It is a thin and soft connective tissue that lines the cavity of long bones such as the humerus and femur.
It acts as a covering for the internal compact bone and the trabeculae of the cancellous tissue.
The osteoprogenitor cells of the preosteoblasts present in this connective tissue lining differentiate into osteoblasts and then into osteocytes, which are bone-forming cells. They also help in the formation of bone matrix secretory cells, also known as bone lining cells.
The endosteum, along with the periosteum, functions for the growth of bone in diameter. The endosteum also contains hematopoietic stem cells or hematopoietic stem cells that are totipotent in nature and play a role in the production of all types of blood cells.
The endosteum is also known medically as the medullary membrane, which is located in the diaphysis (cavity of the long bones). The long bone cavity consists of red and yellow bone marrow with spongy tissue and spongy bones.
Where is the endosteum located?
The endosteum can be seen in the TS of any long bone. This layer of membrane surrounds the spongy tissue, the medullary canal, and the inner lining of the bone cavity and Haversian canal of the marrow.
The endosteum is found in bones such as the femur, humerus, hip bone, thoracic ribs, and sesamoid bones such as the kneecap. It is also seen to cover many walls of different sinuses of the skull.
What are the different types?
Depending on the internal bone area where it is located, 3 types of endosteum are mainly known:
Cortical endoste : The cortical endosteum found on the inner walls of the cortical bone acts as the border of the medullary canal. This cavity stores the yellow bone marrow, which is a reservoir of fat cells or fat cells.
During severe starvation and malnutrition, these fat cells are used as fuel to produce energy for vital body functions.
This, in turn, would reduce the thickness and density of the bone. Therefore, the cortical thickness of the bone reduces the production of a deleterious effect on vital functions such as the storage and release of calcium. Since cortical weight contributes 80% of total body weight, the patient is observed to experience weight loss.
Nosteonal endosteum : located on the internal walls of the osteonal canal, present in compact bones, this endosteum contains the nervous and vascular supply of the bone.
Trabecular endosteum: This endosteum lines the inner walls of the trabeculae, that is, the bony parts that cover the spongy tissue of the bone near the axis. Helps in the absorption and reduction of bone damage caused by an impact.
This part of the bone contains the red bone marrow that contains hemopoietic cells capable of neo-erythropoiesis (production of new red blood cells).
What is the clinical significance of the endosteum?
The exchange of factors such as blood cells and calcium between the bone marrow and blood vessels takes place through osteoblasts in the endosteum called Endosteal Cells. These cells are active participants in bone growth, repair, and remodeling.
The endosteum and periosteum together contribute to bone repair and reconstruction after a fracture. Cells in both layers are capable of rapid mitosis and proliferation, after which they migrate to the site of bone injury.
In certain cases, spinal cord lesions can be seen along the inner aspect of the cortical bone, leading to endostial scalloping that can be associated with both benign and malignant forms of tumors. These can also lead to skeletal metastases in acute stages.
What are the functions of the endosteum?
The endosteum has several important functions in the body, as it is part of the long bones of the human body. Some of its main functions are the following:
1. Appositional bone growth
- The osteoblasts lining the endosteum secrete the bone matrix and form ridges alongside the blood vessels of the periosteum.
- The bony ridges enlarge and join to make the groove into a tunnel that contains the blood vessels.
- The periosteum lining the tunnel becomes an endosteum and, therefore, the osteoblasts lining the tunnel begin to secrete the matrix, narrowing the canal.
- The osteoblasts below the endosteum form new lamellae and a new osteon is formed.
- Eventually, a new circumferential lamella is made below the periosteum and the process is repeated for continuous enlargement of the bone diameter.
- This is seen primarily in children as an accelerated process, which slows down considerably once adulthood is reached.
2. Bone repair
During a traumatic injury or fracture, the appearance of a hematoma within the bone causes rapid multiplication of endosteal cells to reestablish a restorative callus bridge and aid in bone solidification.
3. Bone remodeling
The endosteum stimulates bone resorption on the inner surface of the bone along with the periosteum which stimulates the continued formation of new bone from the outside. This leads to an increase in the diameter of both the medullary canal and the bone as a whole.
What is the difference between endosteum and periosteum?
There are a number of differences between the two linings, endosteum and periosteum. Some of them are listed below:
- The endosteum is located in the medullary canal, the cancellous tissue of the bone, the canal of Volkmann and the canal of Havers while the periosteum is located on the external bony surface of all bones except the sesamoid bones.
- The endosteum is made up of a single-celled layer and is a loose connective tissue, while the periosteum has a bilayer layer itself, has a fibrous and cellular layer called cambium, and is a dense, irregular connective tissue.
- The thickness of the endosteum is in the range of approximately 10 microns, while that of the peritoneum is approximately 0.1-0.5 mm.
- The endosteum primarily aids in bone growth, repair and remodeling, while the periosteum aids in bone sensitivity and nutrition in conjunction with the above activities.