Now for another post on anatomy- this time on the development of the vertebral column! w00t w00t!
1. Evolution of the vertebral column
As you might be aware, humans belong to the phylum Chordata (i.e. the chordates). One of the defining features of chordates is the presence of a notochord during some stage of development.
For a bit more background, let's have a look at some more primitive types of chordates. Many simple chordates pretty much just sit on rocks and filter water to get their food. Chordate larvae can swim, however, and so they have a head, a notochord and a tail that extends past the anus. In these larvae, the notochord acts as a kind of "stiffening rod." When the larva moves, muscles on either side of the notochord contract alternately, allowing the tail to move. Eventually the larvae find rocks of their own to settle down on, and they lose the notochord and tail. They aren't "lost" completely though, as they're passed onto their next children. This phenomenon, in which characteristics only found in the juveniles are passed on, is known as "neotony."
Humans also belong to the subphylum Vertebrata, which as the name suggests, contains organisms that have a vertebral cord. I'm going to cover the development of the vertebral column throughout the rest of this post.
2. Stages in vertebral development
The first stage of vertebral development is the mesenchymatous stage, at around 4-6 weeks gestation. At this stage, neurulation and folding are complete, or pretty much complete. Body wall vessels also begin to appear between the somites.
During the mesenchymatous stage, sclerotome cells begin to migrate. Some surround and enclose the neural tube- these become the neural arch. Some move off to the sides- these become the costal elements. The third type, the perinotochordal cells, surround the notochord. The perinotochordal cells are also closer to the aforementioned body wall vessels and therefore get better nutrition and grow larger. The perinotochordal cells eventually form the vertebral bodies. (Don't worry, I'll explain what the neural arch, costal elements and bodies are later on.)
Since the perinotochordal cells surround the notochord, the notochord kind of gets "pinched off" into the space between somites. This is significant, as we'll see later.
The cartilaginous stage takes place at around 6-9 weeks gestation. During this stage, chondrification begins to occur as the mesenchyme is gradually replaced with hyaline cartilage. Paired centres of chondrification appear in the neural arch, costal elements and centrum (a.k.a. the body). Between vertebrae, fibrocartilage begins to form in a ring (the annulus fibrosus) around the notochordal cells (which will eventually become the nucleus pulposus of the intervertebral disc).
An interesting point to note here is that, since all the chondrification centres are paired, if issues arise with one member of a pair, asymmetrical vertebrae known as "hemivertebrae" can form.
The osseous stage occurs from around 8 weeks onwards. During this stage, the cartilage that formed in the previous stage begins to ossify (i.e. become bone). Once again, centres of ossification start to appear. The only difference this time is that there is only one centre of ossification in the centrum, rather than a pair (the neural arch and costal elements still have paired centres).
As the bone grows, cartilage growth plates continue to separate the three elements. An interlaminar growth plate separates the two neural arch elements (these eventually become the "laminae" of the vertebrae, hence the name "interlaminar"), while the neurocentral growth plate separates the neural arches and centrum (hence "neurocentral"). As for the costal elements, in the thorax they become ribs, whereas in the rest of the spinal column they fuse with the rest of the vertebrae to become part of the transverse processes (more on this in my next point).
Before I move on, though, I'll just make a note of things that can go wrong here. It's during the ossification stage that the location of notochordal cells can have an impact. If too many notochordal cells remain, then the centrum cannot ossify properly, resulting in "butterfly vertebrae" (so-called because apparently they look like butterflies). On the other hand, however, if there are too few notochordal cells between vertebrae, vertebrae can fuse together, forming "block vertebrae."
3. Regional differences in bone element contributions
Here is where I finally get to tell you what the neural arch, centrum and costal elements were all about!
So far, I've been using "centrum" and "body" pretty much interchangeably. That was kinda lazy of me, as the centrum doesn't actually end up forming the entire body of the vertebra. It forms most of it, but not all- the rest is made up of neural arch elements and, in the sacrum, parts of the costal elements as well.
As I've mentioned before, the costal elements form the ribs in thoracic vertebrae. In other vertebrae, they also help form parts of the transverse elements (the bits of the vertebrae that stick out to the sides).
Finally, the neural arch makes up pretty much everything else in the vertebrae, particularly the spinous processes (i.e. the bumpy bits at the back of the vertebrae).
4. Growth of the vertebral column
As I've mentioned before, there are cartilage growth plates around in the developing vertebrae: interlaminar and neurocentral. Growth can continue here until these plates close, which occurs at around 6-8 years of age. However, there is still some cartilage left: the ends of the processes still have cartilage, as does the top and bottom of the vertebral bodies (i.e. the endplates).
During puberty, secondary centres of ossification start to appear at the remaining cartilaginous spots, i.e. at the spinous process, transverse processes and ring apophysis (cartilage surrounding the endplates). The ring apophysis centre in particular is responsible for growth of the vertebral column. All of these growth plates close up by adulthood, so unfortunately we can't keep on getting taller forever :(
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