Hormones in Reproduction

Nc ORMAL 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 arc 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