Hormones are organic chemical substances secreted by ductless glands (or endocrine glands) and are released into the blood stream for onward transportation to their target organs. Examples are: Pituitary, Thyroxin, Adrenalin, Insulin, Estrogens, progesterone, Testosterone, Para hormone, etc.

The major endocrine glands include the two Adrenal glands, the Pituitary gland, the four Parathyroid glands, the Sex glands, and the Thyroid gland. A few hormones are produced by endocrine tissue present in organs that are not primarily endocrine glands. Such organs include the Brain, Stomach and Pancreas.

Generally, hormones regulate a variety of body functions. They may be grouped according to the functions that they control. These functions include:- the way the body uses food; growth; sex and reproduction; the regulation of the composition of the blood; the body’s reaction to emergencies and the control of hormones themselves.

For the sake of this paper, the role of hormones will be viewed in respect of mammalian production with regard to Sex hormones.

During childhood, the features relating to sex are usually undeveloped. When an individual reaches about 14 (10-17) years of age, conspicuous changes take place that lead toward sexual maturity and the ability to reproduce. This time is known as Puberty, when the characteristics that differentiate the sexes gradually appear. In males the beard and body hair begin growth, the shoulders broaden, the voice becomes deeper-pitched, and more attention is paid to the opposite sex. In females the mammary glands and hips enlarge, subcutaneous fat is deposited, the reproductive tract begins a series of cyclic changes, and the attentions of young men becomes of interest.

At the start of Puberty, the Hypothalamus, the portion of the Brain nearest the Pituitary gland, greatly increases its secretion of a hormone called Gonadotropin releasing factor. This Growth-stimulating hormone (GSH) acts on the anterior lobe of the Pituitary. It stimulates the gland to secrete the Gonad tropic hormones – Follicle stimulating hormone (FSH) and Lutenizing hormone (LH). These hormones, in turn act on the gonads (Sex glands) – the Testes in males and the Ovaries in females. If the Pituitary is experimentally removed, the reproductive organs remain infantile, the individual is sterile, and no sexual cycle occurs.

Under the influence of FSH and LH, the Gonads grow and begin to secrete large amount of sex hormones. The male sex hormones, including Testosterone and Androsterone, are called Androgens. The female sex hormones include Progesterone and Estrogens – the most important Estrogens being Estradiol, Estriol and Estrogen. The Cortex of the Adrenal glands in both male and female also secretes some sex hormones, especially Androgens.

The sex hormones regulate the remarkable changes that occur during Puberty. The help trigger a person’s rapid growth in height and weight and at the end of Puberty, the stop this growth. Androgens cause the male sex organ to mature, and the stimulate sexual behaviors. Androgens also stimulate the development of such secondary male characteristics as a deep voice and a beard.

Estrogens cause the female sex organs to develop fully, and they establish female sexual behaviors. They also stimulate the development of secondary sexual characteristics, such as full breast and wide hips e.t.c. In a woman’s body, Follicle stimulating hormone (FSH), Lutenizing hormone (LH), Estrogens and Progesterone work together to control the Menstrual cycle, while Progesterone also regulates processes necessary for pregnancy.

Therefore, the whole series of events making up the breeding cycle in females under the hormones control can be summarize thus:

• A combination of internal and external factors modulated in the Hypothalamus, stimulates secretion of releasing factors. These include production of the Gonadotropins Follicle stimulating hormones (GFSH) and Lutenizing hormone from the pars anterior.
• Follicles stimulating hormone (FSH) stimulates development of the Graafian follicles and induces production of Oestrogens in the Ovary. Lutenzing hormone (LH) activates the Ovulation process, and the feed-back of Oestrogen to the Hypothalamus cuts off the supply of Follicle stimulating hormones (FSH), so that no further Follicle become mature and there is no further Ovulation.
• Lutenizing hormone (LH) stimulate lutenization of the empty Follicle while the Lactogenic hormone (LTH) component stimulates milk production in the mammary gland
• Progesterone from the Corpus luteum ( or later the Placenta) initiate the pregnancy changes and also inhibits further Ovulation, so that in the case of a successful mating, another Oestrous cycle can not commence until the effect of Progesterone ceases

The Placenta, beside serving for exchange of food, respiratory gases, and waste between parent and embryo, also produces hormones:

• Estradiol and Progesterone until near the end of pregnancy
• Gonadotropin which is usually excreted in the Urine. The modern human “Pregnancy Test” depends upon demonstrating the presence of Gonaadotropin in the Urine sample.
• Relaxine, another placental hormone relaxes Pelvic ligaments in anticipation of birth
• Oxytocin ( from Posterior Pituitary) stimulates contraction of Uterine muscle sometimes being injected after child birth for this purpose

Apart from hormones of the Pituitary and Reproductive tract, those from other Endocrine glands (Thyroid, Parathyroid, etc.) have a part in activities related to reproduction. The human Menstral cycle for instance is regulated by Follicle stimulating hormone (FSH) and Lutenizing hormone (LH), as earlier mentioned.

Consequently, in male mammals, reproductive activities also are under hormonal control. The Endocrine of the Testis responsible for Secondary sexual characters is the Testosterone evidently produced by the Leydig or Interstitial cells between the Seminiferous tubules. If this hormone is injected into a Castrated individual, the accessory sex organs enlarge,the Secondary sexual characters develop, and the behaviour becomes that of a normal (Uncastrated animal).

Indeed, both Spermatogenesis and Oegenesis are controlled by the hormone Testosterone and Estrogen respectively. Healthy females from Puberty onward experience a recurrent discharge of blood, mucus, and epithelial cells from the lining of the Uterus, termed Menstration; on the average, this occurs every 28 days ( one lunar month) and lasts from 4 to 5 days. Menstruation is regulated by endocrine secretion (especially the Corpus luteum of the Ovary) and prepares the Uterus for implantation of an Ovum. It usually ceases during Pregnancy and Lactation and stops if the Ovaries are removed. Menstration gradually stops at about age of 45-50 years and its cessation (the Menopause) marks the end of reproduction ability. This “change of life” is a time of physiological emotional distress for many women. There is no comparable sexual cycle in male.

Each Ovary in a young female contains many thousands of immature Ova, the numbers decreasing with age i.e. as Menopause approaches. One enlarged mature Ovum (occasionally more) is released through the effect of Estrogen to pass down the Oviduct some days after each Menstation. This is called Ovulation. Possibly, 400 Ova in all during the life of an average woman are ovulated.

Indeed, .in addition, hormones may be administered to alter a function of the body in some way. Birth control pills, for example, contain synthetic female hormones. By taking these hormone, a woman alters the Endocrine balance that controls the Menstral cycle. This alteration blocks Ovulation, thus making it almost impossible for pregnancy to occur – i.e. another Oestrous cycle cannot commence until the effect of Progesterone ceases. In normal Menstrual cycle, there is first a period when the Follicle is developing up to the time of Ovulation, which corresponds to the periods of Pro-estrus and Oestrus, when the Oestrogen hormone is dominant. During this time, the Uterus lining is thin and unchanged. After Ovulation, the Corpus luteum is formed, Progesterone dominates the cycle and the Uterus wall thickens and becomes highly vascular. This is the Pre- menstrual period and corresponds to Post oestus.

Thus, each normal Oestrous cycle has four main phases:

• Dioestrus or Anoestus phase- a latent period during which there is no visible sexual activity. This may be quite prolonged in animals with a long cycle.
• Pro-oestrus phase- during which follicle develops in the Ovary
• Oestrus phase- the “Period of heat” which is often accompanied by changes in the external and internal genital parts. Only at this time is the female willing to receive male for pregnancy. Also, it is the time that Ovulation occurs in most mammals. (the Rabbit, Cat and Ferret are exceptions, since they ovulate only after copulation).
• Post-Oestrus phase – which is a time merging into pregnancy if copulation is successful, or Pseudo-pregnancy where it is not

If no fertilization is affected, no Placenta is formed, the Corpus luteum degenerates and Progesterone is not produced for long. This results in Endometrium breakdown, which causes the menstrual discharge of blood and cells. This period and the short post-menstrual period, which proceeds, the next development of a follicle corresponds to Anoestrus.

If the union has been successful, the egg or eggs will have been fertilized at the upper end of the Oviduct, cleavage will have commenced and the young embryo (or embryos) will have been implanted in the Uterus wall, which had been prepared by the Progesterone. The mother is then said to be pregnant. Changes which have commenced in the Endometrium of the Uterus soon after Ovulation, becomes very marked at this stage, and a Placenta is formed where tissue of the embryo and mother come into close contact. As the embryo develops further, the Uterus increases in size to keep pace. The high level of Progesterone in the body sustains all these. The critical stages of human development take place quite early in this respect. All the major organs of the body have been established by the end of the third month. The following six months are essentially a period of growth.

By the beginning of the second trimester, the Placenta reaches its full development and secrete Estrogen and Progesterone. During the second and third trimesters the Plasma levels of the Estrogen and Progesterone continuously, reaching a peak at the time of labor. In addition to Estrogen Progesterone, the Placenta secretes a hormone called Human Placental Lactogen (HPL). Placental Lactogen is similar to Growth hormone and Prolactin. It stimulates breast development in preparation for Lactation, supports fetal bone growth, and alters maternal metabolism by substituting Lipids for Glucose for energy. The Placenta also secretes Antidiuretic hormone, Aldosterone, and Renin.

Finally, at birth, which takes place at the end of the third trimester, some 266 days after fertilization in man, changing hormone levels in the developing fetus initiate the process. First, the fetal Pituitary gland secretes the hormone called Adrenocorticotrophic hormone (ACTH), which stimulates the Adrenal glands to release Steroid hormones that induce maternal Placental cells to manufacture Prostagladins that help induce powerful uterine muscle contractions. Acting in parallel, the pressure of the baby’s head against the Cervix sends nerves impulses to the mother’s Brain that trigger the Hypothalamus to release the hormone Oxytocin from the Pituitary. Working together, Oxytocin and Prostagladins stimulate contractions in the walls of the Uterus, forcing the fetus downward – thus resulting in birth.