119 Chapter 3 - Vertebrate Integuments

• Embryonic Origin:

  • Ectoderm proliferates to give rise to multilayered epidermis
  • Stratum basale replenishes the outer cells called the periderm
  • Dermis arises principally from dermatome
  • Neural crest cells migrate between dermis and epidermis, contributing to bony armor and skin pigment cells
• General Features of the Integument:

  • Integument consists of epidermis, dermis, and basement membrane
  • Hypodermis acts as a barrier to prevent pathogen entrance and aids in osmotic regulation and gas movement
  • Epidermis produces basal lamina, while dermis produces reticular lamina
  • Integument forms part of the exoskeleton and helps in thermoregulation, defense, and reproduction
• Epidermis:

  • Produces mucus to moisten skin surface
  • Keratinization process results in a nonliving layer (stratum corneum) to reduce water loss
  • Keratinization involves the production of keratin and keratinocytes
  • Different types of keratin (alpha and beta) are present in various vertebrates
• Dermis:

  • Contains collagen fibers woven into distinct layers
  • Dermal bones are plates of bone formed through intramembranous ossification
  • Collagen fibers in fish skin are regularly organized into plies that allow bending without wrinkling
• Integument of Fishes:

  • Fish integument is covered by mucus that resists bacterial infection and contributes to laminar water flow
  • Epidermal cells in fish are tightly connected and contain secretory vesicles released to the surface
  • Fish scales are coated with enamel of epidermal origin and dentin layer of dermal origin
• Ancestral Fishes:

  • Ostracoderms and Placoderms had prominent bony plates of dermal armor and dermal scales
  • Hagfishes and Lampreys lack dermal bone and possess stacked layers of living epidermal cells with slime glands
  • Cartilaginous fishes have surface denticles (placoid scales) that reduce friction drag
• Dermis of fish: fibrous connective tissue, especially elastic and collagen fibers
• Placoid scale develops in dermis but projects through epidermis to reach the surface
• Cap of enamel forms the tip, dentin lies beneath, and a pulp cavity resides within
• Chromatophores are located in the lower part of the epidermis and upper regions of the dermis
• Dermis of bony fishes: superficial layer of loose connective tissue and deeper layer of dense fibrous connective tissue
• Several types of scales recognized based on appearance
• Cosmoid scale found in ancient sarcopterygians, resides upon a double layer of bone (vascular and lamellar), with dentin on the outer surface and enamel spread superficially
• Ganoid scale has a thick surface coat of enamel (ganoin) without an underlying layer of dentin, foundation formed by dermal bone
• Teleost scale lacks enamel, dentin, and a vascular bone layer, composed of concentric rings (cycloid scale) or with a fringe of projections (ctenoid scale)
• Keratinization is a major feature of integument among terrestrial vertebrates
• Extensive keratinization produces stratum corneum, lipids added during keratinization or spread across the surface from specialized glands
• Multicellular glands are common, reside in dermis and reach the surface through common ducts that pierce the cornified layer
• Amphibians metamorphose from an aquatic form to a terrestrial form, skin is specialized as a respiratory surface for gas exchange
• Salamanders have aquatic larvae with Leydig cells thought to secrete substances that resist entry of bacteria or viruses, terrestrial adults have distinct regions in the epidermis
• Frogs and salamanders have two types of multicellular glands (mucous and poison glands) located in the dermis and open to the surface through connecting ducts
• Reptiles have more extensive keratinization, skin glands are fewer than in amphibians
• Reptilian scales lack bony under-support or any significant structural contribution from dermis, modified into crests, spines, or hornlike processes
• Dermal bone is present in many reptiles, gastralia are a collection of bones in the abdominal area, osteoderms are plates of dermal bone located under the epidermal scales
• Birds have a dermis richly supplied with blood vessels, sensory nerve endings, and smooth muscles, especially near the feather follicles
• Feathers are non-vascular and non-nervous products of the skin, laid out along distinctive tracts (pterylae) on the surface of the body, replaced each year via molts
• Feathers:

  • Flight feathers are long with asymmetrical vanes about the stiffening rachis
  • Remiges (wings) and rectrices (tails) are types of flight feathers
  • Contour feathers (pennaceous feathers) cover the body with symmetrical vanes about a rachis
  • Down feathers (plumulaceous feathers) lack a distinctive rachis and provide insulation
  • Filoplumes are hairlike feathers with soft barbs near the tip, associated with contour feathers for sensory or decorative purposes
• Feathers develop embryologically from feather follicles, invaginations of the epidermis that dip into the underlying dermis

  • The root of the feather follicle, in association with the dermal pulp cavity, begins to form the feather
  • Old feathers are shed (molt), and feather filaments grow out of the follicle due to cell proliferation at the follicular base
  • New epidermal cells form three distinct tissues: a supportive sheath, main feather tissues, and pulp caps to protect the delicate dermal core
• Hairs:

  • Hairs are slender, keratinous filaments with a root at the base
  • The hair shaft consists of a non-living shaft, hair cortex, and hair medulla
  • Hair shafts are produced within epidermal hair follicles rooted in the dermis
  • Chromatophores contribute pigment granules to the hair shaft for color
  • Arrector pili muscle is a thin band of smooth muscle that makes the hair stand erect in response to cold, fear, or anger
• Evolution of Hair:

  • Hair initially arose as surface insulation to retain body heat in primitive mammalian endotherms
  • Hair evolved first as tiny projecting rods between scales and served as tactile devices
  • "Protohairs" could help monitor surface sensory data when animals were hiding or retreating
  • Sensory protohairs might have evolved into an insulative pelage as mammals became endothermic, retaining a sensory function
• Glands:

  • Three main types of integumental glands in mammals are sebaceous, eccrine, and apocrine
  • Sebaceous glands produce an oily secretion, sebum, to condition and waterproof fur
  • Eccrine and apocrine glands are long, coiled invaginations of the epidermis that reach deep into the dermis
  • Sebaceous glands are absent from the palms of hands and soles of feet, but present in other areas like the mouth, penis, vagina, and mammary nipples
• Eccrine Glands:

  • Produce thin, watery fluids
  • Not associated with hair follicles
  • Begin to function before puberty; innervated mainly by cholinergic nerves
  • Associated with the soles of the feet and hands, prehensile tails, and other sites in contact with abrasive surfaces (most mammals)
  • Along with the sebaceous gland, absent in elephants entirely
• Apocrine Glands:

  • Produce a viscous, lipid-containing fluid
  • Associated with hair follicles; begin to function at puberty; innervated mainly by adrenergic nerves
  • Secretions primarily function in chemical signaling
  • Surface evaporation of their products helps to dissipate heat ("sweat glands")
  • Not found in all mammals
  • Human sweat glands derived from eccrine glands
  • Horses from apocrine glands (example of convergent evolution)
  • Sweat contains waste products: elimination of metabolic by-products
• Apocrine Glands:

  • Scent glands
  • Produce secretions that play a part in social communication
  • May be located almost anywhere on the body
  • Secretions are used to mark territory, identify the individual, and communicate during courtship
• Mammary Glands:

  • Produce milk (watery mixture of fats, carbohydrates, and proteins that nourishes the young)
  • Ectodermal mammary ridges, within which mammary glands form, are located along the ventrolateral side of the embryo
  • Lactation: release of milk to a suckling
  • Mammary glands become functional only in females (with rare exceptions)
• Nails:

  • Plates of tightly compacted, cornified epithelial cells on the surface of fingers and toes
  • Products of the keratinizing system of the skin
  • Nail matrix forms new nail at the nail base by pushing the existing nail forward
  • Protect the tips of digits from inadvertent mechanical injury; help stabilize the skin at the tips of the fingers and toes
  • Present only in primates
• Claws:

  • Curved, laterally compressed keratinized projections from the tips of digits
  • Seen in some amphibians and in most birds, reptiles, and mammals
• Hooves:

  • Enlarged keratinized plates on the tips of the ungulate digits
  • Hoof wall: U-shaped and open at the heel, grows out from its base, the germinal region (matrix cells), not from the underlying dermis
  • Wedge-shaped frog
  • Sole: Fills the ground surface space between the wall and triangular frog, consists of epidermis and thickened dermis (corium of the sole), fatty digital cushion or pad deep to the sole; derivative of the hypodermis
• True Horns:

  • Found among members of the family Bovidae (e.g., cattle, antelope, sheep, goats, bison, wildebeests)
  • Both males and females, retained year-round, and continue to grow throughout the life of the individual
  • Unbranched and formed of a bony core and a keratinized sheath
  • Males' are designed to withstand the forces encountered during head-butting combat
  • Present in female of large species; absent in small species
• Baleen:

  • Plates formed by the integument within the mouths of mysticete whales
  • Act as strainers to extract krill from water gulped in the distended mouth
  • Contains no bone; series of keratinized plates that arise from the integument