Vertebrates have developed some intriguing excrescences (projections of the skin). Some of them are feathers, hair and scales—and not much separates them from each other.
Feathers, hair and scales. While it might seem that there’s a lot of difference between these natural “coverings” on animals, they have more in common than you’d probably think.
Despite their multitude of forms, all three of these vertebrate skin appendages share early developmental processes at the embryonic stage.
And now, scientists have achieved a remarkable feat: changing scales into feathers. They’ve discovered how to permanently transform the scales that normally cover the feet of chickens into feathers by modifying the expression of certain genes. This groundbreaking research has profound implications for understanding the evolutionary transitions between species throughout history.
Reptile scales are overlapping layers of skin with an inner and outer coating. Between those layers are more layers that are waxy, which keep reptiles from drying out on land.
A coterie of coverings
The term integument means something that covers or encloses. It applies to any outer covering of an animal. Basically, it means the skin, although many scientists describe the exoskeletons of arthropods as integuments. An exoskeleton is a coating of a hard, protein-type substance that entirely envelops the outside of an animal and provides a place for muscle attachment. In vertebrates, skeletons are internal, and muscles attach from the outside. This permits larger growth of the animal.
Vertebrate animals have developed some very interesting excrescences, or projections of the skin. Some of them are feathers, hair and scales. The primary function of the excrescences is insulation and protection. Claws, hooves and nails are other types of excrescences, but they do not provide the complete body protection an animal gets from the three, main body coverings. All, however, are made from special proteins.
The primary protein used by vertebrates is keratin. It occurs in two forms: alpha (more pliable) and beta (stiffer) keratin. The way in which the keratin is constructed at the molecular level is what determines the structural differences among the various excrescences.
“Archaeopteryx” is a genus of avian dinosaurs. Paleontologists view these winged creatures as an evolutionary link between birds and reptiles, and as a transition between dinosaurs and modern birds.
Let’s look at the three, main body coverings:
• Scales. Scales occur on fish and reptiles and cover their entire bodies. Fish scales are made from more than just keratin. They are often derived from bone in the deeper layers of the skin.
The construction of reptile scales, however, is different. Reptiles have evolved a unique layer in their skins from their aquatic ancestors. For reptiles to live free from water, a waxy layer—the stratum corneum—evolved to keep the animals from drying out on land. These waxy layers lie between the layers that produce the keratin for the scales, which are overlapping layers of skin with an inner and outer coating. A unique adaptation is a basal hinge region from which the scales can be somewhat flexible and fold back without falling off.
• Feathers. It was once believed that only birds had feathers, but later research revealed a striking relationship between birds and dinosaurs. Dinosaur fossils discovered in China near the end of the 20th century revealed a skeleton which is clearly that of a dinosaur, but which has impressions of feathers all over its body.
A feather is a scale in which a long center shaft, called the “rachis,” is the dominant feature. On either side of the rachis, tiny barbs made of keratin hook together and compose the vanes of the feather.
A feather is a scale in which a long center shaft, the rachis, is the dominant feature. On either side of the shaft, the keratin is divided into tiny barbs that, under a microscope, look like the close-knit leaves of a fern. The barbs have tiny hooks on them, which help them attach to one another and keep close together. These interlocked barbs are called the vanes of the feather.
Bird feathers are composed of beta keratin. They occur in tracts along a bird’s skin called pterylae. These tracts are easily seen on plucked chickens or turkeys used for food. It surprises many people to learn that the entire bird is not covered with feathers.
• Hair. Hair is a characteristic of mammals. Hair or fur is made from beta keratin and grows from follicles located all over the epidermis. Its primary function is to provide insulation for warmth. However, there are several kinds of hair that provide sensory functions. For example, whiskers, or vibrissae, are located at places near a mammal’s head. The roots of the vibrissae are connected to nerves that are sensitive to movement and so help an animal to detect its environment.
A hair is basically a cone of keratin. Just like keratin reptilian scales, hair has an overlapping arrangement.
The structure of hair is different from that of scales and feathers. A hair is basically a cone of keratin that is derived from keratinized cells in the dermis, or middle layers of the skin. The hair is generated and formed in a pit in the skin called the follicle. Near the base of each hair, attached to the follicle, is a small muscle called an erector pilli. When stimulated, this muscle contracts and pulls the hair straight up. In humans, this condition is called goose bumps. They come as result of the tightening of the skin, which helps prevent heat loss and serves as a warning gesture in cats, dogs and other mammals.
Whatever the excrescence, the colors and varieties that feathers, hair and scales come in makes spotting and watching animals a thrill and reminds us of Earth’s vast biodiversity.
An ancestor in common
Despite this diversity of forms within and among species, the embryonic development of keratinized skin appendages (hair, feathers and scales) typically begins in a similar way. All these structures develop from cells that produce a localized thickening on the skin surface and that express particular genes.
Brahma chickens have feathered legs, but the genetic determinism of this trait is not yet fully understood. ©Karen Johns, flickr
One of these genes, called sonic hedgehog (Shh), controls a signaling pathway; a communication system that allows the transmission of messages within and between cells. Shh signaling is involved in the development of diverse structures, including the limb buds, the neural tube (the developing brain and spine) and skin appendages.
A research team at the University of Geneva in Switzerland that was studying the biological and physical processes that generate the diversity of skin appendages in vertebrates had previously demonstrated that feathers, hair and scales are homologous structures inherited from a reptilian common ancestor.
Often, feathers of a chicken embryo are used by scientists as a model system to understand skin appendage development. While it’s known that certain breeds of chickens, such as the Brahma and Sablepoot varieties, exhibit dorsal foot surfaces and feathered legs, the genetic determinism of this trait is not fully understood.
While the featherless feet of a chicken look smooth, they are, in reality, covered in scales. The scales are visible, but they sit flat, like a skin. Scientists at Switzerland’s University of Geneva were able to transform the scales on chicken feet into feathers. This confirms that the Shh pathway is involved. ©David Goehring, flickr
So, the University of Geneva research team decided to investigate the potential role of the Shh pathway. Using the classic technique of “egg candling”—in which a powerful light illuminates blood vessels on the inside of the eggshell—the scientists were able to precisely treat chicken embryos with a molecule injected directly into the bloodstream that specifically activates the Shh pathway.
The researchers observed that this single, stage-specific treatment is sufficient to trigger the formation of abundant, juvenile, down-type feathers in areas that would normally be covered with scales. Remarkably, these experimentally induced feathers are comparable to those covering the rest of the body, as they are regenerative and are subsequently and autonomously replaced by adult feathers.
After a comparison with embryos injected with a control solution (without the active molecule), RNA sequencing analysis showed that the Shh pathway is both immediately and persistently activated following injection of the molecule. This confirms that the Shh pathway underlies the conversion of scales into feathers.
We haired humans may not think we have much in common with those covered in feathers or scales. But it appears there’s a common ancestor for all three skin coverings: a reptile that lived 300 million years ago. And that makes us kin, in my book.
Lives that are linked
This research, which was published in the journal Science Advances in May 2023, has important implications for understanding the evolutionary mechanisms generating the enormous diversity of animal forms observed in nature. What we’ve learned is that an evolutionary leap—from scales to feathers—does not require any huge changes in the composition of genes or in how they are expressed.
Instead, a transient change in the expression of one gene, Shh, can produce a cascade of developmental events leading to the formation of feathers instead of scales.
So, while we may not think that we look anything like a chameleon or a cockatoo or that we have any biological link to them, we’re finding out, once again, that we are very much kin.
Here’s to finding your true places and natural habitats,
Candy
