Dwarfed dogs: What exactly is going on in there?

Watching Bronte’s puppies has been nothing but pure joy for me. OK, well, pure joy plus obsessively bugging Kate for weight progress (what can I say; I’m a former Dane breeder who was trained by a NICU nurse–weights are what I wake up at night thinking about). But most of it has been just fabulous.

One of the things that I found EXTREMELY interesting is that the puppies were born with (proportionally) long legs, as long as the legs I saw on any Dane born in my kitchen. But, very quickly, as bodies filled out and lenghthened, as weights soared (sigh of relief on my part) the legs got bigger and thicker but, length-wise, stayed pretty much where they were when they were born. It’s very easy for me to see that by the time the puppies are eight weeks old, they’ll be nearly twice as long as they are high whereas my Danes were about as tall as they were long. 

So what is going on here? Why do Cardigans, along with so many other breeds, end up with such short legs?

The answer lies in a word we throw around a lot but often don’t understand–achondroplasia, or chondrodystrophy. Those are the words that are used to describe what happens in dwarfed dogs, but they’re rather distressingly poorly understood by most. 

Both words–achondroplasia and chondrodystrophy–basically mean “crappy cartilage” or “failure of cartilage.” In humans with dwarfism, achondroplasia is a much more exact word that labels a particular type of dwarfism associated with a particular gene. In dogs, the word is a descriptor rather than a diagnosis–we have no idea what gene or genes make dogs short-legged, and aside from the fact that we’re sure they have deformed cartilage with a bunch of bone problems there’s not a lot of information out there. 

So why is our unique conformation associated with cartilage? For two reasons. The first is that cartilage is where bone growth occurs.

If you want a complex description of what happens, look here. I’m going to try to make it a little easier to understand. 

Look at your arm from your shoulder to your elbow. That’s the humerus bone. It’s the same bone as goes from the center of your Cardi’s body (the “shoulder” joint actually points toward her nose) to her elbow. If you feel your own humerus (if you’re like me you have a healthy fat covering over the bone, but try) it’s pretty long and skinny and straight, with swellings at the shoulder end and the elbow end. If you felt a Lab or Great Dane humerus, it would feel the same way–long and thin, with swellings at both ends. If you feel the same bone in your Cardigan, hers will be mostly lumpy end and other lumpy end, with a tiny section in the middle. Your Cardi’s humerus will also be MUCH shorter than your Lab’s–the long-legged dogs have elbows roughly at the level of the bottom of their chests, while Cardigans’ humerus ends several inches above the bottom of the chest.

The difference between these two lies in how the growth plate works. 

In a normal dog, when it is a puppy, those lumpy ends of the bone have a layer of slippery cartilage that allows the joint to move freely, and then right above that slippery layer is a VERY active layer called the growth plate.

The growth plate has a bunch of round cells in it. As those cells get a little older, they begin dividing rapidly, almost frantically. The result is a whole ton of round cells. Meanwhile, behind this whole ton of round cells, the new baby cells are being born. Then that whole ton of cells move from being round to being veerrrrry long and stretched-out, with lots of little arms that reach out upward and downward. Then the cells die, but they leave their skeletons (veeerrry long and stretched) behind. Then more cells come and build calcium all around those stretched-out arms, making the skeleton of the former cell extremely strong, but still lace-like, with lots of tiny holes. Eventually little blood vessels come and grow all through the holes and laces, and then you have mature bone.

So, basically, the way bone grows is that it pushes from either end, building the long skinny bit in the middle. Eventually hormonal signals that say “I have a grown-up pee-pee, and I like girls!” or “I have a strange feeling that I might like boys” come along, and those hormones gradually shut down the growth plates and the growth of those bones is finished. 

What’s different in dwarfed breeds is that the growth plate is a big pile of FAIL. Instead of having a big thick layer of a ton of those round cells dividing like crazy, it has a thin layer of many fewer round cells. Instead of a steady march of cells from round to long and thin, some cells fail entirely and some go really tall. The lengthening cells turn to mature bone LONG before they are supposed to.

The result is that the bone does not form a long thin straight column; it forms a thick, twisted, short column. And then it shuts down entirely. 

In the next post, we’ll look at what ELSE can go wrong with our cartilage failure, and why this is important to us as breeders.

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