Structure of Bones

00:02 In this lecture, I'm going to talk about bone.

00:05 But I'm going to concentrate on mature bone and the function of mature bone.

00:11 In a later lecture, I'm going to talk about bone formation and bone growth.

00:19 Bone is a very specialized connective tissue.

00:24 Like all connective tissues, it's composed of cells, fibers and matrix.

00:31 But in the case of bone, the matrix is calcified.

00:35 And because of that, it enabled bone to have some very important functions in the body.

00:43 It can protect vital organs of the body, the thoracic cavity protects the heart and the lungs.

00:51 The cranium protects the brain.

00:55 Bone always forms the skeletal components of the body, and in unison with muscle affects movement of the body.

01:07 It's also a very important mineral reserve for both calcium and phosphate.

01:14 And it provides a compartment for bone marrow to form blood cells.

01:20 I'm going to talk about these latter two functions in a lecture later on.

01:28 Let's look at really what I want you to understand with this initial lecture on bone.

01:37 First of all, I want you to appreciate that there are two types of bone, compact bone and spongy bone.

01:45 And I'll describe these two types during this lecture.

01:50 The very fundamental structural unit of bone is the osteon.

01:55 And I will also describe that, and it's something that you really should understand because it will explain to you really how bone forms its functional role in the body.

02:08 And equally important, you must understand how bone cells received their nutrients.

02:15 As I said earlier, the matrix is very hard, it's calcified.

02:20 Unlike other connective tissues, such as loose connective tissue or even cartilage.

02:27 There is no aqueous component of the extracellular matrix.

02:33 So nutrients, including oxygen, cannot diffuse through that matrix.

02:39 So I'm going to explain in this lecture how bone cells get their nutrients from the blood when they're embedded in this very firm calcified matrix.

02:51 So, again, let's look at what characterizes bone.

02:58 First of all, it's vascular.

03:00 Unlike cartilage, it has the blood supply.

03:03 It has a very large blood supply.

03:07 If you break a bone, it bleeds.

03:10 Again, like connective tissues, it has cells, fibers and matrix.

03:18 The cells are called osteocytes.

03:22 And these osteocytes live within the bone matrix.

03:27 And often you see just a small clear space, this is a lacuna.

03:34 And a lacuna is really, a space is occupied by the bone cell or the osteocyte.

03:43 Normally, the osteocyte would occupy the entire space.

03:47 But during processing of bone, often the cell is completely lost from its territorial area in the bone matrix, and just a small little space or lacuna remains.

04:01 Sometimes however, in some of these little lacuna spaces, you can see remnants of the osteocytes.

04:09 Again, like cartilage, the most important functional part of bone is the matrix.