Composition and Structure
It’s all about Keratin
Keratin is a structural, fibrous protein found in vertebrates. Like all proteins, it is formed from amino acid monomers, linked together to form a polypeptide chain molecule. These chains coil together to form macrofibrils, which are deposited inside the cell. Keratin is relatively resistant to chemical and physical damage, in part due to strong disulphide cross-linkages between the monomers in adjacent chains. These linkages stabilize the coils and make keratin insoluble under normal conditions.
Hair, fur and feathers are just a few of many specialized structures composed of keratin. Others include nails, beaks, horns, claws, hooves, quills, whiskers, baleen, turtle shell, and scales. These structures form from the epidermis, the top layer of the skin. New epidermal cells are embedded with keratin that becomes compacted, forming a hardened layer. This layer may be shed, sloughed off, molted, or retained and built up into specialized structures.
Hairs originate from epidermal follicles that grow down into the dermis, the middle layer of the skin. The follicle is separated from the dermis by a membrane that produces cells that are rapidly keratinized to form a hair.
Mammal hairs are composed of three types of cells:
- Scales: flattened, overlapping cells forming the cuticle on the outer surface of the hair. They are highly cross-linked and may crack with extreme stretching and swelling.
- Cortex: long polyhedral spindle-shaped forming the hair’s interior. They are densely packed together and give hairs their flexibility. Hair color is produced by melanin pigments within the cortical cells.
- Medulla: large cells which, if present, form a central open network within the hair that often collapses to form air spaces. They are brittle and ridged, giving stiffness and thermal insulation to hairs.
Fur is made up of guard hairs and underfur. Short, fine, soft underfur is responsible for most of fur’s insulating properties. Long, coarse guard hairs protect the underfur from abrasion and moisture and provide color. In animals adapted for cooler climates such as caribou, Dall sheep, and mountain goats, guard hairs may lack pigment so that they remain hollow to afford further insulation.
Unlike hair, Feathers grow from an outgrowth of the epidermis. Follicles develop as raised papilla filled with dermal cells. Feathers remain attached to the follicle until they are pushed out from below by a new feather.
The anatomy of feathers is surprisingly complex. The central shaft is called the rachis, from which the vane extends outward on either side. The base of the rachis is the calamus or quill. The vane is a sheet composed of parallel barbs that, in some feathers, interlock through the engagement of barbules. Microscopic hooklets on the barbules further enable this interlocking or “zipping” behavior.
Feathers come in diverse forms adapted to different biological purposes. Pennaceous feathers are stiff and mostly flat due to their interlocking barbs. Examples are wing and tail feathers specialized for flight. Plumulaceous feathers are fluffy and disorderly with a short rachis and no hooklets, meaning their barbs do not zip together into a cohesive vane. Examples are downy feathers, which provide insulation for the bird. Pennaceous and plumulaceous barbs often exist together, as on contour feathers, which cover the bird’s body like shingles. The plumulaceous base is insulating while the pennaceous tip is protective and streamlines the animal’s form.
In hair, color is produced by melanin pigments within the cortical cells. In feathers, color is created through two mechanisms that commonly occur together: biopigmentation and structural color.
Biopigments are colored substances found in the feather that are independent of the keratin. These compounds may be produced by the bird directly or derived from the bird’s food.
- Carotenoids are yellow, orange, and red compounds produced by plants and acquired by birds through their diet. For example, certain algae that produce carotenoids are eaten by brine shrimp. Flamingoes eat the brine shrimp and algae, which provides the characteristic pink (red) color to their feathers.
- Melanin granules are produced in skin and feathers and create black, brown, red-brown, and pale-yellow colors. Melanin also gives feathers physical strength and resistance against light damage. Carotenoids and melanin are found throughout many bird species.
- Porphyrins create pink, brown, red, and green feathers. They are found in owls, pigeons, and some poultry.
- Psittacofulvins create bright-red, orange, and yellow feathers. They are found in parrots.
Preen oil sometimes includes biopigments that birds apply to their feathers from their uropygial gland. For example, preen oil that includes a pink pigment will impart a pink tint to white feathers.
Structural colors are optical effects produced as light interacts with keratin tissues.
- Non-iridescent structural colors result when light passing through minute air-filled cavities in the barbs is scattered by pigment particles, producing blues and greens. For example, as its name implies, the indigo bunting is a brilliant blue color. However, if a feather is lit from behind, the blue color disappears because the transmitted light is not scattered.
- Iridescent structural colors are caused by interference of light reflected between layers in highly organized, finely laminated structures in the feather barbules. Hummingbirds and peacocks have iridescent feathers that change colors when rotated.
Non-white feathers with structural color generally still contain biopigments. The structural arrays that scatter light in colored feathers can be made up of melanin, air, or both. For example, greens often contain both structural blue and yellow carotenoids. White feathers are the only feathers without pigment, and they are weaker as a result.
As keratin ages, the disulphide cross-linkages begin to break, leaving the amino acid side chains exposed and the keratin structure more susceptible to chemical and physical damage. Damage is seen as a reduction in tensile strength, an increased tendency for breakage, and an increased swelling response to water.
Hollow hairs and white or light-colored feathers/hairs are particularly susceptible to deterioration. In fur, structural damage manifests most often as broken hairs.
In feathers, damage results in an increase in barb loss and disengagement as barbules are lost, particularly at the tips. Loss may be asymmetrical, meaning that in feathers, the distal barbules are lost preferentially first, resulting in a patchy, thinned appearance. These fine damages contribute to barbs that cannot zip back together.