Physiology of egg laying
The egg production is a process of great complexity and that is intensely related to the reproductive organs physiology of the hen. The reproduction of the birds and of the mammals is very different, so much if we speak about their reproductive system as about their regulatory neuroendocrine mechanism.
Generally, birds or hens are anatomically characterized by the absence or atrophy of both right ovary and right oviduct and for having a very elongated left oviduct.
This way so, unlike the mammals, the hen does not form corpus luteum, this means there is no alternation or variation between follicular and luteal phase.
Therefore, on the one hand, the hen has a reproductive process which consists of ovulation, oviposition and incubation while mammals have ovulation, gestation, birth and lactation.
EMBRYOLOGY OF HENS REPRODUCTIVE SYSTEM
Hen reproductive system is divided in two well differentiated parts: ovary and oviduct.
In embryo, during the 7th day of incubation, the mesonephros starts to develop in to the primitive gonads. Concretely, in female embryos, starts to developing the left gonad, but the right gonad will have a process of atrophy for lack of gonadal epithelium.
Nevertheless, sometimes the right gonad can persist. Since the 11th of incubation, the right oviduct also begins to degenerate.
In the hatch day, the left ovary is small but it already presents follicles which grow slowly until the 16th, at this moment, the ovary reaches 50 grams.
That is to say, that during the previous three weeks before hatching, corresponding to sexual maturity, the hen reproductive system undergoes to a significant morphological and functional development. The growth ovary growth depends on LH and FSH action, which induce the synthesis of the steroid hormones.
The growth of the oviduct is also slow, and it’s not until the hen approaches the sexual maturity, when it already measures 70 centimeters of length. In this stage, cloaca and oviduct are also joined.
OVARY AND OVIDUCT IN HEN
The left ovary is placed in the abdominal left cavity and it is fastened by the broad ligament. The ovary is characterized for having a clustered form due to the presence of numerous follicles, it contains more than 4000 microscopic ovules, and only a limited number will develop and form the yolk. The ovary has great innervation, and the blood irrigation comes from the anterior branch of renal artery. The venous return of the ovary is carried out by the ovarian veins which end in superior vena cava. The oviduct has a pale pink tubular structure that goes from the ovary up to the cloaca and it is fastened by two ligaments (dorsal and ventral).
Sometimes, right oviduct could not be totally degenerated and we could find both oviducts developed. The oviduct is divided in different segments with certain functions and in these there will be settling the different components that will form the egg.
- Infundibulum: the first segment of the oviduct with a reversed funnel form which has thin walls. Place where the yolk or vitellus is collected after the ovulation. ·
- Ampulla or Magnum: segment of major length (40 cm) and with big folds. Magnum has secreting cells and glands which produce albumen (40% of albumen secretion).
- Isthmus: segment of small diameter and less marked folds. Here there will be formed inner shell membrane.
- Uterus (shell gland): it has great diameter and its muscular walls are very thick, it also has a great quantity of folds which go in several directions. In this zone, there are glands which produce calcium settling.
- Vagina: it joins uterus and cloaca. It has an internal wall with longitudinal folds and does not present secreting glands. In this place of the oviduct, the outer cuticle will be formed and thus prevent the entrance of microorganisms.
RELATIONSHIP BETWEEN HORMONES AND THE OVARY
The reproduction of hens has been genetically modified to reach earlier their reproductive maturity and come to have a huge amount of eggs continuously; to achieve this goal the ovulation must be continuous.
The coordinated activity of the hypothalamic-pituitary-gonadal axis regulates the production of endocrine hormones, ovarian hormones and growth factors to initiate and maintain the growth of ovarian follicles and to produce ovulation. The hypothalamus synthesizes the luteinizing hormone-releasing hormone (LHRH), the adenohypophysis synthesizes gonadotropins such as FSH (Follicle-stimulating hormone) and LH (Luteinizing hormone). Finally, the ovary produces estrogens, androgens, progesterone and intraovarian growth factors such as IGF, EGF and BMP.
LHRH is produced in the hypothalamus and regulates the synthesis of LH inside the hypophysis, therefore it influences the ovulation.
Birds express to different forms of LHRH, the LHRH-I and the LHRH-II. Photoperiodism affects the synthesis of LHRH, so when the hours of light are increased, the synthesis of LHRH will be increased.
In hens, light stimuli are captured by the optic nerve and the pineal gland, and these act upon the hypothalamic nuclei that will secrete the releasing factors (LHRH). The hypothalamus will communicate to the anterior lobe of pituitary gland thanks to a one-way vascular system (hypophyseal portal system).
HYPOPHYSEAL OR PITUTITARY HORMONES
Inside the hypophysis are secreted different hormones:
- FSH, responsible of the follicular growth and the production of estrogens. It produces the differentiation of granulosa cells present in the follicles which are not in the quick growth stage. FSH also participates in the destruction of follicles that will never ovulate (follicular atresia).
- LH is also responsible of the ovary development and ovulation. The increase of LH in plasma causes the maturation of the nucleus of the most maturated oocyte which was stopped at germinal vesicle phase. This LH peak also stimulates the progesterone and testosterone synthesis besides the production of prostaglandins that will help to the break the follicle by the stigma tissue weakening, indispensible step to do the ovulation. In the smallest follicles, granulosa cells have FSH receptors, but when this follicle is selected, these cells become LH dependents instead of FSH dependents. So, in the biggest follicles there are only LH receptors. A peak of plasma LH concentration occurred 4-6 hours before the ovulation and it coincides with the increasing of progesterone. The increasing of progesterone and LH is caused because progesterone, LHRH and LH stimulate their releasing themselves. Despite this positive feedback, when the plasmatic concentration of progesterone is too high, LH releasing is inhibited.
- LTH or prolactin is responsible of broodiness (incubation without laying), maternal behavior and the shedding. It also participates in the metabolism of water, and it is FSH and LH antagonist, therefore, its predominance during long time could inhibit the laying.
OVARIAN HORMONES OR STEROID HORMONES
Gonadal hormones are produced in the follicle and its secretion is under the control of the hypophyseal hormones and their production depends on the ovulation phase.
ESTROGENS (estrone, estradiol)
Estrogens level increases while the sexual maturity is appearing. Estrogens are synthesized in theca externa from the androngens. Androgens reach their highest level 4-6 hours before the ovulation and at the same time is produced an increase in plasma of progesterone and LH levels.
Estrogens take part in oviduct development and inhibit prolactin production. Moreover, estrogens participate in the calcium metabolism regulation to form the shell and the medullary bone. They also induce their own receptors production inside the oviduct and the receptors of progesterone in the ovary and other parts of the reproductive tract.
Estrogens also regulate the production of egg compounds in magnum such as ovoalbumin, conalbumin, ovomucoid or lysozyme and the production of vitellogenin in the liver.
ANDROGENS (testosterone, 5α-dihydrotestosterone and androstenedione).
Androgens are synthesized in theca interna and they are estrogen primary precursors. Among other functions, they participate in the crest growth and they are responsible of the appearance of secondary sexual characters and the oviduct development. Androgens also induce the synthesis of proteins in the oviduct and work together with estrogens to produce medullary ossification.
Progesterone is synthesized only in the granulosa cells of the biggest follicles, thanks to their enzymatic activity. That is to say that progesterone is more synthesized while the follicle is growing.
Progesterone takes part in ovulation rhythms, promoting the LH preovulatory surge. It also participates in the oviposition, myometrium contraction and avidin or shell production. Progesterone complicates the oviduct development in comparison with estrogens.
EFFECT OF LIGHT OVER OVULATION
Hens start to evaluate the first ovum of a laying serie during the photophase, meaning the ovulation is synchronized with the duration of light. However, the LH preovulatory peak occurs each 24 hours during the scotophase or dark phase. This little peak will induce a positive feedback over LHRH and progesterone. So, the ovulation will take place when there is a maturate ovum with a high production of progesterone which stimulates LH secretion.
The period between two successive ovipositions is approximately 25 hours, but every egg laying will occur later than the previous egg laying and this delay will continue until the end of the laying serie. This fact is due to the phase difference between the secretion of LH which is produced in the hypothalamic-pituitary axis during the light interruption and the endogenous rhythm of oocyte maturation. That is to say, if the ovulation is not produced one day, then egg laying won’t occur the next day, and this will imply the end of a laying serie.
After the pause, in the ovary will exist again a follicle able to respond to LH peak during the scotophase, thus the new laying serie will begin.
Ovulation also can be synchronized with other circadian rhythms when the alternation light-dark is lost: temperature cycle, alimentation or the ionized calcium concentration in plasma. That is so, for example, a minimum level of ionized calcium is needed in blood to make the progesterone produce a positive feedback with LHRH and the preovulatory LH increases.
The effect of light not only depends on the length or photoperiodism, it is also determined by its intensity and color. Light intensity can oscillate between 5 and 10 lux, this intensity is low but is enough for the hen to capture it. If the intensity was higher possibly it would cause and increase in hens feather pecking. Regarding the color light, hens are more sensitive to colorations with a wave length between red and yellow, while are less sensitive to blue wave length.
Ovulation tend to be produced 15-75 minutes after the previous egg laying, but whenever the age of hens draws on, the number of ovulations is reduced, due to a decrease of gonadotropins and steroids in hens. This fact produces a reduction of egg production, but instead their size is increased and the broken eggs will increase too.
EFFECTS OF SHEEDING IN EGG PRODUCTION
In a natural way, when hens produce eggs during a continuous period they need a break to rest and not laying eggs, this period usually coincides with declining short light periods. During this phase the feathers drop and after a while the feathers grow again and the new laying cycle starts. In modern layers, this shedding process won’t happen until the 12th month of production. Therefore, the replacement of the wasted feathers is produced at the same time as the total regression of the reproductive organs and the interruption of egg laying cycle.
Shedding can be natural or induced. The physiological process observed during induced or artificial shedding are similar to the natural shedding ones. Shedding produces a regeneration or rejuvenation of the reproductive tissues to start the laying cycle again.
Induced shedding permit to enlarge the reproductive period of hens and improves the laying rate, shell quality and the albumen height. Egg size doesn’t change after the shedding and it will continue increasing after restart the production.
Regarding hormones, the shedding is produced when the levels of estrogens, progesterone, LH, prolactin and the growth hormone are low; while the thyroid hormones and corticosterons are increased.
LIGHT PROGRAMS FOR LAYING HENS
The light program used in the laying period will be determined by the light program used in the breeding and rearing.
The light stimulus is initiated at 17-19th week of age. The procedure starts with the suddenly increase of 2 hours in photoperiodism length, and afterwards the time will be increased 15-30 minutes more each week until reach 17 hours of light. These 17 hours will be maintained until the end of the laying cycle. Birds need 8 hours of darkness to have a good rest.
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