Skin histology

Epidermal Layers - Come Get Sun Burned

📌 Mnemonic: Come, Get Sun Burned (Superficial → Deep)

• Stratum Corneum: Topmost layer; anucleated keratin-filled cells. Basket-weave pattern.
• Stratum Granulosum: Contains keratohyaline granules; forms lipid barrier.
• Stratum Spinosum: Desmosomes connect keratinocytes (spiny look). Langerhans cells found here.
• Stratum Basale: Deepest layer; regenerative (stem) cells. Site of melanocytes and Merkel cells.

⭐ Thick skin (palms, soles) has an extra layer, the Stratum Lucidum, between the Stratum Corneum and Granulosum.

Dermis & Hypodermis - The Supportive Framework

• Dermis: Connective tissue layer providing structural and nutritional support.
  • Papillary Layer: Superficial, loose connective tissue with Type I & III collagen, capillary loops, and Meissner's corpuscles (light touch).
  • Reticular Layer: Deeper, dense irregular connective tissue with Type I collagen, elastin, hair follicles, glands, and sensory receptors (Pacinian, Ruffini).

• Hypodermis (Subcutis): Adipose tissue layer for insulation and shock absorption.

⭐ Langer's Lines: Tension lines in the reticular dermis corresponding to the natural orientation of collagen fibers. Surgical incisions made parallel to these lines heal with less scarring.

Specialized Cells - The Epidermal Cast

• Keratinocytes: Main epidermal cell; produces keratin for protection. Journey from basal layer to surface takes ~4 weeks.
• Melanocytes: From neural crest; in stratum basale. Synthesize melanin pigment and transfer it to keratinocytes. Ratio is 1 melanocyte to 10 basal keratinocytes.
• Langerhans Cells: Dendritic, antigen-presenting cells from bone marrow. Found in stratum spinosum. Contain racquet-shaped Birbeck granules.
• Merkel Cells: From neural crest; in stratum basale. Mechanoreceptors for light touch, associated with nerve endings.

⭐ Albinism is due to defective tyrosinase, impairing melanin production, not a ↓ in the number of melanocytes.

Skin Appendages - Glands and Hair

• Glands
  • Eccrine: Simple coiled tubular glands; produce watery sweat for thermoregulation via merocrine secretion. Distributed widely.
  • Apocrine: Large-lumened coiled glands in axilla/genitalia; merocrine secretion of viscous fluid into hair follicles.
  • Sebaceous: Branched acinar glands; holocrine secretion of oily sebum into hair follicles.
• Hair Follicle:
  • Invagination of epidermis; contains hair bulb (matrix), root, and shaft.
  • Associated with sebaceous glands and arrector pili smooth muscle.

⭐ Apocrine sweat glands are misnamed; their secretory method is actually merocrine (exocytosis). True apocrine (decapitation) secretion is found in mammary glands.

• The epidermis is a stratified squamous epithelium; its layers are Corneum, Lucidum, Granulosum, Spinosum, and Basale.
• Keratinocytes are connected by desmosomes (targeted in pemphigus vulgaris); melanocytes are of neural crest origin.
• Langerhans cells in the stratum spinosum are specialized antigen-presenting cells.
• Merkel cells in the stratum basale are neuroendocrine cells essential for light touch sensation.
• The dermis anchors the epidermis and contains collagen, elastin, and adnexal structures.