Anatomy & physiology of skin
Dr. Qassim S. Al Chalabi F.A.B.H.S (Dermatology and venereology)Skin Structure
Skin is the single largest organ in the human body. It weighs an average of 4 kg and covers an area of 2 m2 Three distinct layers Epidermis: Composed of epithelial tissue Dermis: Composed of a combination of connective tissues Hypodermis: usually contains abundant fat.Epidermis
It’s outermost layer of skinIt’s thickness 0.1-1.0 mmIt’s a stratified squmaous epitheliumIt’s ectodermal in originIt’s consist of:Original cell (keratinocyte) 80%Immigrant cells:Melanocyte 10%Langerhan’s cell 2-4%Merkle cellStratum Basale
Basal cell layer: is a single row of cells where mitosis takes place. Not all basal cells have potential to divide, only (30%) Stem cells give rise to transient amplifying cells Once basal cell leaves basal layer in humans, normal transit time to stratum corneum is at least 14 days, and transit through stratum corneum to desquamation requires 14 days, 28 days total.Stratum Spinosum
Stratum Spinosum: Consists of 8-10 layers of Keratinocytes. Cells contain lamellar granules in which contain glycoproteins and lipid precursors which are discharged into intercellular space between granular and cornified layer; forms (ceramide) or “mortar” which acts as intercellular cement for corneocytes (“bricks”) thus contributing to formation of cutaneous lipid barrier.Composed of Keratinocytes attached to each other via desmosomes.Contains langerhans cells that aid in the immune system response.Stratum Granulosum
Stratum Granulosum: The middle layer of 3-5 layers of cells that help form keratin. Contains keratohyline granules that produce a secretion These make up the thick and tough peripheral protein coating of the horny envelope.Stratum Lucidum
Stratum Lucidum: Found only in the thick skin of the palms and soles of the feet Tends to be translucent.Stratum Corneum
Stratum Corneum: The outermost layer of the epidermis.Contains up to 30 layers of the dead, scaly, piled-up layers of flattened dead cells (corneocytes) – the bricks – separated by lipids – the mortar – in the intercellular space.Provides protection.Continuously shed and replaced.Cells of the epiderms
Merckle’s cell Langerhans cellMelanocyte
Few
Next common
Most common
Prevalence
-
2-4%
10%
Percentage
St.Basalis
St. Spinosum
St. basalis
Location in epidermis
Unknown
Bone marrow
Neural crest
Origin
Sensory function
Immune surveillance
Production of melanin
Function
MM, lip, digit
All skin
All skin
Type of skin
Melanocyte
Dendritic cell derived from neural crest localized to basal layer of epidermis Functions include: melanin production. (epidermal melanin unit = approximately 36 keratinocytes in contact with one melanocyte), and transfer pigment to keratinocytes Melanin absorbs and scatters ultraviolet radiation (280-400 nm), and thus protects from UV-induced DNA mutations. Special stains: silver nitrate, S100, HMB-45Skin Color
Skin color is determined by 3 pigments: Melanin: Provides brown, black and tan hues Carotene: Provides yellowish hues Hemoglobin: Found in red blood cells; provides pink and flesh tones.The pathway of melanin synthesis
Melanogensis controlGenetic (constitutional) UV (facultative) MSH
Difference between White & dark individual
SameSame
Melanocyte number
Many
Few
Melansome number
Large
Small
Melanosome size
High
Low
Degree of melanization
Individually dispersed
Grouped in membrane coated complex
Distribution in keratinocyte
Langerhan’s cell Bone marrow –derived dendritic cell.Location in epidermis: St. sipnosumLangerhans cells constitute 2-4% of the total epidermal cell population.Characteristic finding on electron microscopy – Birbeck granules- tennis racket shaped bodies in the cell.Function: Antigen presenting cells, immune surveillance.Functionally impaired by ultraviolet radiation and topical steroid.Special stain: S100, CD1a
Merckel’s cell Shape: Non dendritic cell Site: MM, lip, digit & near hair Location in epidermis: St. Basalis Origin: Unknown Function: Transducer of fine touch
Basement Membrane Zone
Site of interaction between epidermis and dermis Three layers 1.) Lamina lucida, 2.) Lamina densa, 3.) Sub-lamina densa Hemidesmosomal adhesion complex: 1.) hemidesmosomal plaque, 2.) anchoring filaments, and 3.) anchoring fibrilsDermis
Dermis: Composed primarily of connective tissue, mainly collagen, elastic fibers and ground substances. Divides into 2 Categories: Papillary Region: Superficial part that contains the dermal papillae and comprises the wavy boundary between the epidermis and dermis. Houses capillaries, nerve endings, and Meissner corpuscles (touch sensors). Reticular Region: Deeper region made up of dense irregular connective tissue. Contains many of the glands, nail roots, and root follicles.Hypodermis
Hypodermis: Known as the subcutaneous tissue or superficial fascia. The boundary of the dermis is indistinct, but it is clear that there is more areolar and adipose tissue than in the dermis. Primarily serves as a storage site for large blood vessels and fat. Houses Lamellated or Pacinian corpuscles (nerve endings sensitive to pressure).Sebaceous
AppocrineEccrine
Hairy skin, free in eyelid, lip, nipple, genetalia
Axilla, areola, Periumblical, genetalia
All skin
Distribution
Multilobulated
Coiled
Coiled
Acinar part
Short & wide
Straight
Straight
Duct shape
Hair follicle
Hair follicle
Skin surface
Duct opening
Holocrine
Decapitation
Merocrine
Excretion
Oily
Milky & odorless
Isotonic
Secretion
Androgen
Adrenalin
Acetyl choline
Stimulant
Waterproof, lubricant Anti-microbial.
Unknown ,Sex stimulant
Thermoregulation
Function
Epidermal appendages