TBL2 - Osteogenesis

Question 1

How does bone formation begin?

Answer 1

Formation of most bones including the vertebrae begins by chondroblasts producing avascular cartilaginous replicas of the bones

Question 2

Describe hyaline cartilage

Answer 2

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

Question 3

Describe endochondral bone formation

Answer 3

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

Question 4

How are osteoclasts formed?

Answer 4

An osteoclast is created by fusion of multiple macrophages to phagocytize degrading portions of the cartilaginous matrix

Question 5

What are osteocytes and what do they do?

Answer 5

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

Question 6

How can one characterize bone in terms of lacunae structure?

Answer 6

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

Question 7

What is the single layer of osteoblasts covering the bone marrow cavity (medullary cavity) called?

Answer 7

The single layer of osteoblasts covering the medullary cavity is called endosteum, which is essential for continued bone maturation

Question 8

Describe bone matrix

Answer 8

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

Question 9

Describe trabecular (spongy) bone

Answer 9

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

Question 10

What occurs to spongy bone at the periphery of a tissue?

Answer 10

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

Question 11

Describe the periosteum

Answer 11

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

Question 12

How is spongy bone remodeled?

Answer 12

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

Question 13

What are central canals? (Do volkmann canals connect periosteal blood vessels to osteon central canals also?)

Answer 13

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

Question 14

Aside from the ends of bones, where else is spongy bone found?

Answer 14

Trabecular bone remains in the central portion of mature bones and is occupied by bone marrow

Question 15

What is the function of canaliculi?

Answer 15

Canaliculi connect lacunae to the surface of the trabeculae; thus, trabecular osteocytes have access to filtrates from the bone marrow capillaries

Question 16

What is the general distribution of compact and spongy bone in all bones?

Answer 16

All bones, including the vertebral bodies and arches, contain central trabecular bone enclosed by outer compact bone

Question 17

What are interstitial lamellae?

Answer 17

Interstitial lamellae are remnants of osteons after bone remodeling

Question 18

Why do bones remodel themselves throughout life?

Answer 18

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

Question 19

What are the periosteum & endosteum?

Answer 19

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

Question 20

What causes vertebral body osteoporosis and why can it result in increased kyphosis?

Answer 20

1) Vertebral body osteoporosis is a common metabolic bone disease that is often detected during routine radiographic studies. Osteoporosis results from a net demineralization of the bones caused by a disruption of the normal balance of calcium deposition and resorption. As a result, the quality of bone is reduced and atrophy of skeletal tissue occurs
2) Radiographs taken during early to moderate osteoporosis demonstrate demineralization, which is evident as diminished radiodensity of the trabecular (spongy) bone of the vertebral bodies, causing the thinned cortical bone to appear relatively prominent. Osteoporosis especially affects the horizontal trabeculae of the trabecular bone of the vertebral body
3) Radiographs in later stages may reveal vertebral collapse (compression fractures) and increased thoracic kyphosis (Kyphosis is a curving of the spine that causes a bowing or rounding of the back, which leads to a hunchback or slouching posture). Vertebral body osteoporosis occurs in all vertebrae but is most common in thoracic vertebrae and is an especially common finding in postmenopausal females