Problem: Infertility can accompany mastitis in cattle. Involvement of tumor necrosis factor-α (TNF-α) in this phenomenon is suggested by observations that circulating concentrations of TNF-α are elevated after intramammary infection or infusion of endotoxin. It was hypothesized that (1) TNF-α acts on the oocyte during maturation to decrease the percent of oocytes that cleave and develop following fertilization; (2) exposure of embryos to TNF-α after fertilization reduces development to the blastocyst stage; and (3) TNF-α increases the proportion of blastomeres that undergo apoptosis in a stage-of-development dependent manner.
Method of study: In one experiment, oocytes were matured with various concentrations of TNF-α and then fertilized and cultured without TNF-α. In another study, embryos were cultured with TNF-α for 8 days beginning after fertilization. Finally, embryos were collected at the two or four-cell stage (at 28–30 hr after insemination) or when ≥9-cells (at day 4 after insemination) and cultured ± TNF-α for 24 hr. The proportion of blastomeres undergoing apoptosis was then determined by the TUNEL procedure.
Results: Addition of TNF-α to maturation medium did not affect the proportion of oocytes that cleaved. However, the percent of oocytes that developed to the blastocyst stage at day 8 after insemination was reduced (P = 0.05) at all TNF-α concentrations tested (0.1–100 ng/mL). When added during embryo culture, there was no significant effect of TNF-α on the proportion of oocytes that became blastocysts. In addition, TNF-α did not induce apoptosis in two and four-cell embryos. For embryos ≥9-cells, however, 10 and 100 ng/mL TNF-α increased (P < 0.05) the percent of blastomeres labeling as TUNEL-positive.
Conclusion: TNF-α can have deleterious actions on oocyte maturation that compromise development of the resultant embryo. While exposure of fertilized embryos to TNF-α did not inhibit development to the blastocyst stage, TNF-α increased the percentage of blastomeres undergoing apoptosis when exposure occurred for embryos ≥9-cells. Increased blastomere apoptosis could conceivably compromise subsequent embryo survival.