Theriogenology and Veterinary Reproduction Flashcards

A mare presents with a persistent corpus luteum (CL) at day 25 post-ovulation without a detected pregnancy. Which of the following mechanisms most accurately explains why prostaglandin F2α (PGF2α) from the endometrium failed to cause luteolysis?

Answer: Downregulation of oxytocin receptors on the CL prevented the luteolytic cascade from initiating

In the mare, luteolysis depends on a self-amplifying cascade: progesterone first upregulates endometrial oxytocin receptors, then oxytocin (from the CL and posterior pituitary) stimulates PGF2α release in pulses. If oxytocin receptors on the endometrium fail to be upregulated — or if CL oxytocin secretion is insufficient — the episodic PGF2α pulses needed to cause luteolysis do not occur, resulting in a persistent CL. Counter-current transfer (as seen in ruminants and swine) is not the primary mechanism in the mare, which lacks the intimate vascular juxtaposition required for that exchange.

A 4-year-old intact male cat is presented with a history of siring no offspring despite multiple breedings with proven queens. Semen evaluation reveals 85% morphologically normal spermatozoa with good motility. Which diagnostic step is MOST likely to identify the underlying cause?

Answer: Retrograde ejaculation evaluation by post-ejaculatory urinalysis

Cats lack seminal vesicles, so fructose measurement is not applicable. Retrograde ejaculation — where semen is propelled backward into the bladder rather than anterograde — is a well-documented cause of infertility in tomcats despite normal semen quality. Post-ejaculatory urinalysis (centrifugation and sediment examination for spermatozoa) is the key diagnostic step. Testicular FNA would show normal spermatogenesis given the normal ejaculate parameters, and inhibin B is not routinely used as a primary infertility diagnostic in feline practice.

During embryo flushing in a superovulated donor cow on day 7 post-AI, the collection yields 12 structures. Four are classified as compact morulae, five as early blastocysts, two are degenerate, and one is an unfertilized oocyte. If these embryos are to be vitrified rather than slow-cooled, which group of embryos will have the MOST compromised post-thaw survival due to the biophysical properties of vitrification?

Answer: Early blastocysts, because the expanded blastocoele must be artificially collapsed prior to vitrification to prevent ice crystal formation in the cavity

Fully expanded blastocysts (and to a lesser extent early blastocysts with a forming blastocoele) contain blastocoele fluid that, during vitrification, can trap water and become a site for lethal ice crystal formation despite the ultra-rapid cooling. The standard protocol requires artificial collapse of the blastocoele — by micromanipulation needle, laser, or dehydration — before vitrification to eliminate this cavity. Compact morulae, which lack a blastocoele, actually survive vitrification quite well. Degenerate embryos are already non-viable, so survival is moot. Unfertilized oocytes present distinct challenges but in the context of this question the blastocoele issue is the clinically critical point for maximizing transferable embryo survival.

A ewe presents in late gestation with twin fetuses. Transrectal palpation reveals one fetus in anterior longitudinal presentation with dorsoilial position (spine oriented toward the ilium rather than the sacrum). What is the correct obstetrical classification of this positional deviation and the MOST appropriate initial correction maneuver?

Answer: Lateral position; rotate the fetus 90° around its longitudinal axis toward the upper birth canal with counter-pressure on the contralateral hip

In obstetrical terminology, 'position' describes the relationship of the fetal dorsum to the quadrants of the maternal pelvis. Dorsoilial position means the fetal spine faces the maternal ilium (lateral), placing this as a lateral position — not dorsosacral (normal) or dorsopubic (ventral/upside-down). The correction for lateral position is rotation of the fetus approximately 90° around its own longitudinal axis to bring the dorsum to the dorsosacral (normal) position. This is accomplished by pushing on the fetal hip or shoulder while rotating. Repelling 180° would be appropriate for dorsopubic (ventral) position, not lateral.

A stallion undergoes a breeding soundness evaluation. His total sperm output per ejaculate is 12 × 10⁹, with 65% progressive motility and 62% morphologically normal spermatozoa. Using the minimum BSE criteria for stallions, what is the calculated minimum number of progressively motile morphologically normal spermatozoa (PMMN) per ejaculate, and does this stallion pass or fail?

Answer: PMMN = 4.84 × 10⁹; the stallion passes since this exceeds the threshold of 1 × 10⁹

PMMN = Total sperm × %progressive motility × %normal morphology = 12 × 10⁹ × 0.65 × 0.62 = 4.836 × 10⁹. The minimum PMMN threshold for an 'adequate' classification in equine BSE is 1 × 10⁹ PMMN per ejaculate (some references cite 500 × 10⁶ as the absolute floor). With ~4.84 × 10⁹ PMMN, this stallion exceeds the threshold. While morphology at 62% is below the often-cited guideline of ≥70% morphologically normal, BSE classification in the stallion is primarily composite-score driven (PMMN) rather than requiring each parameter to independently reach a threshold — so the stallion passes on the composite metric. The conditional second ejaculate is not indicated when the first ejaculate result is clearly above threshold.

In a bitch with a closed-cervix pyometra, treatment with aglepristone (a progesterone receptor antagonist) is initiated. Which of the following best explains the mechanism by which aglepristone facilitates cervical opening in this condition?

Answer: Progesterone receptor blockade removes the inhibition on estrogen-driven cervical collagen remodeling and relaxin-mediated softening, allowing cervicovaginal relaxation

Progesterone normally keeps the cervix closed, firm, and impermeable by suppressing estrogen-driven collagen lysis and preventing prostaglandin E2-mediated softening. By competitively antagonizing progesterone receptors at the level of the cervix and uterus, aglepristone removes this inhibition, allowing the estrogen-dominant state and prostaglandin/relaxin pathways to soften and dilate the cervix — enabling drainage of the purulent uterine contents. The myometrial contraction effect (choice A) is a secondary benefit but is not the primary mechanism for cervical opening. Aglepristone does not act via glucocorticoid receptors or induce an LH surge.