FEMALE REPRODUCTIVE SYSTEM OF FOWL AND ITS HORMONAL REGULATION

FEMALE REPRODUCTIVE SYSTEM OF FOWL AND ITS HORMONAL REGULATION

Dr. Jitendra M. Bhutediya¹, Dr. Parvej J. Chavda², Dr. Yogendra H. Gadhvi³

• Veterinary Officer, Department of Animal Husbandry- Gujarat,
• Assistant Professor- Polytechnic in animal science, Khadasali- Kamdhenu University

Introduction

Birds have a very different reproductive pattern than mammals, better suited to the peculiar hazards of being a bird. The female bird’s reproductive tract consists of the left ovary and the left oviduct. The left and right ovary and oviduct develop embryonic ally as paired structures, but after hatching, the right ovary and oviduct degenerate. If the left ovary is removed from a chick before 30 days of age, the remnants of the right ovary will develop into an ovitestis, which may be capable of producing sperm.

Female reproductive system

The female reproductive system in the domestic fowl consists of the ovary and the accompanying oviduct. While the female embryo chicken has two sets of reproductive organs, only one of these, the left survives and reaches maturity to produce eggs. The single surviving ovary is located in the laying hen just in front of the kidneys in the abdominal cavity and is firmly attached to the wall of the cavity. The ovary is well endowed with blood vessels to ensure there is no hindrance to the transport of nutrients to the developing yolk.

Ovary

The ovary consists of a mass of yellowish, rounded objects called follicles, each containing an ovum or yolk. There are many such follicles but only a small number in comparison will ever reach maturity to produce an egg. When the hen is in lay the ovary will be active. The size of the follicles will vary from very small to those approaching the normal yolk size in the egg which can be up to 40 millimetres in diameter, and will contain a fully matured yolk ready for release into the oviduct.

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

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

 Yolk

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.

Oviduct

The function of the oviduct is to produce the albumen, shell membranes and the shell around the yolk to complete the egg. It is a long tube well supplied with blood via numerous blood vessels.

There are many glands found in its walls that produce the albumen, the shell membranes and the shell. In the non-layer the oviduct is quite short and small in diameter. However, once the reproductive system becomes active, it grows to a length of 70-80 centimetres with a variable diameter depending on the function of the section being examined.

The oviduct consists of five distinct parts or sections, each having different functions:

1. Infundibulum (or funnel):located adjacent to the ovary and with long segments enclosing the ovary, the infundibulum collects the yolk after its release from the follicle as a funnel and directs it into the oviduct. This section has very thin walls and is 6-9 centimetres long. Fertilisation of the ovum by the male sperm occurs here.
2. Ampulla or magnum: at approximately 40 centimetres long it secretes more than 40% of the albumen.
3. Isthmus: at about 12 centimetres in length, it secretes some albumen and the shell membranes.
4. Uterus or shell gland: at approximately 12 centimetres in length it secretes about 40% of the albumen and the egg’s shell.
5. Vagina: at approximately 12 centimetres in length, it secretes the egg’s outer cuticle and possibly the shell pigment.

Parts of Oviduct	Length of Part	Time Spent There	Function of Part
Infundibulum	2 inches	15 min	Picks up yolk, egg fertilized
Magnum	13 inches	3 hours	40-50% of white laid down-thick albumen
Isthmus	4 inches	1.25 hours	10% albumen shell membrane laid down, shape of egg determined
Uterus (shell gland)	4.2 inches	20.75 hours	40% of albumen, shell formed, pigment of cuticle laid down
Vagina / Cloaca	4 inches	–	Egg passes through as it is laid

Androgen, oestrogen and progesterone

In addition to the production of eggs, the female reproductive system also produces hormones that aid in the control of body functions. These include androgen, oestrogen and progesterone. Androgen causes comb growth and condition, and has a function in the formation of albumen.

Oestrogen causes the growth of the female plumage, mating and nesting behaviour, oviduct development together with the nutrient supply to the ovary/oviduct for egg formation. Progesterone, with androgen, is involved in the production of albumen and the carriage of the message to the pituitary gland to release luteinising hormone.

The female reproductive system remains dormant in the young chicken and growing pullet until she reaches the age when these organs start to prepare for the normal production of eggs. One of the first signs of her developing maturity is the change in the comb development.

This organ starts to grow and to take on a vivid red hue as the hormones produced by the now awakening ovary start to have an effect.

The formation of the hen’s egg

The normal egg consists of the following major parts:

1. Yolk carrying the ovum – produced by the ovary
2. Albumen or white – produced mainly in the magnum
3. Shell membranes – produced in the isthmus
4. Shell – produced in the uterus or shell gland

The ovary and yolk formation

The ovary is attached to the abdominal cavity wall by the meso-ovarian ligament. It carries anything from 2000 to 12000 small ova in miniature follicles on its surface, plus hormone producing cells in its body.

Not all of the ova found on the immature ovary develop and only approximately 200 to 350 reach maturity under normal modern commercial practice. Each yolk or ova takes about 10 days to grow and reach maturity when it is approximately 31% of the weight of the egg.

The composition of the yolk material is as follows:

Component	%
Water	48.0
Protein	17.5
Fat	32.5
Carbohydrate	1.0
Other compounds	1.0

The yolk is laid down in concentric rings of darker and lighter coloured material, the colour being produced by xanthophylls that are yellow/orange/red pigments occurring in many plants, plant products and other naturally occurring materials. The bulk of the yolk material provides a source of food for the developing embryo that originates by the fertilising of the germ disc or blastoderm usually located on the upper surface of the yolk of the broken out egg. It lies in the surface segment of the latebra which is a vase-shaped segment of different yolk with its base in the centre of the yolk, the lips on the surface and the stem joining the base to the lips.

Yolk development in the maturing pullet is initiated by follicle stimulating hormone (FSH) produced by the anterior lobe of the pituitary gland. The compounds in the yolk material are formed in the liver and, on the appropriate signal, are transported by the blood stream to the target follicle and into the yolk. The appropriate signal for this development comes from the hormones oestrogen, progesterone and testosterone which are produced by the ovary after receiving the signal of the FSH. These ovarian hormones also provide the stimulus for the formation of the development of the oviduct.

The yolk is contained in a very thin, transparent membrane called the vitelline membrane. As an egg becomes stale, the vitelline membrane becomes significantly weakened and often breaks to release the yolk contents when the stale egg is broken out. On ovulation the yolk is released and enters the oviduct where, as it passes along that organ, fertilization occurs and the remaining parts of the egg are added around it. The yolk is located in a sack called the follicle, held on the ovary. The follicle, which although quite thin-walled, is extremely well supplied with blood vessels. These are necessary to carry the materials that that have been formed in the liver, that constitute the yolk.

Ovulation

The release of the yolk (the process of ovulation), is the major controlling factor influencing the subsequent steps in the formation and laying of the egg. As a consequence, factors that influence ovulation are of critical importance to the various aspects associated with egg production. The presence of a mature yolk in a follicle causes hormones from the ovary to stimulate the release of luteinising hormone (LH) by the pituitary gland. The presence of LH in the blood stream causes the follicle that contains the mature yolk to split along the stigma thus releasing it into the oviduct abdominal cavity adjacent to the oviduct.

Sexual maturity

Sexual maturity is reached when the hen lays the first egg in her life. Generally sexual maturity is genetically controlled; however, environmental factors play a very significant role. It will be in the age range of 18-24 weeks depending on fowl genotype, but it can be manipulated by controlled feeding practices, light intensity and day length management and other management practices.

Initiation of ovulation

The controlling mechanism setting the time of the day for the first ovulation is not fully understood. However, nervous and hormonal factors are important. Subsequent ovulations are, however, controlled largely by the time of the previous egg passing through the vent (being laid). Subsequent yolk release, if at all, occurs approximately 40-60 minutes after the previous egg has been laid.

Clutches

Eggs laid on successive days are called a clutch. Clutches are separated by days when no eggs are laid. Clutch size is an individual characteristic and may vary in a flock from 2 up to 100 eggs. However, the normal clutch size is significantly less than that and ranges from 3-8 eggs. The larger the clutch size the better will be the total production. Small clutch size indicates an inferior layer and is usually associated with long breaks between them.

Egg formation time

The time taken from ovulation until when the egg passes through the vent varies with individuals within the range 23-26 hours. If the time is longer than 24 hours then the time of laying will be progressively later in the day for each successive egg in the clutch. When eggs are laid at a late hour, an ovulation is missed and the start of a new clutch will be earlier in the next laying day.

Ovulation time

Hens that produce long clutches release the yolk very shortly after first light (whether natural or artificial light). Successive ovulations occur very shortly after the laying of the previous egg. Those that produce short clutches usually release the yolk later in the day and often have longer periods between laying time and the next ovulation.

Laying pattern

When pullets first commence to lay, their hormonal and other controlling systems have not yet reached a state of balance. As a consequence, the first eggs are laid in a somewhat haphazard sequence. However, once these systems have reached a state of balance (usually after 7-10 days), egg production becomes more regular. Peak ovulation is reached 3-5 weeks after first egg. This will be held for a period and then will decline steadily thereafter until the bird moults or some other factor causes a cessation of production for a period.

Hormonal regulation of Reproduction

The Hypothalamus

The hypothalamus is the reproductive controller of the hen. This very small organ deep in the brain is a key component to the hen’s reproductive output. It receives input from other brain centers through tiny neuro-transmitters, as well as signals directly from the environment. There are special cells in the hypothalamus that are thought to receive light energy at photo stimulation in response to the days becoming longer.

The light response involves the stimulation of specialized cells within the brain. That means that light intensity must be adequate to have the light energy penetrate the feathers, the skin, the skull and the brain. There is a network of blood vessels that link the hypothalamus and the adenohypophysis. This means that the message from the hypothalamus (GnRH) is sent directly to the adenohypophysis in a rapid manner.

The Adenohypophysis

The adenohypophysis is the organ that produces the hormones that travel to the gonads to actually stimulate reproductive function in the long term as well as the short term. The adenohypophysis receives messages from the hypothalamus and, if the timing is right, it releases two important gonadotrophin hormones.

1. One hormone is luteinizing hormone (LH) which is essential for sexual maturation and for daily egg production to occur. This hormone is also produced by the adenohypophysis of mammals, but there are some differences in LH make-up between species. The output of LH into the blood stream is directly related to the output of GnRH reaching the adenohypophysis. The role of LH is to stimulate the production of sexsteroids (steroid genesis) in the follicles in the ovary. It also stimulates androgen hormone production from the testes of males. This hormone is an integral component of the day-to-day events of ovulation.
2. The second hormone is follicle stimulating hormone (FSH). As its name suggests, this hormone is thought to be involved in the development of many tiny follicles days or weeks before they become competent to ovulate. The role of FSH in promoting sex steroid production in the ovary is not thought to be significant, as small amounts of LH can do as much as larger amounts of FSH.

The Neuro-Hypophysis

The neurohypophysis lies adjacent to the adenohypophysis. The neurohypophysis (also known as the posterior pituitary) differs from the adenohypophysis in that it communicates with the higher brain by nerves. The neurophypophysis produces argininne vasotocin and mesotocin. Not as much is known about the role of these hormones as there is about adenohypophyseal hormones, but arginine vasotocin is presumed to be involved in the smooth muscle contraction needed at oviposition.

The Ovary

Hens only have a single functional ovary and oviduct (the left one). In a juvenile breeder female, the ovary is small and undeveloped. It normally stays in this condition until the hen approaches sexual maturity.

The ovary is made up of the stroma, which is the base supporting structure. It is also composed of the vast pool of small undifferentiated follicles. The pool of follicles is established prior to the chick hatching. The follicles can be classified by size and color .

The smallest follicles contain “white” yolk. As they are recruited into the pool of larger follicles they become yellow. Most of our attention has been placed on the yellow follicles that are greater than 1 cm in diameter. Some follicles are said to be come “atretic” meaning that they have stopped accruing egg yolk. Each follicle is made up of concentric rings of egg yolk. The wall of the follicle consists of a single layer of “granualosa” cells which lie closest to the yolk.

These cells in the largest (F1) follicle produce the steroid progesterone in response to circulating LH levels. On the outside of the granulosa layer is a layer of “theca” tissue. The theca tissue is a mixture of structural tissue, nerves, blood vessels and other specialized cells that secrete other steroid hormones. This tissue is very important for the production of female sex steroid (estrogen) that are important in the transformation of an immature pullet into a hen. As plasma levels of estrogens increase, externally visible features include reddening and enlargement of the comb and wattles, a prenuptial feather molt (feather drop) and a widening of the pubic bones to permit egg passage.

Internally, estrogen stimulates liver production of egg yolk lipids with a significant change in the color and size of the liver. Finally, the oviduct enlarges and becomes a secretory organ for deposition of albumen. Androgens are thought of as being “male” sex steroids. However, the ovary of the hen produces them as well. The theca cells of the very small follicles produce estrogens and androgens in response to photo stimulation. Androgen hormones are thought to be involved in feather patterns and comb growth in females.

The number of follicles decreases with age, so under normal conditions, hens that have too few follicles at sexual maturity are thought to always have a decreased large follicle component. When a hen ovulates, the layers of cells that surrounded the follicle are known as a post-ovulatory follicle (POF). This structure is not thought to serve a useful purpose after ovulation. There is minimal blood supply to it. It does not produce significant amounts of hormones. It is very unlike the analogous structure (corpus luteum or CL) in mammals in all of these three ways. In about 5 to 7 days, it regresses and disappears. Therefore a laying hen may have four to seven POF sacs in varying stages of regression.

The release of GnRH from the hypothalamus that results in laying sequences. About 6 hours before a hen ovulates she experiences a surge of GnRH which results in a surge of LH. This hormone activity is limited to a finite period of the day. This is called the “open period for LH release.” If there is a mature pre-ovulatory follicle it will respond to this burst of LH release and will produce progesterone. This progesterone will stimulate further LH release and so on. This is a “positive feedback” where the release of one hormone triggers further release of another hormone. Ovulation is the end point, about 6-8 hours after this initial LH surge. Normally ovulation follows oviposition by a period of about 15-45 minutes.