Important bull sperm morphological defects:
Nuclear Vacuoles
- How to detect small apical vacuoles
- What causes nuclear vacuoles
- When to recheck bulls
- Bull prognosis
Nuclear vacuoles are sometimes called crater defects and are categorized as diadem defects, large confluent vacuoles, and small apical vacuoles. All forms of vacuoles have been shown to cause a decrease in fertility rates. These abnormalities do not prevent binding of the sperm to the oocyte; however, vacuole defects decrease fertilization rates and negatively affect development of morulae and blastocysts. (Guajira, 2010) Bulls with levels as low as 20% vacuolated sperm will expect to have reduced fertility. (Barth, 2013)
Large Confluent Vacuoles
Large confluent vacuoles and many diadem defects are easily detected using light microscopes and eosin nigrosin stains. Single small apical vacuoles and some diadem defects are more difficult to detect without the use of microscopes that will create better contrast, these include phase contrast and DIC microscopes. (Perry, et al., 2019)
DIC microscope
Heat stress is a common cause of all types of nuclear vacuoles. The vacuoles are usually detected about 15 days after the insult and may last for as little as 1 week and up to 4 weeks.(Rahman, et al., 2018) A history of heat stress can be very helpful in determining when the best time to recheck the bull. Stress as mimicked by an injection of dexamethasone caused vacuoles to occur from 2 to 4 weeks after the injection. There is very strong evidence that nuclear vacuoles have both an environmental and genetic component. (Guajira, 2010) (Barth, 2013) This occurs when a bull has a genetic predisposition for the defect which only appears when the heat stress occurs. Some bulls do not respond negatively to stress or excessive heat, while other will. There is also evidence that some breeds of cattle will have more of this defect than others. Bull producers should be made aware of these epigenetic concerns, allowing pressure to be place on the genetics to help improve the gene pool.
Other causes can be viral infections, toxins, and a case of a Sertoli cell tumor in a bull. Some bulls will have nuclear vacuoles without any known signs or history to indicate a disturbance in spermatogenesis. (Barth, 2013)
Severely affected bulls may recover over a short amount of time, then reverting to periods of producing a high percentage of vacuolated sperm. Therefore, bulls that produce semen with over 20% nuclear vacuoles should be culled or monitored closely for the rest of his life.
Phase Contrast
phase contrast
phase contrast
References
Barth, A. D., 2013. Bull Breeding Soundness. Third ed. Saskatoon(Saskatchewan): Western Canadian Association of Bovine Practitioners.
Guajira, A., 2010. Diadem/Crater Defect in Spermatozoa of a Brhman Bull: Seminal Traits, Micoscopic Findings IVF Fertility. Genetic Predisposition?. Mol. Reprod. Dev., Volume 77, p. 1000.
Perry, V. A. et al., 2019. Semen Collection and Evaluation (Chapter 7). In: s.l.:Prepublished document obtained through direct communication with Viv Perry, p. 1.
Rahman, M. B., Schellander, K., Luceno, N. L. & Soom, A. V., 2018. Heat stress responses in spermatozoa: Mechanisms and consequences for cattle fertility. Theriogenology, Volume 113, pp. 102-112.