First, and foremost, it holds the animal together. In doing this, the skin forms a boundary between the animal and its environment, and thus helps to regulate what enters the vertebrate body. Good things that might enter include heat, water, O2 and other small inorganic or organic nutrients. Bad things are legion, and include ultraviolet radiation, pathogens, toxins, water, claws, and teeth. The skin also regulates what exits the body, including water and CO2.
The skin protection from predators goes back to the dermal skeleton of the earliest vertebrates and includes both tooth-like scales and bony plates.
In the truly terrestrial tetrapods, the amniotes, the highly keratinized superficial layer of the skin resists water loss. This keritinized skin allowed the evolution of two types of appendage that helped to maintain heat and thus, allow the evolution of endothermy. These appendages are hair and feathers. Modified feathers allow flight. Modified hair forms horns.
The skin, being part of the skeleton, transmits forces from muscles for behaviors like swimming in sharks and frowning in humans. The skin of the fish fin is expanded for swimming while skin of the wings of bats is expanded for flight.
Eccrine (sweat) glands derived from the superficial layer of the skin offer one way to rid the body of head. A different typeof gland derived from the superficial layer of the skin has been modified to supply milk to the nursing infants of mammals.
Finally, several chemical and structural features of skin allow it to act like a billboard, advertising reproductive cues to the opposite sex or warning signals to predators.
The skin is a composite structure:
- epidermis - superficial, multilayer epithelium with cells that synthesize keratin
- basement membrane - material secreted by epidermis and dermis
- dermis - deep layer of dense connective tissue (mostly collagen)
note that the hypodermis is a loose connective tissue between the dermis and the deep muscle. Gross anatomists call this the superficial fascia
Primitive extinct vertebrates present an array of dermal features including:
We don't know which came first but most biologists think it was the odontodes. Dermal bone may just be an odontode without the development of the dentine and enamel.
Odontodes have mostly dissappeared, but the placoid scales of sharks are essentially odontodes without the bony base (see figure).
teeth have only been lost in a few groups (turtles, birds)
dermal bones tend to be restricted to the face and anterior part of the braincase.
Polypterids, a primitive actinopterygian, have modified scales called ganoid scales, which have a layer of dentine formed by many odontode likeelements.
Lepisosteids (gars), also primitive actinopterygians, have ganoid scales but these lack the dentine layer.
Derived actinopterygians, the lungifsh, and the coelocanth have elasmoid scales, which are very thin and constructed of a deeper, dense layer of collagen and a more superficial layer of a calcified (bony) organic matrix. The ganoid scales of polypterus seem to start out developmentally as elasmoid scales. So maybe elasmoid scales are paedomorphic ganoid scales. Importantly all these fish scales are dermal features and not at all like the scales of reptiles and bird feet.
The fins of fishes are supported by bony rays called lepidotrichia, which are also dermal skeletal features but how these are derived (and if) from odontodes is unknown).
The first tetrapods developed a novel feature in the epidermis, the stratum corneum, which is the most superficial layer of the epidermis and is characterized by cells that are filled with keratin and that secrete hydrophobic lipids. This layer is thin in amphibians but is much thicker in amniotes. This creates a tough, largely impermeable barrier to water, which was pretty important for becoming fully terrestrial.
Reptiles evolved an epidermal scale with cells supported by two types of keratin (alpha and beta). Beta keratin is a synapomorphy of reptiles and is not found in other vertebrates. The cornified superficial layer has the beta keratin while the softer, deep layer has the alpha keratin. The soft layer provides a hinge that allows the harder outer layer to overlap with neighboring scales.
One group of therapod dinosaurs elaborated the scales, which became feathers.
http://www.peabody.yale.edu/exhibits/cfd/CFDintro.html
Feathers are epidermal structures with a follicle, which is a part of the epidermis that has invaginated into the dermis and provides a protective house for the germinal layer of cells (the cells differentiating into the feather). This geometry also creates a structure that develops deep and is pushed superficially, out of the body, as it grows.
Development of a feather follicle
We don't know if the earliest synapsids (leading to mammals) had scales similar to those found in reptiles. But at some point, the epidermis evolved the ability to differentiate into hair. Like feathers, hair develops from a follicle, so it grows out of the body.
Mammals also have different epithelial glands of epidermal origin. These incluede the sebaceous gland, which produce an oily secretion (sebum) into the hair follicle, and the sweat glands, which produce sweat. Humand and apes have a particularly large density of sweat glands. Mammary glands are modified sebaceous glands and produce milk, a nutritious mix of water, fat, carbohydrates, protein, and antibodies.
The color of vertebrate skin can be spectacular. For example
Skin color arises from two different mechanisms, 1) pigments, 2) structural properties that scatter light. Pigmented cells are called chromatophores and these are neural crest derived and migrate to the dermis in fishes, amphibians, and non avian reptiles but to the epidermis in birds and mammals. Melanophores are cells with a dark pigment, melanin. Melanins can be either brown or red. Melanin is concentrated on the superficial side of the nuclei of the deeper cells in the epidermis. Melanin absorbs solar radiation and given its location, this allows it to protect the DNA in the nuclei of the deeper epidermal cells. Sometimes red skin occurs from the heme pigment in hemoglobin found in blood cells in superficial artieries.
Many non-mammalian vertebrates have chromatophores other than melanophores that provide a silvery, reflective appearance (iridophores), or yellows and reds (xanthophores and erythrophores). The red pigments come from carotenoids, which are the pigments that make plant leaves turn red in the fall.
Incoherent scattering occurs when short-wavelength (blue) light is scatterd by disordered particles (generally molecules). Scattering is the "reflection" of light in all directions. Longer wavelength light is less easily scattered. The short wavelength (blue) light reflects back to the observer, who will see some hue of blue. This only works well if the longer wavelengths are absorbed by deeper tissues. This phenomenon is called Raleigh scattering and is the reason the sky is blue.
Coherent scatting occurs when ordered particles scatter light and the light that is reflected back is determined by the phase relationship of the different waves. Waves that are out of phase will destructively interfere whereas waves that are in phase will constructively reinforce each other. It is the color associated with these in phase waves that is presented. Often (but not always), the wavelength will change slightly depending on the orientation of the surface and consequently the hue will change slightly. This produces irredescence.
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