integument pt 2

Keratinocytes

Come from ectoderm, most abundant epidermal cell (84%), specialized to produce protein keratin

Cytomorphosis

Changes in anatomy of keratinocytes reflecting changes in cellular activity

Strata naming

Each stratum is a stage in keratinocyte life from basal birth to corneum death

Location

Found from stratum basale to stratum corneum

Stratum Basale

Basal cells divide here, attached to basement membrane, act as stem cells

Basal cell activity

Ribosomes produce cytokeratin → tonofilaments; ribosomes cause basophilic appearance

Stratum Spinosum

Synthesis of cytokeratin continues; tonofilaments form bundles called tonofibrils

Filaggrin

Cytokeratin-binding protein that bundles tonofilaments into tonofibrils

Desmosomes/Hemidesmosomes

Tonofilaments insert into these, reinforcing cell-cell and cell-dermis attachment

Friction resistance

Filament reinforcement counters mechanical stress; Node of Bizzozero = dense desmosome site

Acidophilia

Upper layers increasingly acidophilic due to cytokeratin accumulation

Stratum Granulosum

Cytokeratin synthesis stops; keratohyalin granules and lamellar bodies (Odland bodies) form

Keratohyalin Granules

Deeply basophilic (dark blue), 1-5 μm, non-membrane bound, non-secretory, contains filaggrin, phosphate-rich proteins, RNA

Function

Incorporates surrounding tonofilaments/tonofibrils, large size due to cytokeratin incorporation

Eleidin formation

Keratohyalin + cytokeratin → intermediate eleidin, leading to stratum lucidum

Basophilia disappearance

Cytoplasm becomes acidophilic and transparent, organelles/nucleus disappear in stratum lucidum

Concurrent Events in Stratum Granulosum

Loricrin & Involucrin

Gradually deposited on intracellular membrane, creating physical barrier; inside cell watery, outside dry

Serine protease

Inactive in neutral pH of stratum granulosum, activates at pH 4.5 in stratum lucidum → breaks desmosomes → shedding of dead keratinocytes

Lamellar Bodies (Odland Bodies/MCG)

Small (0.1-0.5 μm), membrane-bound, secretory

Contents

Lipid lamellae + enzymes (including serine protease)

Function

Secrete lipids into intercellular space → build water-resistant physical barrier; intracellular loricrin/involucrin + extracellular lamellae → enhances epidermal barrier

Stratum Lucidum

Only in thick skin, transparent cells, contains eleidin, organelles and nucleus absent

Stratum Corneum

Cell remnants fully converted to keratin, shed via desmosome breakdown

Melanocytes

Located in stratum basale, produce melanin, transfer pigment to keratinocytes via dendrites

Langerhans Cells

Found in stratum spinosum, antigen-presenting cells, involved in immune surveillance

Merkel Cells

Found in stratum basale, mechanoreceptors for touch, associated with nerve endings