Gonadal and Placental Hormones

 

This section briefly discusses the hormonal role of the gonads—the testes and ovaries—which produce the sex cells (sperm and ova) and secrete the gonadal hormones. The roles of the gonadotropins released from the anterior pituitary (FSH and LH) were discussed earlier.

Location of the testes and ovaries, both of which act as hormone secreting structures of the reproductive system.

Figure 1. Location of the testes and ovaries, both of which act as hormone secreting structures of the reproductive system.

The primary hormone produced by the testes is testosterone, a steroid hormone important in the development of the sperm producing and conducting (SPC) organs, the maturation of sperm cells, and the development of secondary sex characteristics such as a deepened voice, body hair, and increased muscle mass. Interestingly, testosterone is also produced in the ovaries, but at a much reduced level. In addition, the testes produce the peptide hormone inhibin, which inhibits the secretion of FSH from the anterior pituitary gland. FSH stimulates spermatogenesis.

The primary hormones produced by the ovaries are estrogens, which include estradiol, estriol, and estrone. Estrogens play an important role in a larger number of physiological processes, including the development of the egg producing and conducting (EPC) organs, regulation of the menstrual cycle, the development of secondary sex characteristics such as increased adipose tissue and the development of breast tissue, and the maintenance of pregnancy. Another significant ovarian hormone is progesterone, which contributes to regulation of the menstrual cycle and is important in preparing the body for pregnancy as well as maintaining pregnancy. In addition, the granulosa cells of the ovarian follicles produce inhibin, which—as in those with SPC organs—inhibits the secretion of FSH.  During the initial stages of pregnancy, an organ called the placenta develops within the uterus. The placenta supplies oxygen and nutrients to the fetus, excretes waste products, and produces and secretes estrogens and progesterone. The placenta produces human chorionic gonadotropin (hCG) as well. The hCG hormone promotes progesterone synthesis and reduces the pregnant individual’s immune function to protect the fetus from immune rejection. It also secretes human placental lactogen (hPL), which plays a role in preparing the breasts for lactation, and relaxin, which is thought to help soften and widen the pubic symphysis in preparation for childbirth. The hormones controlling reproduction are summarized in Table 1.

Table 1. Reproductive Hormones
Gonad Associated hormones Chemical class Effect
Testes Testosterone Steroid Stimulates development of SPC organs, secondary sex characteristics and sperm production
Testes Inhibin Protein Inhibits FSH release from pituitary
Ovaries Estrogens and progesterone Steroid Stimulate development of EPC organs, secondary sex characteristics and prepare the body for childbirth
Placenta Human chorionic gonadotropin Protein Promotes progesterone synthesis during pregnancy and inhibits immune response against fetus

Everyday Connections: Anabolic Steroids

The endocrine system can be exploited for illegal or unethical purposes. A prominent example of this is the use of steroid drugs by professional athletes.

Commonly used for performance enhancement, anabolic steroids are synthetic versions of the sex hormone, testosterone. By boosting natural levels of this hormone, athletes experience increased muscle mass. Synthetic versions of human growth hormone are also used to build muscle mass.

The use of performance-enhancing drugs is banned by all major collegiate and professional sports organizations in the United States because they impart an unfair advantage to athletes who take them. In addition, the drugs can cause significant and dangerous side effects. For example, anabolic steroid use can increase cholesterol levels, raise blood pressure, and damage the liver. Altered testosterone levels (both too low or too high) have been implicated in causing structural damage to the heart, and increasing the risk for cardiac arrhythmias, heart attacks, congestive heart failure, and sudden death. Steroids can also produce side effects including shriveled testicles and enlarged breast tissue in those with SPC organs. In those with EPC organs, their use can cause an enlarged clitoris and growth of facial hair. In both those with SPC and EPC organs, their use can promote increased aggression (commonly known as “roid-rage”), depression, sleep disturbances, severe acne, and infertility.

Chapter Review

The reproductive system is regulated by follicle-stimulating hormone (FSH) and luteinizing hormone (LH) produced by the anterior lobe of the pituitary gland in response to gonadotropin-releasing hormone (GnRH) from the hypothalamus. In those with SPC organs, FSH stimulates sperm maturation, which is inhibited by the hormone inhibin. The steroid hormone testosterone, a type of androgen, is released in response to LH and is responsible for the maturation and maintenance of the SPC organs, as well as the development of secondary sex characteristics. In those with EPC organs, FSH promotes egg maturation and LH signals the secretion of the sex hormones estrogen and progesterone. Both of these hormones are important in the development and maintenance of the EPC organs, as well as secondary sex characteristics and maintaining pregnancy. The placenta develops during early pregnancy, and secretes several hormones important for maintaining the pregnancy.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

  1. Compare and contrast the role of estrogens and progesterone.
  2. Describe the role of placental secretion of relaxin in preparation for childbirth

Glossary

estrogens: class of sex hormones found predominately in those with EPC organs important for the development and growth of the EPC organs, secondary sex characteristics, the menstrual cycle, and the maintenance of pregnancy

inhibin: hormone secreted by the testes and ovaries that inhibits FSH production by the anterior pituitary

progesterone: hormone found predominately in those with EPC organs that is important in regulating the menstrual cycle and the maintenance of pregnancy

testosterone: steroid hormone secreted by the testes and important in the maturation of sperm cells, growth and development of SPC organs, and the development of secondary sex characteristics