PERIOSTEUM AND ENDOSTEUM
As mentioned at the beginning of this chapter, with rare exceptions, almost all bone surfaces are lined by osteoprogenitor cells, osteoblasts, and osteoclasts. The bone articular surfaces are covered by cartilage.
The inner bone surfaces are covered by a very thin cell layer called endosteum (highlighted in light blue). It covers not only the large inner bone cavities (e.g., the medullary canal), but also the surfaces of small cavities present between the trabeculae of the cancellous bone and even the inner wall of the Haversian canals.
The periosteum (highlighted in pink) covers the bone outer surface. It is formed by two layers. The innermost, placed directly on the bone surface is called cambium layer or osteogenic layer. It is very thin, constituted by one cell layer. This layer is responsible for the increase of the width of bones. The thicker, outer layer of the periosteum, is made of dense connective tissue containing connective tissue cells, especially fibroblasts, many collagen fibers, blood and lymphatic vessels and nerves.
The periosteum’s firm attachion to the bone surface results especially to the presence of many bundles of collagen type I fibers called Sharpeys’fibers. They connect the periosteum to the bone surface and reach into the bone extracellular matrix, thus anchoring the periosteum to the bone. Many tendons linked to muscles attach to bones, therefore a strong adhesion of the periosteum to the bones is necessary.
The inner bone surfaces are covered by a very thin cell layer called endosteum (highlighted in light blue). It covers not only the large inner bone cavities (e.g., the medullary canal), but also the surfaces of small cavities present between the trabeculae of the cancellous bone and even the inner wall of the Haversian canals.
The periosteum (highlighted in pink) covers the bone outer surface. It is formed by two layers. The innermost, placed directly on the bone surface is called cambium layer or osteogenic layer. It is very thin, constituted by one cell layer. This layer is responsible for the increase of the width of bones. The thicker, outer layer of the periosteum, is made of dense connective tissue containing connective tissue cells, especially fibroblasts, many collagen fibers, blood and lymphatic vessels and nerves.
The periosteum’s firm attachion to the bone surface results especially to the presence of many bundles of collagen type I fibers called Sharpeys’fibers. They connect the periosteum to the bone surface and reach into the bone extracellular matrix, thus anchoring the periosteum to the bone. Many tendons linked to muscles attach to bones, therefore a strong adhesion of the periosteum to the bones is necessary.
Notice osteocytes (several highlighted in green) inside the bone matrix (highlighted in red). Their distribution in the bone matrix is very regular and they are placed in layers, hence this is a lamellar (mature) bone.
Periosteum and endosteum. Staining: HE. Medium magnification.
