Histology of bone Flashcards
Why is bone a form of connective tissue?
Bone is a form of CT because it has cells (osteoblasts and osteocytes) embedded in an extracellular matrix secreted by the cells. The major difference is that the ECM has become mineralised with calcium salts.
What is the organisation of bone?
Bone is arranged in concentric rings called lamellae. In compact bone these lamellae surround a central Haversian canal. This canal is lined by endosteum and contains blood vessels. Spongy bone has lamellae but not a Haversian canal.
How are collagen fibres arranged in lamellae?
The collagen fibres within the lamellae are arranged in alternative orientations which creates strength.
Periosteum
Lies on the outside of the compact bone. Like the perichondrium, it contains two layers:
- an outer fibrous layer of dense irregular CT (contains fibroblasts, fibrocytes and blood vessels)
- an inner osteoprogenitor (osteogenic) layer of stem cells, osteoblasts and osteoclasts
How is the periosteum anchored to bones?
By Sharpey’s fibers (collagen)
Canaliculi
Cytoplasmic processes that material can enter cells because matrix is solid.
Sharpey’s fibres
Collagen fibres from tendons and ligaments merge with the collagen fibres of the outer fibrous layer of the periosteum. In turn, collagen fibers from the periosteum run into bone matrix at an angle, and become continuous with the collagen fibers of the bone matrix - this helps to anchor the periosteum to the bone matrix and increase the strength of attachment of tendons to bone. There is a continuum of collagen from endomysium –> perimysium –> epimysium –> tendon –> periosteum –> Sharpey’s fibres –> bone matrix
Spongy bone
Irregular projectiosn of bone called trabeculae or spicules make up spongy bone which is found deep to the surface of bones (articular surfaces or compact bones). It consist of several parallel lamellae but with no Haversian canal. It is lined by endosteum consisting of osteoprogenitor cells, osteoblasts, osteoclasts and reticular cells (and blood vessels).
Where are osteocytes found and how do they communicate with one another?
Osteocytes are found within lacuna and communicate with each other via gap junctions on long cytoplasmic processes which run within canaliculi.
Where are osteoblasts found?
In association with the endosteum and periosteum. They secrete ground substance and collagen fibers to form the ECM (osteoid) of the bone.
Where are osteoclasts found and what is their role?
Osteoclasts are found associated with the endosteum and periosteum. Theyd igest and breakdown bone for bone remodeling - a process that continues throughout life. Many may form at the metaphysis in a growing bone.
Osteoclasts
Osteoclasts are multinucleated cell derived from monocytes and macrophages. Fusion of up to 30 cells creates large phagocytic osteoclasts within bone. They play an essential role in remodeling and renewal. New bone is formed by osteoblasts. At the site of bone resorption (Howship’s lacunae) the osteoclast has a ‘ruffled border’ and secrete HCl to break down bone matrix.
Osteoclast activity
Stimulated by parathyroid hormone (PTH) and inhibited by calcitrol (a hormone from the thyroid gland).
Bone ECM
Prodced by osteoblasts and maintained by osteocytes. Composed of:
70% inorganic salts (mostly calcium and phosphate)
30% organic material: 90% type I collagen, 10% proteoglycans and non-collagen molecules (e.g. osteocalcin - involved in binding Ca2+)
Collagen fibres are highly ordered - the orientation altered between each lamellae.
Bone remodelling.
Bone is not a static structure. It is a dynamic tissue that is constantly undergoing modification both in terms of its chemical nature and its physical structure. These changes occur in response to metabolic demands and or environmental factor. Diet, pregnancy, exercise, injury etc.
