12 Integumentary System: Skin Structure and Function

Thermoregulation, Special Senses, Endocrine System, Reproductive System

Integument

  • Integument includes Epidermis and Dermis

Learning Outcomes

  • ALO1: Understand the role of the skin as a significant organ in the body
  • ALO2: Describe in detail the histology of the epidermis
  • ALO3: Discuss the various epidermal derivatives and keratinised structures including nails, hair, sweat glands, and mammary glands
  • ALO4: Describe the dermis and subcutaneous layers of the skin

Skin - Function

  • The skin forms an Integument covering the entire body and performs important functions.
  • Functions:
    • Protection
    • Thermoregulation
    • Sensation
    • Metabolism
  • Protection:
    • Provides protection against physical trauma from mechanical injuries.
    • Forms a continuous line of defence against invasion by pathogens.
    • Aids in water regulation (sweating).
    • Skin pigments prevent the entry of dangerous amounts of UV light.
  • Thermoregulation: Hair and sweating provide insulation and play a major role in temperature regulation.

Skin - Function (Continued)

  • Sensation:
    • The skin appears to be the area of origin of sensory and nervous structures.
    • Embryologically the nervous system still arises in continuity with the skin ectoderm.
    • Sensory nerve endings for pressure, temperature, and pain allow interpretation of the external environment.
  • Metabolism: Produces vitamin D used in calcium and phosphate metabolism.

Skin - Histology

  • One of the largest organs - about 12.5\%-16.7\% of the mass of a normal person.
  • Variable in thickness - very thin eyelids, thick on soles of feet.
  • Continuous with the digestive tract (mouth and anus), and the urinogenital system (penis or vagina/urethra).
  • Composed of two layers - outer epidermis and an inner dermis.
  • Below the dermis is the hypodermis, a third layer of loose connective tissue.
  • Types of Skin:
    • Glabrous: Thick, hairless skin (e.g., palms and soles of feet)
    • Hairy: Thinner skin found in most areas

Epidermis

  • Protective barrier layer: chemical, physical, microbial.
  • Thickness: ~0.06-0.1 mm thick in humans.
  • Arises from the embryonic ectoderm and gives rise to sweat glands, sebaceous glands, and mammary glands.
  • Predominately keratinocytes organized into strata (Stratified, squamous epithelium).
  • Keratinocytes start at the basal membrane, move outwards, mature into corneocytes, and are removed by desquamation.
  • Layers of the Epidermis:
    • Stratum Corneum
    • Stratum Lucidum
    • Stratum Granulosum
    • Stratum Spinosum
    • Stratum Basale
    • Basement membrane

Epidermis (Continued)

  • The entire epidermal cycle takes about 27 days in humans.
  • Stratum corneum: Thick outer layer of flattened, fused cell remnants composed mostly of keratin.
  • Stratum corneum also covered with an oily secretion from the sebaceous glands called sebum.
  • Stratum lucidum: Only in thick skin, a clear homogenous layer of cells without nuclei or organelles.
  • Stratum granulosum: Showing granules within the cells.
  • Stratum spinosum or prickle layer: Growing cells showing early keratin synthesis.
  • Stratum basale (basal layer): A proliferative layer of columnar or cuboidal cells.

Skin Pigmentation

  • The colour of human skin is due to three factors:
    1. An inherent yellowish colour derived from carotene in the subcutaneous fat.
    2. The concentration and oxygenation of haemoglobin in the dermal capillaries.
    3. Special branched cells scattered throughout the stratum basale.
  • The pigment in these cells is called melanin and the cells which produce it are called melanocytes.
  • Melanin functions to protect the body from UV radiation – pigment is able to dissipate over 99.9% of absorbed UV radiation.
  • Number of melanocytes is relatively constant irrespective of skin colour.
  • Melanocytes package melanin into vesicles called melanosomes.
  • These are taken up by keratinocytes and accumulate on top of the nucleus and protect the DNA from ionising UV radiation.
  • A patch of dark pigmentation on the skin (a mole) is a melanocytic nevus, a very small percentage of nevi may become cancerous – melanoma.

Keratinized Structures

  • Most mammals have foot pads, humans and higher primates have friction ridges on the palms of the hand and soles of the feet.
  • At the tips of the fingers they form the genetically determined fingerprint (dermatoglyph).
  • Hair, nails, claws and hooves are other keratinised structures.

Hair

  • An elongated rod of keratinised cells (hair shaft), which grow from a root (hair bulb) lying in the dermal layer of the skin but formed by epidermal cells.
  • Hair bulb is at the base of a pit sunk in the dermis called the hair follicle.
  • Follicle cells are continuous with the epidermis.
  • During early fetal life the epidermis grows down into the dermis to form a cellular shaft.
  • Cells of the hair shaft are dead keratinised epidermal cells except at the base where the growing germinal matrix exists.
  • Germinal matrix forms a cap over a vascularised plug of dermal tissue called the dermal papilla.
  • Germinal matrix is connected to the overlying epidermis by a cellular external root sheath.
  • Entire structure can be called pilosebaceous unit.

Hair Structure

  • Proliferation of cells in the germinal matrix produces hair growth.
  • Initially these cells are pushed up the center of the follicle.
  • As they move further from the papilla blood supply, they become keratinized and die.
  • The mature hair follicle is organised into several layers of cells.
  • These initial cells transform into hair cortex with an outer single layer of cells the hair cuticle.

Hair Follicle Structure

  • In section the hair and follicle consists of:
    • Medulla (seldom seen)
    • Cortex (keratinised cells)
    • Cuticle (single layer of overlapping keratinised cells).
  • The hair shaft is held in position in the follicle by the internal and external root sheaths (ERS, IRS).
  • Internal root sheath (IRS) produced by further division of the germinal matrix cells which pushes up between the External root sheath (ERS) and the hair cuticle.

Hair Growth – The Hair Cycle

  • Hair grows on a cyclical basis
  • Scalp hair growth phase in young adults ~3 years; ~85\% of scalp hair is in growth phase.
  • This phase is followed by degeneration of the follicle - Catagen.
  • This phase is followed by a resting phase - Telogen
  • Old hair shaft loosens and is pushed out by the new shaft – Exogen.
  • In baldness - growth phase does not resume and the shaft eventually falls out.
  • The sebaceous gland is derived from the ERS, secretes sebum into the follicle.
  • Sebum has probably a mild fungistatic and bacteriostatic action.
  • Hair position can be elevated by the arrector pilus muscle which is attached to the hair bulb and the epidermis.

Other Contents Of The Integument

  • Sebaceous (oil) glands
    • Connected to the hair follicles – forms pilosebaceous unit
    • Produce sebum - helps to protect skin and hair
    • Release their secretions via a holocrine mechanism into the hair follicle shaft
    • Located over most of the skin/body except palms of hands or soles of feet
  • Sudoriferous (sweat) glands
    • Apocrine Glands - ducts open in hair follicles
      • Armpits, pubic region, areola of breast
    • Eccrine Glands - ducts terminate as a sweat pore at epidermis
      • Throughout skin except lips, nail-beds, eardrums
    • Coiled tubular glands
    • Secretory portion lies in the fascia with a duct up to the skin surface
    • Secrete sweat - thermoregulation

Other Contents of the Integument

  • Ceruminous glands
    • Modified apocrine glands - ducts open into larger ducts that drain into the hair follicles of the external sudatory canal
    • Secretions combines with sebum secreted by nearby / associated sebaceous glands to form cerumen (ear wax)
    • Lubricates and cleans the external auditory canal, waterproofs the canal, kills bacteria, protects ear from foreign particles e.g. dust, fine sand.

Mammary Glands

  • These are highly modified derivatives of the sweat glands.
  • Growth of functional mammary glands in the female mammal and milk production are under hormonal control.
  • Mammary glands develop by thickening of the epidermis along what are called the "milk lines" in the abdominal wall.
  • Milk composition varies widely in different mammals.
  • Milk Composition (grams/litre) and Days to Double Birth Weight:
    • Human: Protein - 19, Days - 180
    • Cow: Protein - 33, Days - 47
    • Pig: Protein - 37, Days - 18
    • Dog: Protein - 97, Days - 8
    • Seal: Protein - 119, Days - 5

Dermis

  • Papillary Dermis
    • Rich in vasculature
    • Nerve Endings, thermo/ cryo-receptors
  • Reticular Dermis
    • Thicker of the two substrata
    • Dense connective tissue containing elastin network and proteoglycans.
    • Provides the structural support for the skin
  • Dermis is predominately composed of fibroblasts, which secrete and organize large amounts of extracellular matrix (ECM).
  • This ECM is mainly fibrillar collagens- types I, III and V and elastin.

Dermis (Continued)

  • Thickness ~2mm, irregular dense connective tissue.
  • Derived primarily from mesoderm
  • Composition:
    • Collagen
    • Elastin fibers
    • Blood vessels – thermoregulation, immune function
    • Nerves
    • Fibroblasts – form ECM, e.g., collagen & elastin
    • Mast cells – role in inflammatory response to pathogens & injury
    • Lymphatics
    • Epidermal appendages
    • Glycosaminoglycans:
      • hyaluronic acid
      • chondroitin sulfates
      • glycoproteins Retain H20 and provide moisture to epidermis Strength & Elasticity

Subcutaneous Tissue

  • Also called the hypodermis or subcutis, is the lower most layer of the integument in humans.
  • Cells within this layer are predominately adipocytes, fibroblasts and macrophages.
  • Primarily composed of loose connective tissue.
  • Contains large blood and lymphatic vessels and nerves that supply those of the overlaying dermis.
  • Major site of fat storage in the body.
  • Contains collagen and elastin fibres that attach it to the dermis and fibrous bands that anchor it to the deep fascia.

Summary and Learning Outcomes

  • ALO1: Understand the role of the skin as a significant organ in the body
  • ALO2: Describe in detail the histology of the epidermis
  • ALO3: Discuss the various epidermal derivatives and keratinised structures including nails, hair, sweat glands and mammary glands
  • ALO4: Describe the dermis and subcutaneous layers of the skin