Somite Formation – Musculoskeletal System Development

by John McLachlan, PhD

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    00:01 Hi there. Welcome to this lecture on the development of the musculoskeletal system, in which we’ll also look at limb abnormalities for reasons that will become clear during the course of the lecture. So, what we’re going to do is to look at why bone develops in particular places, and how it acquires particular shapes. We’ll be looking at the mechanisms of bone morphogenesis. And then as we focus on the development of the limbs, we’ll look at the major kinds of limb abnormality. And we’ll see how these abnormalities arise from particular defects in the course of development. A usual place to start is right back at early stages of development with the somites. The somites are blocks of mesodermal tissue that lie on either side of the neural tube. So in these very early human embryos, we can see that the neural tube is fusing and extending towards the head and tail, and then either side of it lie the somites. These somites will contribute to the vertebrae, but also to much more in the developing system of the embryo. They’re laid down in sequence from head to tail, and cease to form when you’ve reached the final appropriate number. Here, we’re looking at these in section. You can see in the top picture that we have an open neural tube gradually fusing, and you can see the histological view of the section through the somites on either side of the midline. Gradually, what seems to be a simple block of mesoderm becomes much more complicated, and this is hard to see in the histological section. So, we can label the major parts here. There is the dermatome which will give rise to dermis of the skin, the myotome which will give rise to striated skeletal muscle, and the sclerotome which will give rise to cartilage. But in an illustrated diagram, we can see these regions much more clearly. Again, we can see the sclerotome, which are going to grow round the neural tube as the vertebrae form. We can also see the myotome which is giving rise to striated skeletal muscles, and the dermatome giving rise to the dermis in the body. The somite is responding to signals which are coming from the neural tube, and help regionalize the different parts of the somites. Of course muscle and cartilage are both mesodermal structures. So the muscular skeletal system is largely developing from the mesoderm. So let’s have a look at bones, how our bone is formed, and there are two major roots. The one we’re going to focus on here is replacement of cartilaginous models by bone. I’m not going to go into detail on the histology as best seen in neuron studies of histological development. But essentially, a little cartilage model forms first. It will continue to grow, but gradually begin to be replaced by bone and there’s a kind of race between the cartilage and the bone where the bone is chasing after the cartilage. And when finally, the cartilage is cut off by the bone, then growth will seize at that point. So that may not be until you’re 18, 19, 20 years old, so, a long time after birth. But an obvious question is how do cartilage elements get to be the particular shape they are in making up the skeleton of the body? We can see certain things about them in advance. I’ve chosen a scanning electron micrograph of a human embryo round about 27, 28 days after fertilization as our illustration here. So what we can see about cartilage elements is that, first of all, they arise in mesoderm. And we’ll also know that they’re formed in sequence. So the vertebrae are formed in sequence, but also the bones, for instance, of the limbs of the arm, for example, are formed in sequence from the shoulder out towards the tip of the finger. They’re obviously patterned in three dimensions. They have a three dimensional shape. And we also know that there are various kinds of tissue signalling that are involved in this process. One unexpected observation is that these developmental signals are the same in all vertebrates. In fact, they are common to almost all animals. It’s as if the developmental mechanism only evolved once, and has to be reused again and again no matter what kind of animal you’re actually going to be. So, how can we explore how these elements are formed ? A very successful experimental

    About the Lecture

    The lecture Somite Formation – Musculoskeletal System Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Somite Formation – Musculoskeletal System Development

     John McLachlan, PhD

    John McLachlan, PhD

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