Hormones are those secretions poured out by means of a duct but which are absorbed directly into the blood stream. Such glands are referred to as endocrine glands or glands of internal secretion. As a result of much physiological research it has been found that the internal secretions play the part of governing and regulating the functions of the body. They are linked in their action and may be regarded as the regulators and co-ordinators of bodily growth, nutrition and metabolism.
The whole physiology of reproduction is governed by hormones, and hence it is now recognised that correct functioning of organs of internal secretion is essential to fertility. The hormones chiefly concerned are as follows :
The pituitary body is a small discoid yellowish brown structure at the base of the brain. It secretes a hormone A, which stimulates the development of the ovarian follicles, and another B. which causes the formation of the corpus luteum (yellow body) in the ovary.
The ripening follicle secretes a hormone which is responsible for oestrum or heat. It also prepares the uterus for the reception of the fertilised egg. The yellow body in its turn produces a secretion which prevents the recurrence of "heat" in the pregnant animal and also assists in the preparation of uterus for gestation.
The course of events therefore is as follows:
The pituitary hormone stimulates the ovary and, as a result, one or more follicles develop and become ripe. The follicular fluid in the follicle then causes the animal to come into oestrum, and the uterus becomes congested and is prepared to receive the ovum when fertilised. At the close of oestrum the follicle is ruptured and the ovum passes to the oviduct. The second pituitary hormone then comes into action, and at the site of the ruptured follicle a yellow body (corpus luteum) is developed. This body, by means of its hormone, assists in maintaining the regularity of the oestral cycle if the cow is not fertilised, but if fertilisation does occur, prevents the further development of ripe follicles and the recurrence of heat whilst the animal is pregnant. It also assists in the development of the pregnant uterus and towards the end of pregnancy stimulates the udder to secrete milk.
Should the function of these endocrine organs become deranged, the normal cycle of events is interrupted and hence the following may occur:
Production of too much pituitary hormone A. may lead to sexual precocity, the genital organs developing at a much more early age than usual. In the adult animal the excessive stimulation in the ovary may lead to excessive exhibition of "heat," the ovaries may become cystic, and the animal be a nymphomaniac. In a pregnant animal, "oestrum" may occur.
When insufficient pituitary hormone A. is secreted, sexual development is retarded and the animal may fail to come into oestrum.
In the case of pituitary hormone B., excessive production leads to overgrowth of the corpus luteum formation of persistent corpora lutea and failure of the animal to come into "heat" whilst under-production shortens the periods between oestrum so that at times the animal may be more or less constantly "in season."
When the follicular hormone is produced in excess, there is frequent oestrum and finally nymphomania, whilst if too little is secreted there may be "silent heat" or "no heat." and the uterus will be insufficiently prepared to receive the egg.
Too much luteal hormone causes the animal to fail to come into season, whilst too little shortens the periods between heat and may permit nymphomania.
The hormone of the corpus luteum towards the end of pregnancy stimulates the udder to milk production, so that if insufficient is liberated the udder will fail to function correctly and the milk production be low. It will be noted that the actions of the various hormones are linked, in some cases are antagonistic, and the correct functioning of the genital system is dependent upon the maintenance of the balance between the internal secretions.
|Hormone||Over-Production Causes||Under-Production Causes|
|Pituitary A.||excessive oestrum||diminished oestrum|
|Pituitary B.||diminished oestrum||excessive oestrum|
|Ovarian follicle||excessive oestrum||diminished oestrum|
|Corpus luteum||diminished oestrum||excessive oestrum|
It has been found that certain sex hormones circulate in the blood stream and are excreted in comparatively large amounts in the urine of pregnant animals. That in the blood stream has the effect of stimulating the ovaries to follicle maturation (pituitary A.) whilst that found in the urine is apparently follicular hormone which produces oestrum. The presence of these internal secretions in blood and urine of pregnant females is utilised in the diagnosis of pregnancy. The tests are of especial value in the human field, but are also utilised in animal work. They are useful with the mare but are not dependable in the case of the cow. The injection of the blood serum of a pregnant mare into an infantile laboratory animal causes development of follicles in the ovary, whilst the injection of extracts of urine produces oestrum.
Another endocrine gland of importance is the thyroid, which consists of two lobes, one on each side of the trachea. Its chief function is to govern the general metabolism of the body. There is a direct relationship between the action of all the endocrine glands, and this is well shown in the case of the pituitary and the thyroid, where thyroid deficiency may impair the action of the pituitary. In some recent work carried out by us we are dealing with pedigreed Shorthorn heifers which would not breed, Pituitary hormone A. was admiristered to stimulate ovulation without result. A course of thyroid medication was then given, and of six heifers so treated five bred.