vertebrate integument and its derivatives

INTEGUMENT

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

Different vertebrate taxa have very diverse ways of performing these functions and have evolved many different structures derived from the integument.

·        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.

 

·        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.

Integument of the vertebrate classes
If we again tour through the different taxa that we discussed previously, we find many different forms of integument, based on the different environment that each organism inhabits.

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

v  Agnathans
The skin of living agnathans lacks dermal bone or scales, but the earliest craniate fossils (Ostracoderms) are known from tiny scales of dermal bone found in the Cambrian period.  These scales had
    1. a deepest, thin layer of lamellar bone,
    2. a thick layer of spongy (vascular) bone,
    3. another layer called dentine, and
    4. a surface coat of enamel-like material, often called ganoine.
There was a pore-canal system that likely functioned in electroreception

 

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

v 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
   

v  mucus secreted from fishs skin which seals out water and also prevents invasion by ectoparasites and fungus
   

v  glands are unicellular - derived from a single epidermal cell
Structures associated most with the fishes are scales:
   

v  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
   

v  because they contain compounds that are similar to those in teeth, scales are often compared to teeth
   

v  basal types of scales include :

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

v perform a more protective function, although the protectiveness of the scale is determined by the thickness of the bone

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 .The vane is composed of barbs that help to hold the shape of the feather and can be put back into place during preening.

Birds are not always completely covered in feathers - instead, feathers usually grow along tracts called pterylae, and bare spots are called aptera
Some feathers are modified to perform different functions
    - down feathers are softer feathers because they lack all the barbs of flight feathers
    - bristles and filoplumes are specially modified feathers that are used in catching prey (e.g., bristles around the bill of swallows and flycatchers) and display (filoplumes of grouse)

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 .
    - 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).   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.

THE INTEGUMENT AND ITS DERIVATIVES

 

v   :     Epidermis

v   :     Dermis

 

 

EPIDERMIS AND ITS DERIVATIVES

The epidermis, derived from somatic ectoderm, is the exterior-most covering of the chordate body. 

It provides protection against the invasion of microorganisms, provides flexibility in motion, and seals in moisture.

 As will be seen, it also gives rise to a variety of differentiated structures such as feathers, hair, horns, claws, nails and glands.

 Begin by looking at a cross-section of Amphioxus (Branchiostoma) integument. 

Amphioxus possesses the simplest possible form of epidermis - a single layer of columnar epithelium covered by a thin film of cuticle.

 

All true vertebrates, however, have developed a multi-layered epithelium.

 Note the simple, multicellular, epithelium of the lamprey, which has no scales.

 Fishes and amphibians have a mucus layer for bacterial and mechanical protection and to prevent drying on land.

 See frog skin.Terrestrial vertebrates have replaced the cuticle with keratin. See the snake skin.

 

Epidermal Derivatives of the Integument

 

Keratin Structures

 

New epidermal cells are formed continuously in the lower layers of the epidermis. 

In terrestrial vertebrates, new epidermal cells push more superficial ones to the stratum corneum, the outer-most epithelial layer.

 In the process of self-destruction, these exterior epidermal cells accumulate protein products called keratin.

 Keratinized or cornified skin serves to prevent water escape and to protect against friction and direct mechanical stimulation (e.g. calluses in humans). 

The production of all of the following structures involves keratinization:

 

Epidermal Scales:

                                      a continuous layer of repetitious thickenings of the stratum corneum; you cannot dissect an individual epidermal scale out of the skin.

 These scales may be shed entirely (moulting) or in small flakes. Examine preserved specimen of snake skin and dried specimens of bird legs and feet.

 

Claws and Talons: curved, laterally compressed keratinized projections from the tips of digits. See dried specimen of cat claws and bird talons.

 

 

Hooves: enlarged keratinized plates found on the ends of ungulate digits. Examine the hooves of pig and horse.

Nails: keratinized epithelial cells are produced at the nail base and push the existing nail forward. 

They provide protection from mechanical injury and stabilize skin for better grasping. Found only in primates.

Horns: a tough, cornified layer of the integument covers horns. Their core, however, is bone, which is of dermal origin. 

Horns are found in bovines (cattle, antelope, sheep, goats, bison, wildebeest). They are retained year-round and grow throughout the animal’s lifetime.

 

Baleen: 

It is a filter feeding habit in fishes. found in some whales, baleen is a series of keratinized plates that arise from oral epithelium.

 These sheets hang from the palate along its length and act as a sieve. See the display.

 

Beaks: epidermal structures, jaws are covered by keratinized sheaths in birds and turtles.

Feathers: are believed to have evolved from reptilian scales. Columns of epidermal cells project into the skin initially to form an invagination called the feather follicle.

 Later growth results in a projection out of the skin of a keratinized epidermal sheath with an inner feather shaft.

 These columns then separate and develop into barbs. Feather growth is initiated by dermal papillae, which die in the grown feather to form feather pulp.

 The quill (calamus), which attaches to the body and extends as a rachis. From the rachis project many veins with barbs and barbules to hold them together.

 

Hair: just as in feathers, there is an initial ingrowth of epidermal cells to form the hair follicle, followed by an outward growth of keratinized cells to form the hair shaft.

 Dermal papillae cells of the outer edge die and form the core substance of hair follicles.

 The similarities between hair and feathers both in development and in general anatomy.

 They both possess dermal papillae, shafts, an inner pulp and columns of specialized keratinized cells. Hair is characteristic of mammals.

 

 

 

Gland

Specialized to secrete specific products (oil, sweat, milk, etc.), these structures are derived by an infolding of the epidermis. In many cases they retain a connection to the stratum corneum whereby their secretions can be released at the skin surface.

 

 

• THE DERMIS AND ITS DERIVATIVES

The dermis is generally much thicker than the epidermis and lies more deeply.

 It is made of a fibrous mass of connective tissue (collagen) and is of mesodermal origin.

 It may directly produce dermal (membrane) bone. The dermis is important in defence against injury and in the maintenance of body heat.

 Deeper regions of the dermis often contain fatty deposits, smooth muscle, blood vessels and nerves. Chromatophore cells are sometimes epidermal, but usually dermal in origin. 

They secrete melanin, which can be passed to the stratum corneum of skin and to hair shafts to produce colour and block harmful sunlight.

 

Dermal Bone

 

Once present in some extinct fish - Ostracoderms had a complete head shield, while Placoderms had a broken head shield and body armour.

Now dermal bone is present in turtle dermal bone, antlers, and in the dermal armour of armadillo.

 In antlers the velvet is epidermal in origin and shapes and provides blood to the dermal bone. Once grown, the velvet is shed and only the bone remains. Antlers are found in deer, elk, moose and their relatives, often only in males. They are shed annually.

In most modern vertebrates, dermal bone (membrane bone) is formed from embryonic mesenchyme by intramembranous ossification, and contributes to the skull and skeleton, rather than being manifested externally. An exception is teeth, which are partly derived from dermal bone.

 

 

Fish Scales

 

Fish scales are also called dermal scales since they are derived mainly from the dermis.

 

1.  Cosmoid Scales: Found in Placoderms (extinct) as plates, and also typical of the Lobe Finned Fishes or Sarcopterygii, (Choanichthyes). Extinct fish had scales of enamel, cosmine and bone with pulp cavities. Modern ones, like Coelocanth and the lung fish have calcified fibers so this type of scale is almost extinct. No specimens available.

 

2.  Ganoid Scales: See bioplastic mounts, slides, the plates of sturgeon, called scutes, and the scales of the gar pike on display. Made of multi-layered enamel called ganoin over lamellar bone. Primitive (now extinct) species also had a cosmine layer and vascular bone with pulp, but these were lost in modern day examples.

 

3.  Placoid Scales: See bioplastic mounts and dogfish slides. Made of enamel (epidermal) and the dermal derivatives, dentine and bone with a pulp core. They are typical of cartilaginous fishes. Placoid scales are responsible for the rough feeling of dogfish skin.

 

4) Teleost (bony fish) scales

These are thin scales of dermal bone. They have a thin covering of epidermal tissue over them. It is derived by reduction (loss) of parts of a ganoid scale. There are two types depending on their shape.

 

4 a) Cycloid Scales: See bioplastic mounts and slides. A round ended scale.

4 b) Ctenoid Scales: See bioplastic mounts and slides. A comb shaped end is characteristic of this scale type.

 

 

Referring to the bioplastic mount and slides, make a sketch of the placoid, ganoid, cycloid and ctenoid scales. This sketch is for your own reference so do not copy the drawings but draw what you see under the microscope.

 

TEETH

 

Teeth are composed of three main parts. Enamel, the hardest substance in the body, covers the tooth surface. It is epidermal in origin. Ganoin is a form of enamel. Dentin is similar to bone in structure but is harder. It is located beneath the enamel and forms the walls of the third component of teeth, the pulp cavity. These are of dermal origin. Cosmine is a form of dentin. Dermal bone called cementum is also present in mammalian teeth.

 

Teeth are used to catch and hold prey, to crush hard shells and, in some higher vertebrates, to carry out mechanical digestion of food in the mouth.

Teeth of higher vertebrates are thought to have evolved from bony dermal scales similar to dogfish placoid scales. They have a complex embryonic origin involving both the epidermis and the dermis. Interestingly, their development bears some resemblance to that of hair and feathers. Mesenchyme cells collect in the dermis to form dermal papillae, which are instrumental in the production of dentin and go on to form the pulp of the tooth. Enamel is produced by the epidermis. The tooth in mammals is held in place by cement, which is a non-vascular.