Integument pt 3

Melanocytes

Make melanin, 3rd most abundant epidermal cell (5%), come from ectoderm via neural crest cells

Location

Found in stratum basale (single layer), attached to basement membrane via hemidesmosomes

Basal surface contact

Axon terminals of afferent sensory neurons (pseudounipolar neurons)

Function

Specialized for light touch, part of Merkel’s corpuscle (sensory neuron + Merkel cell), mechanoreceptor

Free nerve endings

Naked/unencapsulated, penetrate to stratum granulosum, broad senses including nociception, most numerous in epidermis

Pacinian corpuscles

Encapsulated, deep in dermis/hypodermis, require strong pressure

Meissner’s corpuscles

Encapsulated, in dermal papillae, sensitive to light touch, not uniform in skin, vary by location

Melanocyte density

Fairly uniform in humans regardless of skin color, higher in forehead (sun exposure), scrotum (decoration) → pigmentation not always UV protection

Cell contacts

Do not form junctions with keratinocytes, extend “arms” into stratum spinosum

Melanosomes

Membrane-bound vesicles containing melanin

Stage 1 Melanosome

0.25 μm vesicle from Golgi, contains structural proteins + enzyme tyrosinase

Stage 2 Melanosome

Pre-melanosome, structural proteins assemble into uniform grid framework

Stage 3 Melanosome

Melanin deposited on grid, tyrosine → DOPA → melanin via tyrosinase

Stage 4 Melanosome

Mature, filled with melanin, enzyme activity ceases

Cytocrine secretion

Direct transfer of mature melanosomes from melanocyte → keratinocyte in stratum spinosum via phagocytosis, never extracellular

Skin color

Result of melanin in keratinocytes, not melanocytes alone

Langerhans Cells / Epidermal Dendritic Cells

Least numerous (~3%)

Location

Middle of epidermal/rete ridges, surrounded by stratum spinosum cells, not attached

Origin

From mesoderm → bone marrow → differentiate into Langerhans → migrate to epidermis

System

Mononuclear Phagocytic System (MPS), related to macrophages & osteoclasts

Function

Antigen-presenting cells, monitor epidermis for antigens

Mechanism

Phagocytize antigen → process → present fragments to lymphocytes in lymph node

Role

Delayed-type hypersensitivity (e.g., contact dermatitis, poison ivy)

Birbeck granules

Unique to Langerhans cells, cytoplasmic, function unknown

Merkel Cells / Epidermal Tactile Cells

2nd most abundant (8%)

Origin

Possibly epithelial, modified basal cells in stratum basale

Attachment

Desmosomes to surrounding keratinocytes

Secretory vesicles

Numerous, <100 nm, dense-core neurosecretory granules containing catecholamines (epinephrine, norepinephrine)

Function

Mechanoreception, light touch sensation, part of Merkel’s corpuscle with sensory neuron