Skin

• Functions of the skin

  • Protection: Physical barrier against mechanical insults and permeability barrier.
  • Sensory: contain receptors that monitor the environment.
  • Thermoregulatory: contains components for insulating and accelerating heat loss for homeostasis.
  • Metabolic: produce vitamin D required for calcium metabolism.
  • Immunologic: contain antigen-presenting cells and lymphocytes that mount immune response.
  • Excretion: excess electrolytes are excreted in sweat.
  • Endocrine
• Components of the skin

  • Primary tissues of skin: epithelial, connective, muscle & nervous tissue
  • Epithelium lining the surface (epidermis)
  • Glands invaginating from the surface; sebaceous & sweat glands (eccrine & apocrine)
  • Connective tissue underlining the epithelium (dermis)
  • Smooth muscle: arrector pili muscle
  • Nervous tissue: sensory receptors & nerves for muscle
  • Embryonic origin of the two main layers: epidermis = ectoderm; dermis = mesoderm
• Thick skin

  Soles of feet/palms of hands, thick keratin layer, absence of hair follicles
• Thin skin

  Covering the body, presence of hair follicle
• Layers of the skin

  • Epidermis (epithelial layer of ectodermal origin)
  • Dermis (layer of mesodermal connective tissue)
  • Underlying hypodermis (loose connective subcutaneous tissue containing adipocytes)
• Thick skin vs thin skin

  • Location: Thick skin = palms & feet soles. Thin skin = rest of body
  • Difference in epidermis: Thick skin = thicker due to 5 layers. Thin skin = thinner due to 4 layers
  • Appendages found: Both = sweat glands(apocrine & eccrine)& sensory receptors (Meissner & Pacinian corpuscles) , Thin skin = sebaceous glands & hair follicles, sensory receptors
• Epidermis

  • Composed of mostly stratified squamous keratinized epithelium, consisting of keratinocytes and some clear cells
  • Irregular junctions occurs between the dermis and epidermis, projections called dermal papillae interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers
  • Epidermal ridges are more prominent in the palmar and plantar surface and form distinctive patterns (fingerprints)
• Layers of the epidermis (thin skin)

  • Stratum corneum
  • Stratum granulosum
  • Stratum spinosum
  • Stratum basale
• Layers of the epidermis (thick skin)

  • Stratum corneum
  • Stratum lucidum
  • Stratum granulosum
  • Stratum spinosum
  • Stratum basale
• Stratum basale


  ❖Single layer of cuboidal or columnar cells resting on the basement membrane

  ❖Intense mitotic activity and contain progenitor cells that contribute to all other epidermal layers

  ❖Large central nucleus with intensely basophilic staining cytoplasm

  ❖Specialized cells include melanocytes that produce brown pigments (melanin)
• Stratum spinosum


  ❖Normally the thickest layer,

  ❖Consist of polyhedral cells with a central nuclei, nucleoli and cytoplasm

  ❖Actively synthesizes keratin

  ❖Keratin filaments assemble and converge into radiating bundles and terminate at numerous desmosomes giving a prickle (spiky) appearance around the cells
• Stratum granulosum


  ❖Consists of 1-5 layers of flattened cells,

  ❖Undergoing terminal process of keratinization, cytoplasm is filled with intensely basophilic keratohyaline granules,

  ❖Contains small ovoid membrane-bound lamella granules that undergo exocytosis and gives rise to a lipid impermeable layer around cells
• Stratum lucidum


  ❖Found only in thick skin

  ❖Very thin single layer of translucent flattened cells,

  ❖Highly refractile layer containing eosinophilic cells,

  ❖Cytoplasm of cells are devoid of nuclei and organelles and are packed with mostly keratin filaments
• Stratum corneum


  ❖Consists of 15 to 20 layers of squamous keratinized cells

  ❖By the end of keratinization, the cells contain only fibrillar proteins with plasma membranes surrounded by the lipid-rich layer,

  ❖Fully keratinized or cornified cells called squames are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes break down
• Cells of the epidermis

  • Keratinocytes
  • Clear cells
  • Melanocytes
  • Langerhans cells
  • Merkel cells
• Keratinocytes

  Predominant cell type, responsible for keratin formation, cell proliferation, epidermal water barrier, and pigmentation
• Melanocytes

  ❖Found in the stratum basale,

  ❖Neural crest derivative,

  ❖Pale-staining, rounded cell bodies attached to the basal lamina but lacking attachments to the neighboring keratinocytes,

  ❖Produce melanin which is stored by neighboring keratinocytes in the basal and spinous layers
• Merkel's cells


  Found in the stratum basale, sensitive mechanoreceptors essential for touch sensation, the basolateral surfaces of the cells contact expanded terminal discs of unmyelinated sensory fibers penetrating the basal lamina, abundant in sensitive skin such as the fingertips
• Langerhans cells

  Usually most clearly seen in the stratum spinosum, mesenchymal origin, are antigen-presenting cells, cytoplasmic processes extend from these dendritic cells between keratinocytes of all the layers, functions as a major component of the skin's adaptive immunity
• The dermis is a layer of connective tissue derived from mesoderm,

  ❖Supports the overlying epidermis and anchors it to the hypodermis,

  ❖The surface is irregular with many projections (dermal papillae) that interdigitates with projections of the epidermis (epidermal pegs),

  ❖Contains microvascular plexus and lymphatic vessels,

  ❖The papillary dermis also contains sensory afferent nerve
• Clear cells


  • Identify Langerhan's cells in the image
  • Functions of Langerhan's cells
  • Identify melanocytes in the image
  • Embryonic origin and function of melanocytes
  • Embryonic origin and function of Merkel's cells
• Dermis


  • Layer of connective tissue derived from mesoderm
  • Supports the overlying epidermis and anchors it to the hypodermis
  • The surface is irregular with many projections; dermal papillae that interdigitates projections of the epidermis; epidermal pegs
  • The dermis contains microvascular plexus and lymphatic vessels
  • The papillary dermis also contains sensory afferent nerve fibres network
• Dermis - Papillary layer

  • Thin layer of loose connective tissue with fibroblast, cell and fibres that anchor to the basal lamina and bind the epidermis to the dermis
• Dermis - Reticular layer

  • Thicker layer of dense irregular connective tissue. Consist of elastic fibres providing elasticity to the skin
• Hypodermis

  • Layer of loose connective tissue that binds skin loosely to subjacent organ (subcutaneous fascia)
  • Contains adipocytes for energy storage and insulation
  • Extensive vascular supply promotes rapid uptake of compounds injected into this tissue
• Nerve supply- free nerve endings

  • Free nerve endings- in the papillary dermis respond to temperature, pain, fine touch itching and are tactile receptors
• Glands

  • Epithelial in origin
  • Found in the dermis
  • Three types of glands are found in the skin: Sebaceous gland, Eccrine sweat gland, Apocrine sweat gland
• Sebaceous glands

  • Found all over the body except in the palms and soles
  • Classified as simple branched acinar
  • Associated with hair follicles
  • The acinar has a holocrine mode of secretion. With flattened basal cells which proliferate and are displace centrally as they undergo autophagy and disintegrate
  • Secretes a mixture of lipids; sebum which maintains the stratum corneum and hair shafts
• Eccrine sweat glands

  • Found in the dermis, abundant in the soles of the foot
  • Classified as simple coiled tubular
  • Pale staining secretory portion produce the sweat
  • Darker cells line the ducts (stratified cuboidal epithelium) granules that undergo merocrine secretion
  • Myoepithelial cells on the basal lamina contract to expel secretion into the duct
• Apocrine sweat glands

  • Found in the skin of the axillary and perineal regions
  • Becomes fully functional after puberty
  • Secretory end pieces consist of simple cuboidal cells and have a larger lumen
  • Ducts open into hair follicles at the epidermis
  • Produces a slightly viscous secretion by exocytosis (apocrine mode of secretion) that may acquire an odor with bacteria activation
• Skin repair and wound healing

  • Coagulation of blood from cut vessel leads to Inflammation reaction by neutrophils and macrophages
  • Epithelization as epidermal basal cells migrate laterally beneath blood clot and desicates
  • Lost epidermis is replaced by new cells migrate from the surviving hair follicle
  • Proliferating fibroblast and new capillaries give rise to granulation tissue and which replace the blood clot
  • The epidermis reestablishes continuity with no hair or glands
  • Granulation tissue is remodeled producing scar tissue at the wound site
• Some structures may have different names, example: Rete pegs=epidermal ridges/pegs, Papillary dermis= dermal papillae
• Nerve supply - encapsulated nerve endings

  • Encapsulated nerve ending
  • Meissner's corpuscle- consists of sensory axons and Schwann cells in the dermal papillae. Initiate impulses for light touch. Found in fingertips, palms and soles
  • Ruffini corpuscle-fusiform capsules with sensory axons stimulated by stretch or twisting
  • Pacinian corpuscle- found in reticular dermis or hypodermis. Lamellated corpuscle for sensing coarse touch, pressure and vibration