E6 - The skin and the skin’s features in domestic birds

Question 1

General features of avian skin

Answer 1

• The skin is divided into a number of separate areas where the skin has been modified to some extent to be able to carry out special functions:
  
  1. Feathered skin
     
     • Subdivided into tracts, where feathers do or do not grow, and other areas where they do not grow but which appear as such because they are covered by feathers
     • Pterylae: where feathers grow
     • Apteria: where feathers do not grow
  2. Scale covered skin on the lower legs and feets
  3. Hard, horny areas of the beak and toe-nails
  4. Pad of the foot
  5. Skin of the comb and vattles
• The skin is thin and loose (because it is protected by feathers
• Tear easily
• It is poorly suppiled with blood vessels and nerves, causing decreased bleeding from wounds
• Skin color:
  
  • Yellowish over the body
    
    • May be more pigmented on the shanks and feet
  • The skin is paler in productive laying hens, in which the pigment is withdrawn and incorporated in the yolk
• Localized changes in the skin occur during the brooding period for more efficient incubation of the eggs
• Brooding pathces:
  
  • At the beginning of brooding the brooding patch develops due to PRL influence in synergism with estrogens
  • The abdominal skin becomes bare, the epidermis thickens, the connective tissue beneath it becomes swollen and abundant venous network proliferates in it
  • On this skin area the heat transmission needed for the hatching is extremely efficient

Question 2

Scale covered skin

Answer 2

• The scales on the shanks and feet are cornified epidermal patches similar to those of reptiles.
• In waterfowl the three foreward-pointing toes are connected by skin (webbed) to make more efficient sculls
• The spur on the caudomedial surface of the rooster´s shank is used as a weapon. It has an osseous core within a cone of horn

Question 3

Feathered skin and feathers

Answer 3

• Subdivided into tracts, where feathers do or do not grow, and other areas where they do not grow but which appear as such because they are covered by feathers
  
  • Pterylae: where feathers grow
  • Apteria: where feathers do not grow
• Highly specialized epidermal structures
• Feather come in many forms, but they all are made up of the same basic parts, arranged in basic structure:
  
  • Calamus (stiff central chaft) extends into a central rachis, which branches into barbs, and then barbules
  • The barbules have small hooks that interlock with neaby barbules
• Type of feathers:
  
  • Wing feathers
  • Down feathers
  • Tail feathers
  • Contour feathers
  • Semiplume feathers
  • Bristle feathers
  • Filoplume feathers

Wing feathers:

• Uniform windproof sufaces
• Asymmetric with a shorter, less flexible leading edge, preventing midair twisting

Tail feathers:

• Fan-shaped arrangement, supporting precision steering in flight
• In some birds, tail feathers have evolved into showy ornaments that are useless in flight

Contour feathers:

• Covers the body
• Arranged in an overlapping pattern
• The waterproof tips are exposed to the elements
• The fluffy base are tucked close to the body
• Coverts: contour feathers on the wing, shaped into an efficient airfoil by smoothening over the region where the flight feathers attach to the bone

Semiplume feathers:

• Hidden beneath other feathers on the body
• Have developed central rachis, but no hooks on the barbules - creating a fluffy insulating structurs

Down feathers:

• Similar to semiplumes, but with an even looser branching structure
• Down feathers look fluffy because they have loosely arranged microstructure
  
  • Flexible barbs
  • Long barbules
• This arrangement are able to trap body heat
• Concealed by the contour feathers, and create an effective dead air space that insulates the body
• The barbs of the down feathers do not unite to form a closed vane

Filoplume feathers:

• Short simple feathers with few barbs
• Function like mammal whiskers - sensing the position of the contour feathers

Bristle feathers:

• The simplest feathes
• Stiff rachis with lacking barb branches
• Found on the head

Question 4

Feathers

Draw the general structure of feathers

Answer 4

Question 5

Skin of the beak

Answer 5

• The beak is the functional counterpart of the lips and teeth of mammals
• A derivative of the skin and provides a horny cover for the rostral parts of both upper and lower jaws that grows continuously to compensate for natural wear.
• The beak varies among species, according to the diet
• A rich innervation makes the beak sensitive
• In parrot-like birds, pigeons and raptors, the base of the beak is covered by a fleshy membrane (cere) that may enclose the nostrils. The cere of the cock is blue, and that of the hen light brown.
• The beak is composed of two parts:
  
  1. Internal skeleton of bone
  2. A closely attached layer of highly keratinized skin over the bony skeleton
• Rhamphtheca:
  
  • ​The outer surface of the beak consists of a thin, horny sheath of keratin
  • There is a vascular layer between the rhamphotheca and the deeper layers of the dermis, which is attached directly to the periosteum of the bones of the beak
• Nail:
  
  • Horny tissue at the tip of the beak
  • Found in duck, geese, swans
  • Often bent at the tip to form a hook
  • Most species use their nails to dig seeds out of mud or vegetation
• Rictal bristles:
  
  • Stiff hair-like feathers
  • Arising around the base of the beak
  • Found in insect-eating birds
    
    • They may function as a “net”, helping in the capture of flying prey
• Nares:
  
  • ​Two holes leading to the respiratory system

Question 6

Skin of the foot pad

Answer 6

• The skin beneath the toes and the foot is specialised to withstand compression caused by the weight of the bird when standing and abrasion caused when the foot comes in contact with rough surfaces as the bird moves about
• Skin of this area is quite thick but retains a high degree of flexibility

Question 7

Skin of comb, wattles and ear lobes

Answer 7

• Soft ornamental outgrowths of the skin about the head.
• Their dermis is thick and vascular, but the covering epidermis is thin
  
  • They are therefore easily injured and potential portals for infection

Question 8

Uropygial gland

Answer 8

• Two-lobed, pea-sized structrue that develops from the epidermis at the base of the tail above the pygostyle or last vertebra
• It produces and discharges a fatty secretion through a duct opening on a small papilla or pimple located on the surface of the skin
• The secretion of this gland is holocrine and is made from disintegrated cells. As new cells are formed they are pushed towards the centre of the gland where they create the gland secretion
• The secretion is similar to that of mammalian sebaceous glands and appears to carry out a similar function. It is collected by the beak during feather preening and is used to oil the feathers. This is very important to aquatic birds for water proofing but not so to terrestrial birds
• The secretion is carried to the wing and body feathers during preening
• In waterfowl the secretion is important for waterproofing the feathers and insulating the body
• It is one of the only two groups of glands found in the skin of the fowl as far as is known (the other group is located in the skin of the outer ear canal)