Locomotor Histology Flashcards
Describe the composition of the osteoid (bone ECM)
Type I collagen fibres, ground substance, structural glycoproteins
Which cells form the osteoid?
Osteoblasts
Describe structure of woven bone
Random collagen fibre organisation
Describe structure of lamellar bone
Highly ordered, layered collagen fibres
Describe the structure of long bones
Thick compact bone on their shafts (strength), towards centre and at ends have spongy/trabecular bone (has spaces filled with bone marrow –> lighter)
What is present at the ends of long bone?
Articular cartilage (if located at joint) or thin layer of compact bone
What is present in the centre of the shaft of long bones?
Medullary cavity filled with bone marrow
Describe the structure of short, flat and irregular bones
Outer surface is compact bone, inner bone is cancellous/spongy
In what type of bone does most blood cell production in adults occur and why?
Flat bones, have greater proportion of red bone marrow where haematopoiesis takes place
What is yellow bone marrow?
Mainly adipose tissue
Why does bone stain a bright pink colour?
Due to presence of type I collagen
Where do osteocytes obtain their nutrients in trabecular bone?
Blood sinusoids in the bone marrow
What lines the surface of the trabeculae?
Endosteum
What is the endosteum comprised of?
Layer of osteoprogenitor cells
What is the purpose of osteoprogenitor cells
Stem cells that are derived from mesenchymal cells which can differentiate into osteoblasts and are important for bone remodelling as well as repair following damage
What are Haversian systems/osteons?
Lengthwise bony coluns in compact bone, where concentric bony lamella surround a neurovascular bundle (Haversian canal)
What is a Haversian canal?
Neurovascular channel in compact bone surrounded by concentric lamellae (Haversian system)
What are Volkmann’s canals?
Neurovascular supply that runs perpendicular to the Haversian canals, and is involved in bringing the neurovascular supply into the compact bone and distributing it to the Haversian systems/osteons
What forms the most outer surface of compact bone?
Several layers of circumferential lamellae
Where is endosteum located in compact bone?
Inside of Haversian system
How do osteocytes distinguish the lamellae of compact bone?
They are arranged in rows, organisation allows you to clearly distinguish between different osteons
Describe osteoclasts
Multinucleate, macrophage-like cells that resorb bong in response to blood calcium requirements and changing mechanical stresses
What are resorption canals?
Tunnels through bone created by osteoclasts
How do osteoblasts respond to a resorption canal?
Build a new osteon there (deposition)
What is an intact osteon indicative of?
Being most recently formed by osteoblasts, as there hasn’t been an opportunity for osteoclast degeneration yet
What are canaliculi in bone?
A system of tiny canals that allow osteocytes to be able to access the nutrients carried in the capillaries of the Haversian canals or from marrow spaces in trabecular bone - appear as fine lines in ground bone extending from capillaries to osteocytes in their matrix
How are osteocytes interconnected?
By canaliculi through which they send fine cytoplasmic processes and communicate via gap junctions
What is the periosteum?
Fibrous capsule that covers the outer surface of a bone
What is the function of the periosteum?
Carries a blood and neural supply to the bone
Describe the structure of the periosteum
Periosteal blood vessels penetrate into bone from the fibrocollagenous layer and the inner periosteum is lined by osteoprogenitor cells (stem cells –> osteoblasts)
What are the two types of ossification?
Intramembranous ossification and endochondral ossification
What is intramembranous ossification?
Formation of bone from within a membrane of primitive mesenchymal tissue (occurs in flat bones)
What is endochondral ossification?
Formation of bone from a cartilage model (typified by long bone development)
Outline the 4 stages of intramembranous ossification
1) Within mesenchyme, mesenchymal cells –> osteoblasts and form numerous centres of ossification
2) Osteoblasts –> osteoid at centres –> mineralisation of osteoid –> spicules of bone
3) Ossification centres grow (due to osteoblast differentiation) –> grow around foetal blood vessels in mesenchymal membrane until centres meet –> fuse –> spongy bone –> mesenchyme condenses on outer surface –> vascularised periosteum
4) Further fusion of outer trabeculae and periosteum formation –> outermost bone –> continuous sheet of woven bone –> remodelling –> outer woven to lamellar compact –> inner bone –> lamellar trabecular, mesenchymal cells within spaces of spongy bone –> haemopoietic marrow cells
What is a diaphysis?
Shaft of dumb-bell hyaline cartilage model
What is an epiphysis?
End portions of dumb-bell shaped hyaline cartilage model
Describe the formation of the primary ossification centre in endochondral ossification
1) Perichondrium around shaft of cartilage model –> periosteum –> mesenchymal cells here –> osteoblasts –> lay down collar of bone (sides of diaphysis)
2) Cartilage cells in centre of diaphysis hypertrophy –> thin intervening cartilage matrix –> matrix calcifies –> cells degenerate
3) Nutrient artery perforates bony collar around diaphysis –> carries primitive mesenchymal and osteoprogenitor cells –> osteoblasts –> osteoid –> form bone of calcified cartilage remnants –> trabecular bone formed in centre of diaphysis –> primary ossification centre
What covers the dumb-bell shaped structure at the beginning of endochondral ossification?
Perichondrium
What is the necessary precursor to endochondral ossification?
Hyaline cartilage model (dumb-bell shape)
What is the primary ossification centre in endochondral ossification?
Trabecular bone in the centre of the diaphysis
How does the diaphyseal bone grow in diameter?
By adding to the periosteal surface
Describe the formation of the secondary ossification centre in endochondral ossification
At epiphyseal ends: cartilage cells hypertrophy and die, intervening matrix –> calcifies, epiphyseal arteries carry osteoprogenitor cells into area –> osteoblasts –> form trabecular bone in centre of epiphyses –> secondary ossification centres
What is the secondary ossification centre in endochondral ossification
Trabecular bone in the centre of the epiphyses
Describe the formation of the epiphyseal growth plate
Trabecular bone continues to replace cartilage in epiphysis –> cartilage only remains on articulating surfaces between diaphysis and epiphysis –> epiphyseal growth plate
What is the role of the epiphyseal growth plate?
Allows bone to continue to grow in length until the end of puberty
Describe appositional growth
More bone is laid down by osteoblasts in the periosteum and more cartilage is laid down on articulating surfaces by proliferation of surface chondroblasts and matrix expansion
What are the 5 zones of endochondral ossification
1) Zone of resting/reserve cartilage
2) Zone of chondrocyte proliferation
3) Zone of maturation and hypertrophy
4) Zone of degeneration and calcification of matrix
5) Zone of osteogenesis (bone formation)
Describe the histology of how muscles can be directly attached to bone
Collagen of endomysium of skeletal muscle interweaves with the collagen of the bone periosteum
What are Sharpey’s fibres?
Dense collagen fibre bundles that are inserted into the bone matrix which attach the periosteum to the bone (extend deeply into bone at such sites)
