foot, palm of hand).

• Energy Storage: Stores fat for metabolic needs.

• Thermal Insulation: Helps maintain body temperature.

Glands of the Skin

Types of Skin Glands

• Eccrine Sweat Glands: Distributed over most of the body; regulate temperature by secreting watery sweat.

• Apocrine Sweat Glands: Found in axilla, areola, anus, and external genitalia; secrete a thicker fluid, may function in pheromone release.

• Sebaceous Glands: Associated with hair follicles; produce sebum (oily secretion) for skin lubrication and barrier function.

Modes of Secretion

• Merocrine: Product released by exocytosis (eccrine sweat glands).

• Apocrine: Product released with a portion of the cell membrane (apocrine sweat glands).

• Holocrine: Entire cell disintegrates to release product (sebaceous glands).

Structure of Glands

• Eccrine Sweat Glands: Coiled tubular glands with simple epithelium; ducts lined by stratified cuboidal epithelium (2 layers).

• Apocrine Sweat Glands: Coiled tubular glands with wider lumen; ducts also lined by stratified cuboidal epithelium.

• Sebaceous Glands: Typically connected to hair follicles; holocrine secretion of sebum, regulated by androgens.

Hair and Sensory Receptors

Hair Structure

• Hair is an appendage of the epidermis, growing from hair follicles located in the dermis.

Skin as a Sense Organ

The skin contains various sensory receptors for detecting environmental stimuli.

• Free Nerve Endings: Intraepidermal; detect temperature, pain, and touch.

• Merkel Cells: Basal layer; pressure receptors.

• Meissner's Corpuscles: Stratum papillare; touch and movement receptors, especially in fingers.

• Pacinian Corpuscles: Large, located in deeper dermis/hypodermis; detect vibration and pressure.

• Ruffini's Corpuscles: Dermis; detect stretch and tension.

Summary Table: Layers and Cell Types of the Epidermis

Layer	Main Cell Type	Key Features
Stratum basale	Keratinocytes, Melanocytes, Merkel cells	Stem cells, cell division, melanin synthesis
Stratum spinosum	Keratinocytes, Langerhans cells	Desmosomes, antigen presentation
Stratum granulosum	Keratinocytes	Keratohyalin granules, lamellar bodies
Stratum lucidum	Keratinocytes (dead)	Present only in thick skin
Stratum corneum	Keratinocytes (dead)	Keratinized, main barrier

Additional info:

• Melanin protects against DNA damage from UV radiation, reducing the risk of skin cancer.

• Desmosomes are specialized cell junctions providing mechanical strength to the epidermis.

• Lamellar bodies release lipids that contribute to the water barrier function of the skin.

• Regeneration and shedding of keratinocytes is a continuous process, essential for skin health.