The breeding soundness examination (BSE) involves a complete and systematic evaluation of the reproductive potential of a given male, including mating ability and libido, general physical examination and inspection of the genital organs, and assessment of sperm production and quality. The BSE is not a direct evaluation of fertility: this can be confirmed only by successful production of offspring after breeding a fertile female. The specific male animal must be properly identified, and a detailed history is important because sub- or infertile males might require more exhaustive evaluation. The evaluation of mating ability and libido is possible only when collecting semen via artificial vagina or manual stimulation in the presence of a female in estrus. Therefore, mating ability is seldom evaluated in bulls and rams for routine BSE in which semen is typically collected via electroejaculation.
The components of semen quality evaluation are: 1) semen volume and sperm concentration, which allow for the calculation of the total number of sperm in the ejaculate; 2) sperm motility, including gross motility (ruminants only) and percent individual sperm motility (total and progressive) of a diluted sample; and 3) the percent morphologically normal sperm. A Romanowsky-stained cytology sample also allows for evaluation of red (hemospermia) or white (pyospermia) blood cells in the ejaculate. When collecting semen from ruminants via electroejaculation, the sperm production potential is estimated by measuring scrotal circumference. Scrotal circumference is correlated with daily sperm output and therefore the serving capacity of a bull or ram (ie, number of females he can settle in a limited time).
After the BSE is complete, the male is classified as a satisfactory, questionable, or unsatisfactory prospective breeder. An animal with physical defects that may be inherited (including cryptorchidism) should be declared unsatisfactory. Guidelines for the BSE in each domestic species are included in the following species-specific discussions.
Spermatogonial Stem Cells and Prospects for Preservation of Male Fertility
Spermatogonial stem cells (SSCs) are stem cells located against the basement membrane of the seminiferous tubules. As such, they can regenerate themselves but also give rise to daughter cells that follow the replication and differentiation path toward the generation of mature sperm. Of particular interest is that SSCs are present throughout the life of a given male and hence may provide an avenue to preserve or even restore male fertility.
Although much work is still needed for SSCs to become a reality in the management of male infertility, these cells have been isolated and partially characterized in boars, bulls, tom cats, and stallions. Most knowledge derives from studies in the mouse, in which SSCs can be cultured indefinitely, cryopreserved, genetically modified, and transplanted into the testes of sterile recipient mice for generation of mature sperm cells. Studies are underway to pursue these goals in domestic species. In the future, SSCs may be used to preserve fertility in valuable domestic species as well as in their endangered wild counterparts (eg, felids). Moreover, SSCs could also become an integral tool in the assessment of male fertility or ability to restore fertility in a given male. In addition, and given some of the advances in assisted reproductive technologies in domestic species, it is feasible that SSCs could be differentiated into haploid germ cells in vitro for intracytoplasmic sperm injection and the generation of offspring from valuable azoospermic males.