Male Puberty and Spermatogenesis

Puberty

Puberty in a bull happens at around 11 months and is determined when ejaculate contains 50 million spermatozoa with a minimum motility of 10%. It is also determined when scrotal circumference reaches 28 to 29 cm. To determine number of sperm cells, motility and abnormalities, a breeding soundness exam can be done (3). Along with that, bulls should be at a weight between 250 kilograms to 300 kilograms when entering puberty (4).

Four markers that can help identify when puberty has been reached include: Age when behavioral traits are expressed, Age at first ejaculation, Age when spermatozoa first appear in the ejaculate, and Age when the ejaculate contains a threshold number of spermatozoa. Along with scrotal circumference and weight, nutrition also plays a big role in achieving puberty.

Spermatogenesis

Spermatogenesis consists of three phases; proliferation, meiosis, and spermiogenesis. Starting with proliferation or spermatocytogenesis, this phase includes all mitotic divisions of spermatogonia. In order to replenish spermatogonia, stem cell renewal occurs which will provide a continual renewal of stem cells so that new spermatogonia can develop. Next is the meiotic phase where primary spermatocytes undergo meiosis I and start meiosis II where it is arrested in metaphase II until fertilization occurs and meiosis II is complete (5). At the end of the meiotic phase, haploid undifferentiated spermatids are produced. Finally, the last phase is spermiogenesis or the differentiation phase. Here, full differentiation and highly specialized spermatozoon are created from the spermatids. These spermatozoa contain a head, flagellum, midpiece, and principal piece. The bull has eight stages per cycle of seminiferous epithelium. A cycle is a progression through a complete series of cellular stages at one location along a seminiferous tubule. Each cycle length in a bull is 13.5 days which means it takes 13.5 to progress from stage I, spermatogonia A, to stage VIII, where mature spermatids are present. The total length of spermatogenesis is 61 days.

Endocrinology

Leydig cells are located outside of the seminiferous tubules and are sensitive to LH and are responsible for the production of testosterone while Sertoli cells are located inside the seminiferous tubules. Sertoli cells are sensitive to FSH and use the testosterone created by Leydig cells to produce dihydrotestosterone (DHT) and estradiol. DHT is necessary for sexual differentiation duruing embryonic development and promote prostate growth, sebaceous gland activity, and hair growth. Estradiol is responsible for modulating libido, erectile function and spermatogenesis (6). Testosterone is responsible for sex drive, sperm production, normal reproduction, and secondary sex characteristics. Both testosterone and estradiol have negative feedback on the hypothalamus. GnRH is responsible for controlling and stimulating the anterior pituitary to release LH and FSH. Inhibin is synthesized by the Sertoli cells and inhibits synthesis and release of FSH in order to lower the levels of hormones. In the male, LH is released in pulses instead of continuously. LH is regulated by pulsatile discharge which is responsible for keeping testosterone high in the seminiferous tubules and low in systemic circulation. LH is released in pulses rather than continuously. This is to prevent Leydig cells from becoming less responsive to LH and becoming refractory.

Maturation and transport of spermatozoa

When spermatozoa are released into the lumen, they then travel to the epididymis through the efferent ducts. In the epididymis, spermatozoa is stored but more importantly gains the ability to be motile. Along with that the epididymis is responsible for protecting sperm cells by releasing antioxidant enzymes and providing a blood-epididymis barrier to protect sperm from the immune system and other harmful substances. Once sperm is mature and fertile, it is then stored in the tail of the epididymis (7). When ejaculation occurs, sperm is forcefully expelled from the tail into the vas deferens. From the vas deferens, sperm travels through the spermatic cord into the pelvic cavity where it joins with the seminal vesicle to form the ejaculatory duct which will lead into the urethra. This movement is all attributed to rhythmic smooth muscle contractions which help propel the mature sperm (8).

Changes In the Penis during Copulation

Erection

Erection and protrusion happen when the bull finds a cow that is in estrus, or sexually receptive, and her behavior signals him that she can be bred. Stimulation from the cow, or erotogenic stimuli, causes neurons in the bulls "behavior center" in the hypothalamus to become excited. These neurons then trigger parasympathetic neurons, which release NO gas and cause the relaxation of smooth muscles in the penis. The increase in blood flow that comes as a result of this also compresses the emissary veins in the penis preventing the outflow of blood and allowing the maintenance of the erection.

Emission

Emission is when seminal fluids are moved from the accessory glands into the pelvic urethra and mix with spermatozoa. This process happens before and during ejaculation.

Ejaculation

After intromission, stimulation of the glans penis triggers reflexes that move semen into the urethra. Impulses from the glans penis trigger nerves in the spinal cord, which then trigger nerves in the hypogastric plexus and innervate the target muscles that will ultimately cause ejaculation. These muscles include the urethralis muscle, the bulbospongiosus muscle, and the ichiocavernosus muscle. In the bull, this process happens 1-3 seconds after intromission.

References

1. Hall, J. B. (2019). Capturing the Value of Artificial Insemination in Commercial Herds. University of Idaho Extension. https://doi.org/https://beef.unl.edu/documents/RBC-Symposium/2019/RBCS-2019-03-Capturing-the-Value-of-Artificial-Insemination-in-Commercial-Herds.pdf
2. Heat detection and timing of artificial insemination. Select Sires. (2020, November 3). Retrieved October 28, 2022, from https://www.selectsires.com/article/ss-blog/2020/11/03/heat-detection-and-timing-of-artificial-insemination#:~:text=Cows%20should%20be%20inseminated%20within,be%20within%20this%20time%20period.
3. Barthle, C., & Reiling, B. A. (n.d.). Developing Young Bulls . Department of Animal Science. Retrieved October 29, 2022, from https://animal.ifas.ufl.edu/extension/beef/
4. Lunstra, D. D., Ford, J. J., & Echternkamp, S. E. (n.d.). Puberty in Beef Bulls: Hormone Concentrations, Growth, Testicular Development, Sperm Production and Sexual Aggressiveness in Bulls of Different Breeds'. Retrieved October 29, 2022, from https://www.ars.usda.gov/
5. Cooper, G. M. (2000). Meiosis and Fertilization. National Center for Biotechnology Information . Retrieved October 29, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK9901/
6. Schulster, M., Bernie, A. M., & Ramasamy, R. (2016). The role of estradiol in male reproductive function. Asian Journal of Andrology. Retrieved October 29, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4854098/#:~:text=Estradiol%20in%20men%20is%20essential,organs%20important%20for%20sexual%20function
7. James, E. R., Carrell, D. T., Aston, K. I., Jenkins, T. G., Yeste, M., & Salas-Huetos, A. (2020, July 29). The role of the Epididymis and the Contribution of Epididymosomes to Mammalian Reproduction. International Journal of Molecular Sciences. Retrieved October 29, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7432785/