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THE INTEGUMENT

THE EPIDERMIS

     The integument is the largest organ of the human body.  It can be divided into accessory structures (such as hair, nails, and glands) and the cutaneous membrane.  The cutaneous membrane is composed of a superficial epidermis (composed of stratified squamous epithelia) and a dermis (composed of connective tissue).  The epidermal cells are filled with the intermediate filament keratin (which also composes hair, feathers, scales, nails, and horns) and are joined by a number of junctions.  Melanocytes of the cutaneous membrane and hair synthesize pigment molecules.  Under the integument, the hypodermis stores adipose.  Comparative anatomy suggests that the human integument evolved through a long series of transitional stages.

     The outer covering of the body of cnidarians is composed of a tissue called epithelia, as is the case in all bilateran animals.  The epithelia which covers the body, known as the epidermis, can be ciliated in primitive animals and used for locomotion. This trait of a ciliated epidermis is known in diverse invertebrates, including basal deuterostomes and chordates.  Acorn worms possess a ciliated epidermis which secretes a mucus film.  (Benito, form Harrison 1997, p. 16).  During the development of enteropneusts, all epidermal cells begin as ciliated cells although later in life variations in the typically dense cilia can occur. (Benito, form Harrison 1997, p. 23)  The epidermis is ciliated in the embryos of Amphioxus, some actinopterygians (including the primitive gar), lungfish, and amphibians (Kemp, 1996). 

   Turbellarian flatworms possess a basement membrane which joins their epithelia to the deeper layer of connective tissue (Rieger, from Harrison, 1991).  In tunicates tight junctions, gap junctions, and desmosomes exist in the epidermis (Burighel, from Harrison, 1997, p. 232).

Planaria

WORM

In Amphioxus, simple cuboidal or columnar epithelial cells form the superficial epithelium over a deeper dermal connective tissue layer which is rich in collagen. Epidermal cells are joined by desmosomes, hemidesmosomes, and perhaps gap junctions.  A basement membrane separates the epidermis from the dermis and the dermis contains fibroblasts (Ruppert, from Harrison, 1997, p. 364-9).  The lamprey epidermis contains keratin proteins as does that of higher vertebrates (Alarcon, 1994).  The pattern of keratinization in lungfish and amphibians is similar to that of amniotes (Alibardi, 2001b).     While in lungfish and actinopterygians, keratin exists throughout the epidermis, there is evidence of increased production in the outermost layers of the epidermis.  However, keratin associated proteins filaggrin and loricrin, are not present in fish as they are in tetrapods (Alibardi, 2003).  The structure and keratinization embryonic skin in alligators is similar to birds (Alibardi, 2001). 

     The epidermis and nervous system are derived from the same embryonic tissue, neuroectoderm.  In mammals, both these tissue types express intermediate filaments: keratins in skin and a variety of other proteins in the nervous system.  Although lampreys possess glial cells which are morphologically similar to astrocytes, they lack the intermediate filaments normally expressed in astrocytes (vimentin and GFAP) and express keratins instead (Merrick, 1995).  

     Gibbons possess one chromosomal site at which keratinocyte growth factor genes are located.  Orangutans have two chromosomal sites, gorillas have four, and chimpanzees and humans both have 5.  The locations of these genes are homologous in the African apes and humans (Zimonjic, 1997).  Birds and most mammals (including Old World monkeys, orangutans, and gibbons) lack multiple copies of KGF-like genes.  The amplification of the KGF genes occurred at the base of the African ape clade (Kelley, 1992). 

    Although fingerprints (dermatoglyphics) can identify each human as unique, these epidermal ridges are not unique to humans.  All primates possess dermatoglyphics on areas they rely on for grip/friction such as hands, feet, the tails of New World monkeys, and the knuckles of apes (Montagna, 1972).

Monkey hand and foot

HAND FOOT

MELANOCYTES

     Although melanocytes function in the skin of vertebrates to determine pigmentation, these cells exist in a number of sites in the body, such as the choroid layer of the retina and the substancia nigra of the brain.  The characteristics of epidermal and dermal melanocytes in many primates are unique to that species (Montagna, 1972).  Melanocytes can be found in viscera in different groups of vertebrates, although their significance is not understood (Montagna, 1971).

      The melanocytes of the epidermis are typically inactive in the potto, a prosimian (Yun, 1966). Old World monkeys vary in their pigmentation from the Celebes ape with its large melanocytes and heavy pigmentation to the rhesus monkey whose small melanocytes contain little if any melanocytes.   In both rhesus monkeys and the prosimian the greater bushbaby, melanocytes are active during development but are inactivated before birth (rhesus) or shortly after (bushbaby).  Chimps can have light or dark skin (Yun, 1966). 

Pigmentation in mammals is controlled by the MC1R, agouti, encoding and agouti-signaling protein (ASIP) genes (Schioth, 2005).

HAIR

spidercaterpillar

     There are a number of groups of organisms which have evolved an epidermal covering of hairlike structures.  Some frogs possess hairlike filaments on their legs.  In some snakes, the ends of scales are tapered and appear hairlike.  There are two families in the fish order  Miripinniformes, one of which (Miripinnidae) possesses a “hair cover” over the surface of its body (Lindbergh, 20).

hair cover
pteranodon
Some pterosaur fossils reveal a coating of fine hair, such as Pteranodon.
In the mammalian lineage, hair is thought to have evolved in the late cynodonts, given that fossils suggest that the amount of adipose and vasculature of the snout was similar to mammals which possess whiskers (Kemp, 1982, p. 248).  In mammals, there are many examples in which different members of a certain group differ in the degree to which they are covered by hair.  While most rhinos and elephants have very little hair, the wooly rhinoceros and wooly mammoths were covered with it.
mammoth
wooly rhino

There are pigs which are covered in hair such as the peccaries of today (a fossil peccary is depicted below); other pigs have less hair.

Some animals are virtually hairless although their relatives possess a normal complement of hair.  Among the rodents, naked mole rats have very little hair.
peccary naked mole rat

Porcupines have obviously specialized some of their hairs to function as defensive quills.  Some porcupines have more quills than others.

porcupine

A number of breeds of dogs have little to practically no hair such as the Xoloitzcuintli (Mexican hairless), American hairless terrier, and Peruvian Inca Orchid hairless dogs.  The hairless Chinese crested breed possesses hair on its head, its tail, and feet but is hairless otherwise. The little lion dog breed has a ruff on its head and neck which is similar to a lion’s mane.  The Shar-pei breed has a pronounced wrinkling of the skin (Cohen, 1989).  Some breeds of cat are also hairless.  Mutations in the zinc finger gene hairless can cause the absence of hair (alopecia universalis) in both humans and mice (Ahmad, 1998).

     Humans are the only primates which lack vibrissae (specialized sensory hairs near the nose, often referred to as whiskers).  Monotremes lack vibrissae and thus it is possible that these structures evolved in therian mammals.  Some have argued that there is a gradual reduction of vibrissae through the higher primates.  For example, the lack of skeletal muscle attachments and smaller follicle size in rhesus might be a sign of reduction which began long before the hominid lineage (Van Horn, 1970).  Some squirrels possess vibrissae on the abdomen while some lemurs and marmosets posses them on the wrist (Montagna, 1971).

     In general, the less hair cover in a mammal, the thicker the epidermis (Montagna, 1971).  Higher apes have reduced the density of the hair cover and chimps and humans have reduced hair cover further (Gibbs, 2002).  Humans are unique in the number of hair follicles innervated and thus the sensation; equivalent to sex skin of nonhuman primates (Montagna, 1972).

There are a diversity of genetic syndromes which involve abnormally high levels of hair growth known as hypertrichosis. In some, the hair can cover the face, ears, shoulders, and other portions of the body and reach the length of about a foot unless shaved. The hair may be short and curly or long and fine. Portraits dating back to the 1500s were painted of individuals with this syndrome (also known as Ambras syndrome). Hypertrichosis universalis, Ambras type has been linked topericentric inversions of chromosome 8. Different genetic syndromes are known which cause hypertrichosis of the elbows (typically associated with short stature), around nipples and other body regions (amaurosis congenital), and on the hands, feet, and face in Bazex-Dupre-Christol syndrome (OMIM, 2007).

 

FEATHERS

parrotpeacock

   The presence of feathers is perhaps the easiest way to identify a bird: all birds have them and no other organism on earth possesses them.  Feathers serve a number of functions: they allow for flight in most birds, they serve as insulation which helps birds maintain their high body temperatures, and their coloration/arrangement may function in courtship or camouflage.  Ruby throated hummingbirds may have as few as 940 feathers while whistling swan had 25,216 (Van Tyne, 1976).  Feather color and arrangements can vary greatly even among closely related birds.

     Most birds arrange the bases of feathers in tracts with spaces in between.  When hatched, some birds possess no feathers (such as kingfishers and woodpeckers), others a small number, and others are coated in down. Young birds develop a juvenile plumage that they will molt to attain their adult plumage. 

Birds do possess scales on their legs and it seems that feathers evolved from modified scales.  The early development of feathers is similar to that of scales  The time of this molt is often about 3 months after hatching but it can be longer (four years in herring gulls).  In adult birds, feathers molt at least once a year.  Feathers are usually molted gradually but some birds (typically aquatic birds which can escape without flight) can molt their feathers all at once.  Some feathers form very fine filoplumes which are hairlike (Van Tyne, 1976).

       How did feathers evolve?  Anatomical, embryological and genetic evidence indicate that birds are more closely related to reptiles (especially crocodilians) than they are to other groups such as mammals.  Birds do possess scales on their legs and it seems that feathers evolved from modified scales.  The early development of feathers is similar to that of scales  (Van Tyne, 1976).

The development of feather branching patterns is controlled by the interaction between Sonic Hedgehog and Bmp2 (Harris, 2005).

A number of theropod fossils indicate that they possessed feathers.

sinosauropteryx protoarchaeopteryx beipiasaurus
longisquama
Feathers also appear on the odd reptile Longisquama whose relationship to birds, if any, has not been established.

GLANDS

   Most epidermal glands of fish are unicellular and secrete mucus. This mucus protects from bacterial infection and allows water to pass over the fish body more smoothly. A few fish, such as sharks and catfish have multicellular glands associated with their spines (Webster, p. 156).   Most glands in amphibians are also simple, mucus secreting glands (Webster, p. 156).  Reptiles possess a variety of glands, although their function is poorly understood.  Higher apes possess an apical lingual gland and higher apes have lost ancestral sternal glands (Gibbs, 2002).  Sudoriferous and apocrine glands only exist in mammals. (Webster, p. 158).

Eccrine/Sudoriferous Glands

     In humans, eccrine/sudoriferous glands secrete a watery sweat allowing for the cooling of the body.  Most mammals have few if any eccrine glands.  Tree shrews and primitive primates have eccrine glands in their hairy skin, Old World monkeys have numerous eccrine glands, and apes have even more with the greatest numbers (other than humans) present in chimps and gorillas.  Their original function was to increase grip and keep epidermis pliable (Montagna, 1971; Montagna, 1972).  Only humans and horses sweat for cooling (Montagna, 1971).  

 

Sebaceous Glands

     Sebaceous glands secrete an oily sebum which functions as a conditioner for both skin and hair.  Sebaceous glands are similar in all mammals.  Smegma and cerumen are stale compacted sebum (Montagna, 1971).  Groups of sebaceous glands can form specialized structures in certain groups such as harderian glands behind the eye, preputial, and inguinal glands in rodents, abdominal glands in gerbils, costovertebral glands of hamsters, inguinal glands of tamarins and marmosets, labial glands of tarsiers, brachial glands of some lemurs, and the meibomian glands on eyelid in all mammals.  Mammalian sebaceous and apocrine glands can secrete pheromones (Montagna, 1971).  Variations in the number of sebaceous glands; only lemurs have as many as humans.  Ringtail lemurs have large sebaceous glands over their clavicles.   (Montagna, 1972). 

Apocrine Glands

     Humans and pigs possess apocrine sweat glands which open onto the skin as adults but as embryos they possess the typical arrangement of apocrine glands opening into hair follicles.

Apocrine sweat glands are clustered over the sternum in orangutans and some New World monkeys, the antecubital region in lorises, and many prosimians around genitalia.  Only gorillas, chimps, and humans possess an axillary organ with apocrine glands in the armpit (Montagna, 1971). 

    The breasts and navel have apocrine glands, in addition to those found in the armpit and groin.  Although apocrine glands develop in embryos, they aren’t functional until after puberty.  Apocrine secretions are odorless until microbes metabolize them (Hohl, 2001).

In humans, a large number of apocrine sweat glands develop all over the body early in life which later degenerate (Montagna, 1972). 

In mammary glands of pregnant mice, adipocytes are able to transdifferentiate into secretory epithelial cells.  The reverse change occurs after pregnancy (Moroni, 2004). In mice, males produce pheromones from their extraorbital lacrimal gland which stimulate vomeronasal neurons in females (Kimoto, 2005).

cats

     Milk is a form of modified apocrine sweat which is rich in proteins and fats.  In the platypus, milk simply oozes onto the surface of the abdomen and the young lick it off of tufts of hair. 

     Enamel matrix proteins (EMPs), caseins, and proteins in saliva belong to a family of genes known as secretory Ca-binding phosphoproteins, most of which are located in the same cluster .  EMPs include amenogenin, ameloblastin, and enamelin.  Milk may originally have served in the protection of eggs from microbes.  Casein is only known from mammals. As milk began to be used as a food source in mammals, other genes such as α-lactalbumin (which resulted from a duplication of lysozyme) were also expressed in milk (Kawasaki, 2003).

 

SCALES

Scales are folds of the integument. Epidermal scales are primarily composed of epidermal keratinwhile dermal scales are primarily comosed of dermal tissues (such as bone). Ancestral jawless fish evolved a series of scales along their bodies which were comosed of a dermal bone base, covered with a layer of dentine (originally called "cosmine") and enamel (originally called "ganoine").

Shark scales are classified as placoid scales: they lack ancestral dermal bone and are composed of enamel and dentin around a pulp cavity. Jawless fish known as thelodonts possessed scales similar to shark scales and are thought to be close to gnathostomes. The earliest thelodont scales are known from the Late Ordovician. Scales seemed to have lined the oral cavity of at least some thelodonts and it is possible that the cartilaginous arches of these fish, lined with tooth-like scales, were the precursors for jaws. Early sharks and acanthodians also seem to have possessed these branchial scales which may or may not have been homologous to true teeth. In strata dated at about 450 million years ago, there are the remains of tiny scales which are so similar to modern shark scales that they are attributed to primitive sharks. The first shark teeth are not known until strata which are 40 to 70 million years younger. Although the earliest shark-like scales are known from the Early Siluran, shark teeth are not known until later suggesting that the fish from which these scales originated may have lacked teeth or even jaws. The first fossils of true teeth are known from the shark fossil Doliodus of the Late Siluran/ Early Devonian (Long, 1995; Turner, 2004).

 

In bony fish, the scales do not penetrate the epidermis, leaving an intact epidermis which typically secretes mucus. Sarcopterygians developed

cosmoid scales which are composed of two layers of bone (vascular and lamellar), a thick layer of dentin (cosmine), and a thin layer of enamel.

lungfish

Primitive actinopterygians developed ganoid scales which possessed a thick layer of enamel which made them more shiny.

sturgeon

Teleosts lack enamel, dentine, and vascular bone in their scales. These scales only possess lamellar bone which are acellular and not calcified. These teleost scales can be classified if they are concentric or ctenoid if they possess projections at their posterior margins. They grow annually, allowing estimates of their age (Kardong, 2002).

Some lineages of fish, such as eels and catfish, have lost their scales and can even breathe through their skin.

catfsh

A number of catfish lineages have developed armor, apparently independent of one another (Geerinickx, 2006).

catfish

In the family Loricariidae of suckermouth armored catfish, the armor of the skin is composed of odontodes made of dermal bone, covered by dentin and enamel and, attached by either bone or connective tissue. In some species, the odontodes around the opercle are spikelike and muscular contraction can elevate them (Geerinickx, 2006). Some possess dermal armor composed of two longitudinal rows of plates (Shimabukuro-Dias, 2004).

Early amphibians retained dermal scales on their undersides. There were still small dermal scales on the undersides of the first reptiles  (Clack, 2002).  Amphibians lost their ancestral scales, allowing them to supplement the respiration occurring at their lungs with cutaneous respiration through their skin. All salamanders of the family Plethodontidae breathe solely through their skin and lack lungs. 

frogsalamander

Most reptilian scales lack any bony base and are epidermal folds of keratin. While reptiles cannot breathe through their skins as amphibians do, their skin is resistant to water loss and allows them to live in drier environments. When reptiles shed their skin, the outer layer of scales is separated from a deeper layer which forms the next set of scales.  Before shedding, lipids accumulate between these two layers, making it easier for one to be shed. Snake skins are usually shed entire, with a snake beginning by rubbing its head.  A new inner layer then forms.  The dermal bones of turtle shells are covered by horny scutes whose margins tend to lie over the sutures between shell bones.  Leatherbacks and softshell turtles have lost the horny covering over their shells and have reduced their shell bones.  Turtles lack teeth and use a horny beak to bite their food (Ernst, 1994).

turtleturtle

Only the most primitive snakes (blind snakes) possess uniform scales which cover their entire bodies, higher snakes developed ventral scales which are larger than other scales.     Scaly skin can vary greatly in its permeability to water depending on a snake’s habitat; scales are less permeable in snakes which live in dryer habitats.  The permeability of the skin can vary over course of year as snakes vary lipids produced in their integument.  The heads of snakes may also possess a number of modified scales forming “horns”, “eyelashes”, and “tentacles”.  In the snake family Acrochoridae, the scales are covered in short hairlike filaments and raised scales give a few snakes a hairy appearance.   Hognose snakes have a modified rostral scale.  Snakes lack eyelids and in some primitive snakes, scales cover the eyes.  In most snakes, a specialized scale called the brille covers the eye.  The number of scales on the underside of the body varies from under 100 in slug eaters to more than 500 in seasnakes.  (Greene, 1997; Mattison, 1995). 

snakelizaRD

Some rodents have evolved scales on their tails, such as some porcupines and squirrels.  Armadillos possess epidermal scales over their bony armor.

porcupines squirrel

DERMAL BONES

Bony plates (scutes) serving as protection are not unique to turtles: they existed in pareisaurs, aetosaurs, crocodiles, and several Mesozoic groups of reptiles.  The rib cage and vertebral column help form the shell.  There are about 50 bones which compose the carapace and 23 in the plastron (Ernst, 1994).    Many ornithischian dinosaurs had dermal armor as did many of the earlier thecodonts and some sauropods.  It is possible that some dermal armor was an ancestral condition for dinosaurs which was lost in many lineages.

Armadillos possess bones in their skin which typically form five shields (cephalic, scapular, dorsal, pelvic, and caudal).  These bony scutes are covered by a horny epidermal scales.  Giant ground sloths also possessed dermal armor.

armadillo

CLAWS

     Only 2 amphibians are known which possess keratin at the tips of their digits: the toad Xenopus and spade-foot toads. (Webster, p. 167).  All amniotes have keratinized tips of their digits.  Mammal claws are equivalent to reptilian claws but are composed of a softer keratin. (Webster, p. 167).  Primates evolve flattened claws, called nails although claws still exist in some prosimians (the aye-aye) and in some New World monkeys (family Callithricidae).  Old World monkeys lack the toilet claw known in some prosimians and New World monkeys.  The number of layers which compose the nail is reduced to one single layer in apes (Soligo, 1999).

Opossum

opossum fingers

Rhesus Monkey

nails

ADIPOSE

Turbellarian flatworms store energy as lipid (Rieger, from Harrison, 1991).  Frog adipose is depicted below.

frog adipose

THE DERMIS

     In many mammals, ranging from insectivores to humans, the collagen of reticular layer of dermis exists in horizontal layers at angles to each other, although masked in humans (Montagna, 1971).

In most mammals, the dermis has fewer blood vessels than found in humans (Montagna, 1971).  Anthropoid primates have a thicker papillary layer of the dermis.  There is an enormous amount of dermal pigment in New World spider monkeys (Montagna, 1972).  Chimpanzees possess a dermis most similar to humans (Montagna, 1972).

HORNS

Only mammals possess true horns and antlers. Horns are usually present in both genders of cattle, antelope, sheep, goats, bison, and wildebeest and are composed of a bony core surrounded by a keratin sheath made by the epidermis.

bison

 

Antlers exist in deer where they are usually found only in males (although they can be produced by females, as in caribou).

The bony core is covered by a thin layer of skin, known as velvet, which can later be shed. In pronghorns, the bony core of the horn is unbranched but the keratinized coverings may branch. In giraffes, small knobs of cartilage ossify but remain covered by living skin.

giraffeokapi

Rhino horns are not true horns because they lack a bony core, being composed instead by compacted keratin.

rhino