Histology and Ontogeny of
Scales, Feathers and Hair
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
Integument - Structure and Development

Epidermis
– Stratified, squamous
– Ectoderm

Basement
Membrane
– Fibrils

Dermis
– Fibrous connective
tissue
– Mesodermal
dermatome of the
somites
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
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”
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
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
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


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”)
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
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
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
Types of Bony Scales

Cosmoid plates and scales
– Ostracoderms
– Cosmine – “dentine”

Ganoid plates and scales
– Actinopterygian fishes
– Bichirs (Polypterus), Garpikes
Types of Bony Scales

Modern
– Cycloid and Ctenoid
– Most teleosts
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
Feathers

Most conspicuous
integumental derivative

Keratin

Function
– Flight
– Heat Conservation
 Reduced convective and
evaporative heat loss
 Increased insulation
Feathers

Types
– Contour feathers
 Flight feathers on tail
and wings
– Down
– Filoplumes
Rachis
Calamus
 Barbs reduced or lost
 Rictal bristles
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
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
Feather morphogenesis

Contour feather
movie from
John Fallon’s
lab at UWisc
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
Hair

Keratinized derivative
of mammalian skin

Function
– Protective
– Insulation
– Tactile
Hair

Structure
– Shaft of dead,
cornified cells
– Base is
embedded in
follicle in the
dermis
– Follicle wall
composed of
epidermal cells
Hair

Development






Shaft
Follicle
Epidermis
Dermis
Root
Hair papilla
Cuticle
 Cortex
 Medulla
 Air vacuole

Hair
Hair
Hair derivatives
• Antlers – Cervidae
– Bony outgrowths, shed
annually.
• True horns – Bovidae
– Bony cores covered
with horny sheaths;
permanent.
•
•
•
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.