Hormones in Reproduction

by S. R. HALL and J. F. SYKES

NORMAL reproduction and lactation in domestic animals are regulated by hormones, a group of chemical materials that are secreted by the ductless or endocrine glands directly into the blood stream. The blood carries these chemical messengers to the parts of the body where they produce their effects.

Of the endocrine glands, the pituitary is the most important. In the cow it is about the size of an acorn and is located at the base of the brain. It has two main parts, the anterior and posterior pituitary. It is complex, the anterior portion secreting at least six hormones that affect sexual development and lactation; the posterior part secretes another hormone. The pituitary regulates the activity of the other endocrine glands. The ovary—an endocrine gland—secretes the female sex hormones, estrogen and progesterone. It also produces the egg, or ovum, that, when fertilized, develops into a new individual. The testis manufactures the sperm and the male sex hormone, testosterone. From the adrenal gland arises the adrenal cortical hormones. The thyroid secretes the hormone thyroxine.

A great deal of work has been done in recent years to determine the fundamental action of the hormones and to apply the findings to practical problems on the farm.

There are also synthetic—that is, laboratory-made—hormones. Two substances, known as diethylstilbestrol (or stilbestrol) and hexestrol and some of their derivatives, while not related chemically to estrogen, have the same general effect but are several times more potent. They are also Much cheaper than natural estrogen and have been used to replace estrogen for several purposes. A material containing the thyroid hormone, thyroxine, has also been produced by adding iodine to casein under controlled conditions. That material, usually called thyroprotem, is several times more active than dried thyroid gland and is cheaper to produce.

The growth of the ovaries and testes and their function after maturity is reached are determined by the secretion of the gonad-stimulating hormones from the anterior pituitary gland and by the interaction of those hormones with hormones produced by the ovary and testis. Successful pregnancy is also largely a result of hormone activity.

If the pituitary gland is removed from very young animals, the ovaries and testes fail to develop to normal size and do not produce eggs or sperm. If the glands are removed from mature animals that are producing eggs or sperm, those functions cease, and the ovaries and testes degenerate. Such effects may be prevented by injecting the gonad-stimulating hormones, FSH and LH, into the animals.

It is now known that the follicle-stimulating hormone (FSH), produced by the pituitary gland, is necessary for the formation of eggs in the ovary and for the production of sperm in the testis. It also stimulates the ovary to produce the female sex hormone, estrogen.

The luteinizing hormone (LH), acting with the follicle-stimulating hormone (FSH), causes the follicle, which contains the egg, to rupture at the heat period and release the egg. It is also necessary for the growth of the corpus luteum, or yellow body, which forms after ovulation. Prolactin causes this structure to secrete progesterone. LH also stimulates certain cells in the testis to produce the male sex hormone, testosterone. Thus the production of eggs and sperm and the secretion of the male sex hormones are controlled by secretions of the anterior pituitary gland.

Both the male and female sex hormones produce marked effects on the reproductive organs and on the reproductive behavior of animals.

Estrogen causes the external signs of heat and is responsible for the female sex characters. It also produces changes in the lining of the uterus, which partly prepare it for pregnancy.

Progesterone, which is produced when the yellow body is present, further increases the changes in the uterus that were started by the estrogen, and it is by this combined action of estrogen and progesterone that the uterus is maintained in a highly active state during pregnancy so that the fetus can be nourished. Abortion usually occurs if the yellow body is removed during pregnancy.

The male sex hormone is similarly responsible for the male sex characters and for the sex urge. The seminal vesicles and the prostate gland, which produce fluids that combine with the sperm to form semen, are maintained in working condition under the influence of the male sex hormone.

In the nonpregnant female the secretion of the sex and pituitary hormones occurs in cycles rather than continuously, as is the case with the hormones concerned in reproductive processes in the male. The secretions occur in a regular order and in varying amounts in such a way that heat and ovulation occur at precise intervals. The regularity of this sequence is determined by the interaction of the sex and pituitary hormones with each other.

For instance, when estrogen is produced by the ovary under the stimulus of the follicle-stimulating hormone, its concentration in the blood increases to the point where it can keep the pituitary from secreting FSH. The amount of estrogen that prevents the release of the follicle-stimulating hormone by the pituitary, on the other hand, stimulates the Pituitary to produce the luteinizing hormone, which, together with FSH, causes ovulation and growth of the yellow body. If enough estrogen is not present to cause release of LH, ovulation will not occur even though heat may be exhibited, and the animal cannot possibly conceive even though she usually accepts service to the male. When the yellow body is present and is producing progesterone, this latter hormone also affects the pituitary gland and prevents it from secreting LH in a manner similar to the way in which estrogen affects FSH. The whole cyclic nature and the normalcy of reproduction therefore depend on a delicate balance in the amounts of these hormones that are secreted at various times.

Success in treating reproductive disorders depends on recognition of this fact. Treatments that upset this balance are almost always failures.

During pregnancy much larger amounts of certain hormones may be produced than at other times. The extra hormones are not formed in the pituitary gland or the ovary but in the structures associated with the developing fetus. In all species studied, large amounts of estrogen appear in the blood and urine. That is particularly true of mares, whose urine, therefore, is the usual commercial source of estrogen.

Progesterone likewise increases during pregnancy. The blood of mares also contains very large amounts of hormones that have an action similar to the gonad-stimulating hormones of the pituitary gland. Mares can be bled several times during pregnancy, and the serum containing the hormones is processed by several companies and sold as pregnant-mare serum (PMS), which is used to treat sterility. A woman during pregnancy likewise secretes in her urine a hormone similar to the luteinizing hormone of the pituitary gland. This hormone is generally known as chorionic gonadotropin. It is sold under various trade names.