Full transcript
0:02Besides providing structure and support for the body,
0:04and allowing for mobility, bones also protect various organs, produce blood cells,
0:09and store minerals. These functions are possible thanks to the tissues that make up the bones.
0:15There are two types of bone tissue – cortical bone and cancellous bone. Bones also feature other
0:20tissue types, including periosteum, endosteum, bone marrow, cartilage, blood vessels, and nerves.
0:28Cortical bone, also called compact bone, makes up the hard outer layer of bones. It gives bones
0:34their smooth and white appearance and makes up 80% of the total bone mass of the skeleton.
0:39An osteon, also called a Haversian system, is the primary anatomical and functional unit of
0:44cortical bone. It is a microscopic column that tends to run parallel to a bone’s long axis.
0:51Osteons have an osteonic or Haversian canal running through their center,
0:55surrounded by concentric rings of matrix called lamellae. Haversian canals allow nerve fibers
1:01and blood vessels to pass through and supply the bone. Between the lamellae, you have bone cells
1:07called osteocytes in small, oblong spaces called lacunae. I’ll describe osteocytes further later
1:13in the video. Canaliculi are tiny passageways that radiate from the Haversian canal to the lacunae.
1:19Transverse vessels called Volkmann canals run perpendicular to the osteons and connect
1:24adjacent osteons. They also connect blood vessels within osteons to the periosteum.
1:30Osteons are densely packed, and spaces between adjacent osteons are filled with
1:34interstitial lamellae, which are layers of bone that are generally remnants of previous osteons.
1:40Why are there previous osteons? Well, this is due to bone remodeling, or bone metabolism,
1:46which is a process in which mature bone tissue is removed through resorption and new bone
1:50tissue is added through ossification. Bones are constantly remodelled, because this helps repair
1:56microdamages and allows bones to adjust their structure to meet changing mechanical needs.
2:02This remodelling is done by specialized cells. Osteoblasts secrete new bone,
2:07while osteoclasts break bone down. In addition, there are cells called osteocytes,
2:12which result when osteoblasts get trapped in the mineral matrix of bone they’ve created
2:17and develop specific features. The space each osteocyte occupies is called a lacuna. Osteocytes
2:24can send signals influencing the activity of osteoblasts and osteoclasts and have many other
2:29functions. Cortical bone is covered by periosteum on its outer surface, and endosteum on its inner
2:36surface. The endosteum forms the boundary between cortical bone and cancellous bone.
2:41Cancellous bone, also called trabecular or spongy bone, consists of a porous network. It’s weaker
2:48and less dense but more flexible than cortical bone. Cancellous bone accounts for 20% of total
2:54bone mass but has nearly 10x the surface area of compact bone. Cancellous bone is highly vascular
3:01and often contains red bone marrow where hematopoiesis, the production of blood cells,
3:05occurs. It also has a higher surface area to volume ratio compared to cortical bone,
3:11which means it’s better for metabolic activities such as the exchange of calcium ions.
3:16Anyway, let’s discuss the structure of cancellous bone.
3:20Cancellous bone is made up of a network of trabeculae, which are its primary
3:24anatomical and functional units. Openings on the trabeculae are called canaliculi
3:29and these connect to adjacent cavities, instead of to a haversian canal, for blood supply. Within
3:35cancellous bone spaces we find bone marrow and hematopoietic stem cells, which are stem cells
3:40that give rise to other blood cells, including platelets, red blood cells and white blood cells.
3:46Bone marrow is a nutrient-dense, spongy tissue located in the cavities of bones.
3:50It can be found in almost any bone with cancellous tissue and produces both red and
3:55white blood cells, as well as platelets. Bone marrow also contains stem cells. In newborns,
4:01all such bones are filled exclusively with red marrow or hematopoietic marrow.
4:06Red bone marrow produces blood cells. As children age, red bone marrow decreases in quantity and
4:11yellow bone marrow increases in quantity. By the age of 25, red bone marrow achieves the
4:17final adult distribution. In adults, red marrow is mostly found in the bone marrow of the femur,
4:22ribs, and pelvic bones. Yellow bone marrow is located in the hollow cavity of long bones. It
4:29is typically found at the center surrounded by red bone marrow. Yellow marrow stores
4:34fat and can be called on in life-threatening situations to produce more red blood cells,
4:39specifically if you experience rapid blood loss. During these kinds of situations, yellow
4:44bone marrow can transform into red bone marrow, producing more blood cells to keep you alive.