The egg production is a process of great complexity and that is intensely related to the reproductive organs physiology of the hen. Reproduction in birds and mammals is very different one another when we look at 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.

Unlike mammals, hens do not form corpus luteum. That means there is no alternation or variation between follicular and luteal phase.

Therefore, hens go through 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 7^th day of incubation, the mesonephros starts to develop into the primitive gonads. Concretely, in female embryos, left gonad develops but the right gonad will experience a process of atrophy due to lack of gonadal epithelium.

Nevertheless, sometimes the right gonad can persist. Since the 11^th day 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 day, right when the ovary reaches 50 grams.

Previous three weeks before hatching, which correspond to the sexual maturity stage, hen reproductive system undergoes a significant morphological and functional development. Ovary growth depends on LH and FSH action, which induce the synthesis of the steroid hormones.

Although ovary growth is slow, it will approximately reach 70 centimetres of length when the hen approaches its sexual maturity. In this stage, cloaca and oviduct join.

Ovary and Oviduct in adult hens

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 up in the 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, the right oviduct develops completely, so both oviducts are functional. The oviduct is divided in different segments, each one with a specific functionality, which will settle the different components that will form the egg.

Infundibulum

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 

Long segment (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. The inner shell membrane is formed here.

Uterus (shell gland)

It has great diameter and its muscular walls are very thick. It also presents a large amount of folds, which go in several directions. Here glands which produce calcium settling.

Vagina

It joins the uterus and the 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, preventing the entrance of microorganisms.

Relationship between hormones and the ovary

Hen reproduction has been genetically modified to reach its reproductive maturity earlier and continuously lay a big amount of eggs; to achieve this goal ovulation must be constant.

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 an hormone that secretes the luteinizing 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.

Hypothalamic hormones

LHRH is produced in the hypothalamus and regulates the synthesis of LH inside the hypophysis, therefore it influences the ovulation.

Birds express two different forms of LHRH: the LHRH-I and the LHRH-II. Light exposure affects the synthesis of LHRH,. When light hours are increased, the synthesis of LHRH will be also increased.

In hens, light stimuli is 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).

Pituitary Hormones

Different hormones are secreted inside the hypophysis:

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, granular cells have FSH receptors, but when this follicle is selected these cells become LH-dependent instead of FSH-dependent. 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 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

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 ovulate the first egg of a laying series according to light-phases, meaning that the ovulation is synchronized with day-light duration. However, the LH preovulatory peak occurs every 24 hours during the dark phase. This little peak will induce a positive feedback over LHRH and progesterone. So, the ovulation will take place when there is a mature 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 series. 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 series.

After the pause, in the ovary will exist again a follicle able to respond to LH peak during the scotophase, thus the new laying series will begin.

Ovulation also can be synchronized with other circadian rhythms when the alternation light-dark is lost: temperature cycle, feeding 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 shedding in egg production

When hens naturally produce eggs during a continuous period, they need a break to rest and not laying eggs. This period usually coincides with declining day-light hours. During this stage, the feathers drop and grow again, then the new laying cycle starts. In modern layers, this shedding process will not happen until the 12^th month of production. Therefore, feathers replacement takes place simultaneously with 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 lengthens the reproductive period of hens and improves the laying rate, shell quality and the albumen height. Egg size does not change after the shedding and it will continue increasing after resuming 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 corticosterone 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 the 17-19th week of age. The procedure starts with the sudden increase by 2 hours of light exposure,  and afterwards the time will be increased 15-30 minutes more each week until reaching 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.

You can read more about this topic at the following links:

The broodiness

Egg shell formation

Egg shell quality problems

Eggshell problems in layers: causes and solutions

The egg yolk color and pigments