But researchers are still working to understand the complexities of bone growth in the womb. That’s why van Weeren and colleagues recently completed a study on the topic.
Bone Development 101
The “exercising” and “loading” of bones and their inner grid structure—composed of small beams, the so-called trabeculae—are what make them grow strong and able to support body weight and the forces put upon them, he said. That happens in mature animals and in most young animals that can’t walk just after birth, like dogs, cats, and humans. In those species, that loading happens in the days, weeks, and months after birth, building up to walking and running.
But equids (and other “precocial” species in which the young are relatively mature and mobile from the moment of birth) don’t have time for that. Van Weeren said this is partially because of movements and loading in the uterus, but other reasons remain unclear.
“The loading in utero is different from what it is outside the womb, mainly because the foal is more or less floating, so you don’t get that influence of gravity that you would see after birth,” he said. “So, there must also be something like a ‘genetic blueprint’ in those animals.”
This special pre-birth bone preparation is what the researchers call a “mechanism of anticipation” to ensure the bone is strong enough to keep up with the herd just hours after foaling, van Weeren said.
Trabecular loading involves not only intensities of pressure that cause the grid to react by strengthening but also angles of pressure that steer the trabeculae’s orientation, he said. It results in the horses having greater bone strength in some directions but not others.
“That’s why your horse can snap his leg if he drops his foot into a hole,” he explained. “The bone might be strong, but not able to resist forces at that particular angle.”
In horses and other precocial species, the determinants of bone strength—the trabeculae’s density and angle—get a significant head start in the uterus.
Interestingly, however, not all precocials prepare for loading after birth in the same way, said van Weeren. And the differences in that pre-birth development might give insight into post-birth bone development disorders.
“Ponies have much denser bones, which are, hence, much stronger,” he said. “They also have fewer incidences of OCD,” or osteochondritis dissecans, a common developmental orthopedic disease (DOD) characterized by loose cartilage and/or bone fragments in the joints.
Recent Research Results
In a recent study, van Weeren and his fellow researchers used micro-computed-tomography (micro-CT) imaging to analyze trabecular bone parameters. They focused on hock and tibia (the bone above the hock) trabeculae in newborn calves and foals that were stillborn but had grown to full term.
They found an average bone volume fraction in Shetland ponies ranging from 49% to 74% (100% would represent solid, nonporous bone mass). In Warmblood foals they studied, the rate was significantly lower, ranging from 28% to 51%. Calves had a lower average bone volume fraction, ranging from 22% to 28%, but this species had a trabeculae orientation that was better adapted to the direction of loading’s angle.
“We see a different way of coping with the forces demanded shortly after birth in the different loading styles,” van Weeren said. “Horses and ponies anticipate with greater thickness, cows with angles of resistance that are better aligned with the direction of loading.”
The differences between ponies and horses could give the most useful clues in dealing with DOD, however. “Shetlands have far slower growth rates compared to horses—especially certain fast-growing breeds like Warmbloods,” he said. “But we have to consider the time it takes to lay down all the bone. We are pushing our horses to the limits in growth rates.”
Selective breeding for taller horses has resulted in Warmbloods’ average height increasing by 1 mm per year, he said. “That doesn’t seem like a lot, but if horses had been growing at that rate since they first appeared 65 million years ago as fox-sized animals, they’d be 64 kilometers (roughly 40 miles) tall today,” he said. “That’s what we’ve done with selection.”
Fast growth in height redirects the available calcium to building height rather than strength, said van Weeren. “You can make more bone that makes them taller or makes them stronger, but you can’t do both,” he said. “And that’s already starting before birth.”
The study, “Trabecular bone of precocials at birth; Are they prepared to run for the wolf(f)?” was published in the Journal of Morphology.