Anatomy and Physiology of the Skin
Skin largest organ in body. Surface area of 1.5-2m2. 16% of body weight.
Embryonic development:
• appears as early as third foetal week as a single layer of cells (periderm)
• several days later the germinativum layer develops
• during 2nd month, the connective tissue elements begin to become apparent
Three layers:
1) Epidermis – thin, avascular external part – varies from 0.8-1.4mm thickness
– consists of actively dividing cell population (ectodermal cells)
– protective barrier
– main cell is keratinocyte produces keratin
– keratin is a filamentous protein that forms protective layer (stratum corneum); several keratin genes have been identified
– basement membrane zone (BMZ)– extracellular material that separates dermis from epidermis.
– epidermal turnover of about 4 weeks
Four* layers:
1) Stratum Germinativum/Basale (basal cell layer)
– deepest layer
– columnar/cuboidal cells – well defined nucleus – mostly keratinocytes adhered to basement membrane by hemidesmosomes
– melanocytes make up 10-15% of cells
– at any one time, only a small number are actively undergoing mitotic division – rest are in a ‘resting phase’
2) Stratum Spinosum (prickle cell layer)
– basal cells move upwards – become flattened
– cells polyhedral in shape connected by prominent desmosomes – contain one or two nuclei and keratinohyaline granules develop
– keratin tonofibrils form supportive mesh of cells in cytoplasm
3) Stratum Granulosum (granular cell layer)
– cells become flattened and diamond shaped – nucleus bulges start to loose nuclei
– keratinohyaline granules coalesce
– tonofibrils mesh with granules
– dehydration occurs and cell structures degenerate
4) Stratum corneum (horny layer)
– outermost layer of dead keratinocytes
– composed of flat sheets of overlapping polyhedral ‘cornified’ cells with no nuclei that are constantly shedding – ‘nonliving’ layer for protection from injury and penetration of microorganisms and chemicals
– thicker on plantar and palmer surfaces
– time from basal layer to shedding is normally 27-30 days
– estimated that 50 billion keratinocytes are shed per day “house dust”
*Fifth layer (?) in soles and palms – Stratum lucidum – translucent line of flat cells – assumed to act as a barrier for prevention of water loss (some consider this layer as an artefact of the fixation process used for light microscopy).
2) Dermis – varies from 0.5mm to 3mm+
– supportive connective tissue matrix
– 70% is collagen
– elastin
– mesodermal in origin
Two layers:
1) Papillary dermis (pars papillaris):
– thin upper layer (about 1/3)
– interdigitates with epidermal rete ridges
– composed of loosely interwoven collagen
2) Reticular dermis (pars reticularis):
– deeper and thicker (about 2/3)
– coarser and horizontally running bundles of collagen
3) Subcutaneous/subcutis layer
up to 3cm thick (abdomen)
consists of loosely packed connective tissue and adipose tissue – lobules are divided by fibrous connective tissue septae which contain blood supply
slow turnover of tissue
important in heat insulation, cushioning against trauma – also functions as an energy reservoir
considerable amount of morphological heterogeneity depending on body site (thickness, function, metabolic activity of cells)
Langerhans Cells:
Dendritic antigen presenting cell – ‘scattered’ in epidermis, mostly in stratum spinosum layer; makes up to 6% of epidermal cells; not connected to adjacent keratinocytes; derived from stem cells in bone marrow as part of monocyte-macrophage line; involved in immunity crucial for processing antigens; play a role in graft rejection and immunosurveillance.
Dermal Dendrocytes:
Resemble Langerhans cells, but located in dermal stroma
Traditionally labelled as fibroblasts
Found in perivascular network
Antigen presenting cells – express high levels of MHC class 2 molecules
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