Histology and Ontogeny of Scales, Feathers and Hair

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Histology and Ontogeny of Scales, Feathers and
Hair
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The Integument

• Jack-of-all-Trades organ system
• Forms interface b/w the internal and external
  environment
• Homeostasis
• Protection from abrasion
• Buffers drastic environmental change
• Na, HOH, UV
• Exchange of chemicals and gases (receptors)
• Protective
• Communicative
• Nutritive
• Coloration
• Protection
• Communication
• Structure and Function correlated with an
  organisms life style and environment

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Integument - Structure and Development

• Epidermis
• Stratified, squamous
• Ectoderm
• Basement Membrane
• Fibrils
• Dermis
• Fibrous connective tissue
• Mesodermal dermatome of the somites

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Integument - Structure and Development

• Stratum germinativum
• Cuboidal
• Mitotic Division
• Migrate Distally
• Differentiate
• Sloughed off
• Synthesis of keratin
• Water insoluble protein that fills cells
• Stratum corneum of Vertebrates

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Integument - Structure and Development

• Beneath the Basement Membrane (Dermis)
• Proximal Migration and differentiation of of
  collagen fibers and other structures
• Two layers
• Stratum laxum (spongiosum)
• Stratum compactum
• Blood vessels, nerves, pigment cells
• Endotherms bases of hair and feathers erector
  muscles

• Ancient and persistent potential to form
  bone

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Interaction between dermis and epidermis

• Bony scales
• Horny scales
• Feathers
• Hair
• Other skin derivatives claws, antlers, horns,
  baleen
• Cells that form the skin layers
• Respond to inductive influences of adjacent cells
  and to environmental influences.
• Neural crest cells migrating between epidermis
  and dermis signal the building of these
  derivatives

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Integument of Fishes

• Fish are aquatic
• Conservation of structure and function among the
  different fish groups.
• Bony scales
• Characterize the skin of most fishes
• Substances (tissues) that contribute to bony
  scales
• Bone
• Dentine
• Enamel

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Function of bony scales

• Protection from parasites, other predators
• Positioning
• Slide one atop the next
• Allow for the distortion of body
• Hydrodynamic function to reduce drag
• Feeding

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• Cellular Bone
• Extra-cellular matrix of collagen fibers
• Embedded in polysaccharide ground substance
• Matrix laid down by osteoblasts
• Differentiate from mesenchyme cells of the dermis
• Calcium phosphate crystals (hydroxyapatite) bind
  to fibers
• During osteogenesis
• Osteoblasts mature and become entrapped in matrix
  osteocytes
• Osteocytes are located in lacunae
• Small cavities interconnected by canals
  (canaliculi)
• Cell processes are extended through these canals
• Bone of fish scales
• dermal bone (cf. cellular bone)

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Bony scales

• Bone is deposited by osteocytes on the periphery
  of a developing scale
• Osteocytes move centrifugally away from center of
  scale
• No bone cells or processes are left behind
• Then dentine and enamel layers can be added to
  the surface of the bone for increased hardness of
  scale

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Dentine and Enamel

• Mesenchymal aggregation (papilla) beneath
  basement membrane
• Basal cells above the papillae respond and
  differentiate into ameloblasts
• Ameloblasts collectively called the enamel
  organ
• Secrete enamel
• Retreat
• Underlying dermal cells differentiate into
  odontoblasts
• secrete dentine
• Retreat in direction opposite of ameloblasts
• Leave long cytoplasmic processes dentine tubules

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Types of Scales

• Four general types
• Placoid scales paleozoic sharks, elasmobranchs
• Also called dermal denticles
• Spinous process
• From dermis
• Dentine, surrounding a vascular pulp cavity and
  capped by enamel
• Become teeth at jaws

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Types of Bony Scales

• Cosmoid plates and scales
• Ostracoderms
• Cosmine dentine
• Ganoid plates and scales
• Actinopterygian fishes
• Bichirs (Polypterus), Garpikes

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Types of Bony Scales

• Modern
• Cycloid and Ctenoid
• Most teleosts

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Horny Scales

• Scales of reptiles
• scale
• Many layers of cells
• Thick stratum corneum
• Keratin with phospholipids
• Outer epidermal generation
• Inner epidermal generation
• Function
• Protection
• Reduce water loss

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Feathers

• Most conspicuous integumental derivative
• Keratin
• Function
• Flight
• Heat Conservation
• Reduced convective and evaporative heat loss
• Increased insulation

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Feathers

• Types
• Contour feathers
• Flight feathers on tail and wings
• Down
• Filoplumes
• Barbs reduced or lost
• Rictal bristles

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Feathers

• Development triggered by an interaction b/w
  epidermis and dermal mesenchyme
• Formation of dermal papilla (placode)
• Mitotic divisions in a collar zone of the stratum
  germinativum near the base of the papilla form a
  crown of barbs
• Covered by a horny sheath of epidermis

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Feathers

• As development proceeds
• Differential cell division on one side of the
  papilla
• Timing of expression of two proteins Shh Bmp2
• These cells form a shaft away from the body
• carrying the barbs that are formed in the collar
• The base of the feather recedes into the skin
• Accompanied by layers of epithelial cells
• Feather follicle
• Degeneration of epidermal sheath

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Feather morphogenesis

• Contour feather movie from John Fallons lab at
  UWisc

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Cross-section of feather follicle

1. Barb ridges of epithelial
2. Surrounding dermal core of connective tissue
3. Space of the follicle
4. Epithelial tissue of follicle
5. Associated musculature

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Hair

• Keratinized derivative of mammalian skin
• Function
• Protective
• Insulation
• Tactile

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Hair

• Structure
• Shaft of dead, cornified cells
• Base is embedded in follicle in the dermis
• Follicle wall composed of epidermal cells

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Hair

• Development

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• Shaft
• Follicle
• Epidermis
• Dermis
• Root
• Hair papilla

• Cuticle
• Cortex
• Medulla
• Air vacuole

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Hair
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Hair
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Hair derivatives

• Antlers Cervidae
• Bony outgrowths, shed annually.               
• True horns Bovidae
• Bony cores covered with horny sheaths permanent.

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• SLIDE 35. BMP2 and SHH expression in the scale
  and feather rudiments. (Harris et al., 2002)
• What about the creation of a new morphological
  structure--a novelty. Avian feathers have long
  been proposed as an evolutionary novelty. But the
  mechanism to produce feathers has remained
  elusive. However, a paper recently published from
  John Fallon's laboratory provides a developmental
  mechanism by which feathers can be generated from
  scales. They provide evidence that the
  differences in the expression of sonic hedgehog
  and BMP proteins separate the feather from the
  scale. Both the scale and the feather start off
  the same way, with the separation of BMP2 and
  SHH- secreting domains. However, in the feather,
  both domains shift to the distal region of the
  appendage. Moreover, this pattern becomes
  repeated serially around the proximal distal
  axis. The interaction between BMP2 and Shh then
  causes each of these regions to form its own
  axis--the barb of the feather.
• Matt Harris and others in the Fallon
  laboratory have shown that when you alter the
  expression of Shh or BMP2, you change the feather
  pattern. The results correspond exceptionally
  well to a proposed mechanism of feather
  production from archosaurian scales.