PERIOSTEUM AND ENDOSTEUM
As mentioned at the beginning of this chapter, with rare exceptions the majority of the bone surface are lined by cells: osteoprogenitor cells, osteoblasts, and osteoclasts. As an exception, the bone articular surfaces are covered by hyaline cartilage.
The inner bone surfaces are covered by a thin cell layer, the endosteum (highlighted in blue). It covers not only the large inner bone cavities (such as the medullary canal), but also the surfaces of small cavities placed between cancellous bone trabeculae and even the inner surface of Haversian canals.
The periosteum covers the outer surface of bones. It has two layers, the innermost, apposed over the bone surface is the cambium layer or osteogenic layer (highlighted in yellow). It is very thin, constituted by one layer of cells, responsible for most of the growth in width of bones.
The thicker, outer layer of the periosteum, (highlighted in green) is made of a dense connective tissue containing connective tissue cells, especially fibroblasts, many collagen fibers, blood and lymphatic vessels and nerves.
The periosteum’s firm attachment to the bone surface results mainly to bundles of collagen type I fibers called Sharpey’s fibers that connect the periosteum to the bone surface and reach into the bone extracellular matrix, thus anchoring the periosteum tightly to the bone. Many tendons linked to muscles attach to the periosteum, therefore a strong adhesion of the periosteum to the bones is necessary.
The inner bone surfaces are covered by a thin cell layer, the endosteum (highlighted in blue). It covers not only the large inner bone cavities (such as the medullary canal), but also the surfaces of small cavities placed between cancellous bone trabeculae and even the inner surface of Haversian canals.
The periosteum covers the outer surface of bones. It has two layers, the innermost, apposed over the bone surface is the cambium layer or osteogenic layer (highlighted in yellow). It is very thin, constituted by one layer of cells, responsible for most of the growth in width of bones.
The thicker, outer layer of the periosteum, (highlighted in green) is made of a dense connective tissue containing connective tissue cells, especially fibroblasts, many collagen fibers, blood and lymphatic vessels and nerves.
The periosteum’s firm attachment to the bone surface results mainly to bundles of collagen type I fibers called Sharpey’s fibers that connect the periosteum to the bone surface and reach into the bone extracellular matrix, thus anchoring the periosteum tightly to the bone. Many tendons linked to muscles attach to the periosteum, therefore a strong adhesion of the periosteum to the bones is necessary.
Notice osteocytes inside the bone matrix (both highlighted in magenta color). The distribution of the osteocytes is very regular and they are placed in layers, hence this is a lamellar (mature) type of bone.
Periosteum and endosteum. Staining: HE. Medium magnification.
