The integumentary system consists of the skin, the feathers and the appendages (claws and beak). The skin covers the majority of the body and contains glands in the outer ear canal and the preen gland at the base of the tail, that the bird uses to preen its feathers. The integumentary system is very important in providing protection to the bird from a number of potentially dangerous situations. The functions provided by the integumentary system include:
The skin is composed of three tissues or layers of cells:
The fowl’s skin is divided into a number of separate areas where the skin has been modified to some extent to be able to carry out special functions. These areas are:
The feathered skin is also divided into a number of special areas or tracts – those where feathers actually do grow and other areas where they do not grow but which appear as such because they are covered by feathers. The areas where they do grow are called pterylae and the areas where they do not grow are called apteria.
The skin is composed, in the main, of two different tissues:
The different skin types contain different amounts and distribution of special compounds mainly collagen, elastin and keratin.
The epidermis consists of three separate layers:
The dermis is relatively thin and shows a uniform, microscopic structure. This is different to many other animals that have both densely packed and loose layers of cells. The main component of the dermis is collagen with a small amount of elastin. The hypodermis contains fewer cells and is more loosely arranged than the dermis. Fat is stored here in special adipose cells (adipose tissue is fat tissue). Air spaces found in the hypodermis connect with air sacs of the respiratory system thus enhancing the ability to fly.
Skin surface cells are replaced continuously through life by mitosis in the epidermal germinative layer. As an outer or surface layer wears away or is lost, the cell layer beneath replaces it. The tissue lost in this way forms part of the scurf or dander that provides nutrition for many parasites such as lice and a place for the survival of disease causing viruses such as Marek’s Disease. Almost all of the activity of the epidermis is devoted to keratinisation that involves two processes:
The skin has many nerve endings located in it. These are mainly located in the outer layers of the dermis and provide good protection from potentially harmful situations by making the bird aware of their presence. The dermis is also quite well supplied with blood vessels that do not reach into the epidermis. Nutrients, respiratory gases, hormones and other compounds pass by diffusion into the live inner cells of the epidermis.
The skin usually covered by the feathers is normally protected and hence is thinner. Over the wings and thighs, it is more closely joined to underlying tissue than over the rest of the body. The epidermis is about 12 cells thick with the horny outer layer being about 5 cells, the transitional layer being about two and the inner, germinative layer being about 4-6 cells.
The lower legs including the upper sides of the toes are covered by skin carrying scales attached to the epidermis. In those breeds with feathered legs the feathers grow out of the margins of the scales. The scales are considered to be a carry over from the bird’s reptilian ancestors. Scales are very rich in keratin.
The skin beneath the toes and the foot is specialised to withstand compression caused by the weight of the bird when standing and abrasion caused when the foot comes in contact with rough surfaces as the bird moves about. The skin of this area is quite thick but retains a high degree of flexibility.
The beak is composed of two parts:
The epidermis of the beak develops a thick, horny layer (called rhampotheca) made of sheets of flattened cells firmly attached to each other. The strength of the beak keratin is partly due to its special make-up. The hard egg tooth that a hatching chicken uses to break out of the shell is made of strongly calcified keratin cells. It is not a true tooth.
The claws are keratinised epidermal appendages of the digits or toes. The claw root epidermis gives rise to columns of flattened keratinised cells that remain firmly stuck together.These are accessory sexual epidermal appendages on the head and neck. They develop as a result of the production of hormones by the sex organs of the male and female when these are functional. The very rich supply of blood vessels of these appendages is the cause of the bright red colour. In very high temperatures the bird dips these organs in the water supply as an aid to cooling.
This is a two lobed, pea-sized structure that develops from the epidermis at the base of the tail above the pygostyle or last vertebra. It produces and discharges a fatty secretion through a duct opening on a small papilla or pimple located on the surface of the skin. It is one of the only two groups of glands found in the skin of the fowl as far as is known. (The other group is located in the skin of the outer ear canal). The secretion of this gland is holocrine i.e. is made from disintegrated cells. As new cells are formed they are pushed towards the centre of the gland where they create the gland secretion that contains:
The secretion is similar to that of mammalian sebaceous glands and appears to carry out a similar function. It is collected by the beak during feather preening and is used to oil the feathers. This is very important to aquatic birds for water proofing but not so to terrestrial birds.
Feathers are confined to precise tracts or areas called pterylae. Areas devoid of feathers are called apteria. Smooth muscles attached to the feather follicle from which the feather emerges cause erection of the feather during cold periods thus increasing the volume of air trapped therein. This in turn increases the insulation capacity of the feather layer. This ability to control the feathers is also utilised in periods of hot weather when the birds release hot air from among the feathers by erecting them as an aid to the elimination of animal heat.
In fowls there are five types of feathers:
These consist of:
All contour feathers are not the same. The main wing feathers, called remiges have the posterior portion of the vane broader than the anterior portion (anterior = front; posterior = rear or back). They are divided into the primary and secondary flight feathers separated by the smaller axial feather. The primary flight feathers are located on the trailing edge of the outer part of the wing and the secondaries are similarly located but closest to the body. There are 10 primary feathers on each wing of the normal adult fowl. The large feathers of the tail are called reticles. With these feathers the vanes are of equal width. Special feathers called coverts cover the bases of the remiges and rectices e.g. wing coverts and tail coverts.
Plumules are found beneath the contour feathers where they form a soft, downy undercoat. They have a short shaft with radiating, free barbs and barbules. They have no interlocking hooklets or barbicels. These feathers provide the depth to the coat and play a large part in trapping the air and holding it still – thus improving their ability to conserve warmth.
The filoplumes are very small and hairlike with rudimentary barbs and barbules. The barbs are confined to the apex. These feathers are often quite troublesome during the processing of live poultry into poultry meat as they are very difficult to remove. If they are dark in colour they are easy to see and tend to detract from the appearance of the carcass.
When a chicken hatches it has a coat of short, fluffy down which closely resembles plumules i.e. it has multiple radiating barbs. At first moult the juvenile feather emerges from the same follicle to replace the fluff. The outermost juvenile feathers resemble adult contour feathers but with a softer texture. The order of emergence of juvenile feathers varies with different pterylae and is related to when the feather follicles appear in these areas during embryonic development. Juvenile plumage is eventually replaced by definitive (final) plumage. The number of moults before adult contour feathers are formed is specific for each follicle, but varies between pterylae.
Bristles are found around the mouth and eyes of chickens and are thought to be primarily used to aid the sensory ability of birds and protect sensitive areas. A comparison between bird bristles and mammalian eylashes could be used here.
Feathers are keratinised epidermal derivatives related to epidermal scales. Each follicle develops in the embryo as a dome shaped projection of the epidermis over a raised dermal papilla or pimple. The dome later sinks into the skin by a downward growth of its rim so that eventually it lies in a short tube beneath the skin surface. The base of the rim – called the collar continues to show rapid cell division during feather formation and constitutes the germinal region of the follicle. The cells forming the feather by differential growth become arranged into structural patterns of calamus and rachis, barbs, barbules and barbicels.
During the development phase of the feather, the dermis within the follicle is highly vascularised (many blood vessels to supply the nutrients required for feather formation). But after the feather’s cells become keratinised, this part of the dermis (in the follicle) dries up and forms the pulp found inside the calamus. Finally, the feather sheath (formed by the horny layer of the epidermis) splits and the new feather unfurls. No new follicles develop after hatching.
Black, brown and reddish brown colours are due to the granular pigment, melanin, deposited in the keratinised feather cells. Melanin is formed from tyrosine (a non-essential amino acid) by special pigment cells called melanocytes. These cells enter the dermis during embryonic development and congregate beneath the feather follicles. Melanin granules move into epidermal cells prior to keratinisation. The melanin colour may be black or reddish (called phaemelanin) – the chemical differences between the two are unknown.
Carotenoids are other important skin pigments. These are yellow to orange lipid pigments contained in free fats and are found in some dietary ingredients. Carotenoids in the skin, beak and legs are responsible for the yellow colour of these tissues and organs; they are also responsible for the yellow colour of egg yolk. Carotenoids are very commonly found in many plants e.g. maize, carrots and lucerne leaf.
Colour patterns are largely dependent upon the distribution of melanin and other pigments. Colour bands of light and dark as seen in barred plymouth rocks are due to cyclic changes in melanin deposition during feather growth. The colour patterns of some breeds e.g. brown leghorns show sex dimorphism (depends on the sex with the males and females being different). Genes are also responsible for mutant feather types e.g. frizzle feather types and woolly feathers. Colours and colour patterns are controlled by two groups of genes:
Each feather follicle undergoes cyclical changes of growth called anagen and rest, called telogen. The growth phase occurs prior to the moult of old plumage. Once growth has occurred the completed feather remains attached to the base of the resting follicle by its quill or calamus.
At the end of the telogen period, the germinal cells again enter an anagen phase. As the new feather develops the quill of the old is pushed out of the follicle canal but remains attached to the horny sheath of the new feather. When the sheath splits to reveal the new feather the old inevitably falls away i.e. is moulted. Thus the moulting of feathers is a mechanical process dependent upon the growth of a new feather. Completed plumage is always in the telogen phase. Plucking feathers from the bird can induce the anagen phase.
There is variation in the age of chickens when feather growth is initiated and down replaced. Sex-linked genes control this and chickens are described as being fast or slow feathering or, more correctly, as early or late feathering. There is some form of local control in the moulting of feathers in close proximity – probably a hormone. In wild birds there is a growth centre in each pterylum (pterylae) providing an ordered sequence to the normal moult. This control is not so evident in fowls although the proximal flight feathers moult first.
There is also a degree of control between pterylae. Bilaterally symmetrical pterylae have synchronised moulting – an important feature in flying birds. The sequence of pterylae starts with the wing feathers spreads to the body and ends at the neck and head. In fowls, the moult is a seasonal occurrence in autumn. However, environmental conditions may initiate a moult at other times. Fowls in good production are prone to a moult when stress is applied. Males moult before females.
Thyroxine, the hormone of the thyroid gland plays an important part in the growth, differentiation and patterning of feathers by increasing the metabolic activity of the feather forming cells. Chicken down is not influenced by thyroxine and only the feathers of juveniles and adults are dependent on having adequate levels available. Wing feather growth is not dependent as well and, after the removal of the thyroid gland, all feather growth in adults ceases except on the wings. Giving thyroxine induces a moult by speeding up the anagen phase with females being more sensitive than males.
High levels of oestrogen (female sex hormone) e.g. as occurs during egg laying, retard feather formation and maximum egg production occur when the follicles are in the telogen phase. A reduction in production is usually associated with the anagen phase. Oestrogen and testosterone influence feather development and are responsible for differences occurring between males and females. In the absence of androgens, males develop “female” type feathers. Removal of the gonads will produce a situation where the feathers are being replaced continuously rather than on an annual basis i.e. an annual moult.
The light pattern to which the birds are subjected also affects feather development and moulting because of its effect on the pituitary gland. This, in turn, controls the production of gonadotropins by the ovaries and testes, and thyroxine by the thyroid gland. Increasing day length stimulates the production of the gonadotropins and thyroxine while decreasing day length causes a reduction in their production.
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