Chicken Feather and Skin Development
From Full Plumes to Naked Necks, Genetics Determine What Today’s Chickens Look Like
Feathers are actually a very complex part of the bird; the development of the feathers and the feather follicles is extremely involved.
By Doug Ottinger – Most of us as children probably enjoyed picking up feathers when we were outdoors playing or walking home from school. It seems that almost every child does. Some of us may have had feather collections or proudly taken feathers to show-and-tell time when we were very young. And there are those of us who never got over that childhood curiosity. We still have to stop and examine feathers when we find them on the ground. I know. I am one of those people.
Feathers are actually a very complex part of the bird. While they will eventually stop growing and fall off of the bird (only to be replaced by a new, growing feather), they start out as a living, growing appendage. There are many diverse types of feathers, each serving a specific purpose.
The development of the feathers and the feather follicles is extremely involved. The follicles, feathers, and skin of the chicken, as well as other birds, start forming during the first few days of embryonic growth. Complex chemical interactions, all dictated by the genes in the newly forming cells, take place in these regions, giving rise to what will become the feathers, in all of their shapes, colors and individual purposes in the life of the bird.
In this series of articles, I will often refer to how frequently avian research (often meaning research on chickens) is carried out as a way to help us understand human medical issues, as well as avian issues. Much of this research links directly to genetics and tissue similarities in many animals, including humans. Researchers are now concentrating on the molecular structures within the cells, in the newest branch of genetics, more commonly known as “genomics.”
In 2004, a group of researchers from two combined departments at the Keck School of Medicine at the University of Southern California, Los Angeles, led by Yu Mingke, published a comprehensive research paper on the entire process of feather follicle development in birds. This group of researchers actually went so far as to term the feather “a complex epidermal organ.”
The feather follicles, which form in conjunction with complex protein and chemical interactions taking place between the layers of forming skin during early stages of embryonic growth, are also semi-complex organs. When looked at under a microscope, you will see many components and parts to each follicle. Each part serves a unique function in the development of the new feather.
So, as we just learned, feathers start as small living organs. There are numerous layers and parts to each feather. Different species of birds may have feathers that differ somewhat, chemically, as well as in physical form to serve the specific needs of that species. The newly forming feather contains a small artery in the middle, as well as several veins, which are all responsible for supplying blood, oxygen and nutrition to the new “feather-organ.”
The different types of feathers on the body, as well as the colors or pigments they have, are all regulated by genetic information, which is permanently implanted into each feather follicle when they are formed..
The feather patterns of a bird are regulated by complex genetic components. These include numerous genes as well as numerous modifying genes on many different chromosomes. Feather growth in birds is also partially regulated by sexual hormones. This is why one will see brightly colored breeding plumage fade out to lighter hues later in the season, or may infrequently see one sex of a bird species develop temporary, or sometimes permanent feathering, of the opposite sex, if there is a disruption in normal hormone balances within the bird.
Feathers serve many purposes for a bird. One obvious purpose is for protection of the skin. Another is for heat retention and insulation in cold weather. The longer wing feathers (primaries and secondaries, for example), as well as the retrices, or tail feathers, make flight possible. Feathers are also used for communication between birds. They can be used to signal welcoming advances, such as in courtship, or can be used to display anger, aggression and repulsion to other birds. One example would be two angry roosters with raised hackle feathers, facing each other, ready to fight.
Color of Feathers and Skin
It would probably be safe to say that no area of poultry genetics has been more studied, or had more articles and books written on it, than the area of color in the feathers, plumage and skin. After all, it is one of the first things we see that draws us to the beauty of a particular breed, or individual bird.
Color, and color patterns, have been, and still are, one of the easiest areas to study and make clear-cut predictions of the outcome. After all, we have almost immediate fruit from our labors. Based on simple dominant and recessive genetic patterns, it only takes a few generations, all workable within just a few years, to usually get what we want. The results may not be perfect, and may require more years of breeding work, but we can usually see where the project is going. The heredity of color and color patterns have been extensively studied and cataloged for well over 100 years. Numerous genetic and breeding books have been written. Many of these contain large sections on color and color-pattern genetics. There are also very nice and informative websites that are almost entirely dedicated to feather and plumage colors and patterns.
It is for these exact reasons that I am not dealing with this in this article. Instead of replicating what has been printed time and again, it is my desire to share information that is lesser known, but can be used as examples of discoveries researchers have found out in more recent years.
Feathers and Skin
Genetic traits like the genetic dominance of feather-barring, sex-linkage and certain color patterns of a bird’s feathers and skin are already well known to many poultry keepers. In this article, I am going to diverge from some of these more common subjects, and talk about two traits — one dominant and one recessive — that give examples of the biochemistry involved in the development of the bird’s feathers and skin. I will keep it as simple as possible. The first example is the dominant Na, or “Naked Neck” gene, found in the Transylvanian Naked Neck breed of chicken. The second example is a lesser-known, recessive gene, sc, or scale-less trait, that causes homozygous carriers (birds that have two of these genes) to be almost bald, over their entire bodies.
In most breeds of chickens, the feathers are distributed in 10 major feather tracts or pterylae. The spaces between these tracts are called “apteria”. In most birds, these apteria carry scatterings of down feathers and semiplumes. However, in the Transylvanian Naked Neck Fowl, there are no down patches or semiplumes in the apteria.
Furthermore, the head tract is free of feathers, as well as feather follicles, except for an area around the comb. There are no feathers on the dorsal surfaces of the neck, except for a few on the spinal tract. The ventral tract is virtually absent, except for the area around the crop, and the lateral feather tracts on the breast are very reduced. When the bird matures, the naked skin area of the neck turns a red color. One researcher, L. Freund, found many similarities between the breed’s bare neck tissue and that of the wattles.
Back around 1914, the first records of genetic studies with these fowl were reported in research papers. A researcher, named Davenport, determined that a singled, dominant gene caused the trait. Later, a researcher, named Hertwig, in 1933, assigned the gene symbol, “Na.” Later, the gene was reclassified by some researchers as semi-dominant.
More recently, the Naked Neck effect was found to be the result of one gene, plus another modifying segment of DNA, or gene, both working together. Two researchers from the University of Edinburgh, Chunyan Mou and Denis Headon, completed much of this later work, most of it within the past 15 years.
Early, it was known that the naked-neck effect was a dominant trait, but the exact biochemical process was not known. After many years and much research in this area, we now have some answers as to what causes this.
From a chemical or molecular perspective, it was determined that the Na gene was the result of a genetic mutation. This mutation causes the overproduction of a feather-blocking molecule, called BMP 12 (short for Bone Morphogenic Protein, number 12). At one point it was thought that that the Na gene acted alone. However, more recent research, mainly done by Mou and his group, found that another segment of DNA, on the same chromosome, working as a modifier, helps cause the overproduction of this chemical. To show how much our understanding of genetics is changing, an increasing number of researchers now refer to the “BMP 12 gene” in research, instead of just referring to the “Na” gene, as has been done for some 80 years.
Here’s some trivia about BMPs: There are at least 20 identified BMPs. Many of these proteins have been determined to be crucial in the development, growth and repair of various body tissues, including connective tissue, skin, tendons and bones. They are also crucial to the development and functioning of the central nervous system. Interestingly enough, BMP 12 is a member of the human BMP family of proteins, and is found in humans, as well as our little friends, the chickens. Essential to the development of tendons and other connective tissues, BMP 12 also works as one of the agents that help retard over-development of hair and feathers in mammals and birds.
Researchers were mystified why the overproduction of BMP 12 only affected certain feather-tracts in the Naked Neck Fowl. Through continuing research, led by Dr. Headon, it was found that retinoic acid, derived from vitamin A, is produced in the skin of the chicken’s neck, head and some of the lower areas surrounding the neck. This acid enhances the molecular effect of BMP 12, causing development of feather follicles to cease. This overproduction happens during the first week of embryonic development while the baby chick is still in the egg. Just this brief period is enough to stop the feather follicle growth and formation.
Here’s just a little more trivia: For any readers interested in the health sciences, intensive studies have been done with BMP 12 within the past 15 years. Extensive research has been done in the areas of using this substance in the healing and repair of the tissues in the tendons. Injections of BMP 12 have been used, and studied in the healing and regeneration of completely severed chicken tendons. In at least one case, tensile strength of the repaired tendon was double that of the normal tendon. These types of studies have given great hope for the repair and healing of human tendon injuries. Again, the lowly little chicken has been used as a forerunner in human medicine.
Back to the Naked Neck fowl: Transylvania Naked Necks are a very interesting breed from the perspective of environmental genetics. They are a bird that has been found to thrive well in hot areas of the world, partially due to a lack of feathers that would otherwise retain excessive body heat. Interestingly enough, they also seem to thrive and do well in cold climates. The nation of Hungary, not exactly known for mild winters, considers the Transylvania Naked Neck, along with five other indigenous breeds, to be national historic and genetic treasure. Flocks of Mottled Naked Neck have been known to exist in this region of the world, for some 600 years. Intensive genetic testing of these indigenous breeds in Hungary, have indicated that they belong to a very well-kept and stable population of birds, that has been fairly free from outside influences or other introduced breeds, for a very long time.
It is not believed by researchers, however, that the breed originated in Hungary. Throughout many of the indigenous chicken populations in the hot and tropical areas of Asia, the Naked Neck, or Na gene, is often found. Some research indicates that the breed may have been brought into the Caspian Basin, from Asia, sometime in the ninth century. As with all studies into these types of things, however, there is more that we don’t know than what we actually do, and many times we can only make educated guesses, or hypothesis, as to what the real story is.
Back in 1954, at least one little featherless baby chick showed up in a hatching of some New Hampshire chicks at the University of California at Davis. To say the least, this happening would become an almost unlimited gold mine for researchers for many years to come.
In my research for this article, I was not able to find how many featherless baby chicks originally hatched, or what the survival rate was. Some of the sources I drew from indicated that there was at least a small group. One other source seemed to indicate that it was only one lone little mutant that inspired the whole breeding project. (Consequently, it is easy to see how even the most basic of information can be lost or skewed in tracking or writing about scientific subjects.) I would suspect that this original information is still somewhere in the research archives at U.C. Davis. If anyone reading this article (including anyone at U.C. Davis) has any information on this original brood, I am asking you to send a short letter to the editor and let us know a little more about it
Many times, mutations such as this prove to be lethal to the animals involved. In this case, however, these birds lived, bred, reproduced, and the offspring are still a major source of study to this day.
This particular strain of chicken is fairly smooth skinned with few feather follicles. The skin develops a red-color in many of the adult birds, similar to the exposed skin of the Naked Neck Fowl. The rudimentary feathers that do exist seem to be concentrated in the thigh area and wing tips. Most of these feathers are severely mutated, however, and are not fully developed. There are a number of other differences present in these birds also. Besides not having feathers, the shanks and feet do not develop scales. It is because of this trait that the responsible gene, as well as the birds, were called “Scale-less.”
Spur growth on the legs is non-existent. The bodies of most of these birds also lack much of the normal body fat, including fat normally found in feather follicles, that other breeds and strains of chickens have. Footpads on the bottom of the feet are also reportedly non-existent in most birds. Because the sc gene is recessive, birds that have these traits, or phenotype, must have two of the genes present in their genome, or genetic makeup (sc/sc).
The gene that causes this condition is a prime example of a mutated gene, and the difference such a mutation can make. By any standards, the change in this gene, as well as the resulting phenotype of the birds, is greater than most mutations that are normally seen. This gene, known as the FGF 20 gene, is responsible for production of a protein called FGF 20 (short for Fibroblast Growth Factor 20). FGF 20 is necessary in the production of both feather and hair follicles in developing birds and mammals.
In naked scale-less having the sc/sc genotype, the FGF 20 genes are actually mutated to the point that the production of 29 essential amino acids is halted, keeping the FGF 20 from interacting with other proteins, all necessary for development of feather follicles in the growing chicken embryo. (These extreme types of mutations that cause a breach in genetic communications are called nonsense mutations.)
The normal interaction between skin layers during embryonic growth is thwarted, thus causing the lack of follicle growth. Due to this, particular strain of bird and the molecular interactions of this genetic abnormality are being studied, in order to gain a better understanding of how skin forms during embryonic growth in many other animals, including human beings.
One of the foremost researchers with these fowl is Professor Avigdor Cahaner, at the Rehovot Agronomy Institute, near Tel Aviv, Israel. Dr. Cahaner has spent years developing birds that can survive and function in extremely hot areas of the world. Many of his genetic trials involve these birds. One benefit cited is the fact that the growing birds can cool down and get rid of body heat more easily. Rapidly growing broilers produce vast amounts of body heat. In extremely hot areas of the globe, even brief periods of additional heat can cause mortality losses between 20 and 100 percent. Reported feed consumption is also markedly less, due to the fact that feathers are almost all protein, and it takes a lot of protein in the feed just to make the feathers. Another benefit cited: is the water conservation during feather removal. Commercial plucking uses voluminous amounts of water. This can be a significant waste of resources in arid regions of the world.
The birds’ lack of extra body fat is also of interest to some of those interested in creating healthier food sources.
Experimental work with birds holding the Naked Neck gene is also being conducted by the same researchers. This genetic trait also holds promise for extremely hot areas in the world.
Dr. Cahaner and his colleagues are not without their share of critics, however. Some see the entire idea of mutated featherless birds as a demented project of mad scientists run amok. There are some definite problems that the birds experience. One is potential sunburn if raised in outdoor areas. Another comes from problems present in natural mating.
There are definite mobility problems for the rooster when mounting the hen. Feathers on the hen’s back also protect her from skin damage from the rooster’s claws during the mating process.
Some critics have concerns about skin damage to all birds. There are also no feathers to protect the birds from insect bites. And such birds raised in small free-holder systems in the developing world cannot fly, and thus are more prone to be killed by predators. There is also concern about mobility problems in the legs and feet because of the absence of cushioning footpads.
Will we ever see featherless chickens become an item of interest and fancy, eventually gaining enough support, to be admitted to the American Standard of Perfection? Who knows? I won’t even venture a guess on that one. There are already hairless dogs and hairless cats, both of which currently hold a place in the showring. My best remark on that one is to just say, “Never say never.”
This article has been a little longer than some, so I think it is time to stop. No matter how deep things get scientifically, the most important aspect of keeping poultry, in my view, is the enjoyment we each get from the beauty of our birds, and watching their cute little antics. If your birds are like mine, they rarely complain. However, if they do, you may want to remind them that some chickens don’t even have feathers to wear to bed.
If they don’t believe you, you can read them this article as proof.
Here are a few terms you may encounter in this series of articles, and an explanation for each term:
These are the segments of DNA (deoxyribonucleic acid) found within the nucleus, or center, of a cell. The “genes” are attached to these. These are in pairs in all cells, except the sex cells of an organism. Sex cells (eggs and sperm) only have one chromosome from each pair. The term “chromosome” literally means “colored body,” since these show up under a microscope after a preparatory staining or coloring.
These are actually just shorter appendages of DNA that are attached along the edges of the chromosomes, in a linear order. Working together, the genes hold the blueprint or “instructions” that make up all of the traits in an organism while it is developing — color, skin color, feather color in birds, hair color in mammals, types of combs that chickens have, or color of flowers on a plant.
LOCUS (PLURAL: LOCI)—
This is simply the “location” of where a gene sits on a chromosome. This is a little more technical term, and under most circumstances, most people, including scientists, could really care less where that gene sits along the strand of DNA. In some recent works or reports, one will sometimes see the word locus being substituted for gene. Sometimes you may read something like, “The locus responsible for hair growing in the chicken’s nostrils …” (Hey! I know hair doesn’t really grow in a chicken’s nostrils … it’s just another one of my silly examples.)
Most often used as just another word for “gene.” More correctly, allele refers to a gene that is part of a pair of genes, at the same locus on a chromosome, or pair of chromosomes.
DOMINANT GENE OR DOMINANT ALLELE—
A gene that by itself will cause an organism to have a certain trait. In nomenclature or writing about genetics, they are always designated with a capital letter.
RECESSIVE GENE OR RECESSIVE ALLELE —
Always designated by small letters in nomenclature, these genes require two of them, working together to give an organism a certain trait.
This means that only one of the genes for a given trait is carried by the animal or plant.
Two genes for the same trait, carried by the animal or plant.
The chromosomes that determine an organism’s sex. In birds, designated by Z and W. Males have two ZZ chromosomes, females have one Z and one W chromosome.
A gene attached to either the Z or the W sex chromosome. In birds, most sex-linked traits are due to a gene on the male, or Z chromosome.
Any chromosome, other than a sex chromosome.
This refers to differing sex chromosomes carried by an organism. For example, in chickens, the female is heterogametic. She has both a Z (“male” sex chromosome) and a W (“female” sex chromosome) in her genome, or genetic makeup.
This means that the organism carries two of the same sexual chromosomes. In chickens, males are homogametic, as they carry two Z chromosomes in their genome.
A reproductive cell. Can be either an egg or a sperm.
Same as a gamete.
A change in the actual molecular structure of a gene. These changes can be either good or bad. Such a mutation may then make a physical change in the actual structure of the new organism.
These are genes that, when present in a homozygous state, usually cause the organism to die during development, or shortly after hatching or birth.
The whole big picture of all the genes and chromosomes put together, in an animal or plant.
The study of genetics and a cellular and molecular level.
This refers to the total number of chromosomes in an organism. For example, chickens have 39 pairs of chromosomes in all cells, except the gametes. Since chromosomes normally come in pairs, the scientific “diploid” number for the chicken is 78.
This refers to the number of chromosomes in a sex cell or gamete. There is only one half of each chromosomal pair in an egg or sperm. Consequently the “haploid” number of the chicken is 39.
This is a gene that, in someway, modifies or changes the effects of another gene. In reality, many genes work on each other, to a certain extent, as modifiers.
This refers to the actual genetic makeup in an organism’s cells.
This refers to what the animal or plant actually looks like.
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Yong, Ed, How the Transylvanian Naked Neck Chicken Got Its Naked Neck, blogs.discover magazine.com March 15, 2011.
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Hall, Brian K., Bones and Cartilidge: Developmental and Evolutionary Skeletal Biology, second edition, Academic Press, Elsevier, Inc., 2015.
http://genesdev.cshlp.org/content/27/450.long FGF 20 governs formation of primary and secondary dermal condensations in developing hair follicles.
Yu, Mingke, et al., The developmental biology of feathered follicles (2004), http://www.hsc.usc.edu/~cmchuong/2004/DevBiol.pdf.
Ajay, F.O., Nigerian Indigenous Chicken: A Valuable genetic Resource for Meat and Egg Production, Asian Journal of Poultry Science, 2010, 4: 164-172.
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