Full transcript
0:04Bone remodeling is when old, brittle bone tissue is removed or resorbed and gets replaced
0:09by new bone tissue.
0:12Remodeling also occurs when reshaping your bones after a fracture or when repairing micro-cracks
0:17which form during ordinary activities, especially when your bones are under stress, like after
0:22lifting heavy weights.
0:25Now the bones’ surface is covered by a layer called the periosteum except at the articular
0:31cartilages - the parts involved in the joints.
0:34The periosteum consists of an outer fibrous layer which protects the bones and provides
0:38attachment for the tendons and the ligaments, and it also has an inner cellular layer which
0:43houses progenitor stem cells.
0:46These progenitor stem cells develop into both osteoblasts which secrete the bone matrix,
0:51and chondroblasts - which produce cartilage.
0:55Now let’s look at the femur - the longest bone in the body.
0:59The two ends of the bone that forms the joints are called epiphysis, while the shaft of the
1:03bone is called the diaphysis.
1:05Looking at the diaphysis; or the bone shaft, it has an external part; the cortical bone,
1:09which consists of many tiny cylinders known as osteons.
1:12Each osteon is made of many lamellae, which are these concentric layers made of an organic
1:17part - mostly collagen, and an inorganic part called hydroxyapatite, which is mostly calcium
1:23phosphate.
1:25In the center of every osteon is a Haversian canal, which contains the blood supply and
1:29innervation for the bone cells.
1:32In the center of the bone, is the medullary canal - a hollow space lined by a honeycomb-looking
1:37structure called the spongy or cancellous bone.
1:40The medullary canal contains the bone marrow, which is the site of blood cell production.
1:45Now the epiphysis is made of a lot of spongy bone.
1:48And when you look closer at the spongy bone, it’s made of crosslinking tiny roads called
1:53trabeculae, which make your bones resistant to mechanical stress, so that they can bear
1:58weights without caving in.
2:00And just like the medullary cavity, the spaces in the spongy bone of the epiphysis are occupied
2:05by bone marrow.
2:06Now, let’s jump into the bone marrow.
2:09That’s where we find the hematopoietic stem cells, the blood-making cells of the bone
2:14marrow, which give rise to the lymphoid progenitor cells - which mature and differentiate into
2:19lymphocytes like T and B cells, the main cells involved in your adaptive immunity, and the
2:24myeloid progenitor cells, which differentiate into red blood cells, platelets, and myeloblasts
2:30– the progenitors of basophils, neutrophils, eosinophils and monocytes.
2:35There are a number of growth factors that help these cells develop.
2:39For example, osteoblasts release a substance called M-CSF - Macrophage colony-stimulating
2:45factor, which helps stimulate myeloid cells like monocytes.
2:50Now in bone remodeling, the process begins when osteoblasts sense micro cracks at their
2:55location, like when your bones are bearing much weight.
2:59The osteoblasts produce a substance called RANKL - receptor activator of nuclear factor
3:05κβ ligand, which binds to RANK receptors on the surface of nearby monocytes.
3:10RANKL induces those monocytes to fuse together to form a multinucleated osteoclast cell.
3:17RANKL also helps the osteoclast mature and activate so that they can start resorbing
3:22bones.
3:23The osteoclast starts secreting lysosomal enzymes – mostly collagenase, which digests
3:29the collagen protein in the organic matrix.
3:32This drills pits on the bone surface known as the Howship’s lacunae.
3:37Osteoclasts also start producing hydrochloric acid - HCl, which dissolves hydroxyapatite
3:41into soluble calcium – Ca2+ and phosphate – PO42- ions, and these ions get released
3:45into the bloodstream.
3:46There are also a scattering of osteocytes which are trapped within the bony matrix.
3:52When these get freed up by the dissolving of bone, they get eaten up or phagocytosed
3:56by the osteoclasts.
3:58What a way to go!
4:00Now to keep bone resorption under control, the osteoblasts also secrete Osteoprotegerin,
4:06which binds to RANKL and prevents it from activating RANK receptors.
4:10This slows down the activation of osteoclasts.
4:14Finally, once osteoclasts complete their job, they commit suicide by means of apoptosis.
4:20Following bone resorption, osteoblasts start secreting osteoid seam, a substance mainly
4:26made of collagen, to fill in the lacunae created by the osteoclasts.
4:31Calcium and phosphate begin to deposit on the seam, forming hydroxyapatite.
4:35Also, as osteoblasts keep producing new bony material, many get trapped within tiny lacunae
4:41within the bony matrix, and turn into osteocytes.
4:46Bone remodeling is affected by various hormones.
4:49The parathyroid glands, the four small pea-like structures located on the thyroid gland in
4:54the neck, release parathyroid hormone in response to a drop in blood calcium levels.
4:59The parathyroid hormone travels to the bones and stimulates the osteoblasts to release
5:04RANKL, which triggers bone resorption.
5:07This allows calcium ions-Ca2+ to be released into the bloodstream, and that corrects the
5:12deficiency.
5:13Now, when the blood calcium level is higher than normal, the parathyroid gland releases
5:17less parathyroid hormone to have less bone resorption.
5:21Now, in addition, parafollicular cells in the thyroid gland produces a hormone called
5:27calcitonin.
5:28High calcitonin levels inhibit bone resorption which results in lower blood calcium levels.
5:35Another factor on bone remodeling is mechanical stress.
5:38That’s why bones that bear a lot of weight remodel at such a high rate - a phenomenon
5:42called Wolff’s law.
5:45Next there’s Vitamin D which stimulates intestinal absorption of calcium, which then
5:50causes calcitonin levels to increase and that inhibits bone resorption.
5:56Alright, as a quick recap, we have seen that bone remodeling is a continuous process by
6:03which bones are resorbed by osteoclasts, and remade by osteoblasts.
6:09Osteoblasts release RANKL to initiate remodeling, and osteoprotegerin to help turn it off.
6:15Bone remodeling is involved in repairing those tiny cracks in your bones due to normal activities,
6:20and in helping bones heal after a fracture.