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Aina Meducci 2012


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Avian female reproductive system

During my second industrial training at chicken breeder farm, I always wondering how the reproductive system if avian works. I studied carefully about it, and it was very short of time that I forgot to post about it. This is very important because of the bird's unique reproductive system compared to other mammals.


Avian female reproductive system

The avian reproductive system is VERY different from that of mammals. While 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 receivet heir 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 provide in the egg before it is laid.

Incubated egg

Interior view of chicken's egg before and after incubation

The female reproductive system of the chicken is shown below. It 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. In some birds, such as hawks, it is the right, and not the left, ovaryand oviduct that typically develops. Kiwis are unique in that both the left and right ovaries develop, though it is only the left oviduct thatdevelops. Ova from both ovaries will passdown the same oviduct, though not typically atthe same time.

Female reproductive tract

Chicken female reproductive system

Diagram of the oviduct

1. Ovary

The ovary is a cluster of developing yolks or ova and is located midway between the neck and tail of the bird, attached to the back. The ovary is fully formed although very small when the female chick is hatched. It is made up of 13,000 – 14,000 ova which grow by the addition of yolk fluid. Each ovum (singular 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 pigments called xanthophylls contained in the hen’s diet.

It is possible to find five stages of development in the active ovary:

Primary follicles – follicles that have not yet commenced to grow
Growing follicles
Mature follicles – follicles ready or nearly so for release
Discharged follicles – where the yolk has just been released
Atretic follicles – those from which the yolk has been released some time ago

It takes approximately 10 days for a yolk to develop from the very small to the normal size found in eggs and during this time it is contained in the follicle. The follicle acts as a sack during this period of development supplying it with the nutrients required for its growth. When a mature follicle is examined an elongated area virtually free of blood vessels will be found on the distal surface of it. This area, called the stigma, is where the follicle normally splits to release the yolk into the oviduct. If, for some reason, the follicle splits at other than the stigma, the numerous blood vessels that rupture will result in free blood being found in the egg i.e. a blood spot will form.

Ovulation is 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. This release of the ova occurs 30-75 minutes after the previous egg has been laid.

2. Oviduct

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, shell gland, and vagina.

Infundibulum- The first part of the oviduct is 3-4 inches long, and it engulfs the ovum released from the ovary. The ovum or yolk remains in the infundibulum 15-18 minutes. The infundibulum also serves as a reservoir for spermatozoa so that fertilization can take place.

Magnum- 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.

Isthmus- The third section of the oviduct is the isthmus which is 4 inches long. The ‘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.

Shell gland- 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 is also done in the shell gland.

Vagina- 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 egg out of the hen’s body. There are also glands located in the vagina where spermatozoa are stored.

Near the junction of the vagina and the shell gland, there are deep glands lined with simple columnar epithelium. These are the sperm host glands, so called because 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 lumen of the tract, so that they can migrate farther up the oviduct to fertilize another egg. This is one of the really remarkable things about birds; the sperm remain viable at body temperature.

In 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.

Ovulation for the next egg of a clutch occurs within an hour of laying the previous egg, and so that each day the hen gets later and later in her timing; she "runs behind," like a clock that is improperly adjusted. Eventually she gets so far behind schedule that she would have to ovulate later than 3:00 PM. Since hens do not typically ovulate after 3:00 PM, the next ovulation is delayed and egg laying is interrupted.

Occasionally, a hen will produce double-yolked eggs. This phenomenon occurs primarily due to the age of the hen, but can also be related to genetics. Young hens sometimes release two follicles from the ovary in quick succession. The highly active ovary due to high activity of reproductive hormones in peak egg production can also be a factor.

Double-yolked eggs are 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. It has happened, but it is rare.

It is rare, but not unusual, for a young hen to produce an egg with no yolk at all. Yolkless eggs are usually formed when a bit of tissue is sloughed off the ovary or oviduct. This tissue stimulates the secreting glands of the different parts of the oviduct and a yolkless egg results.

Things occasionally go wrong when an egg shell is being developed. The most obvious relates to shell texture. Occasionally the shell becomes damaged while still in the shell gland and is repaired prior to being laid. This results in what is known as a ‘body check.’ Occasionally there will be ‘thin spots’ in the shell or ‘ridges’ will form. The shells of such eggs, though not cracked, are weaker than ‘normal’ eggs and should not be used as hatching eggs.

Body checkRidgesThin spot

From left: body check, thin spot, ridges

A second category of problems is abnormal shape. To be considered a hatching egg, the egg should be the typical ‘egg shape.’ Abnormally shaped eggs should not be used as hatching eggs. In many cases it is not clear which is the large end (and eggs should be incubated large end up) or they may not properly fit in the egg trays.

Egg shapeEgg shape

From left: Football shaped and pear shaped eggs.

Ps: Maybe you're not familiar with the terms, click here for more details

Sources: Reproduction;PoultryHub, Female reproductive system, poultry production manual, the university of Kentucky

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