Skin: Histology

Continually renewing stratified squamous epithelium-keratinizes & gives rise to derivative structures called appendages (pilosebaceous units, nails & sweat glands)

Cell Cycle-G0-G1-S-G2-M

Keratinocyte-ectodermally derived cell constituting 80% of epidermal cells-keratin filaments are hallmarks of keratinocytes & other epithelial cells

Immigrant cells- Melanocytes& Langerhans cells(migrate into epidermis during embryonic development), Merkel cells(differentiate in situ)

Melanocytes- located in stratum basale(stratum germinativum)- project their dendrites into epidermis- transfer their melanosomes (melanin packed membrane bound organelles) to keratinocytes which contains majority of melanin pigment - 36 keratinocytes per melanocytes- epidermal melanin unit

Keratinization- genetically programmed, carefully regulated, complex series of morphological changes & metabolic events that occur progressively in postmitotic keratinocytes & involve-

• increased cell size & flattening
• appearance of new cellular organelles & structural reorganization of those present
• change from generalized cellular metabolism to a more focussed metabolism associated with the synthesis of molecules related to keratinization(structural proteins & lipids)
• alterations in properties of plasma membrane, cell surface antigen & receptors
• eventual degradation of cellular organelles including internucleosomal chromatin fragmentation characteristic of apoptosis
• dehydration

Epidermal Layers

• Basal layer/stratum germinativum-attached to basement membrane-contains mitotically active keratinocytes-contains house keeping organelles(RER,golgi complex, mitochondria, lysosomes & ribosomes)-gives rise to superficial layer
• Spinous layer/Stratum spinosum-named for spine like appearance of cell margins in histological sections. Spines are abundant desmosomes, calcium dependent cell surface modifications that promote adhesion of epidermal cells & resistance to mechanical stress.Upper spinous layer cells have organelles called lamellar granules
• Malpighian layer-includes both basal & spinous layer
• Granular layer/stratum granulosum-characterised by buildup of components necessary for the process of programmed cell death & formation of a superficial water impermeable layer.The most apparent structures within these cells are-basophilic keratinohyaline granules-composed of profilaggrin,keratin intermediate filament & loricrin. Conversion of profilaggrin to filaggrin(filament aggregating protein)-occurs during transition of granular to cornified cells
• Corny layers or Stratum corneum- formed of cornified or horny cells which is the largest of epidermis & have highest concentration of free amino acids,esp in mid layers.Stratum corneum cells retain some metabolic function (not just an inert covering)

Normal turnover time of epidermis/skin doubling time-- 4wks

VLBW/premature infants lack-stratum corneum