Lab: Skin Histology

Lab: Skin Histology

Overview of Skin Structure

Skin is the largest organ of the body. It consists of two main layers: the epidermis (epithelial tissue) and the dermis (connective tissue), overlying the hypodermis (subcutaneous layer).

Epidermis

The epidermis is a self-renewing stratified squamous epithelium. From deep to superficial, its layers are: (1) Stratum basale: single layer of columnar keratinocytes and stem cells resting on the basement membrane (basal lamina + anchoring fibrils); contains melanocytes (produce melanin, which accumulates over nuclei in an "umbrella" protecting DNA from UV) and Merkel cells (sensory, light touch, hemidesmosome connection to basal lamina). (2) Stratum spinosum: 8–10 layers of polygonal keratinocytes linked by desmosomes and containing cytokeratin (tonofibrils); Langerhans cells (dendritic immune cells) are present. (3) Stratum granulosum: 3–5 layers; keratohyalin granules (protein for keratin cross-linking); glycophospholipids secreted between cells to form the water barrier (prevent transepidermal water loss). (4) Stratum lucidum: only in thick skin (palms and soles); clear, anucleate transition layer. (5) Stratum corneum: 20–30 layers of dead, anucleate, keratin-filled corneocytes; cells desquamate from surface. Desmosomal connections weaken progressively allowing shedding.

Cells: keratinocytes (predominant; undergo continuous renewal from basal stem cells, ~30-day turnover); melanocytes and Merkel cells lack desmosomes to adjacent keratinocytes but have hemidesmosome connections to basal lamina; Langerhans cells lack desmosome connections.

Dermis

Papillary layer (superficial): loose irregular connective tissue; contains Meissner's corpuscles (touch, rapidly adapting) in dermal papillae; rich vasculature for epidermal nutrition (epidermis is avascular — nutrients reach it by diffusion). Reticular layer (deep): dense irregular connective tissue; type I collagen (tensile strength) + elastin (recoil); contains Pacinian corpuscles (pressure/vibration), hair follicles, sebaceous glands, and sweat glands. Fibroblasts produce ECM; the dermis also contains macrophages, plasma cells, mast cells, blood vessels, lymphatics, and nerves.

Hypodermis

Loose connective tissue, primarily adipose; thermal insulation, energy storage, shock absorption, skin mobility. Blood vessels, nerves, and lymphatics pass through it.

Epidermal Appendages

Hair follicles (sensory information, thermoregulation), sebaceous glands (produce sebum; open into hair follicle; can cause acne), eccrine sweat glands (thermoregulation, whole body), apocrine sweat glands (axilla and perineum), arrector pili muscle (smooth muscle, connects follicle to dermis; contraction causes piloerection/goosebumps; located near sebaceous glands — use this to identify them in histology slides).

Pressure Ulcers

Localised soft tissue injury caused by prolonged pressure. Sustained pressure compresses blood supply in deeper dermis → hypoxic changes → skin breakdown. Occurs over bony prominences; staged by depth (Stage I–IV).