Layers of the Skin — 10 Mark Answer

Labeled Diagram

Introduction

1. Epidermis (outermost)
2. Dermis (middle)
3. Hypodermis / Subcutaneous tissue (deepest)

1. EPIDERMIS

(i) Stratum Basale (Germinativum)

• Single layer of cuboidal to low columnar cells resting on the basement membrane
• Contains mitotically active stem cells (keratinocytes) that continuously renew the epidermis — hence called "stratum germinativum"
• Also contains melanocytes (secrete melanin for UV protection) and Merkel cells (mechanoreception/touch)
• Cells are connected to the basement membrane by hemidesmosomes and to each other by desmosomes
• Function: Cell renewal and proliferation; melanin synthesis for UV protection

(ii) Stratum Spinosum (Prickle Cell Layer)

• Several layers of polyhedral cells with prominent spiny/prickle-like processes (actually desmosomal connections — an artifact of preparation)
• Rich in tonofilaments (keratin intermediate filaments)
• Contains Langerhans cells — dendritic antigen-presenting cells involved in immune surveillance
• Function: Mechanical strength; immune surveillance

(iii) Stratum Granulosum (Granular Layer)

• 3–5 layers of flattened cells packed with keratohyalin granules (precursors to keratin)
• Also contains lamellar bodies (Odland bodies) that secrete lipids forming the water-impermeable barrier
• Nuclei and organelles begin to degenerate here (onset of keratinization)
• Function: Initiates keratinization; forms the permeability/water barrier

(iv) Stratum Lucidum

• A thin, translucent/clear band found only in thick skin (palms and soles)
• Composed of dead, densely packed cells filled with eleidin (a precursor to keratin)
• Considered a subdivision of the stratum corneum
• Function: Additional protection in areas of high friction

(v) Stratum Corneum (Horny Layer)

• The outermost layer, composed of 15–20+ layers of dead, anucleate, flattened corneocytes filled with hard keratin
• Cells are continuously shed (desquamation) and replaced from below
• The extracellular space is filled with lipids forming a "brick-and-mortar" waterproof barrier
• Function: Primary barrier — protects against physical, chemical, and microbial agents; prevents transepidermal water loss

Key cells of the epidermis: Keratinocytes (80%), Melanocytes (UV protection), Langerhans cells (immune), Merkel cells (touch sensation)

2. DERMIS

(i) Papillary Layer

• Thin, superficial zone of loose connective tissue (types I and III collagen)
• Contains fibroblasts, mast cells, dendritic cells, and leukocytes
• Houses Meissner's corpuscles (fine touch receptors) in dermal papillae
• Anchoring fibrils of type VII collagen bind it to the basement membrane
• Function: Structural support to epidermis; houses superficial sensory receptors; nutritive capillary loops

(ii) Reticular Layer

• Thicker, deeper zone of dense irregular connective tissue (mainly type I collagen bundles)
• Contains a network of elastic fibers (provide skin elasticity and resilience)
• Houses Pacinian corpuscles (deep pressure receptors), hair follicles, sweat and sebaceous glands
• Rich in proteoglycans (dermatan sulfate)
• Contains two vascular plexuses:
  • Subpapillary plexus (between papillary and reticular layers)
  • Deep plexus (at dermis-hypodermis junction)
• Arteriovenous anastomoses regulate thermoregulation
• Function: Mechanical strength and elasticity; thermoregulation; houses appendages and sensory receptors

3. HYPODERMIS (Subcutaneous Tissue)

• Located below the dermis; corresponds to the superficial fascia of gross anatomy
• Composed of loose connective tissue with lobules of adipocytes (fat cells) separated by fibrous septa
• NOT part of the skin strictly, but closely associated
• Contains larger blood vessels, lymphatics, and nerves
• Functions:
  • Insulation — thermal regulation
  • Energy storage — fat depot
  • Mechanical cushion — protection against trauma
  • Attachment — binds skin loosely to underlying muscles and bones

Summary Table

Overall Functions of Skin

1. Barrier — protects against physical, chemical, and microbial agents
2. UV protection — melanin in epidermis absorbs/blocks UV radiation
3. Immunologic — Langerhans cells process and present antigens
4. Thermoregulation — dermal vasculature and sweat glands regulate heat
5. Sensory — various mechanoreceptors (Meissner's, Pacinian, Merkel cells, free nerve endings)
6. Metabolic — synthesis of vitamin D₃ via UV action on precursors
7. Excretion — sweat glands excrete water, salts, urea
8. Absorption — lipid-soluble substances (e.g., steroid hormones, nicotine patches) can be absorbed transdermally