Flashcards in TBL2 - Osteogenesis Deck (20):
How does bone formation begin?
Formation of most bones including the vertebrae begins by chondroblasts producing avascular cartilaginous replicas of the bones
Describe hyaline cartilage
1) Except where hyaline cartilage serves as articular cartilage and is exposed to synovial fluid, it is enclosed by a layer of dense connective tissue—the perichondrium—which is essential for cartilage growth
2) Cartilage consists of chondrocytes in lacunae dispersed within a noncalcified matrix enclosed by perichondrium
3) This connective tissue investment is rich in fibroblasts, undifferentiated mesenchymal cells, blood vessels, and nerves
4) A meshwork of thin Type II collagen fibers creates the translucent, glassy appearance of the matrix that designates the tissue as hyaline cartilage
5) O2 and nutrients diffuse from capillaries in the perichondrium into the avascular matrix to supply the chondrocytes
Describe endochondral bone formation
1) Mesenchyme cells begin to condense and differentiate into chondrocytes
2) Chondrocytes form a cartilaginous model of the prospective bone
3) Blood vessels invade the center of the cartilaginous model, bringing osteoblasts and restricting proliferating chondrocytic cells to the ends (epiphyses) of the bones
4) Chondrocytes toward the shaft side (diaphysis) undergo hypertrophy and apoptosis as they mineralize the surrounding matrix
5) Osteoblasts bind to the mineralized matrix and deposit bone matrices. Later, as blood vessels invade the epiphyses, secondary ossification centers form
6) Growth of the bones is maintained by proliferation of chondrocytes in the growth plates
How are osteoclasts formed?
An osteoclast is created by fusion of multiple macrophages to phagocytize degrading portions of the cartilaginous matrix
What are osteocytes and what do they do?
1) Osteoblasts produce randomly organized Type I collagen fibers that entrap most of the osteoblasts in lacunae. These cells in lacunae are called osteocytes
2) Osteocytes induce deposition of hydroxyapatite crystals on the collagen fibers that subsequently align in linear parallel rows
How can one characterize bone in terms of lacunae structure?
Parallel alignment of the collagen fibers places the lacunae in concentric rows; thus, bone is characterized by rows of lacunae separated by parallel type I collagen fibers for strength and hydroxyapatite crystals for hardness
What is the single layer of osteoblasts covering the bone marrow cavity (medullary cavity) called?
The single layer of osteoblasts covering the medullary cavity is called endosteum, which is essential for continued bone maturation
Describe bone matrix
1) Bony matrix can be likened to reinforced concrete and consists of both organic and inorganic components
2) The organic portion, comprising 30%-40% of the matrix, consists mostly of type I collagen and associated glycoproteins and provides tensile strength and resilience
3) The remaining 60%-70% of
the matrix is inorganic and consists of minerals, mostly crystals of hydroxyapatite, for hardness and rigidity
Describe trabecular (spongy) bone
1) Maturation organizes the mineralized fibers and lacunae into three-dimensional meshworks designated trabecular (spongy) bone
2) Compared to compact bone, spongy bone has a higher surface area to mass ratio because it is less dense. This gives it softer, weaker, and more flexible characteristics
3) The greater surface area in comparison with cortical bone makes spongy bone suitable for metabolic activity e.g. exchange of calcium ions
4) Spongy bone is typically found at the ends of long bones, proximal to joints and within the interior of vertebrae
5) Spongy bone is highly vascular and frequently contains red bone marrow where haematopoiesis, the production of blood cells, occurs
What occurs to spongy bone at the periphery of a tissue?
1) At the periphery, trabecular bone is transformed into compact (dense) bone, which is covered on its outer surface by periosteum
2) Remodeling occurs by osteoblasts laying down bone on one part of a trabecula, while osteoclasts resorbs another part. Its trabecular framework also protects the marrow cells. Compact bone can form or resorb beneath the periosteum
3) The inner surfaces of primary osteons are lined by endosteum, which is separated from the centrally positioned capillaries (and sensory nerves) by interstitial fluid
4) Compact bone also remodels by forming osteons, which are all aligned in the same direction to resist bending forces. The outer periosteum provides a route for vessels and nerves and actively participates in bone growth and repair after fracture
Describe the periosteum
The periosteum consists of two
poorly demarcated layers that differ histologically:
1) An outer layer of dense, irregular, connective tissue consists mostly of fibroblasts interspersed with type I collagen fibers and a smaller proportion of elastic fibers. It contains many large blood vessels, nerves, and lymphatics (outer layer is firmly anchored to compact bone)
2) An inner (cambium) layer of loose, richly vascularized connective tissue contains osteogenic cells and osteoblasts in direct contact with the bone surface
How is spongy bone remodeled?
1) Endosteal osteoblasts produce a bony matrix with its characteristic concentric rows of lacunae separated by parallel mineralized fibers
2) Observe progressive deposition of the bony matrix encloses the central capillaries (and sensory nerves) and thereby transforms the 1° osteons into 2° osteons
What are central canals? (Do volkmann canals connect periosteal blood vessels to osteon central canals also?)
1) In 2° (mature) osteons, central canals enclose the capillaries and sensory nerves
2) Central canals are lined by endosteum and the concentric dotted lines represent osteocyte-filled lacunae created by progressive inward deposition of parallel mineralized fibers
3) Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal
4) In each osteon, bony canaliculi (tiny canals) connect the central canal to the innermost row of lacunae and interconnect the concentric rows of lacunae. Thus, lacunae throughout the osteon have access to capillary filtrates in the central canal thereby insuring osteocyte viability and active maintenance of the mineralized bony matrix
5) Central canals are joined at right angles by Volkmann canals thereby enabling capillaries of the periosteum to extend into the central canals
6) Osteons are oriented parallel to the longitudinal axes of bones and interstitial lamellae reside between the clustered osteons
Aside from the ends of bones, where else is spongy bone found?
Trabecular bone remains in the central portion of mature bones and is occupied by bone marrow
What is the function of canaliculi?
Canaliculi connect lacunae to the surface of the trabeculae; thus, trabecular osteocytes have access to filtrates from the bone marrow capillaries
What is the general distribution of compact and spongy bone in all bones?
All bones, including the vertebral bodies and arches, contain central trabecular bone enclosed by outer compact bone
What are interstitial lamellae?
Interstitial lamellae are remnants of osteons after bone remodeling
Why do bones remodel themselves throughout life?
1) Understand remodeling occurs throughout life to adapt bones to changing physical stresses e.g., weight gains or new physical training regimens
2) Bone vascularity ensures good regenerative potential for remodeling, which includes matrix resorption and deposition of new bony matrix adapted to support the altered physical stresses
What are the periosteum & endosteum?
1) Endosteal and periosteal osteoblasts are responsible for the deposition of new bony matrix
2) Deposition rates must match resorption rates to maintain skeletal integrity
3) Periosteum is a membrane that covers the outer surface of all bones, except at the joints of long bones
4) Endosteum lines the inner surface of all bones