BIOLOGY OF CHICKEN
INSIDE OF YOUR CHICKEN
Biology of chicken
OUTSIDE OF YOUR CHICKEN
Biology of chicken
An understanding of the avian respiratory system is essential to developing a health monitoring plan for your poultry flock. A knowledge of chicken anatomy, and what the parts normally look like, will help to recognize when something is wrong and to take the necessary actions to correct the problem. The respiratory system is involved in the absorption of oxygen (O2), release of carbon dioxide (CO2), release of heat (temperature regulation), detoxification of certain chemicals, rapid adjustments of acid-base balance, and vocalization. While the function of the avian respiratory system is comparable to that of mammals, the two are quite different anatomically Birds don’t breathe the same way mammals do. Like mammals, birds have two symmetrical lungs that are connected to a trachea (windpipe). But here the similarity ends. Mammalian lungs contain many bronchi (tubes), which lead to small sacs called alveoli. Because alveoli have only one opening, air can flow into and out of them, but it cannot flow through them to the outside of a lung. In comparison, the avian lung has parabronchi which are continuous tubes allowing air to pass through the lung in one direction. They are laced with blood capillaries and it is here that gas exchange occurs. The avian respiratory tract starts with the glottis. The glottis closes when feed is passing down the throat so that the feed does not enter the lungs.The trachea is made up of cartilaginous rings that prevent its collapse from the negative pressure caused by inspiration of air. The syrinx is the voice box. The chicken’s ‘voice’ is produced by air pressure on a sound valve and modified by muscle tension. It is not possible to remove the syrinx to prevent roosters from crowing. Both roosters and hens are able to ‘crow.’ The reason hens don’t normally crow is because they ‘don’t feel like it’ due to the effects of the female hormone and the absence of sufficient levels of the male hormone. When the ovaries become diseased and the level of female hormones decrease, many hens will start to show male characteristics, including crowing. Chicken lungs are relatively small and do not expand. Instead, they are firmly attached to the ribs. Birds have an incomplete diaphragm and the arrangements of the chest musculature and the sternum do not lend themselves to expansion in the same way that the chest of mammals does. Consequently they can’t inflate and deflate lungs in the same way as mammals do. Instead, birds pass air through the lungs by means of air sacs, a uniquely avian anatomical feature. The air sacs are balloon-like structures at the ‘ends’ of the airway system. In the chicken there are nine such sacs: an unpaired one in the cervical region; two interclavicular air sacs, two abdominal air sacs, two anterior thoracic air sacs and two posterior thoracic air sacs.
Biology of chicken
The key to the avian respiratory system is that distention and compression of the air sacs, not the lungs, moves air in and out. At any given moment air may be flowing into and out of the lung and being ‘parked’ in the air sacs. The lungs are stiff and fixed, not at all like the distensible lungs of mammals. The air sacs act as ‘bellow’s to suck air in and blow it out and also to hold part of the total volume. With each breath, the chicken’s respiratory tract is exposed to the inside environment of a poultry house. Poor environments normally do not cause disease directly but they do reduce chickens’ defenses, making them more susceptible to infection from existing viruses and pathogens. The air of poultry houses can contain aerosol particles or ‘dust’ originating from the floor litter, feed, dried manure, and the skin and feathers of the chickens. These aerosol particles can have a range of adverse effects on poultry. They act as an irritant to the respiratory system and coughing is a physiological response designed to remove them. Excessive coughing lowers the chicken’s resistance to disease. The chicken’s respiratory tract is normally equipped with defense mechanisms to prevent or limit infection by airborne disease agents; to remove inhaled particles; and to keep the airways clean. Chicken health is affected by the function of three defensive elements: the cilia; the mucus secretions; and the presence of scavenging cells which consume bacteria. Cilia are tiny hair-like structures in the trachea. Cilia are responsible for propelling the entrapped particles for disposal. Mucus is produced in the trachea. Mucus secretion and movement of cilia are well developed in chickens. The consistency of the mucus produced is important for the efficiency of the ciliary activity. Cilia cannot function when the mucus is too thick Scavenging cells in the lungs actively ‘scavenge’ inhaled particles and bacteria that gain entrance to the lower respiratory tract. These cells consume bacteria and kill them, thus preventing their further spread. It is the integrated function of cilia, mucus and scavenging cells that keeps chicken airways free of disease-producing organisms. The impairment of even one of these components permits an accumulation of disease agents in the respiratory tract and may result in disease.
Biology of chicken
As this term indicates, it is associated with the regulation of the temperature of the bird. Fowls are homeothermic animals. However, while every attempt is made to achieve a stable body temperature, there are times when, under extreme conditions, the birds’ temperature will vary up or down. When this variation is too great the bird is likely to die.
Fat and heat loss
The layer of fat usually found under the skin plus the coat of feathers provide very good protection from low temperature and it is unlikely that birds other than young chickens will die of hypothermia. For a constant deep body temperature to be maintained, heat production must equal heat loss. Heat is produced by metabolic processes or may be gained from the environment when environmental temperature is above bird temperature. Birds lose heat to the atmosphere when the environmental temperature is below the birds body temperature, and when temperatures are approaching the birds’ body temperature if the relative humidity is low. Heat is lost by the bird as sensible heat directly to the atmosphere when the temperature gradient is sufficiently great and as insensible heat by the evaporation of water from the respiratory system and skin when the temperature gradient is less but relative humidity is low. At high temperature the birds increase their respiration rate to increase the amount of air passing through the respiratory system to increase the cooling by evaporation. This panting also involves gular flutter which is the rapid movement of the upper throat tissues to increase evaporation. The movement of air around the body of the bird will assist in removing heat from the bird as sensible heat and as insensible heat.
High respiratory rate and eggshell formation An increase in the respiratory rate at high temperatures will increase the loss of carbon dioxide from the body. While carbon dioxide is removed from the body because of its toxicity, a certain amount is used in the formation of eggshells. Eggshell is essentially calcium plus carbon dioxide that forms calcium carbonate. At higher environmental temperatures, layers often lay eggs with weaker shells because their panting to cool themselves removes more carbon dioxide from the body. Biology of chicken
MALE REPRODUCTIVE SYSTEM OF CHICKENS The male reproductive organs in the domestic fowl consist of two testes, each with a deferent duct that leads from the testes to the cloaca. The testes are bean shaped bodies located against the backbone at the front of the kidney. Their size is not constant and they become larger when the birds are actively mating. The left testis is often larger than the right. On the inside of each is a small, flattened area that is believed to correspond to the epididymis of mammals. The deferent duct starts at this flattened area.
The deferent duct transports the sperm from the testes where they are formed to the cloaca from which they enter the oviduct of the female when mating. The deferent duct enters a small pimple-like structure in the cloaca. This structure equates to the mammalian penis and is much larger in ducks to form a penis like organ. The deferent duct is quite narrow at first but widens as it approaches the cloaca. Testes and sperm
In the testes very twisted tubes called seminiferous tubules are found. It is in these tubules that a special process of cell division called meiosis and transformation produces the sperm. One cubic millimetre of the fluid called semen produced by the male contains on average 3-5 million sperm. Under a microscope the sperm of the fowl will be seen to have a long pointed head with a long tail. The testes also produce hormones called androgens that influence the development of what are called
Biology of chicken
secondary sex characteristics such as comb growth and condition, male behaviour and mating.
FEMALE REPRODUCTIVE SYSTEM
We begin with the ovary. The ovary is a cluster of various sizes of developing follicles. The follicle is a sack that contains the developing yolk. It takes about 10 days for a yolk to grow from a very small size to the normal size found in eggs. The oviduct is a long tube containing many blood vessels and glands. The function of the oviduct is to produce the albumen, shell membranes and the shell around the yolk to complete the egg. Normally, a yolk is released when the follicle ruptures (breaks). Then the yolk enters a thin-walled infundibulum, the first part of the reproductive tract (oviduct). It is in the infundibulum where the egg can become fertilized if sperm are present. The egg then passes to the magnum where albumen (egg white) is placed around the yolk. The egg then passes to the isthmus where the shell membranes are placed around the egg. The egg then moves to the shell gland (uterus) where a hard calcified shell is placed around the developing egg. The egg passes quickly through the vagina just before it is laid.
FEMALE REPRODUCTIVE SYSTEM
Biology of chicken
The female reproductive system of the chicken is divided into two separate parts: the ovary and the oviduct. In almost all species of birds, including chickens, only the left ovary and oviduct are functional. Although the embryo has two ovaries and oviducts, only the left pair (i.e., ovary and oviduct) develops. The right typically regresses during development and is non-functional in the adult bird. There have been cases, however, where the left ovary and oviduct have been damaged and the right one has developed to replace it Mammals typically give birth to their offpsring, the offspring of birds develop outside the body of the parents - in eggs. When carried in the womb, mammalian embryos receive their daily requirement for nutrients directly from their mother via the placenta. For birds, however, all the nutrients that will be needed for the embryo to fully develop must be provided. The ovary is a cluster of developing yolks or ova and is located midway between the neck and the tail of the bird, attached to the back. The ovary is fully formed when pullet chicks hatch, but it is very small until the chicks reach sexual maturity. At hatch, pullet chicks have tens of thousands of potential eggs (i.e., ova) which theoretically could be laid. Most of these, however, never develop to the point of ovulation. So the maximum number of eggs a hen can lay is determined when she hatches since no new ova are added once the chick has hatched. Each ovum (singular form of ova) starts out as a single cell surrounded by a vitelline membrane. As the ovum develops, yolk is added. The color of the yolk comes from fat soluble Biology of chicken
pigments called xanthophylls contained in the hen’s diet. Hens fed diets with yellow maize, or allowed to range on grass, typically have dark yellow yolks. Hens fed diets with white maize, sorghum, millet or wheat typically have pale yolks. The color of the yolks from these hens can be ‘improved’ by the addition of marigold petals to provide the desired level of xanthophylls in the yolk. Ovulation is the term used for the release of the mature ovum from the ovary into the second part of the female reproductive system, the oviduct. The ovum, which is enclosed in a sac, ruptures along the suture line or stigma. The second major part of the female chicken’s reproductive system is the oviduct. The oviduct is a long convoluted tube (25-27 inches long when fully developed) which is divided into five major sections. They are the: Infundibulum or funnel, magnum, isthmus, uterus or shell gland, and vagina.
The first part of the oviduct, the infundibulum or funnel, is 3-4 inches long, and engulfs the ovum released from the ovary. ‘Funnel’ is an inaccurate choice of name for this part since it gives the vision of the infundibulum waiting for the ovum to fall into it, which is not the case. Instead the released ovum stays in place and the muscular infundibulum moves to surround it. The ovum or yolk remains in the infundibulum for 15-18 minutes. Fertilization, if it is going to occur, takes place in the infundibulum. The next section of the oviduct is the magnum which is 13 inches long and is the largest section of the oviduct as its name implies (from the Latin word for ‘large’). The ovum or yolk remains here 3 hours during which time the thick white or albumen is added. The third section of the oviduct is the isthmus which is 4 inches long. The developing egg remains here for 75 minutes. The isthmus, as its name implies, is slightly constricted (The term ‘isthmus’ refers to a narrow band of tissue connecting two larger parts of an anatomical structure). The isthmus is where the inner and outer shell membranes are added. The next section of the oviduct is the shell gland or uterus. The shell gland is 4-5 inches long, and the ‘egg’ remains here for 20 plus hours. As its name implies, the shell is placed on the egg here. The shell is largely made up of calcium carbonate. The hen mobilizes 47% of her body calcium from her bones to make the egg shell, with the diet providing the remainder of the required calcium. Pigment deposition, if there is any, is also done in the shell gland. The last part of the oviduct is the vagina which is about 4-5 inches long and does not really play a part in egg formation. The vagina is made of muscle which helps push the Biology of chicken
egg out of the hen’s body. The bloom or cuticle is also added to the egg in the vagina prior to oviposition (the laying of the fully formed egg). Near the junction of the vagina and the shell gland, there are deep glands known as sperm host glands. They get their name from the fact that they can store sperm for long periods of time (10 days to 2 weeks). When an egg is laid, some of these sperm can be squeezed out of the glands into the oviduct so that they can migrate farther up the oviduct to fertilize an ovum. This is one of the really remarkable things about birds; the sperm remain viable at body temperature. Birds lay eggs in clutches. A clutch consists of one or more eggs laid each day for several days, followed by a rest period of about a day or more. Then another egg or set of eggs is laid. For commercial egg layers clutch size is typically quite large. In chicken hens, ovulation usually occurs in the morning and under normal daylight conditions, almost never after 3:00 PM. The total time to form a new egg is about 25-26 hours. This includes about 3½ hours to make the albumen, 1½ hours for the shell membranes, and about 20 hours for the shell itself. Occasionally, a hen will produce double-yolked eggs. This phenomenon can be related to hen age but genetic factors are also involved. Young hens sometimes release two yolks from the ovary in quick succession. Double-yolked eggs are typically larger in size than single yolk eggs. Double-yolked eggs are not suitable for hatching. There is typically not enough nutrients and space available for two chicks to develop to hatch Even rarer is an egg within an egg. This occurs when an egg that is nearly ready to be laid reverses direction and moves up the oviduct and encounters another egg in process of being put together. The results is that the first egg gets a new layer of albumen added and two ‘eggs’ are encased together within a new shell. Such eggs are so rare that no one knows exactly why they happen. Another egg problem that is commonly noted Blood spots are normally found on or around the yolk. The main cause is a small break in one of the tiny blood vessels around the yolk when it is ovulated. High levels of activity during the time of ovulation can increase the incidence of blood spots. Meat spots are usually brown in color and are more often associated with the egg white. They are formed when small pieces of the wall of the oviduct are sloughed off when the egg is passing through. In commercial operations, eggs with blood or meat spots are typically identified during candling and removed. It is rare, therefore, to see these eggs in stores. The incidence is higher in brown shelled eggs, and it is harder to identify them when candling the darker colored shells.
Biology of chicken
DOUBLE YOLKED EGG EGG
MUSCULAR SYSTEM OF CHICKENS
The muscular system provides the mechanical activity for the animal in the form of mobility of the different parts of the skeleton or its appendages, the movement of materials along tubular organs such as the alimentary canal, air passages and blood vessels, and the pumping of the blood through the circulatory system by the heart. Muscles are structured from special muscle cells in the form of fibres that have the ability to contract or shorten. When they relax the muscle lengthens. Muscle types
There are three types of muscle found in the animal body. These are:
Involuntary muscles found in the walls of the alimentary canal, blood vessels, air passages and other tubular structures. These muscles are beyond the control of the will and are called involuntary muscles. The fibres of these muscles do not carry transverse striation or stripes and are therefore said to be ‘unstriped’ or ‘unstriated’.
Cardiac muscle of the heart. This too is involuntary muscle but is striated an and is structured differently to other muscle. It is nucleated, contains many Fibres and
Biology of chicken
forms a syncytium with many nucleii but no differentiation of the protoplasm into cells.
The striated or striped, voluntary muscles of the body that move the various parts of the skeleton or appendages. These consist of very minute thread-like muscle fibres in bundles enclosed by sheaths of fibrous tissue.
There are white and red types of skeletal muscle fibers found in birds and all muscles have some white fibers and some red fibers. However, the proportion varies and some muscles are predominantly white and others predominantly red or dark. White fibers lacks a compound called myoglobin, but store more glycogen and have a fast contraction of short duration. They have little staying power. The breast muscles of fowls, the muscles of flight, are predominantly white fibers and fowls have very poor flying ability. They fly very short distances with a very rapid wing movement. Red fibers have myoglobin and other cellular structures for continuous production of energy for contraction. These fibers have a slow contraction of long duration. The flight muscles of flying birds consist mainly of red fibers.
The poultry meat we eat is skeletal muscle. The breast meat of chicken often is referred to as white meat. White meat results from muscles that are used less frequently. Chickens usually do not fly. Consequently, they do not use their breast muscles as often as they would if they flew more frequently or for longer distances on a regular basis. The leg meat, such as thigh meat, typically is referred to as dark meat. Dark meat results from muscles that are used for sustained activity. Chickens use their legs for walking. The higher activity of the leg muscles increases the muscles need for oxygen. The darker color of more active muscles comes from a chemical compound in the muscle called myoglobin, which is important for oxygen transport. Other species of poultry that are capable of flight (such as some ducks, geese, and guinea fowl) have dark meat throughout their bodies (that is, in the breast, thigh, and drumstick). Consumers, in general, tend to prefer white chicken meat, which typically is used in value-added products, such as chicken nuggets and chicken fingers. White meat often is considered the healthier of the two types of chicken meat because it has less fat and
Biology of chicken
more protein than dark meat. The higher fat content of dark chicken meat gives it more flavors.
NERVOUS SYSTEM OF CHICKENS
The nervous system and the important sensory organs play a key role in the day-to-day functioning of the animal. The nervous system integrates and controls the various functions of the body, while the sensory organs detect the various stimuli in the bird’s environment that it reacts to. Functions are actively (voluntarily) or automatically (involuntarily) controlled: Voluntary control occurs where the animal, in its response to some activity or stimulus, has a choice in what actions (if any) It may take. It chooses to respond in one way or another, such as to move a particular part(s) of its anatomy. The nerves that make up this voluntary part of the nervous system are called voluntary, or somatic, nerves. Involuntary control exists where the animal has no choice and the response to an activity or event occurs without the animal having any conscious control. Examples of the functioning of this part of the nervous system are the regulation of heart beat and circulation, digestion, and respiration. It is obvious that the animal cannot afford to have to remember to keep its heart beating, its digestive system functioning or even just to Biology of chicken
breath. The part of the nervous system that regulates such important functions is called the autonomic, or involuntary, nervous system. The Brain
The brain is located in the head and is well protected by the bones of the cranium. The brain consists of a number of parts, which in turn consist of various special cells that have the ability to detect, recognize, remember and direct. Thus the brain is the control centre for the many functions and activities of the many systems, organs and tissues that make up the bird’s body. The parts and major regions that make up the avian brain are as follows:
The forebrain which consists mainly of the cerebral hemispheres and the olfactory lobes. The hypothalamus and pituitary gland are located on the lower side of the forebrain.
The midbrain which mainly consists of the optic lobes.
The hindbrain which consists mainly of the cerebellum and the medulla oblongata.
The olfactory lobes of the forebrain are the receptor areas for the olfactory nerves and are the centre for smell, while the optic lobes are the receptor areas for the optic nerve and are the centre for sight. The optic lobes of poultry are very large when compared with those of other species in relation to total brain size. This indicates that sight plays a major role in the normal behaviour of fowls. The very small gland called the pituitary gland, or hypophysis, is associated with the hypothalamus. This gland is of major importance as an endocrine gland, so much so that it is often called the “master” gland. Many of its secretions regulate the operation of many other glands as well as the functions of the systems, organs and tissues of the bird. SPINAL CORD The spinal cord, as the name suggests, is a cord of nerve tissue that extends from the medulla oblongata of the brain along almost the full extremity of the vertebral column through the canal provided for that purpose. The spinal cord and the brain constitute the Central Nervous System (often referred to as the CNS). Like the brain, the spinal cord is well protected, firstly by its spinal fluid and the sheath that encloses it, all of which is fully enclosed within the bones of the vertebral column. The various nerves that provide for the control of the various systems, organs and tissues of the body leave the spinal cord through appropriate openings located in the joints between the different vertebrae. If the spinal cord should be broken, the connection between the brain and the parts that it controls will be broken and hence control of the effected parts will be lost. For
Biology of chicken
example, a break of the spinal cord in the lower back will result in the loss of control (paralysis) of the legs and other functions that take place below the break. Nerves
The nerve cell or neuron consists of a cell body with one or more elongated projections extending from it. The cell body contains the nucleus. The projections are part of the cytoplasm and are called axons if they are long and singular, or dendrites if the are short and branched. Nerve endings or receptors at one end carry the sensors that respond to the stimuli, while on the opposite side, the stimulus is transferred ultimately to the brain. The nerve endings are the means by which stimuli are perceived or control exercised. The remainder of the nerve cell acts as a message carrier to the brain in much the same way that telephone line carries a message between two telephones. These messages are in the form of very weak electrical currents. There are a number of different types of nerve endings on nerve cells depending on the task they have to carry out, such as smell, hear, see or perceive touch. The bird may then react to these stimuli and response is directed via the nerves of a particular system, organ or tissue. Those that send messages to the muscles to respond are often called “motor nerves” or “motor neurons”. THE CIRCULATORY SYSTEM
The circulatory system is responsible for the transport of the various essential compounds and other factors around the body, as well as the removal of the metabolic wastes that accumulate in the tissues from body activities, to the appropriate places. The compounds and other factors transported around the body are blood, nutrients, medications and antibodies to fight infection, the residue of worn out cells and the wastes of metabolism. There are times when undesirable compounds and factors are found in the system as well, such as poisons or toxins and disease causing organisms. The circulatory system consists of a number of organs and an associated transport system. This mainly includes heart, the blood vessels, the spleen, the bone marrow, the blood and the lymph vessels. The blood and vascular system develops very early in the life of the embryo as the nutrition of the rapidly developing embryo is urgent and a transport system is required to transport the nutrients to where they are needed. Evidence of the system can be seen within about an hour and the system is clearly defined and operating within two days. By day three, it is possible to see the beat of the embryonic heart with the naked eye. BLOOD CIRCULATION Biology of chicken
In broad terms the heart acts as the pump that pumps in two directions: 1.
To the lungs where the carbon dioxide in the blood is removed and the oxygen replaced
To the rest of the body to deliver the nutrients and oxygen to the cells and to collect wastes and carbon dioxide
The blood leaves the heart via arteries called the aorta (to the body) and the pulmonary artery (to the lungs). The blood always enters the heart via the vena cava vein (from the body) and the pulmonary vein (from the lungs). The heart is located in the thoracic cavity between the two lobes of the liver and mainly in front of that organ. It is relatively large and is enclosed in a thin membrane called the pericardium. The avian heart has two atria and two ventricles (four chambers), as is found in mammals. In general shape, it is typically conical with its apex, or pointed end, directed to the rear and slightly left of middle. The walls of the atria are thin while those of the ventricles are quite thick. This is because of the atria only have to move the blood from the atria to the ventricle while the ventricles are responsible for pumping blood around the body. Microscopically, the muscle of the heart is similar to that of mammals as the fibres are nucleated, striated and form a syncytium (a mass of cytoplasm with numerous nuclei).
Biology of chicken