There is nothing more conspicuous about an organism than its skin.  It is our primary means of identifying the organism, and is what defines the boundary of its body.  Skin is also the primary means through which an organism interacts with its environment.

Because of its importance as the primary interface between an organism and its environment, the skin is designed to perform many functions.  These functions include:

• support and protect soft tissues against abrasion, microbes
• reception and transduction of external stimuli - i.e. heat, chemical, tactile
• transport of materials involved in excretion, secretion, resorption, dehydration, rehydration
• heat regulation
• respiration
• nutrition/nutrient storage - i.e. storage of vitamins, synthesis of Vitamin D
• locomotion
• coloration - cryptic or display

Basic structure of the integument:
The integument consists primarily of the skin and its derivatives.  Skin is a functional unit composed layers of fairly distincy epidermis (derived from ectoderm) and dermis (derived from the dermatome of somites) that are separated by the basement membrane (Fig. 6.1, p. 199).

• Epidermis
    - is relatively thin in most animals
    - the upper layer composed of mostly dead, differentiated cells (stratum corneum) with a lot of keratin which helps the skin maintain some protection against water loss and bacteria
    - continually produced by the most basal layer of the epidermis (stratum germinativum) and consists of cuboidal cells that are generalized and move toward the upper layer as they differentiate
    - as the cells move outward, most synthesize keratin, a water-insoluble protein, the cells become flattened, die, and are sloughed off.  Other epidermal cells form multicellular glands or isolated glandular cells.
• Dermis
    - is more of a connective tissue than protective
    - irregularly-shaped connective tissue cells that produce the extracellular matrix, including collagen and elastic fibers
    - the upper layer (stratum laxum) lies directly below the basement membrane and is mostly loosely-packed cells
    - the stratum compactum lies below and contains more tightly-packed cells
    - the presence of elastin in the dermis is a synapomorphy of Gnathostomata - in part, the dermis anchors the skin to the underlying musculature
    - also includes dermal scales, blood vessels, nerves, pigment cells, the bases of feathers and hairs, and their associated erector muscles.

Amphioxus has an epidermis with a single layer of cells.  A synapomorphy of Craniata is the presence of a stratified (multilayered) epidermis.  The horny teeth of lampreys are keratin - most other fishes have little or no keratin in the skin.

There are three major types of hard tissue associated with skin:

Enamel
    - the hardest tissue in the body
    - made of hydroxyapatite and has no cells or tubules within it; only about 3% of it is organic
    - ectodermal in origin and is produced by accretion of layers
    - generally it is the most superficial of hard tissues and is found on teeth and the outer layers of denticles, scales and dermal armor - one type of enamel is ganoine
Dentine
    - is softer than enamel and has about 25% organic fibers
    - usually contains tubules occupied by the processes of the mesodermal cells
    - found on the same structures as enamel, but is always deep to the enamel layer
    - some types of dentine are osteodentine, orthodentine, and cosmine, the last of these has characteristic types of canals
Bone
    - has about the same level of organic component as dentine
    - may have osteons (Haversian systems) as does osteodentine, or may be deposited in layers like orthodentine
    - unlike enamel and dentine, bone may undergo drastic reorganization

Chondrichthyes
The skin is covered with denticles or placoid scales with layers of dense lamellar bone, dentine, and enamel
Teeth are modified placoid scales

Bony fishes
Integument of fish is characterized by structures that help the organism maintain its water balance
    • generally characterized by thin epidermis, with little or no keratinized cells at the stratum corneum
    • mucus secreted from fish’s skin which seals out water and also prevents invasion by ectoparasites and fungus
    • glands are unicellular - derived from a single epidermal cell
Structures associated most with the fishes are scales:
    • composed of three basic compounds: bone, dentine and enamel (moving from inside to outside); the outside layer, enamel, is the hardest tissue in the body, and therefore can be very protective
    • because they contain compounds that are similar to those in teeth, scales are often compared to teeth
    • basal types of scales include (Fig. 6.11):

cycloid scale - thin bony scale having a smooth surface and rounded margins
ctenoid scale - thin bony scale having comblike processes on its outer part and a serrate margin
placoid scale - scaly outgrowth of the skin, that is thicker and more embedded in the skin
cosmoid scale - thick bony plates that are embedded into the skin, that act more like a bony armor

Amphibians
The earliest tetrapods had dermal scales, which probably functioned as armor.  Among living amphibians, caecilians have tiny dermal scales called osteoderms.  Their homology with dermal armor is not clear.
Amphibians mark the transition between the aquatic and terrestrial environment.  Skin remains similar to its aquatic roots and resembles the skin of the fish; however, scales are not present.
To prevent water loss, amphibians utilize mucus, which is a similar mechanism that fish use to prevent taking on additional water.  However, the mucus in amphibians is secreted by multicellular glands rather than the unicellular glands in fish.
Because the integument of amphibians makes them somewhat vulnerable, many amphibians also secrete toxins that prevent them from being eaten by other organisms.  The primary gland responsible for the secretion is the parotid gland, located behind the ear of amphibians.

Reptiles
Reptiles show more advanced integumental adaptations to the terrestrial environment because they are more far-removed from the water.  In contrast, the cells are more highly keratinized.
The integument is modified into horny scales in snakes and lizards.  In snakes, the scales on the ventral surface can be further modified into scutes, which can be used in locomotion.  In turtles the epidermis is strongly modified into plates that cover the shell, and because they increase in diameter each year, they can be used to age the animals.

Birds
The integument of birds reflects some reptilian ancestry and some new developments of the class.  Scales are present on the legs and feet of most birds, and the bill is covered in a tough skin that is highly keratinized.  The remaining skin is relatively thin.
The defining characteristic of bird integument is feathers:

- derived originally from scales, so that scales and feathers are homologous
- function in flight (flight feathers) as well as temperature regulation (contour feathers)
- basic structure of feather calamus, rachis and vane, which are derived from a feather follicle (Fig. 6.16).  The vane is composed of barbs that help to hold the shape of the feather and can be put back into place during preening.

Mammals
Mammals generally have skin that conforms to the basic structure described previously, with the epidermal layers of the skin being especially thick in areas such as the soles and the palms of the feet, where proection is needed.
Hair is the distinctive characteristic of mammals, and it provides insulation as well as some additional protection to the animal
    - grow in folllicles derived from the stratum germinativum of the epidermus but are rooted in the dermis (Fig. 6.20)
    - hair growth continues until the mitosis in the root stops - individuals in which mitosis completely stops at the hair root are usually the ones that go bald.
The fine structure of an individual hair consists of three layers: medulla, cortex and cuticular scale (which contain a lot of keratin).  (Fig. 6.20).  Softer hairs (such as our fine body hairs) lack a medulla, whereas our scalp hair contains a medulla and is usually very strong.
Modifications of hair include guard hairs (that protect the undercoat hair), quills (such as in hedgehogs and porcupines) and vibrissae (the tactile whiskers on the snouts of mammals).
Other modifications of mammalian skin includes blubber, which is found in many cetaceans and marine mammals.  Blubber is a highly thickened subcutaneous fat layer that adds to the insulation of marine mammals and also acts as a food source for the body.

Glands of the skin:  Glands associated with the skin that help to protect the skin and its associatedd structures, aid in heat regulation, and give off scent.  Include:
    - sebaceous glands which lubricate and waterproof hairs - special case in birds the uropygial gland located at the base of the tail which secretes a waxy substance that is used to waterproof and clean feathers.
    - two types of sweat glands in mammals aid in heat regulation: eccrine and apocrine sweat glands
    - eccrine sweat glands secrete a watery solution that assists in evaporative cooling on the entire body
    - apocrine sweat glands have thicker secretions that contain more odor, and are sometimes modified into scent glands in some species to use for scent marking (dogs) or defense (skunks); also the wax gland, which secretes the wax in mammalian ears.
    - the mammary gland (related to sebaceous glands) which contain fatty tissue in addition to secretory cells that produce milk; usually only become active under hormonal influences, such as the secretion of prolactin by the body that occurs in females during pregnancy and lactation.

Nails, claws, hoofs, horns and antlers: all are integumental derivatives.
    - nails grow from the nail bed located in the epidermis at the distal part of the phalanges; the nail is higly cornified in ungulates whereas in clawed animals the nail is elongated and thickened for defense or predation
    - horns are supported by a bony structure growing out from the skull; surrounding the bony core is a highly keratinized layer of the epidermis which is generally permanent
    - antlers are not present throughout the year, and are shed during the non-breeding season; develop under a protective covering of skin (velvet), which is lost as the antlers mature
    - rhinoceros horns are simply hairlike keratin fibers that are woven together without a bony core - similar to baleen in whales that is used for feeding

Integument coloration - Pigment cells
Pigment cells (chromatophores) are derived from neural crest cells that break off from the ectoderm during neural tube formation and are usually found in the dermis
    - in the epidermis of mammals and birds, the pigment cells are usually melanophores which contain the pigment melanin.  Melanin is red or blackish brown.  Melanophores in the epidermis are usually responsible for slow color change, such as that related to aging or seasonal changes.
    - in groups other than mammals and birds the chromatophores are mostly in the dermis:
    - melanophores are like those of the epidermis
     iridophores have organelles that contain platelets of guanine pigment, which reflects or scatters light
    - xanthophores and erythrophores have yellowish pteridine pigments and reddish carotenoid pigments
    - dermal chromatophores are responsible for rapid, physiological color change.
Coloration can be of many types, including cryptic (providing blend into the environment) and aposematic (warning coloriation, that occurs in some snakes)
 

Definitions:
Aposematic coloration - a form of coloration that serves to advertise the presence of dangerous, venomous or distasteful species
Chromatophore - a vertebrate cell of neural crest origin that carries pigment or reflective granules
Cosmoid scale - thick bony plates that are embedded into the skin, that act more like a bony armor
Ctenoid scale - thin bony scale having comblike processes on its outer part and a serrate margin
Cycloid scale - thin bony scale having a smooth surface and rounded margins
Erythrophores - pigment cells that contain red pigments
Fibroblast - irregularly-shaped connective tissue cell that produces the extracellular matrix, including collagen fibers
Iridophores - pigment cells that confer a silvery appearance
Keratin - a horny protein synthesized by the epidermal cells of many vertebrates
Macrophages - large cells that phagacytose, or ingest, foreign material
Placoid scale - scaly outgrowth of the skin, that is thicker and more embedded in the skin
Sebaceous gland - branched alveolar gland that produces oily and waxy secretions
Uropygial gland - an oil-secreting gland of birds located dorsal to the tail base
Vibrissae - long tactile whiskers found on the snouts of mammals
Xanthophores - pigment cells that contain yellowish pigments