Gonads

Gonads refer to the testes and ovaries present in male and female animals, respectively. They are responsible for the production of gametes and hormones, which are crucial elements governing sexual reproduction in animals (Table 17.4).

17.6.1 Ovary

The follicle and corpus luteum developed sequentially are the transient endocrine organs that secrete two major steroids: estro­gen and progesterone, respectively. The granulosa cells present in the follicle produce estrogen from androgens synthesised in the theca cells. Estrogen plays a vital role in the estrous cycle, development of female secondary sexual characteristics, ductular growth in the mammary gland, and parturition. Apart from the steroidal hormones, peptide hormones such as inhibin, activin, follistatin, and anti-Mullerian hormone are produced from the ovarian follicle. Whereas progesterone secretion to support gestation and lobuloalveolar development in the mam­mary gland is the major endocrine function of the corpus luteum. In addition, it also secretes relaxin and oxytocin that are responsible for the widening of the birth canal and luteal demise in cyclical animals.

Table 17.4 List of gonadal hormones and their functions

17.6.2 Testes

The Sertoli and Leydig cells are the somatic cell types with endocrine function in testes.

Endocrinology to Rescue Wild Animals...

• Several wild animal species are staring at an impending threat of extinction more than ever in the history. The ex situ breeding and managemental practices play a huge role in reviving their popula­tion and curtail them from getting extinct. Endo­crinological approaches such as measuring the plasma levels of hormones such as estrogen, proges­terone, and cortisol help in assessing the reproduc­tive status and well-being of different wild animals. They also greatly contribute in understanding the reproductive behavior of animals and aid in devising intervening strategies of breeding in captive wild animals.

Learning Outcomes

• Natriuretic peptides: ANP, BNP, and CNP are the major natriuretic peptides secreted from cardiac tis­sue and CNS. They are secreted due to the stretch of cardiac cells due to an increased blood volume. The natriuretic peptides (ANP and BNP) are responsible for the excretion of sodium and water from kidneys to regulate the circulatory volume to a normal level. They cause an increase in the glomerular filtration via their vasodilatory effect on afferent arteriole. Decreased tubular absorption of sodium and water leads to an increased excretion with a simultaneous decrease in the blood volume. The natriuretic and diuretic effects of natriuretic peptides antagonize the antidiuretic action of aldosterone, which is known as the aldosterone escape.

• Erythropoietin: It is a glycoprotein hormone released from the kidneys in response to tissue hypoxia. The bone marrow as its target organ, EPO acts on the erythropoietic stem cells to increase their proliferation, differentiation, and subsequent trans­formation into the erythrocytes.

• Thymus: A major organ determines the cell- mediated immunity in animals by acting as the site of differentiation and production of T-cells. Addi­tionally, the thymic epithelial cells possess the endo­crine activity and secrete hormones such as thymulin, thymopoietin, and thymosins. These hormones primarily act to regulate the activation and differentiation of T-cells and modulate the immune status of the animals.

• Gut hormones: The enteroendocrine cells present in the GIT produce a wide variety of peptide hormones. The enteroendocrine cells act as nutrient sensors and secrete the hormones that control the appetite, gastrointestinal motility, exocrine secretion of the pancreas, liver, and GIT. All these hormones work in concert to harvest appropriate amounts of nutrients by tightly regulating the process of diges­tion and metabolism.

• Placenta: Recognized as a crucial transient organ formed during the early gestation period and respon­sible for supplying the nutrients to the fetus. Fur­thermore, it also secretes important hormones such as progesterone, estrogen, chorionic gonadotropins, placental lactogen, and relaxin. Together, they ensure the maintenance of pregnancy, fetal growth, and progression of parturition.

• Gonads: The production of gametes and hormones that control sex-specific behaviors in animals are two important functions of gonads. The testes in male animals produce testosterone, responsible for libido, secondary sexual characteristic, anabolic effects on bone and muscles. Estrogen is the major circulatory steroid produced by the ovarian follicles responsible for reproductive cyclicity, increasing bone mass, growth of reproductive tract and mam­mary gland in female animals. In addition, corpus luteum formed after ovulation produces progester­one, which helps in the growth of endometrium and immunotolerance to support the fetal growth in the uterus.

Exercises

Objective Questions

Q1. ___________ is a natriuretic peptide that cannot pro­

mote the excretion of sodium

Q2.

to initiate the signal transduction.

Q3. The antagonistic effects of natriuretic peptides on aldo­sterone are collectively known as

Q4. The secondary messenger produced in the target cells when bound with natriuretic peptides is

Q5. Erythropoietin is a____________ type of hormone, and

released primarily due to_______________.

Q6. The thymic hormones are produced by cells present in the thymus

Q7. Which thymic hormone requires zinc for its biological activity?

Q8. What is the first hormone to be discovered?

Q9. The cells in GIT that produce gut hormones are collec­tively called as

Q10. Which hormone is responsible for migrating myoelec­tric complex?

Q11. The gut hormones that stimulate insulin secretion from the pancreas are known as

Q12. Which hormone is responsible for the relaxation of pelvic symphysis before parturition?

Q13. The specialized areas in equine placenta that secrete eCG are called as

Q14. Which placental hormone has structural homology of both GH and PRL?

Q15. Which gonadal hormone is responsible for the lobuloalveolar development of the mammary gland?

Subjective Questions

Q1. Enlist the different types of natriuretic peptides and their major production sites.

Q2. Briefly describe the biological effects of ANP.

Q3. What is EPO? Describe its effect on the bone marrow.

Q4. Explain the mechanism of production of EPO from kidneys.

Q5. Describe briefly the endocrine role of the thymus

Q6. What are the orexigenic hormones produced by the GIT?

Q7. What are the biological effects of gut hormones?

Q8. Enlist various placental hormones and briefly describe their biological effects in animals

Q9. What are the cyclical endocrine structures developed in the ovary and the respective hormones secreted from them?

Q10. What are the different somatic cells present in the testes and list their hormones?

Answers to Objective Questions

A1.

A2. Guanylyl cyclase

A3. Aldosterone escape

A4. cGMP

A5. Glycoprotein, tissue hypoxia

A6. Thymic epithelial cells

A7. Thymulin

A8. Secretin

A9. Enteroendocrine cells

A10. Motilin

A11. Incretins

A12. Relaxin

A13. Endometrial cups

A14. Placental lactogen

A15. Progesterone

Answers to Subjective Questions

A1. ANP, BNP, CNP, cardiac muscle, and CNS

A2. Natriuresis, diuresis, vasodilation in arterioles, and inhibit aldosterone secretion

A3. Erythropoietin, glycoprotein, kidney, and erythropoiesis

A4. Tissue hypoxia, HIF1α, oxygen-sensitive

hydroxylases, and hypoxia-response element

A5. Thymic epithelial cells, thymulin, thymopoietin, and thymosins

A6. Ghrelin, motilin, and insulin-like peptide 5 (INSL5)

A7. Regulating GIT motility, exocrine pancreatic secretion, gastric acid secretion, and appetite

A8. Progesterone, estrogen, placental lactogen, chorionic gonadotropins, and relaxin

A9. Follicle, corpus luteum, estrogen, and progesterone

A10. Sertoli cells-estrogen, inhibin; Leydig cells- testosterone

Further Reading

Textbooks

Hafez ESE, Hafez B (eds) (2013) Reproduction in farm animals. Wiley McDonald LE, Pineda MH, Dooley MP (2003) McDonald’s veterinary endocrinology and reproduction. 5th ed./edited by M.H. Pineda, with the editorial assistance of Michael P. Dooley. Ames, Iowa: Iowa State Press. Print

Norman AW, Henry HL (2022) Hormones. Academic Press. https://doi. org/10.1016/B978-0-08-091906-5.00007-0

Takei Y, Ando H, Tsutsui K (eds) (2015) Handbook of hormones: comparative endocrinology for basic and clinical research. Aca­demic Press. https://doi.org/10.1016/C2013-0-15395-0

Articles

Ahlman H, Nilsson O (2001) The gut as the largest endocrine organ in the body. Ann Oncol 12:S63-S68. https://doi.org/10.1093/annonc/ 12.suppl_2.S63

Alvarez-Oxiley AV, De Sousa NM, Beckers JF (2008) Native and recombinant bovine placental lactogens. Reprod Biol 8(2):85-106. https://doi.org/10.1016/S1642-431X(12)60006-0

Goetze JP, Bruneau BG, Ramos HR, Ogawa T, de Bold MK, de Bold AJ (2020) Cardiac natriuretic peptides. Nat Rev. Cardiol 17(11):698-717. https://doi.org/10.1038/s41569-020-0381-0

Hall CM, Glaser S, Alpini G (2017) Gastrointestinal Hormone (GI) Regulated Signal Transduction. https://doi.org/10.1016/B978- 0-12-809324-5.03384-8

Latorre R, Sternini C, De Giorgio R, Greenwood-Van Meerveld B (2016) Enteroendocrine cells: a review of their role in brain-gut communication. Neurogastroenterol Motil 28(5):620-630. https:// doi.org/10.1111/nmo.12754

Savino W, Dardenne M (2000) Neuroendocrine control of thymus physiology. Endocr Rev 21(4):412-443. https://doi.org/10.1210/ edrv.21.4.0402

Schuler G, Furbass R, Klisch K (2018) Placental contribution to the endocrinology of gestation and parturition. Anim Reprod 15(Suppl 1):822-842. https://doi.org/10.5167/uzh-167451