Relationship between stages of the estrous cycle and bone cell activity in Thoroughbreds.

The study examines how stages of the estrous cycle affects bone cell activity in Thoroughbred horses, finding that bone formation and resorption markers had increased levels during the luteal phase of the estrous cycle.

Methods

• The study involved forty-seven 2-year old Thoroughbred mares in training for racing. These mares had blood samples drawn monthly from April to September.
• They determined the stage of estrus in each mare through an assessment of serum progesterone concentration, a commonly used indicator of stages in the estrous cycle.
• Bone cell activity was gauged by measuring concentrations of 2 markers of bone formation (osteocalcin and the carboxy-terminal propeptide of type I collagen [PICP]) and a marker of bone resorption (the cross-linked carboxy-terminal telopeptide of type I collagen [ICTP]) in the sera.

Results

• When examining the relationship between the stage of the estrous cycle and markers of bone cell activity, it was found that serum concentrations of both osteocalcin and ICTP were significantly higher in mares that were in the luteal phase, compared with mares that were at other stages of the estrous cycle.
• There was no effect on the concentration of serum PICP due to the stage of estrus.

Conclusions

• Findings suggest that bone cell activity in Thoroughbred mares fluctuates during the estrous cycle; the serum concentrations of markers of bone formation and bone resorption are increased during the luteal phase.
• However, further studies are required to determine whether these changes are of clinical importance and increase the risk of injury for mares in training during the breeding season.
• Similar to humans, the stage of estrus may influence serum bone marker concentrations in horses, introducing a source of uncontrollable variability. This is an important consideration in any future research or clinical applications involving horses.