General characteristics

• Outermost layer of the skin: keratinizing, stratified, squamous epithelium
• Total epidermal turn over time from the basal layer to the stratum corneum is approximately 30 days
• Thickness is approximately 0.075 mm to 0.15 mm

Cell types

• Keratinocytes
  • 80% of epidermal cells
  • Originate in the basal layer
  • Contain keratin for structure and support
• Melanocytes
  • Found in skin, inner ear, choroid, and iris of the eye
  • Neural crest derived, pigment-producing, dendritic cells with clear cytoplasm 
  • Only in stratum basale
  • Produce melanin to protect the mitotically active basal keratinocytes from ultraviolet radiation induced DNA damage
    • Melanin production
      • Melanin is packaged in melanosomes in the melanocyte
      • Melanosomes are transported through dendritic projections from the melanocytes to adjacent keratinocytes by phagocytosis
  • Number of melanocytes does not differ between races; the number and size of melanosomes is greater in people with darker skin
• Langerhans cells
  • Bone marrow-derived, antigen-processing and antigen-presenting cells
  • Found mainly in the spinous epidermal layer but also in the normal dermis
  • Characterized by dendritic processes (similar to melanocytes)
  • Identified on electron microscopy by intracytoplasmic, small, tennis racket-shaped structures called Birbeck granules 
• Merkel cells 
  • Slow-reacting type I mechanoreceptors of neural crest origin involved with touch sensation
  • Found in basal layer of palms, soles, oral and genital mucosa, nailbeds, and the follicular infundibulum
  • Contain granules of neurotransmitter-like substances
  • Identified by immunohistochemical identification of cytokeratin-20 (CK-20)
  • Origin of Merkel cell carcinoma 

Layers of epidermis

• Stratum corneum (cornified superficial layer)
  • Terminal differentiation of keratinocytes into anucleate, flattened keratinocytes surrounded by extracellular lipid matrix
  • Highly variable in thickness
  • Thickest on the palms and soles; thinnest on the eyelids and genitalia
  • “Barrier activity” providing mechanical protection and preventing water loss
• Stratum lucida 
  • Eosinophilic acellular layer seen beneath the stratum corneum on acral skin (palms, soles)
• Stratum granulosum (granular layer)
  • Larger and flatter than cells in the stratum spinosum 
  • 1-4 cells thick
  • Deeply basophilic keratohyalin granules which contain proteins important for the cornification and barrier function of the stratum corneum
• Stratum spinosum (spinous layer)
  • Several cells thick
  • Polygonal cells with abundant eosinophilic cytoplasm 
  • Contain lamellar granules which contain lipids important for integrity of the epidermis
  • Named for small, spiny intercellular desmosomal attachments seen under light microscopy
• Stratum basale (basal layer)
  • Deepest layer
  • Single mitotically active layer of cuboidal to columnar-shaped basophilic keratinocytes that attach to the basement membrane zone and give rise to the more superficial epidermal layers 
  • Attached to underlying basement membrane zone by hemidesmosomes 
  • Melanocytes are interspersed between basal cells

Epidermal appendages

• Hair follicle
  • 3 regions
    • Infundibulum – from skin surface to opening of the sebaceous duct into the follicle
    • Isthmus – between the opening of the sebaceous duct into the follicle and the bulge
    • Inferior most portion – inferior to the bulge and includes the hair bulb
  • Internal organization of hair follicle is conceptualized as series of distinct concentric layers. From peripheral to inner:
    • Outer root sheath
      • Contiguous with the epidermis and lined by the dermal epidermal junction
      • Follicular epithelium in the dermis of the skin is additional source of germinative cells for re-epithelialization of partial thickness wounds
    • Inner root sheath
      • Three layers: Henley’s layer, Huxley’s layer, cuticle
    • Hair shaft
      • Three layers: cuticle, cortex that forms most of the hair shaft, variable central medulla
  • Hair follicles undergo independent cycles of growth, involution, and rest
    • Anagen (growth) – keratinocytes in the bulb proliferate and produce growing hair 
    • Catagen (involution) – keratinocytes stop proliferating and the inferior portion of hair follicle involutes
    • Telogen (rest) – inferior portion of the hair follicle is lost and hair is typically shed from the follicle
• Sebaceous glands 
  • Connect to the hair follicle by a squamous epithelial duct 
  • Secrete sebum through the duct into the follicle and onto the surface of the skin 
    • Controlled by androgens and associated with acne
  • Enlarge and become active during puberty
  • Found everywhere on the body except the palms and soles
  • Most abundant on face and scalp
  • Not always associated with a hair follicle (Fordyce’s spots on the lip and meibomian glands on the eyelids)
• Eccrine sweat glands
  • Found everywhere on the skin except the mucus membranes
  • Eccrine sweat unit = coiled secretory gland + duct 
    • Opens directly on the skin 
  • Innervated by cholinergic nerve fibers which are stimulated by thermal, mental, and gustatory stimuli
• Apocrine sweat glands 
  • Axilla, areola, perineum, eyelids (Moll’s glands), and external auditory canal (ceruminous glands)
  • Become functional just before puberty
  • Respond to emotive stimuli by adrenergic innervation
  • Odorless secretions
  • Apocrine sweat unit = secretory gland + duct
    • Opens into hair follicle

References

1. Bichakjian CK, Johnson TM. Anatomy of the skin. In: Baker SR (ed) Local Flaps in Facial Reconstruction, 2nd ed, Philadelphia, Mosby, 2007, p 3-13. 
2. Leithauser LA, Collar RM, Ingraffea A. Structure and function of the skin. In: Papel ID, Frodel JL, Holt GR, et al. (eds) Facial Plastic and Reconstructive Surgery, 4th ed, New York, Thieme, 2016, p 1-5. 

General characteristics

• Extracellular dermal matrix: collagen, elastin, ground substance
• Gives the skin pliability, elasticity, and tensile strength
• Regional variation in thickness: <1mm on eyelid, 1.5mm in temple, 2.5mm in scalp, and >4mm on the back

Layers of the dermis

• Papillary dermis – superficial, thin
• Reticular dermis – deep, thick

Structural components of the dermis 

• Collagen – principle component 
  • Synthesized by dermal fibroblasts
  • Tensile strength and elasticity 
  • 85% is type I collagen – thick fibers, dense bundles, predominantly found in reticular dermis
  • 10% is type III collagen – thin fibers, small/loose bundles, predominantly found in papillary dermis
  • <5% is type V collagen
  • Type IV and VII collagen are predominately in the basement membrane zone.
  • Degraded by matrix metalloproteinase (MMPs) (e.g. collagenase) and replaced by new fibers
• Elastic tissue
  • Composed of a protein elastin, microfibrillar matrix with fibrillin, a glycoprotein, and other components 
  • Synthesized by fibroblasts
  • In papillary dermis – thin fibers, perpendicular to the skin
  • In reticular dermis – thick fibers, parallel to the skin
• Ground substance 
  • Proteoglycans, glycosaminoglycans, filamentous glycoproteins
    • Hyaluronic acid is important glycosaminoglycan 
  • Synthesized by fibroblasts
  • Plays a role in skin hydration/water reservoir and preserves the tensile elasticity of compressed skin by redistributing the pressure forces

Cellular components of the dermis

• Fibroblasts 
  • Synthesize collagen, elastin, and ground substance
• Phagocytic cells: monocytes, macrophages, dendrocytes
• Mast cells 

Vasculature 

• Two vascular plexuses connected by communicating vessels
  • Superficial vascular plexus
    • At the superficial aspect of the superficial reticular dermis 
  • Deep vascular plexus 
    • At the junction of the dermis and subcutaneous fat 
  • Lymphatic system similar to vascular plexuses 

Nerves

• Myelinated and nonmyelinated free nerve endings
  • Temperature, pain, itch
• Meissner’s corpuscles
  • Fine touch 
• Pacinian corpuscles
  • Deep pressure and vibration
• Efferent nerves
  • Innervate blood vessels and appendageal structures to regulate function

References

1. Bichakjian CK, Johnson TM. Anatomy of the skin. In: Baker SR (ed) Local Flaps in Facial Reconstruction, 2nd ed, Philadelphia, Mosby, 2007, p 3-13. 
2. Leithauser LA, Collar RM, Ingraffea A. Structure and function of the skin. In: Papel ID, Frodel JL, Holt GR, et al. (eds) Facial Plastic and Reconstructive Surgery, 4th ed, New York, Thieme, 2016, p 1-5. 

• Examination findings of photoaging
  • Epidermal and dermal dyspigmentation (“sun spots”)
  • Dermal telangiectasias
  • Rough surface texture
  • Fine rhytids, crepe-like changes 
  • Deep rhytids 
  • Pallor 
  • Cream-yellow discoloration 
  • Loose, sagging, and folded skin 
• Epidermal pathology associated with aging
  • Actinic keratoses
  • Lentigines 
  • Seborrheic keratoses 

Glogau classification of facial photoaging

Fitzpatrick skin types

References

1. Bichakjian CK, Johnson TM. Anatomy of the skin. In: Baker SR (ed) Local Flaps in Facial Reconstruction, 2nd ed, Philadelphia, Mosby, 2007, p 3-13. 
2. Weiss RA1, McDaniel DH, Geronemus RG, Weiss MA, Beasley KL, Munavalli GM, Bellew SG. (2005). Clinical experience with light-emitting diode (LED) photomodulation. Dermatol Surg, 31(9 Pt2):1199-1205.
3. Airan LE, Hruza G. Current lasers in skin resurfacing. Facial Plast Surg Clin N Am. 2005;13:127-139.
4. Glogau RG, Matarasso SL. Chemical face peeling: patient and peeling agent selection. Facial Plast Surg. 1995;11(1):1-8