Concept 35.4: Hormones Regulate Mammalian Physiological Systems

Hormones are involved in controlling and coordinating a wide range of mammalian physiological systems, and these regulatory processes in mammals often have parallels in other vertebrates. Here we take a brief look at the some of the major roles of hormones in the lives of mammals.

FIGURE 35.10 shows the locations of the principal mammalian endocrine glands and summarizes their major hormones and actions. In addition to the principal glands, many organs—such as the heart and stomach—are not named as glands but nonetheless contain endocrine cells. The box in the figure lists some of these. We discuss several endocrine control systems in other chapters, as FIGURE 35.11 indicates.

Figure 35.10: The Human Endocrine System

Go to ACTIVITY 35.1 The Human Endocrine Glands

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Figure 35.11: Several Endocrine Control Systems Are Discussed in Other Chapters Endocrine control is of such widespread importance that it features in many chapters in Part 6.

The thyroid gland is essential for normal development and provides an example of hormone deficiency disease

The thyroid gland is found in the neck, wrapped around the front of the trachea (windpipe) (see Figure 35.10). Two different cell types in the thyroid gland produce two different hormones. One hormone is thyroxine (T4), described earlier, and the other is calcitonin (involved in calcium metabolism). Here we focus on T4, an amine hormone that contains four iodine atoms (see Figure 35.4C). The presence of iodine in the T4 structure is the reason for most of our dietary iodine requirement. The thyroid also releases a relatively small amount of T3 (triiodothyronine)—a molecule formed by removal of an iodine from T4—that has an iodine atom at only three of the four positions where T4 does. T3 is more active in affecting target cells, and after T4 has been secreted into the blood, peripheral tissues convert it to T3, as noted earlier. When biologists speak of “thyroid hormones,” they mean T4 and T3 unless stated otherwise.

The thyroid hormones are vital during development and growth. They promote cellular amino acid uptake and protein synthesis. They enter cells and bind to an intracellular receptor that stimulates transcription of numerous genes. Insufficiencies of the thyroid hormones in a human fetus or growing child greatly retard mental and physical development. The thyroid hormones also are noted for elevating metabolic rate in mammals and birds.

Soils deficient in iodine occur in many parts of the world. Plants that grow in these soils tend to be low in iodine, and people who depend on local foods in these areas often suffer from iodine deficiency. Iodized table salt (salt containing traces of iodine) was invented to solve this problem. If people purchase iodized salt and use it in a customary way to salt their food, they get enough iodine regardless of whether or not they live in an iodine-deficient part of the world.

Impaired mental development caused by childhood deficiency of thyroid hormones—resulting from iodine deficiency—remains a massive worldwide problem because in many poverty-stricken regions, iodized salt is not readily available. Global human mental performance would be raised in a generation if iodine deficiency during pregnancy and childhood could be eliminated.

Goiter is another hormone deficiency disease that (in its most common form) results from inadequate dietary iodine. It illustrates that the proper operation of feedbacks in endocrine control systems can be critical for health. Goiter refers to an enlarged thyroid gland (FIGURE 35.12). It was common in many parts of the United States until use of iodized salt became the norm.

Figure 35.12: Goiter

Adults develop goiter when they get too little iodine in their diet and, as a result, their thyroid produces inadequate amounts of thyroid hormones. When thyroid hormones are present in sufficient amounts in the blood, they exert negative feedback on the production of thyroid-stimulating hormone (TSH) by the anterior pituitary. This feedback does not take place in people producing only small amounts of thyroid hormones. Their blood levels of thyroid hormones are low, and accordingly such people produce high levels of TSH due to a lack of negative feedback on TSH production. Their steady, high production of TSH stimulates their thyroid gland to grow to excessive size.

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Sex steroids control reproductive development

Many hormones, including the thyroid hormones, play critical roles in mammalian development. In certain ways, however, the hormones of greatest importance for development are the sex steroids and the hypothalamic and pituitary hormones that control them.

The gonads—the testes of males and the ovaries of females—produce steroid hormones as well as gametes (sperm and ova). Many of these hormones are described as androgens or estrogens. Masculinizing steroid hormones are called androgens (Greek, “male-makers”). Testosterone is an androgen and is the principal hormone produced by the testes. It was named testosterone when it was discovered in 1935 for the two-fold reason that it was collected from testes and it belongs chemically to the family of cholesterol-like compounds (see Figure 35.4B).

Go to MEDIA CLIP 35.1 The Testosterone Factor

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Feminizing steroid hormones are called estrogens. Two or more types of estrogen molecules are typically secreted, one often being estradiol (see Figure 35.4B). The ovaries produce estrogens and another female sex steroid, progesterone, that is involved principally in coordinating processes associated with pregnancy. During pregnancy, the placenta also secretes estrogens and progesterone. We will discuss these hormones—and the hormones that control their secretion—at length in Chapter 37.

Androgens are not exclusive to males, and estrogens are not exclusive to females. Both sexes make both androgens and estrogens. However, the relative blood concentrations differ in the two sexes. In some cases androgens are simply precursors in the synthesis of estrogens, to which androgens are converted by a process called aromatization.

During embryonic development, the sex steroids play a key role in controlling whether a human embryo develops phenotypically into a male or female. Androgens are required for male phenotypic development, and in their absence an embryo develops as a female. The sex of an individual mammal is determined genetically at the time of conception. Females inherit two X chromosomes, whereas males inherit an X and a Y chromosome (see Concept 37.1). At the start of embryonic development, the gonads and external genital structures are the same whether an individual is XX or XY. In males, early in development a gene on the Y chromosome, the SRY gene, causes the gonads to differentiate as testes (otherwise they become ovaries). As the differentiation into testes proceeds, the gonads of a male embryo begin to produce testosterone. In humans, starting 2–3 months after conception, testosterone increases from a low blood concentration to a high blood concentration, remains high for about 2 months, and then falls to a low concentration again because the gonads stop producing it rapidly. This period of high blood testosterone causes the external genital structures to differentiate into the male form. Without testosterone, the structures differentiate into female form.

Particular external genital structures diverge into male or female form during this phenotypic sexual differentiation. The scrotum of males and certain labial folds of females develop from the same early structures (FIGURE 35.13). The head (glans) of the penis and the head of the clitoris also have the same embryonic origins. The gonads are initially located in the abdomen in both sexes. In males, the testes migrate out of the abdomen into the scrotum. The ovaries remain in the abdomen in females.

Figure 35.13: Sex Steroids Direct the Prenatal Development of Human Sex Organs The external sex organs of early human embryos are undifferentiated. Testosterone promotes the development of male external sex organs. In its absence, female sex organs form.

Just as hormones play a crucial role in embryonic development, they again play a crucial role at puberty. Recall that luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both anterior pituitary tropic hormones that act on the gonads—are together called the gonadotropins. Their secretion by the anterior pituitary is under the control of hypothalamic neurosecretory cells that produce gonadotropin-releasing hormone (GnRH). As we have been doing, let’s focus on humans as we discuss the process of puberty.

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When a person is between about 1 year of age and the start of puberty, the GnRH neurosecretory cells in the hypothalamus are quiet, producing a GnRH pulse only once every few hours. Puberty begins when these cells suddenly shift to producing GnRH pulses at a far higher rate, a change that results in a sustained increase in the average concentration of GnRH in blood flowing into the anterior pituitary. Despite many theories, researchers remain uncertain about the causes of the change in activity of the GnRH neurosecretory cells. The consequences, however, are well known and dramatic.

The anterior pituitary responds to the increase in the pulse rate of GnRH secretion with a large increase in FSH and LH secretion. In males, the elevated blood concentration of LH stimulates cells in the testes to produce testosterone vigorously. A large increase occurs in the blood concentration of testosterone, which (along with other androgens) brings about many changes. Testosterone enters cells, binds to intracellular receptors, and alters gene expression throughout the body. As a consequence, a boy’s voice deepens, hair begins to grow on his face and body, his skeletal muscles increase in mass, and his testes and penis grow larger. The increased blood concentration of FSH stimulates the production of sperm.

In females, increased blood levels of LH stimulate the ovaries to increase production of estrogens. Concentrations of estrogens in the blood rise and initiate development of many traits: a girl’s breasts, vagina, and uterus become larger; her hips broaden; she develops increased subcutaneous fat; and her menstrual cycles begin. Increased FSH stimulates the maturation of ovarian follicles, which are necessary for production of mature eggs (see Concept 37.1).

Sex steroids continue to play essential roles in reproductive cycles and sexual functioning throughout adulthood. We will discuss these roles in Chapter 37.

CHECKpoint CONCEPT 35.4

  • Boys are occasionally born with a testicle missing from the scrotum. The missing testicle is nearly always in the abdomen, and its position can be corrected surgically. But why might a testicle be in the abdomen at birth?
  • Explain why the thyroid gland can grow to great size when it secretes inadequate amounts of thyroid hormones because of iodine deficiency.
  • What endocrine event in the hypothalamus initiates puberty?

Now that we’ve examined vertebrate endocrine systems, let’s conclude with a look at endocrine systems in invertebrates. Of all the invertebrates, the insects are the best understood in this regard. A striking parallel is that endocrine controls are of enormous importance in insect development, just as they are in vertebrate development.