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Anatomy of the Avian Integument

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Anatomy of the Avian Integument 2/5/2008 I. Skin highly specialized, Anatomy of the Avian semisemi--transparent,transparent, Integument elastic organ. In birds the skin is very thin, particularly in Caprimulgiformes, and Strigiformes. A. Layers Skin composed of two tissue layers 1 2/5/2008 1. Epidermis a. Stratum germinativum Derived from embryonic ectoderm separated from dermis by stratum basale 4 layers layer whose cells continuously divide Deep layer of dividing cells, stratum these cells enlarge and form intermediate germinativum,,p and superficial lay er of layer keratinized cells, stratum corneum Stratum intermedium – middle epidermal As cells mature, carried toward surface of skin layer, signs of keratinization (stratum and flatten granulosum name applied in mammals) Once flattened cells become keratinized and die forminggy horny stratum corneum b. stratum corneum Feathered areas – layer 2-3 cells thick horizontally flattened keratinized cells other areas (feet) – much thicker and greatly modified Living cells synthesize keratin 2 2/5/2008 Keratin – 2 types of keratin found in birds • highly resistant to chemical or physical Theta keratin – feathers, scales of legs breakdown Alpha keratin – epidermis between feather • functions as a water barrier follicles •amiidino acid sequence si iilfllbidmilar for all birds 2. Dermis lies between epidermis and subcutaneous Superficial layer tissue divided into superficial and deep layers Deep layer Derived from embryonic mesoderm Dermis very pliable but scattered elastin consists of and collagen throughout. • connective tissue Also thick type of elastin joined together • adipose to form tendons which run to smooth muscle • blood vessels Muscles run throughout dermis and move • smooth muscles feathers • sensory structures 3 2/5/2008 B. Cutaneous Glands • small wax secreting glands in external wall of auditory meatus • small mucous secreting vent glands • large uropygial gland bi-lobed gland located dorsally along the Each gland is lined with secretory cells midline of trunk above base of pygostyle that open into central cavity Shape varies between species and familes Absent in Ostrich, Amazon Parrots and close relatives. When massaged, gland produces droplet of oil. Bird wipes bill on papilla and spread over body 4 2/5/2008 Secretion – lipid based, some proteins and ions Function – • lubricate scales on bill, tarsus • keep feathers pliable Secretion can be • provides waterproofing colored • vitamin D precursors Yellowish in Hornbill’s • antibacterial/antifungal properties Some birds substance can be odorous Green Woodhoopoes use secretion as predator deterrent II. Feather Structure and C. Sweat glands Function Birds do not possess sweat glands Feathers extremely good insulators! Feather phylogeny, ontogeny and Evaporative cooling occurs through molt respitiratory sys tem 5 2/5/2008 A. Phylogeny Origins of feathers puzzling Hampered by false leads!! LiLongisquama New evidence for idea that developmental Long held – processes can provide window into origin of structures 1.feathers evolved from scales through elongation, 2.then scales split 3.finally developed hooked barbules (see previous version of Gill pg 28; now revised pg 91) Recent research in developmental biology To understand this hypothesis first need to refutes this hypothesis understand feather structure and development! Prum and Brush 2002. The evolutionary origin and diversification of feathers. Quarterly Review of Biology 77: 261-295 6 2/5/2008 Distal B. Feather Structure Hooks --barbicelsbarbicels Proximal C. Feather Types 1. Contour – Flight feathers, body feathers, some have aftershaft Owls - ends of flight feathers lack barbules, the edges are softer and reduces the noise they make 7 2/5/2008 2. Semiplumes 3. Down feather - Soft plumaceous No central rachis barbs, no barbicels 4. Bristles 5. Filoplume Tactile structures Provide sensory information - Positioning of feathers - Airspeed 6. Powder feathers Barbs disintegrate into fine powder as feather matures. Thoug ht to a id bi rd i n grooming/waterproofing. Specific function unknown 8 2/5/2008 D. Feather Ontogeny 1. 5th day of incubation Thickening in epidermis – mitotic activity Feathers grow from base not tip (unlike plant growth) Placode Region of actively growing cells base of feather next to body. 2. 8th day placode forms unique elongated tube, papilla (feather germ) begins to form and continues to grow into cone. Base begins to sink creating a circular moat. “Moat” becomes feather follicle. Stratum corneum – feather sheath Stratum germinativum – rachis, barbs, and barbules 9 2/5/2008 Down Feather Development 3. Day 13 – 19 keratinization begins, rachis begins developing on dorsal surface of papilla. Dermal pulp begins to withdraw Barbs begin forming Aftershaft (if present) begins forming Pennaceous Feather Development Dermal papilla remains as regeneration zone. Thyroid and its hormones very important in regulating process. E. Evolution of Feathers – Revisited Feather constructed of epidermal cells but, Prum et al. dermal cells determine type of feather produced! Mechanisms of development can help understand evolutionary sequence. 10 2/5/2008 st Model proposes – unique characteristics Stage 1 – 1 feather, hollow cylinder evolved through series of evolutionary novelties (each stage essential for subsequent stages). Stage 2 – Tuft of unbranched barbs Stage 3a – Feather with unbranched barbs attached to a calamus fused to central rachis Stage 4 – Closed pennaceous vane Stage 3b – Feather with barbs and barbules attached at the base to a calamus 11 2/5/2008 Stage 5 – Closed asymmetrical vane Support for Model 1. Diversity of feathers in modern birds All hypothesized stages are within developmental capacity of feather follicles (no theoretical structures needed!) 2. Molecular findings Shh and Bmp2 – function as signaling molecules Pattern formation genes – Shh, Bmp2 Shh induces cell proliferation genes important for growth of vertebrate limbs, digits, integumentary appendages Bmp2 regulates extent of proliferation (hair, teeth and nails) and fosters cell differentiation i. Shh and Bmp2 expression begins in feather placode (anterior-posterior pattern) – mediates elongation of placode 12 2/5/2008 ii. evolution of a novel pattern of longitudinal stripes of Shh and Bmp2 results in differentiation of epithelium into barb ridges = branched structure Each stage in development has distinct 3. Paleontological findings pattern of Shh and Bmp2 signaling Findings of feathered theropods consistent with model subsequent events dependent on earlier stages! • New evidence damaging to classic hypothesis (– feathers evolved from elongated scales) • rejects hypothesis that feathers evolved for flight • feathers evolved as series of developmental innovations (each may have evolved for different original function) 13 2/5/2008 ¾ insulation F. Molt ¾ water repellency 1. Function ¾ courtship Prime function – replacement of worn ¾ camouflage feathers. ¾ defense As feathers become worn, they loosen in follicle and are dropped Feather forms inside sheath which grows Secondary function – from follicle. Partial molt just before breeding season provides brightly colored courtship plumage. Typical pattern in adults – replace all However, many variations on typical feathers 1x per year (following breeding pattern. season) Some species have partial molts before breeding season HY birds may grow feathers continuously during 1st year. 14 2/5/2008 2. Energy Adelie Penguin – molt all feathers at once. Energy demands are heavy during molt Premolt fattening followed by inactive period. Birds ppgartition molting so it doesn’t Wght loss during molt - overlap with other energy demanding activities. 45 – 53% of premolt mass 190 African species - < 4% showed overlap Migratory species often molt at faster rate 3. Patterns of molt than nonmigratory birds. Feathers don’t fall out randomly but in a symmetrical pattern Long distance migration never performed as long as primary or secondary is Pattern constant within species growing. Primary and secondary feathers numbered. Typical pattern – descending or centrifugal molt (from center out). 15 2/5/2008 Many exceptions to pattern – 4. Terminology Ascending molt (rails) Humphrey-Parkes system Synchronous molt (Anseriformes) Complete molt – molt in which all feathers are replaced Chaotic molt (turaco ’ s) Prebasic molt (late summer early fall) Æ basic plumage (renewed each year) Some species undergo additional molt where some feathers replaced (head, Definitive stage – adult plumage body) often in late winter Prealternate molt Æ alternate plumage New feathers found with old feathers F. Feather Care Feather Care 16.
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