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Endosteum

Written by Dr.Mary

What is Endosteum?

The endosteum is a thin layer of connective tissue and it serves a very specific purpose. Also known as the medullary membrane, the endosteum practically represents the interior lining of the walls of different cavities that the bone marrow is organized of. The endosteum lines the interior walls of the haversian canals that constitute compact bones, plus it covers the small trabeculae. The interesting thing is that the endosteum is resorbed when one suffers from severe or prolonged malnutrition, thus leading to a reduced cortical thickness.


Endosteum is composed of endosteal cells or ‘bone lining’ cells as they are also called. Among these cells, you can find the bone stem cells, the ones that are going to further develop into osteoblasts and osteoclasts. The first ones are cells that contribute to the formation of bone, while the latter represent cells that actually dissolve the bone. Apart from that, the endosteum contains connective tissue fibers, in a small amount and blood vessels that provide the nourishment it needs. The endosteum nervous function is regulated through the existent autonomic nerve fibers.

Function of Endosteum

Research has demonstrated that the endosteum has osteogenic properties, contributing directly to the process of bone growth or repair. In fact, both endosteum and periosteum play an active role in the healing of different types of fractures. The endosteum has a highly reparative function, especially after the harvesting of bone marrow. In such situations, the bony trabeculae are damaged during the harvesting process (mechanical fracture) and the endosteum contributes to the repair process, by stimulating the production of osteoblast and osteoclast cells. The endosteum is also believed to possess hematopoietic properties, containing a series of factors that stimulate the production of hematopoietic cells.

Location

The endosteum is located on the internal surface of the bone, being the membranous layer that covers the medullary cavity, the bony trabeculae (spongy part of the bone), the haversian canals and internal walls of the compact long bones. It is found in bones such as the humerus and the femur, in the bone that are flat (hip bones), the thoracic cage (ribs) and the patella (or other sesamoid bones). At the level of the skull, endosteum is located as an internal lining within the various cavities.

Types

There are three different types of endosteum, as it follows:

  • Cortical endosteum
    • Function – internal lining of the cortical bone walls
    • Separates the marrow cavity from the other structures of the bone
  • Osteonal endosteum
    • Function – internal lining of the osteonal canals
  • Trabecular endosteum
    • Function – internal lining of the bony trabeculae.

What does endosteum do?

The endosteum is more than a simple membranous layer that coats the internal walls of the bone cavities. This is what the endosteum is capable of:

  • Stimulates the growth of the bones
    • The thin layer of connective tissue stimulates the bones to grow (in width)
    • The endosteum allows for the appositional bone growth, by stimulating the osteons that are part of the cortical bone
  • Remodeling of the bones
    • The endosteum plays an active part in the resorbing of the bone, working side-by-side with the periosteum in order to stimulate the formation of new bone
  • Repairing of the bones
    • In case of a bone fracture, the endosteum contributes to the repair process. When the hematoma occurs inside the bone, the endosteal cells proliferate and contribute to the formation of the reparatory callus. They also play an active part in the consolidation process, followed by the above-mentioned remodeling.

Pictures

endosteum pictures

Picture of Endosteum : showing the diagram of Periosteum, Endosteum, Osteon and Lamella.

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Endosteum vs Periosteum

There are a series of differences between endosteum and periosteum; as you will have the opportunity to read in the paragraph below, these two membranous layers have different locations, structures, thickness and functions.

Let’s start with the location. As it was already mentioned, the endosteum is located within the medullary canal, in the spongy part of the bone, the haversian canals and inside the cortical walls of the compact bones. The periosteum, on the other hand, it is found on the outer surface of the bone, with the exception of the articular surfaces. It is also important to highlight that the periosteum is not to be found on the sesamoid bones, such as the patella of the knee.

The structure of the endosteum and periosteum is also different. The endosteum has a characteristic structure, being comprised of a single layer of endosteal cells, with the connective tissue being quite loosely organized. The periosteum has a more complex structure, containing two layers of periosteal cells. One of the layers is fibrous, while the other one is known as cambium, containing large quantities of periosteal cells. As opposed to the endosteum, the connective tissue that the periosteum is comprised of is dense and irregularly organized.

When it comes to the thickness of both membranous layers, you should know that the endosteum is less thick than the periosteum. In fact, the endosteum has a thickness of 0.01 mm, while the periosteum is obviously thicker, with a thickness that varies between 0.1 and 0.5 mm.

The functions of the endosteum and periosteum are somewhat similar and complementary. The endosteum, as you have read in the paragraphs above, contributes to the appositional bone growth, playing an active part in the remodeling and repair processes after the occurrence of bone fractures. The periosteum, working in harmony with the endosteum, contributes to the growth of the bone, playing its part in the remodeling and repair processes as well. Apart from that, the periosteum also contributes to the bone sensitivity and delivers the necessary nutrients to the bone.

As you can see, the endosteum plays a series of important roles, contributing to the growth and repair of the bone. It works in conjunction with the periosteum, maintaining the health of the bone, especially after fractures or bone marrow harvesting. It is also subjected to different disorders, such as the endosteal sclerosis that is encountered in patients diagnosed with sclerosing osteomyelitis. Scientists are still working to discover more information on the endosteum and the other roles it may have, especially when it comes to the hematopoiesis process.

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