Puberty in Males

Puberty can be defined as the age at which the male gonads are capable of releasing the gametes. In males, it is characterized by the development of secondary sexual characteristics ability to copulate and produce sperm.

Table 19.9 Age for the onset of puberty and sexual maturity of male domestic and wild animals

^a Zoo animals attain puberty earlier than wild

^b Male cheetahs attain puberty earlier than their females

contain about 50 million spermatozoa with more than 10% mortality around puberty.

19.4.1 EndocrineBasisofPuberty

The attainment of puberty is a consequence of HPG activa­tion in response to external and internal factors. The activa­tion of the HPG axis results in the secretion of a large quantity of testosterone for spermatogenesis and the devel­opment of characteristics sexual behavioural patterns. The concentration of testosterone from pre-pubertal to pubertal

Fig. 19.24 Effect of various factors on puberty

period can be increased to the tune of 70-890 pg/mL in bulls. The detailed mechanism of HPG activation around puberty has been discussed (see Sect. 19.2.6 Endocrine regulation of puberty).

19.4.2 Factors Affecting the Puberty

The physiological events of puberty are the result of the integration between the environment and the HPG axis. Therefore, it depends on an individual’s seasonal rhythms, genetic makeup, body growth, nutritional status, social status, and stress response (Fig. 19.24 and Table 19.9).

19.4.2.1 Genetic Factors

The onset of puberty is a multigenic trait with high heritabil- ity (0.2-0.48 in heifers). Generally, species of smaller size experience puberty at an early age compared to larger-sized species, particularly in cattle and horses. In rabbits, the mini­ature breeds reach sexual maturity at 4-5 months; medium breeds at 5-6 months; large breeds at 7-8 months; and giant breeds after 8 months.

19.4.2.2 Nutritional Factors

The role of nutrition and body growth in puberty attainment has been established for a long time. According to the ‘criti­cal fat mass hypothesis’, critical fat is essentially required to attain puberty. As a thumb rule, the onset of puberty gener­ally occurs when an animal reaches its 55-65% of adult body weight, depending on breed. The animals with a low protein and low total digestible nitrogen (TDN) diet experience delayed puberty. The deficiency of some specific elements like iodine, phosphorus, calcium, copper, iron, cobalt, man­ganese, zinc, vitamin A, and vitamin E also suppresses the pubertal onset. Metabolic cues such as glucose, insulin, and leptin have been shown to regulate the HPG axis directly or indirectly through a complex neuronal network (see Sect.

19.2.6.1 in Energy metabolism and HPG axis activation).

Know More...

The Evolutionary Advantage of Seasonal Breeder

Wild animals have various physiological adaptations and uniqueness in reproductive perfor­mance. In male animals, puberty generally attains later than in females as the spermatogenesis process is relatively longer, comparing the follicular maturation and ovulation in its complementary female. Male also required more energy than females for courtship, com­peting with others in the herd. Animals with short life expectancy are very ‘opportunist’ in sexual perfor­mance throughout the year, while animals with more extended life expectancy become ‘strategic’ in repro­duction to give birth in the favourable season with plenty of food availability to facilitate the growth of

the young.

19.4.2.3 Environmental Factors

The geographical location, season, and photoperiod are the important environmental determinants to regulate the onset of puberty. Animal of tropical region attains puberty earlier. Photoperiod is an important environmental cue that determines puberty onset in seasonal breeders (see Sect.

19.2.6.2 Photoperiod and HPG axis activation). The onset of puberty is determined by the birth timing in the seasonal breeders. The lambs born during spring attain puberty in the autumn of next breeding season, but the lambs born during autumn reach puberty 10-12 months later in the autumn of the next breeding season. The delay may be due to prolonged steroid feedback over the hypothalamus. In cattle and buffaloes, the long day of photoperiodic exposure increases body weight gain but hastens the pubertal onset. Animal of tropical region attains puberty later. The animals exposed to heat stress exhibit delayed puberty. Increasing day length (including artificial light) reduces the age of puberty and sexual maturity in birds.

19.4.2.4 Interaction with the Opposite Sex

The interaction with the opposite sex causes early puberty and maturity due to bio-stimulation by pheromone. Male pheromone can induce early puberty in cows, sheep, goats, and pigs. The introduction of males can induce LH surge and ovulation in a flock of anestrus sheep and goats during the non-breeding season. This phenomenon is called the male effect and is used widely to induce oestrus in these species.

19.4.2.5 Stress

Prolonged or chronic stress has a negative effect on the HPG axis, thus delaying the onset of puberty. Stress-induced HPG axis suppression is mediated through the HPA axis detailed in the previous chapter (Sect.

19.4.3 Manipulation of Puberty

Age at puberty and sexual maturity are important economic traits in farm animal practices. Delayed puberty causes huge economic losses. The age of puberty in farm animals can be manipulated through genetic selection, nutritional management, improvement of the microenvironment through housing and other managemental practices, and hormonal interventions. The scrotal circumference is an important selection marker of males to get offspring with lower age of puberty. Crossbreeding between Zebu and exotic cattle is also recommended for the same reason. Balanced nutrition together with good husbandry practices helps to achieve better growth and early sexual maturity. Proteins and energy are the most important nutrients that influence the growth of animals. Adequate mineral supplementation is also required for optimum metabolic process of the body and improves the pubertal onset.

Learning Outcomes

• Testes: The primary sex organ of the male repro­ductive system is the testes which serve two impor­tant functions, viz. spermatogenesis and steroidogenesis. Spermatogenesis occurs within seminiferous tubules of the testes with the active support of Sertoli cells and Leydig cells. The Leydig cells are primarily concerned with steroidogenesis by modulating its function by containing steroido­genic enzymes such as cytochrome P450 oxidase (s) (P450scc). Functional features of both the cells are unique in an individual animal’s pre-pubertal, adult and senile state. The specialized thermo­regulatory system and blood-testis barriers help to create a congenial environment for spermatogenesis.

• Excurrent tract: The excurrent tract consists of rete testis, efferent ducts (vasa efferentia), epididymis, vas deferens, and urethra. It involves in maturation, storage, and passage of spermatozoa. The spermatozoa gain their maturity during their passage through the excurrent tract, particularly epididymis.

• Accessory sex glands: The secretions of accessory sex glands, viz. ampulla, seminal vesicles, prostate, and bulbourethral glands (Cowper’s glands) are col­lectively called seminal plasma that acts as a vehicle for spermatozoa during its transport. The seminal plasma acts as a buffer and protects the spermatozoa from the harsh acidic environment of the female genital tract. The functional morphology of acces­sory sex glands varies between species and results in

distinct properties of semen and spermatozoa between species.

• Ancillary organs: The penis and prepuce, two accessory structures of the male reproductive sys­tem involved in the ejaculation of semen into the female genital tract. The penis is a fibroelastic struc­ture covered by a mucocutaneous tissue called the prepuce. The morphological features of the penis and prepuce vary between species, resulting in dif­ferent ejaculatory features between species.

• H-P-G axis: The gonadotropin-releasing hormone (GnRH) of the hypothalamus stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinising hormone (LH). The FSH and LH, in turn, control spermatogenesis and ste­roidogenesis. The H-P-G axis remains quiescent till puberty. The pubertal activation of the H-P-G axis is mediated by the nutritional factors and photoperiod with the involvement of neuroendocrine factors. The H-P-G axis is influenced by age, stress, envi­ronmental factors (season), and circadian rhythm.

• Androgens: The major androgens are testosterone, dihydrotestosterone (DHT), and androstenedione, out of which testosterone is most abundant in blood. The testosterone is synthesized in the Leydig cells and transported in conjugation with plasma proteins. After metabolism, it is excreted through faeces and urine. Testosterone is primarily involved in spermatogenesis, and it also helps to develop the secondary sex characteristics of a male.

• Puberty: Puberty is the age at which the male gonads are capable of releasing the gametes. In males, it is characterized by the development of secondary sexual characteristics ability to copulate and produce spermatozoa. The onset of puberty requires complex and integrated biological sequences that lead to progressive sexual maturation and full reproductive capacity through the activation of the H-P-G axis.

Exercises

Objective Questions

Q1. Which part of the testis shows exocrine activity, and which part has an endocrine role?

Q2. Extreme ambient temperature causes temporary infer­tility in certain species, termed as ?

Q3. Which cell population has a direct physiological role in the seasonal breeding activity of males?

Q4. Which endocrine axis has ‘auto-control’ in the Sertoli cell function?

Q5. Which cells of the male reproductive system utilize vitamin A (retinol) in the spermatogenesis process?

Q6. What is the basis of the functional relationship between GnRH secretions with FSH and LH from gonadotropes?

Q7. In the hypothalamus, which neuroendocrine

bio-molecule acts like the RFamide-Related Peptide (RFRP)?

Q8. When a male can produce its offspring successfully, it is called ?

Q9. In any stress, which hormone reduces the

responsiveness of the receptors for Luteinizing hor­mone in Leydig cell?

Q10. Which is the biologically most potent form of testosterone?

Q11. Which enzyme is responsible for converting testoster­one to oestradiol?

Q12. Which is the major site of production of sex steroid- binding globulin (SHBG)?

Q13. Which forms of testosterone are mostly available in urine and faeces?

Q14. In which form of testosterone facilitates brain development?

Q15. Which peptide hormones have a role in epididymal migration?

Q16. Which part of the epididymis is mostly responsible for morphological and ionic changes of spermatozoa?

Q17. Fertility-associated proteins are released mostly from which part of the male reproductive system?

Q18. Which part of the excurrent tract acts as a common pathway for transmission of both urine and semen?

Q19. Which glandular secretions clean the passage of the reproductive tract before ejaculation of semen?

Q20. In a dog, which part of the penis favours locking during mating?

Subjective

Q1. Why ablations of the gene for INSL3 cause cryptorchidism?

Q2. Write the adoptive characteristics features of avian species in testicular thermoregulation.

Q3. Why is the blood-testis barrier significant to maintaining fertility?

Q4. Why can’t the exogenous administration of testoster­one alone influence the spermatogenesis process?

Q5. Write the functional differences between FLC and ALC.

Q6. Write the various factors that can control the HHG axis.

Q7. Write the factors affecting puberty.

Q8. How is obesity related to spermatogenesis?

Q9. Why does spermatogenesis initiate only after puberty? Q10. Write the biological functions of testosterone.

Q11. Write the role of the epididymis in sperm maturation.

Q12. Write the role of seminal vesicles in semen production in a bull.

Q13. Write the role of specific biomarkers to access the prostate gland activity.

Q14. Describe the functional and morphological features of the penis during erection?

Q15. Write the events of erection and ejaculation in the bull?

Answer to Objective Questions

A1. The exocrine part is the seminiferous tubules, and the endocrine part is the interstitial or Leydig cells

A2. Summer sterility

A3. Sertoli cell

A4. Inhibin-activin-follistatin axis

A5. Peritubular myoid (PTM) cells

A6. Characteristics of pulsatile release of GnRH

A7. Gonadotropin-inhibiting hormone (GnIH)

A8. Sexual maturity

A9. Glucocorticoids

A10. Dihydrotestosterone (DHT or 5α-DHT)

A11. Aromatase

A12. Liver

A13. Glucuronidated or sulphated products of testosterone

A14. Oestradiol

A15. Oxytocin and vasopressin

A16. Body or corpus of the epididymis

A17. Epididymis

A18. Urethra

A19. Bulbourethral (Cowper’s) glands

A20. Bulbus glands

Keywords for the Answer to Subjective Questions

A1. Testicular descent, Role of genetic factors, Cryptorchidism

A2. Testis position, period of spermatogenesis, genetical adaptability of avian spermatozoa

A3. Morphological structure of blood-testis barrier, micro­environment, damage of barrier, and development of immune response

A4. Mechanism of action of testosterone, role of Sertoli cells in spermatogenesis, the functional relationship between testosterone and Sertoli cells

A5. Steroidogenesis, organogenesis, relation with puberty

A6. Axis activators, axis suppressors, physiological modu­lation in various rhythms, including seasonal breeding

A7. Genetical variation, environmental variation, hormones involves in the HHG axis

A8. Functional relationship with obesity and HHG axis, role of oestrogen, leptin, and testosterone in spermato­genesis, leptin and puberty relationship

A9. Role of adult Leydig cells, activation of cytochrome P450 oxidases, role of steroidogenic acute regulatory protein (StAR)

A10. In foetal life, before puberty, after puberty

A11. Biochemical changes of seminal plasma, structural and functional changes in spermatozoa, secretory role of the epididymis

A12. Major secretory role of seminal vesicles, secretion of proteins, modulation of glandular function

A13. Relationship with various hormones and benign pros­tatic hyperplasia (BPH), prostate-specific antigen (PSA), prostatic acid phosphatase (PAP)

A14. Erectile tissues, penile erection, sigmoid flexure, and species variations

A15. The rigidity of the penis, emission, and expulsion, spiralling of the penis

Acknowledgement We are grateful to Prof. Sagar Sanyal, Ex. Professor and Head, and Dr. Prabal Ranjan Ghosh, Associate Professor, Department of Veterinary Physiology, West Bengal Univer­sity of Animal and Fishery Sciences, Kolkata, West Bengal, India, for their valuable guidance and assistance during the preparation of the chapter. We are also greatly indebted to the help of Dr. Parthib Roy, Master Scholar, Department of Veterinary Gynaecology and Obstetrics, for the preparation of the figures.

Further Reading

Textbooks

Liverman CT, Blazer DG (2004) In: Testosterone and Aging: Clinical Research Directions. Institute of Medicine (US) Committee on Assessing the Need for Clinical Trials of Testosterone Replacement Therapy. Washington (DC): National Academies Press (US); pp 1-240. https://www.ncbi.nlm.nih.gov/books/NBK216175/

Melmed S, Polonsky KS, Larsen PR, Kronenberg HM (2017) Williams textbook of endocrinology, 13th edn. Elsevier, pp 1855-1916. https://doi.org/10.1016/C2013-0-15980-6

Noakes DE, Parkinson TJ, England GCW (2019) Veterinary reproduc­tion and obstetrics, 10th edn. Elsevier, pp 1-837. https://doi.org/10. 1016/C2014-0-04782-X

Research Articles

Bonneau M, Carrie-Lemoine J, Prunier A, Garnierm DH, Terqui M (1987) Age-related changes in plasma LH and testosterone concen­tration profiles and fat 5α-androstenone content in the young boar. Anim Reprod Sci 15(3-4):241-258

Byrne CJ, Fair S, English AM, Urh C, Sauerwein H, Crowe MA, Lonergan P, Kenny DA (2018) Plane of nutrition before and after 6 months of age in Holstein-Friesian bulls: II. Effects on metabolic and reproductive endocrinology and identification of physiological markers of puberty and sexual maturation. J Dairy Sci 101(4):3460-3475

DePalatis L, Moore J, Falvo RE (1978) Plasma concentrations of testos­terone and LH in the male dog. J Reprod Fertil 52(2):201-207. https://doi.org/10.1530/jrf.0.0520201

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