A Feather's Tale From tube to plane and back again

Abstract

All birds have feathers and feathers are unique to birds.]!"]" Birds rely upon their feathers for thermoregulation]""], flight[1115], waterproofing]"], communicationj ":"] and more [911.12·11·15]. To an aviculturist, a bird's feathers represent not only the distinctiveness and beauty of the Class Aves but also a significant investment of a bird's nutritional and physical resources[1].

Feathers are important to birds - and to aviculture.

What are feathers and what are they made of? What makes them and how are they made? How are the patterns on feathers made? Summarized here** are some of the amazing answers to these questions, which begin with a unique form of protein and a very special ring.

What is a Feather?

A feather is an epidermal appendage]':"], which is a structure that is formed by the skin. Examples of other kinds of epidermal appendages are claws[1] and beaksf'[trharnphotheca).]"]. Feathers are the most complex and highly organized]"] epidermal appendages found in vertebrates.]"]

What are Feathers Made Of? [0']

"There are no genes for feathers. The information for their production resides in the timing for production of a set of unique protein molecules. This occurs only in feather follicles during well-defined periods of molt. The conversion of the protein into feathers involves a production-linelike series of events. "[1]

Feathers are about 90% protein, most of which is in the form of keratin]"], There are two basic types of keratin:

Alpha-keratin, which forms a helical strand (such as is found in hair) and is very flexible, and beta-keratin, which forms a pleated and folded sheet and is more rigid. All avian epidermal appendages, including feathers, are made from a family of beta-keratin*** molecules called phi-keratins. Phi-keratins have the special ability to self-assemble into filaments (not all members of the beta-keratin family have this ability).

Genes direct the types of phi-keratins produced and the timing and sequence in which they are made; however it is the physical properties of the phi-keratins that determine how they may assemble from the molecular level to filaments to feather fibers]"] (hierarchical organization). By taking advantage of the physical ways in which components 

may assemble, fewer genes are necessary to form complex structures such as feathers]"],

Phi-keratins exist in two sizes, one with larger polypeptides and one with smaller. The larger phi-keratins are found in alligator claws]':"] and in avian beaks and claws. The smaller are found only in downs and feathers of birds[1·8]. Feather phi-keratins have the special quality of being more flexible than other beta-keratins while retaining their strength]' T

The organization of the phi-keratin filaments in epidermal structures is different: In claws and beaks it is interwoven; in feathers it is linear and branching. The components and structure of beaks and claws combine to make them rigid, durable and resistant to wear. Feathers are flexible but are more easily abraded. This susceptibility may have contributed to the evolution of regular replacement of feathers via molt (as opposed to continuous growth, such as occurs in beaks and claws).

Phi-keratins, like other keratins, are manufactured within specialized cells in the skin called keratinocytes]"], The purpose of these cells is to produce and fill with a keratin, bond to each other to form solid structures (such as developing feather barbs) then die (apoptosis), leaving their keratin behind.]"], This is called keratinization. A unique quality of feather keratinization is that the keratinocytes fuse and assume their final shape prior to their death. [5]. The keratin is not shaped after apoptosis as occurs in hair.[5]

What Makes a Feather?[A5] Formation of a Feather Follicle [A5]

"The feather follicle is the complex organ that provides the spatial organization required to growfeathers. 'TJ

When a bird is an embryo, feather follicles form]':"] in tracts (pterylae) in the epidermis. The follicles form by the time the embryos of most birds are twelve days old.[9] In the pterylae, cells of the epidermis and dermis (second layer of skin, below the epidermis) are chemically signaled[4·9] to organize into thickened regions called placodes. [8'9] Placodes elongate into fingerlike projections called feather buds]"] (or feather germsf'[). At this stage, the primary growth zone of the feather bud is at its tip (distal end).[31113]

Proliferation of cells in a ring around the feather bud creates a cylindrical dimple (invagination) into the dermis with the feather bud at its base.]?"] As the dimple descends,

the growth zone and stem cells of the germ]":"] migrate from its distal to its proximal (base) end.]""] The feather germ is now called the dermal papilla, the growth zone is now the follicle collar[13] and the dimple is now the follicle cavity or wall]"]. Together, these are known as the feather follicle[3]. The feather follicles are now permanent features of a bird's skin. Because follicles protect thedermal papillae and stem cells within, they are able to regenerate a virtually unlimited numbers of feathers over time.]"]

The feather follicle might be described as looking something like a test tube with a rounded w-shaped base instead of the typical U-shaped base (the central point being the dermal papilla). It might also be described as looking like a cylinder within a tube. The concept of a feather follicle as a cylinder within a tube may be helpful when visualizing how a feather forms.

Parts of a Feather Follicle["5]

"The structure of the follicle creates a unique cylindrical sandwich of epidermal and dermal tissue layers. This structure permits: (1) continuous interaction between the epidermis and the dermis, (2) indeterminate growth of the epidermis, and (3) continuous nourishment of the epidermis by the dermis ... "[9]

Because the feather follicle and its dermal papilla (tube and cylinder) are formed by the skin, they have layers, as the skin does: outer epidermis, inner epidermis (that interfaces with the dermis), and dermis. Each of these layers is modified to serve a unique function in the formation of a feather:

1 ). The "tube" is the layer of outer epidermis]"] that descends into the dermis and lines the feather follicle (follicle wall)[3].

2a). The "cylinder" is the dermal papilla, which consists of a layer of epidermis covering a core of dermis, "like a thimble covering a finger''[5]. The cells of the outer layer of epidermis will form the feather sheath]"]. The cells of the inner epidermis will form the feather!"]. The epidermal layers are termed the "ramogenic zone" [8.u1•15] from "rami" which means "branch" and "genie" which means "forming."

The ramogenic zone and the growth zone are collectively termed the follicle collar[9].

2b). In addition to generating the ring-like growth zone at its base, the dermal papilla is the source of the feather pulp. As such, the core of the dermal papilla (and the feather pulp) is an extension of the dermis; it is nourished]"] by an artery and has nerve]"] and other connections to the underlying dermis layer of the skin. (So it is easy to understand why blood feathers are sensitive and bleed profusely when broken. The pulp is actually a living part of the dermis).