Histology and Ontogeny Histology and Ontogeny of Scales, Feathers and of Scales, Feathers and

HairHair

The IntegumentThe Integument ““Jack-of-all-Trades” organ system Jack-of-all-Trades” organ system

Forms interface b/w the internal and Forms interface b/w the internal and external environmentexternal environment

– HomeostasisHomeostasis– Protection from abrasionProtection from abrasion– Buffers drastic environmental changeBuffers drastic environmental change

Na, HOH, UVNa, HOH, UV– Exchange of chemicals and gases Exchange of chemicals and gases

(receptors)(receptors) ProtectiveProtective CommunicativeCommunicative NutritiveNutritive

– ColorationColoration ProtectionProtection CommunicationCommunication

Structure and Function correlated with an Structure and Function correlated with an organisms life style and environmentorganisms life style and environment

Integument - Structure and Integument - Structure and DevelopmentDevelopment

EpidermisEpidermis– Stratified, Stratified,

squamoussquamous– EctodermEctoderm

Basement Basement MembraneMembrane– FibrilsFibrils

DermisDermis– Fibrous connective Fibrous connective

tissuetissue– Mesodermal Mesodermal

dermatome of the dermatome of the somitessomites

Integument - Structure and Integument - Structure and DevelopmentDevelopment

Stratum germinativumStratum germinativum

– CuboidalCuboidal– Mitotic DivisionMitotic Division– Migrate DistallyMigrate Distally– DifferentiateDifferentiate– Sloughed offSloughed off

Synthesis of keratinSynthesis of keratin

– Water insoluble Water insoluble protein that fills cellsprotein that fills cells

– Stratum corneum of Stratum corneum of VertebratesVertebrates

Integument - Structure and Integument - Structure and DevelopmentDevelopment 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, Blood vessels, nerves, pigment cellspigment cells

– Endotherms – bases of hair Endotherms – bases of hair and feathers + erector and feathers + erector musclesmuscles

““Ancient and persistent potential to form Ancient and persistent potential to form bone”bone”

Interaction between dermis and Interaction between dermis and epidermisepidermis

Bony scalesBony scales Horny scalesHorny scales FeathersFeathers HairHair Other skin derivatives – claws, antlers, horns, Other skin derivatives – claws, antlers, horns,

baleenbaleen

Cells that form the skin layersCells that form the skin layers

– Respond to inductive influences of adjacent cells and to Respond to inductive influences of adjacent cells and to environmental influences.environmental influences.

Neural crest cells migrating between epidermis and dermis Neural crest cells migrating between epidermis and dermis signal the building of these derivativessignal the building of these derivatives

Integument of FishesIntegument of Fishes

Fish are aquaticFish are aquatic– Conservation of structure and function among Conservation of structure and function among

the different fish groups.the different fish groups.– Bony scalesBony scales

Characterize the skin of most fishesCharacterize the skin of most fishes Substances (tissues) that contribute to bony scalesSubstances (tissues) that contribute to bony scales

– BoneBone– DentineDentine– EnamelEnamel

Function of bony scalesFunction of bony scales

Protection from parasites, other predatorsProtection from parasites, other predators Positioning: Positioning:

– Slide one atop the nextSlide one atop the next– Allow for the distortion of bodyAllow for the distortion of body

Hydrodynamic function to reduce dragHydrodynamic function to reduce drag FeedingFeeding

Cellular BoneCellular Bone

– Extra-cellular matrix of collagen fibers Extra-cellular matrix of collagen fibers – Embedded in polysaccharide ground substanceEmbedded in polysaccharide ground substance– Matrix laid down by osteoblasts Matrix laid down by osteoblasts

Differentiate from mesenchyme cells of the dermisDifferentiate from mesenchyme cells of the dermis

– Calcium phosphate crystals (hydroxyapatite) bind to Calcium phosphate crystals (hydroxyapatite) bind to fibersfibers

– During osteogenesisDuring osteogenesis Osteoblasts mature and become entrapped in matrix – Osteoblasts mature and become entrapped in matrix –

osteocytesosteocytes Osteocytes are located in Osteocytes are located in lacunaelacunae

– Small cavities interconnected by canals (canaliculi)Small cavities interconnected by canals (canaliculi)– Cell processes are extended through these canalsCell processes are extended through these canals

Bone of fish scales Bone of fish scales – ““dermal bone” (cf. “cellular bone”)dermal bone” (cf. “cellular bone”)

Bony scalesBony scales

Bone is deposited by osteocytes on the Bone is deposited by osteocytes on the periphery of a developing scaleperiphery of a developing scale

Osteocytes move centrifugally away Osteocytes move centrifugally away from center of scalefrom center of scale

No bone cells or processes are left No bone cells or processes are left behindbehind

Then dentine and enamel layers can be Then dentine and enamel layers can be added to the surface of the bone for added to the surface of the bone for increased hardness of scaleincreased hardness of scale

Dentine and EnamelDentine and Enamel Mesenchymal aggregation (papilla) beneath basement Mesenchymal aggregation (papilla) beneath basement

membranemembrane Basal cells above the papillae respond and differentiate into Basal cells above the papillae respond and differentiate into

ameloblastsameloblasts– Ameloblasts – collectively called the enamel organAmeloblasts – collectively called the enamel organ– Secrete enamelSecrete enamel– Retreat Retreat

Underlying dermal cells differentiate into Underlying dermal cells differentiate into odontoblastsodontoblasts – secrete dentinesecrete dentine– Retreat in direction opposite of ameloblastsRetreat in direction opposite of ameloblasts– Leave long cytoplasmic processes – dentine tubulesLeave long cytoplasmic processes – dentine tubules

Types of ScalesTypes of Scales

Four general types: Four general types:

Placoid scales – paleozoic Placoid scales – paleozoic sharks, elasmobranchs sharks, elasmobranchs

– Also called dermal denticlesAlso called dermal denticles

– Spinous processSpinous process From dermisFrom dermis

– Dentine, surrounding a Dentine, surrounding a vascular pulp cavity and vascular pulp cavity and capped by enamelcapped by enamel

– Become teeth at jawsBecome teeth at jaws

Types of Bony ScalesTypes of Bony Scales Cosmoid plates and Cosmoid plates and

scales scales – OstracodermsOstracoderms– Cosmine – “dentine”Cosmine – “dentine”

Ganoid plates and scalesGanoid plates and scales– Actinopterygian fishesActinopterygian fishes– Bichirs (Bichirs (Polypterus)Polypterus), ,

GarpikesGarpikes

ModernModern– Cycloid and CtenoidCycloid and Ctenoid– Most teleostsMost teleosts

Types of Bony ScalesTypes of Bony Scales

Horny ScalesHorny Scales Scales of reptilesScales of reptiles

– ““scale”scale” Many layers of cellsMany layers of cells Thick stratum corneumThick stratum corneum

– Keratin with phospholipidsKeratin with phospholipids Outer epidermal generationOuter epidermal generation Inner epidermal generationInner epidermal generation

– FunctionFunction ProtectionProtection Reduce water lossReduce water loss

FeathersFeathers

Most conspicuous Most conspicuous integumental derivative integumental derivative

KeratinKeratin

FunctionFunction– FlightFlight– Heat ConservationHeat Conservation

Reduced convective and Reduced convective and evaporative heat lossevaporative heat loss

Increased insulationIncreased insulation

FeathersFeathers TypesTypes– Contour feathers Contour feathers

Flight feathers on Flight feathers on tail and wingstail and wings

– DownDown– FiloplumesFiloplumes

Barbs reduced or Barbs reduced or lostlost

Rictal bristlesRictal bristles

Calamus

Rachis

FeathersFeathers

Development triggered by an interaction Development triggered by an interaction b/w epidermis and dermal mesenchymeb/w epidermis and dermal mesenchyme

Formation of dermal papilla (placode)Formation of dermal papilla (placode)

Mitotic divisions in a collar zone of the Mitotic divisions in a collar zone of the stratum germinativum near the base of stratum germinativum near the base of the papilla form a crown of barbsthe papilla form a crown of barbs

Covered by a horny sheath of epidermisCovered by a horny sheath of epidermis

FeathersFeathers As development proceeds:As development proceeds:

– Differential cell division on one side of the papillaDifferential cell division on one side of the papilla Timing of expression of two proteins: Shh & Bmp2Timing of expression of two proteins: Shh & Bmp2

– These cells form a shaft away from the bodyThese cells form a shaft away from the body

carrying the barbs that are formed in the collarcarrying the barbs that are formed in the collar

– The base of the feather recedes into the skin The base of the feather recedes into the skin Accompanied by layers of epithelial cellsAccompanied by layers of epithelial cells Feather follicleFeather follicle

– Degeneration of epidermal sheathDegeneration of epidermal sheath

Feather morphogenesisFeather morphogenesis

Contour feathContour feather movie froer movie from John Fallonm John Fallon’s lab at ’s lab at UWiscUWisc

Cross-section of feather Cross-section of feather folliclefollicle

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

HairHair

Keratinized Keratinized derivative of derivative of mammalian skinmammalian skin

FunctionFunction– ProtectiveProtective– InsulationInsulation– TactileTactile

HairHair

StructureStructure– Shaft of Shaft of

dead, dead, cornified cellscornified cells

– Base is Base is embedded in embedded in follicle in the follicle in the dermisdermis

– Follicle wall Follicle wall composed of composed of epidermal epidermal cellscells

HairHair

DevelopmentDevelopment

ShaftShaft FollicleFollicle EpidermisEpidermis DermisDermis RootRoot Hair papillaHair papilla

CuticleCuticle CortexCortex MedullaMedulla Air vacuoleAir vacuole

HairHair

HairHair

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.