Formation of the Egg PDF by Donald D. Bell 2002

5 Formation of the Egg by Donald D. Bell The avian egg consists of a minute reproductive cell quite comparable to that found in mammals. But in the case of the chicken, this cell is located on the surface of the yolk and surrounded by albumen, shell membranes, shell, and cuticle. The ovary is re...

5 Formation of the Egg by Donald D. Bell The avian egg consists of a minute reproductive cell quite comparable to that found in mammals. But in the case of the chicken, this cell is located on the surface of the yolk and surrounded by albumen, shell membranes, shell, and cuticle. The ovary is responsible for the formation of the yolk; the remaining portions of the egg originate in the oviduct. 5-A. OVARY At the time of early embryonic development, two ovaries and two ovi- ducts exist, but the right set atrophies, leaving only the left ovary and oviduct at hatching. Prior to egg production, the ovary is a quiet mass of small follicles containing ova. Some ova are large enough to be visually seen; others require magnification. Several thousand are present in each female chicken, many times the number that will eventually mature into full-size yolks necessary for egg production during the life of the bird. Formation of the Yolk The yolk is not the true reproductive cell, but a source of food material from which the minute cell (blastoderm) and its resultant embryo partially sustain their growth. When the pullet reaches sexual maturity, the ovary and the oviduct un- dergo many changes. About 11 days before she is to lay her first egg, a sequence of hormonal changes occur. The follicle-stimulating hormone ( FSH) produced by the anterior pituitary gland causes the ovarian follicles 59 D. D. Bell et al. (eds.), Commercial Chicken Meat and Egg Production © Springer Science+Business Media New York 2002 60 FORMATION OF THE EGG to increase in size. In turn, the active ovary begins to generate hormones: estrogen, progesterone, and testosterone (sex steroids). Higher blood plasma levels of estrogen initiate development of the medullary bone, stimulate yolk protein and lipid formation by the liver, and increase the size of the oviduct, enabling it to produce albumen proteins, shell mem- branes, calcium carbonate for shell formation, and cuticle. The first yolk (ovum ) to begin maturing does so as major amounts of the yolk material produced in the liver are transported by the circulatory system directly to the developing ovary. A day or two later, the second yolk begins to develop, and so on, until at the time the first egg is laid, five to ten yolks are in the growth process. About 10 days are required for an individual yolk to mature. Deposits of yolk material are very slow at first and light in color. Eventually the ovum reaches a diameter of 6 mm at which time it grows at a greatly increased rate, with the diameter increasing about 4 mm per day. A greater number of yolks are under de- velopment at one time in the broiler breeder hen than in the egg-type hen, but the broiler breeder hen does not have the ability to produce as many complete eggs. The color present in the yolk is xanthophyll, a carotenoid pigment de- rived from the diet. The pigment is transferred first to the bloodstream, then quickly to the yolk, as well as other parts of the body. Consequently, more is deposited in the yolk during the hours when the hen is eating than during dark hours when she is not. This gives rise to deposits of dark and light layers of yolk material, depending on the dietary pigment available. From seven to eleven concentric rings are found in each yolk. Yolk formation is rather uniform and the total thickness of both dark and light deposits during 24 hours is about 1.5 to 2.0 mm. Egg yolk is composed mainly of fats (lipids) and proteins, which com- bine to form lipoproteins, of which 60% of the dry yolk weight is of low density lipoproteins (LDL), and are known to be synthesized by the liver through the action of estrogen. In the laying hen, LDL is removed from the blood plasma as intact particles for direct deposition in the developing ova. What influences growth rate of the yolk ? Yolks vary greatly in size be- tween individual chickens in the flock at the same age, and are usually associated with body weight differences. Yolk size is not associated with rate of lay, but probably more with the length of time required for the ova to reach maturity. The yolks from an individual hen in- crease in size over the production cycle. Furthermore, the first egg laid in a clutch will usually contain a larger yolk than the remaining ones. Eggs laid later in the day are 0.5 grams lighter for each addi- tional hour in the day; this is also associated with smaller yolks. The inclusion of added fat and protein in the diet has also been shown to increase the size of the developing yolk.  5-A. OVARY 61 Location of the germinal disc. The yolk material is laid down adjacent to the germinal disc that continues to remain on the surface of the globu- lar yolk mass. Once the egg is laid, the yolk rotates so the germinal disc remains in the large end of the egg. Ovulation At maturity the ova are released from the ovary to enter the oviduct by a process known as ovulation. Each ovum hangs on the ovary by a narrow stalk containing the arteries that supply the blood to the developing yolk. The arteries undergo much branching in the surface membranes of the yolk and the follicle appears highly vascular except for the stigma, a nar- row band surrounding the yolk that is almost void of blood vessels. When an ovum is mature, the hormone progesterone, produced by the ovary, stimulates the hypothalamus to cause the release of the luteinizing hormone (LH) from the anterior pituitary, which, in turn, causes the ma- ture follicle to rupture at the location of the stigma releasing the ovum from the ovary. The yolk is then surrounded only by the vitelline mem- brane ( yolk membrane). Delaying first ovulation. Sexual maturity, as indicated by the first ovula- tion, may be accelerated or retarded. Restricting feed or decreasing day lengths during the pullet's growing period are the two main pro- cedures used (see Cage Management for Raising Replacement Pullets, Chapter 51, and Managing the Breeding Flock, Chapter 34). What initiates ovulation ? It is not known what sets the hour for the bird 's first ovulation, but both the nervous system and hormonal se- cretions are of primary importance. The second ovulation is regulated by oviposition (laying) of the first egg and occurs about 15 to 40 min - utes after the first egg passes through the vent. Future ovulations oc- cur at about the same frequency after subsequent eggs are laid. Eggs laid in clutches. Chickens lay eggs on successive days known as clutches, after which none are laid for one or more days. The length of the clutch may vary from 2 days to more than 200 before a day is missed, but most commercial egg-type chickens can produce more than 50 eggs in succession without a pause during the early stages of production in the first lay cycle. The length of clutches is quite consis- tent with individuals; poor producers have shorter clutches, good pro- ducers have longer clutches. Once the clutch length is established, the hen will not ovulate for one or more days and then will produce an- other clutch. Poor egg producers have a longer rest period between clutches than do good producers. Time necessary to produce an egg. The time necessary for an egg to trans- verse the oviduct varies with individuals. Most hens lay successive eggs with time intervals of 23 to 26 hours. If the time is greater than 62 FORMATION OF THE EGG 24 hours, each successive egg will be laid later in the day, and the ovulation of the yolk for the next egg will also occur later in the day. Eggs laid in the afternoon have spent several more hours in the ovi- duct than those laid in the morning. Eventually eggs are laid so late that the rhythm is broken and an ovulation is skipped. Time of ovulation. Hens that produce long clutches lay their first egg of a clutch early in the day, an hour or two after the sun rises or the artificial lights are turned on. Ovulation of the next yolk comes quickly after an egg is laid, with only a slight time lag. Those hens with shorter clutch lengths lay their first egg of the clutch later in the day, ovulation of the next yolk is slower, and the time lag for laying is greater. Most ovulations occur during the morning hours, as it is not natural for ovulations to occur in the mid to late afternoon. Egg production at start of lay. During the first week of lay, ovulation is quite irregular; as the hen's hormonal mechanism is not in balance. Often, only two to four eggs are produced in the first clutch. But by the second or third week, ovulation is progressing at its peak rate, only to drop slowly each week throughout the remainder of the laying cycle. Light and ovulation. Light, either natural or artificial, has an effect on the pituitary gland, stimulating it to secrete an increased quantity of the follicle stimulating hormone (FSH), which in turn, activates the ovary. Both duration and intensity of light are important. The proce- dure for correctly lighting a flock of laying hens is complicated and is discussed in Cage Management for Layers, Chapter 52, and Fundamen- tals of Managing Light for Poultry, Chapter 10. Nesting as an indication of ovulation. On most occasions the hen seeks a nest about 24 hours after ovulation, leading scientists to theorize that nesting can be used as an indicator of ovulation. Evidently, the presence of a fully formed egg in the cloaca has nothing to do with the hen's desire to seek a nest. For example, some hens will ovulate, but because of a malfunction, or for some other reason, the ovum does not reach the oviduct, these hens will still seek a nest a day later. Double ovulation. Normally, only one yolk is ovulated per day, but oc- casionally two may be released and on rare occasions there may be three. If two are ovulated at the same time normally only one enters the oviduct, but if both are picked up simultaneously by the oviduct, a double yolk egg will result. About two-thirds of the double-yolk eggs are the result of ovulations within 3 hours of each other. If there is a great difference in ovulation time, two eggs may be produced on the same day, but usually the second is soft-shelled. Double-yolk eggs are more common during the first part of the egg production period because of an overactive ovary, and are more often associated with meat-type strains than with egg-type ones. The inci- dence is an inherited trait since some birds produce higher percent-  5-A. OVARY 63 ages of double-yolk eggs than others. Spring- and summer-housed pullets also produce a greater number of double-yolk eggs than fall- and winter-housed pullets. Defective Eggshells When the normal interval of about 23 to 26 hours between ovulations is broken, more eggs are produced with defective shells, including those with sandpaper texture, white bands, calcium splashing, and chalky white deposits. The occurrence is greater in meat-type than in egg-type breeds. From 5 to 7% of the eggs produced have some form of defective shells. These defects are mostly associated with the age of the flock, with some strains more prone to the problem than others. Various egg shell defects are described in Egg Handling and Egg Breakage, Chapter 56. Yolk Size Affects Egg Size The size of the completed egg is more closely associated with yolk size than with any other factor, although variations in albumen secretions in the oviduct have some influence. The yolk-albumen relationship changes throughout the laying cycle. Eggs produced at the beginning of the laying period have yolks that comprise about 25% of the total weight of the egg, while yolks make up about 30% of egg weight when hens are near the end of their laying period. In other words, as egg size increases, yolk weight increases more rapidly than the weight of albumen. In younger flocks when egg size is small, increasing the level of protein in the diet may increase the total weight up to 1.5 oz / doz (3.5 g / ea ). Blood Spots and Meat Spots Often, when the yolk sac ruptures along the stigma, small blood vessels near the area of the rupture are broken, leaving a clot of blood attached to the yolk. The frequency of hemorrhages can be related to a number of factors: genetics, feed, age of the hen, and others. Blood spots are two to three times more common in brown-shelled than in white-shelled laying hens. Any tissue sloughed from the follicular sac or the oviduct can be in- cluded in the developing egg as it passes through the oviduct. These bits of tissue darken with age and are known as meat spots. Many blood spots darken too, and are often incorrectly classified as meat spots. This problem is especially prevalent in brown-shelled eggs where 15% or more of the 64 FORMATION OF THE EGG eggs can be affected, compared to less than 1% in white shell eggs (Carey, 1988). 5-B. PARTS OF THE OVIDUCT The oviduct is a long tube through which the yolk passes and where the remaining portions of the egg are secreted. Normally, the oviduct is relatively small in diameter, but with the approach of the first ovulation its size and wall thickness expand greatly. The segments of the oviduct and their purpose are summarized below and are illustrated in Figure 5-1. A OVARY 1 Mature yolk within yolk sac or follicle 2 Immature yolk 3 Empty follicle 4 Stigma or suture line B OVIDUCT 1 Infundibulum 2 Magnum 3 Isthmus 4 Uterus 5 Vagina 6 Cloaca 7 Vent Figure 5- 1. Ovary and Oviduct  5-B. PARTS OF THE OVIDUCT 65 1. Infundibulum The funnel-shaped upper portion of the oviduct is the infundibulum. When functional, its length is approximately 3.5 inches (9 cm ). Normally inactive except immediately after ovulation, its purpose is to search out and engulf the yolk causing it to enter the oviduct. After ovulation, the yolk drops into the ovarian pocket or the body cavity, from which it is picked up by the infundibulum. The yolk remains in this section for only a short period of about 15 minutes, then is forced along the oviduct by multiple contractions. Malfunction of the infundibulum. To be completely functional, the infun- dibulum should pick up all the yolks dropped into the body cavity. However, it has been found that an average of 4% are not drawn into the infundibulum, but remain in the body cavity where they are reab- sorbed within a day. The percentage varies with strains of chickens, some of which retain up to 10% of their yolks in the body cavity. Meat- type birds are more often affected than egg-type strains. Internal layers. Sometimes the infundibulum loses its ability to pick up a high proportion of the yolks, and they accumulate in the body cavity faster than they can be reabsorbed. Such hens are known as "internal layers," although the term does not define the condition well. The abdomen in such layers becomes distended, and the hen stands in an upright position. 2. Magnum The magnum is the albumen-secreting portion of the oviduct, and is about 13 inches (33 cm) long in the average laying hen. It takes approxi- mately 2 to 3 hours for the developing egg to pass through the magnum. Albumen. The albumen in an egg is composed of four layers (see Shell Eggs and Their Nutritional Value, Chapter 57). The names and percent- ages are: Chalazae 2.7% Dense white 57.3% Liquid inner white 16.8% Outer thin white 23.2% While all four are produced in the magnum, the outer thin white is not completed until water is added in the uterus. Chalazae. Upon breaking an egg, one notices two twisted cords, known as chalazae, extending from opposite poles of the yolk through the albumen. The chalaziferous albumen is produced when the yolk first enters the magnum, but the twisting to form the two chalazae seems 66 FORMATION OF THE EGG to occur much later as the egg rotates in the lower end of the oviduct. Twisted in opposite directions, the chalazae tend to keep the yolk cen- tered in the egg after it is laid. Liquid inner white. As the developing egg passes through the magnum, only one type of albumen is produced, but the addition of water plus the rotation of the developing egg gives rise to the various layers, one of which is the liquid inner white. Dense white. The dense white makes up the largest portion of the egg albumen. It contains mucin that tends to hold it together. The amount of thick white generated in the magnum is large, but the breakdown of mucin and the addition of water as the egg moves through the oviduct tend to reduce the amount of thick white while increasing the amount of thin white. At the time the egg is laid, it has about one- third of its original content of thick white, but what remains still com- prises over half the albumen in the egg. Egg quality deterioration. After laying, there is a constant change in the internal contents of the egg. The thick white gradually loses its viscous composition and its volume decreases, while the thin white becomes more watery, and the amount increases. These conditions are affected by holding temperature, relative humidity, time and certain diseases. The increasing amount of thin white is one of the best indicators of the age (freshness) of the egg. 3. Isthmus Next, the developing egg is forced into the isthmus, a relatively short section approximately 4 inches (10 cm) in length, where it remains for about 75 minutes. Here the inner and outer shell membranes are formed in such a manner as to represent the final shape of the egg. The contents at this time do not completely fill the shell membranes, and the egg resem- bles a sack only partially filled. The shell membranes are a papery material composed of protein fibers. The inner membrane is laid down first, followed by the outer membrane, which is about three times as thick as the inner membrane. The two mem- branes are held closely together until the egg is laid; then at the large end of the egg, the two membranes separate to form the air cell. In a small percentage of the eggs, the air cell will form in the small end or on the side. Air cell is important. When the egg is first laid there is no air cell. How- ever, it soon appears and increases in diameter to about 0.7 inches (1.8 cm). As the egg ages, moisture within the egg evaporates through the shell pores and the air cell increases in diameter and depth. The size of the air cell can be affected by various storage conditions. High surrounding temperature and / or low humidity increase the size of  5-B. PARTS OF THE OVIDUCT 67 the air cell. The size of the air cell, as determined by candling, is used in grading programs to judge the age of the egg. Larger air cells are indicators of poorer interior quality. Shell membranes act as a barrier. The shell membranes act as a barrier to the penetration of organisms such as bacteria. Eggs laid by young hens have thicker shell membranes than eggs laid by older hens. 4. Uterus (Shell Gland ) The uterus is from 4.0 to 4.7 inches (10 to 12 cm ) long in the laying hen. The developing egg normally remains in the uterus from 18 to 20 hours, much longer than in any other section of the oviduct. Outer thin white deposited after shell membranes. When the egg first enters the uterus, water and salts are added through the shell membranes by the process of osmosis to plump out the loosely adhering shell membranes and to liquefy some of the thin albumen to form the fourth layer, the outer thin white. The shell. Eggshell calcification begins just before the egg enters the uterus. Small clusters of calcium appear on the outer shell membrane just before the egg leaves the isthmus. These are the initiation sites for calcium deposition in the uterus. Their number is probably inher- ited and plays a part in the amount of calcium deposition later. They disappear a short time after the egg enters the shell gland. The first shell is deposited over the initiation sites to form the inner shell, a layer composed of calcite crystals, a sponge-like material. This layer is followed by the addition of the outer shell which is made up of a layer of hard calcite crystals that are chalky and about twice as thick as the inner shell surface. The longer the calcite columns, the stronger the shell. The completed eggshell is composed almost en- tirely of calcium carbonate (CaC03), with small amounts of sodium, potassium, and magnesium. Source of calcium for eggshell. There are only two sources of calcium for eggshell production: the feed and certain bones which act as storage sites in the body. Normally, most of the calcium for egg formation comes directly from the feed, with some being derived from the med- ullary bone which serves as the calcium reservoir. The reservoir is particularly important at night when the bird is not eating and egg- shell is being deposited. Formation of calcium carbonate. Calcium carbonate is formed when cal- cium ions from the blood and carbonate ions from both the blood and the shell gland combine in the shell gland. Anything that reduces the supply of either of these ions interferes with CaC03 formation and eggshell development, many times resulting in poor shell quality. It 68 FORMATION OF THE EGG is thought that high environmental temperatures may also contribute to this problem inasmuch as eggshells are thinner during hot weather. Poor shell quality. Many factors may cause a deterioration in eggshell quality, and their influence may or may not be due to an inadequate supply of calcium or carbonate ions. Shell quality is generally defined as the shell's ability to withstand shock, its overall appearance, and smoothness. Shell strength can be measured by several different tech- niques, including resistance to breaking, specific gravity, shell defor- mation, and shell thickness (see Shell Egg Quality and Preservation, Chapter 60). Several factors lower eggshell quality; for example: 1. Quality is reduced as the bird ages and continues to lay, as the hen cannot produce as efficiently an adequate quantity of calcium carbonate to cover the larger eggs produced during the latter part of the laying cycle. 2. Increased environmental temperatures. 3. Eggs laid in the morning have poorer shell quality than those laid in the afternoon. 4. Stress experienced by birds in the flock. 5. Practically all misshapen eggs and eggs with body checks are laid between 6:00 and 8:00 a.m. 6. Certain poultry diseases (Infectious Bronchitis, Newcas- tle disease). 7. Certain drugs. Calcium requirements are high during production. The demand of the lay- ing hen for calcium is extremely high. A 4-lb (1.8-kg) hen producing 250 2-oz (56.7 g) eggs per year requires about 1.25 lb (0.56 kg) of cal- cium. Since this is about 25 times the amount of calcium in the bird 's skeleton, it is evident that the dietary need for calcium is great. Most laying rations contain from 3 to 4% calcium to meet the requirements and to allow for the inefficiencies of absorption. Pores in the eggshell. Both the inner and outer shell layers contain small openings called pores. There may be as many as 8,000 per egg. Through these pores, air passes into the egg to supply oxygen to the developing embryo. Also, carbon dioxide and moisture is removed from the egg by passing through these same pores. In the freshly laid egg, the pores are almost completely closed, but as the egg ages or is washed, the number of open pores is greatly increased. Color of eggshell. Eggshells are predominantly white or various shades of brown. However, a South American breed, the Araucana, produces eggs with green or blue shells. Pigments produced in the uterus at the time the shell is produced are responsible for the color. The shade of coloring is quite consistent for each bird, with the color intensity  5-6. PARTS OF THE OVIDUCT 69 being a derivative of the genetic makeup of the individual. Some strains of birds lay eggs with very dark brown shells, while others may vary all the way to pure white. The brown pigment in eggshells is porphyrin, uniformly distributed throughout the entire shell. The cuticle. The cuticle is laid down on the outside of the shell in the uterus and represents the last of the concentric layers of egg forma- tion. The cuticle is composed primarily of organic material. Con- taining a high percentage of water, it acts as a lubricant during the laying process. But once the egg is laid the cuticle material soon dries, sealing many of the pores of the eggshell to help prevent too rapid an exchange of air and moisture and to aid in preventing bacteria from entering the egg. Various shell cleaning processes (washing and sanding) will reduce the effectiveness of the cuticle. To counteract this, egg processors commonly apply a coating of mineral oil to the shell's surface during processing. The mineral oil helps to slow down the loss of moisture and maintain interior egg quality (see Processing and Packaging Shell Eggs, Chapter 58). 5. Vagina The final section of the oviduct is the vagina, which is about 4.7 inches (12 cm) in length in a bird during egg production. Normally, the egg is held in the vagina for only a few minutes, but in some instances may be held there for several hours. The vagina has no role in egg formation and only serves to expel the egg once it leaves the shell gland. Eggs are laid large end first. Although the egg transverses the oviduct small end first, if the hen is not molested or frightened, the egg will rotate horizontally just prior to oviposition and will be expelled large end first. The rotation requires less than 2 minutes, and makes it possi- ble for the uterine muscles to exert greater pressure on more surface area during oviposition. However, if something disturbs the bird prior to rotation, the egg will be laid quickly and forced through the vent small end first. See Shell Eggs and Their Nutritional Value, Chapter 57, for more information on the composition and characteristics of the chicken egg.