Skin and Nerve Tissue Histology

Lecture 2 covers the histological organisation of the integumentary system (skin) and nervous tissue.

SKIN HISTOLOGY

The skin is the largest organ (~1.5-2 m2 in adults), consisting of two main layers: the epidermis (epithelial) and dermis (connective tissue). Beneath the dermis lies the hypodermis (subcutaneous tissue — adipose + loose CT), which is not technically part of the skin.

EPIDERMIS

The epidermis is a stratified squamous keratinising epithelium. Its cells (keratinocytes, ~90% of epidermal cells) undergo a continuous process of terminal differentiation from the base (proliferating layer) to the surface (dead, flattened cells). The layers from deep to superficial are: (1) Stratum basale (germinativum): single layer of cuboidal/columnar cells on the basement membrane; contains keratinocyte stem cells; mitotically active; also contains melanocytes (dendritic, neural crest-derived; produce melanin via tyrosinase; transferred to keratinocytes via melanosomes) and Merkel cells (mechanoreceptors — slow-adapting touch). (2) Stratum spinosum: multiple layers of polyhedral cells connected by desmosomes (intercellular bridges visible on LM — hence "spiny"); contains keratinocyte lamellar granules (Odland bodies). (3) Stratum granulosum: flattened cells with basophilic keratohyalin granules (containing filaggrin — key to cornification) and lamellar granules that exocytose lipids (forming the water barrier). (4) Stratum lucidum: only in thick skin (palms, soles); homogeneous eosinophilic band. (5) Stratum corneum: most superficial; dead, anucleate, fully keratinised squames; barrier function. Transit from basal layer to shedding ~30 days.

Non-keratinocyte cells: Melanocytes (S1 to S4 in basal layer): produce eumelanin (brown/black) and pheomelanin (red/yellow); melanin number equal in all skin types — only melanin production differs (accounts for variation in skin colour). Langerhans cells: derived from bone marrow, in stratum spinosum; dendritic antigen-presenting cells (APCs) of the skin; key in contact hypersensitivity and immune surveillance.

Thick vs thin skin: Thick skin (palms, soles): has stratum lucidum; no hair follicles or sebaceous glands; abundant sweat glands; Meissner's corpuscles (lamellated, fast-adapting mechanoreceptors in dermal papillae) and Pacinian corpuscles (deep dermis/hypodermis — vibration and pressure, rapidly adapting). Thin skin (most of body surface): lacks stratum lucidum; has hair follicles, sebaceous glands; Meissner's and Ruffini endings present.

DERMIS

Papillary dermis: superficial loose CT directly beneath epidermis; dermal papillae project into epidermis (forming epidermal ridges/fingerprints); contains capillaries and nerve endings including free nerve endings (pain, temperature) and Meissner's corpuscles. Reticular dermis: deeper, dense irregular CT; collagen fibres (type I and III) in interlacing bundles providing tensile strength; elastin fibres for recoil; contains hair follicles, sweat glands, sebaceous glands, and Pacinian corpuscles.

NERVE TISSUE HISTOLOGY

Neurons: highly specialised, electrically excitable cells. Structure: cell body (soma/perikaryon) — contains nucleus (large, euchromatic, prominent nucleolus — reflecting high transcriptional activity), Nissl substance (rough ER + free ribosomes — for protein synthesis), Golgi apparatus; axon hillock (where action potentials are initiated); axon (single, conducts impulses away from soma); dendrites (multiple, receive input). Satellite cells surround neuronal cell bodies in ganglia.

Myelination: Schwann cells myelinate peripheral nervous system axons (one Schwann cell: one internodal segment of one axon). Oligodendrocytes myelinate CNS axons (one oligodendrocyte: up to 50 axon segments). The myelin sheath is a multilayered wrapping of Schwann cell membrane; it increases conduction velocity by saltatory conduction — action potentials jump between nodes of Ranvier (gaps where the myelin sheath is absent, with high Na+ channel density). Unmyelinated axons are simply embedded in Schwann cell cytoplasm without wrapping.

Peripheral nerve organisation: individual axons + Schwann cells are surrounded by endoneurium; fascicles (bundles of axons) are bound by perineurium; the entire nerve trunk is covered by epineurium. The blood-nerve barrier is formed by tight junctions in the perineurium and endoneurial capillary endothelium.

Wallerian degeneration: after axon injury, the distal segment undergoes anterograde degeneration — axon fragments, myelin breaks down, Schwann cells proliferate and phagocytose debris (myelinophages), and form tubes (Bands of Bungner) to guide axon regeneration. Regeneration can occur in PNS (Schwann cells provide guidance and trophic support — NGF, BDNF); CNS regeneration is limited (inhibitory myelin-associated proteins — Nogo-A; scar formation by astrocytes/microglia).