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What are two main types of bones?
- the end of the bone containing the articular cartilage, the 2o center of ossification, and the epiphyseal disc
- where all growth occurs during development and up until puberty
the funnel-shaped region starting with the "bottom" of the epiphyseal plate, and extending to the shaft (diaphysis) of the bone
the shaft (tubular portion) of the bone; extending from the metaphysis of one end to the metaphysis of the opposite end
External surfaces of bones
- either covered (over most of their surface) by a dense connective tissue (the periosteum) or by cartilage (over their articular surfaces)
- they can also be bare (eg. at the sites of attachments of ligaments and tendons)
macroscopically bone is either:
compact or spongy (cancellous)
- dense and, for example, makes up the wall of the diaphysis of a long bone
- gives the bone it's strength
cancellous (spongy) bone
- forms a porous meshwork consisting of branching and anastomosing spicules (trabeculae) of bone
- In mature bones it is generally located on the interior (in the marrow cavity)
Which type of bone are the respective arrows pointing to?
- slivers of bone in spongy bone filled with bone marrow
- (anastomosing bony spicules in spongy bone that form a meshwork of intercommunicating spaces)
What type of bone is this and what are these little holes?
- it's compact bone
- the little holes are blood vessel canals - bone is HIGHLY vascular
- Woven bone
- “young” bone (produced early in development) or made early on when healing fractures
- it's characterized by coarse, interlaced, and randomly dispersed collagen fiber bundles
- it's not organized around blood vessels, but rather is nourished by blood vessels from adjacent connective tissue
- picture is of fractured bone undergoing repair: can see numerous cells in surrounding endosteum
In woven bone, there is a ________ proportion of osteocytes to matrix compared to lamellar bone.
There is a GREATER proportion of osteocytes to matrix in WOVEN bone than in lamellar bone
- Lamellar bone
- highly organized with the collagen fibers oriented oppositely each other and positioned in parallel sheets
- lamellar arrangement is most highly ordered in compact, mature bone where there is a concentric arrangement of lamellae (Haversian)
- can see in picture matrix organized as lamellae, seen faintly with the circles surrounding osteonic canals
What part of lamellar bone imparts strength onto the tissue?
the part where successive sheets of collagen fibers differ in their direction of orientation
organized concentric lamellar bone as long rod like structures around blood vessel channels
haversian cannal in the middle encasing vascular plexus, surrounded by layer upon layer of lamellar bone
periosteal vs. endosteal lamellae
periosteal (outer) lamellae lie beneath the periosteum while endosteal (inner) lamellae lie beneath the endosteum
- a thin layer of connective tissue that lines the medullary cavity
- endosteum have osteogenic capability (stem cells are available on all surfaces of bone tissue)
- a tough dense connective tissue that surrounds the outer surface of bone organs EXCEPT at articular surfaces
- it's anchored to the underlying bone tissue by bundles of collagen fibers (Sharpey fibers) which penetrate perpendicularly into the bone
Which cells of the periosteum have osteogenic potential?
- the innermost cells of the periosteum
- they can become osteoblasts during normal growth, or when they're activated in surface remodeling injury
- Volkman's Canals
- vascular channels that interconnect adjacent Haversian canals
- they can be distinguished from Haversian canals because they cut across the lamellae
- Interstitial Lamellae
- angular regions of parallel lamellae that lie among neighboring osteons
- they're derived from previous generations of osteons partially replaced during the bone reorganization
- they have NO associated blood vessels
- they do contain osteocytes in lacunae & canaliculi, which connect to adjacent osteons
- occur as two types based on their location
- 1) Outer circumferential lamellae: found just beneath the periosteum & encircle the entire bone
- number of them depends on size of the bone organ
- 2) Inner circumferential lamellae: found immediately beneath the endosteum & encircle the marrow cavity
What "bands" osteons and interstitial lamellae for added strength?
the circumferential lamellae below the endosteum covering the marrow cavity
Osteoid (preosseous tissue)
newly synthesized, UNCALCIFIED bone matrix, termed osteoid or
In adult bone tissue, what is always present between osteocytic and mineralized matrix?
- a slight “buffer” zone that remains unmineralized
- the osteoid-like zone serves as an important diffusion zone for oxygen and nutrients, since the blood vessels are only found in the Haversian and Volkmann canals or in the bone marrow
Unlike cartilage, which can grow both appositionally and interstitially due to its flexible ECM, what is the one mechanism bone tissue can grow by?
intramembranous ossification (bone formation)
- when primitive mesenchymal cells differentiate immediately into bone
- early on, mesenchymal cells differentiate into osteoblasts and lay down bone matrix
- as it is initially laid down, the bone is spongy & woven
- mesenchyme --> bone
- how flat bones of skull/jaw form
What are the two initial fates of intramembranous bone?
- 1) it may become compact bone
- in this case appositional growth continues until virtually all of the CT (except blood vessels) are replaced by bone
- spongy bone is gradually converted to compact bone
- *this is still woven bone; lamellar and Haversian bone do not appear UNTIL remodeling occurs
- 2) it may stay cancellous bone
- it's not filled in by bone, and instead remains a framework for hematopoietic bone marrow or fat
endochondral ossification (bone formation)
- when primitive mesenchymal cells differentiate first into chrondrocytes of hyaline cartilage
- they enlarge then die and leave behind cartilage matrix that becomes calcified (calcified MATRIX)
- this calcified matrix induces other mesenchymal cells to differentiate into osteoblasts
- these secrete bone matrix over the surface of calcified cartilage spicules
- mesenchyme --> hyaline cartilage --> bone
- how the long bones form
How can one differentiate endochondral bone from intramembranous bone?
endochondral bone spicules have a calcified basophilic cartilage matrix core (it's the hallmark of newly made endocondral bone)
Just read and know this: spicules and trabeculae of newly formed intramembranous bone tissue have a homogeneous eosinophilia due to the fact that bone tissue matrix is mainly type I collagen. This homogeneous appearance is in contrast to the heterogeneous appearance of the early endochondral trabeculae (see below) and should be used as an important diagnostic feature between the two. However, when adult bones are fully formed and bone tissue has been remodeled, the histology of mature bone is identical, whether formed by intramembranous or endochondral mechanisms.
- new bone tissue formed from stem cells in the periosteum surrounds the bone organ model around its mid-circumference and resembles a napkin ring
- serves as the surface for bone tissue apposition that is responsible for the continued growth in girth of the bone organ.
With the exception of _______________, hyaline cartilage is surrounded by a fibrous CT capsule called the perichondrium. Type I collagen occurs here, but not in the ___________.
- articular surfaces!
- perichondrium has type I cartilage, the cartilage matrix does NOT
periosteum surrounds ____
- perichondrium encapsulates ________
- periosteum surrounds BONE (peri = around, osteon = bone)
- while the perichondrium encapsulates cartilage (peri = around, chondros = cartilage)
- red thing is an EPIPHYSEAL PLATE
- it's a hyaline cartilage plate in the metaphysis at each end of a long bone found in children and adolescents
- in adults who have stopped growing, the plate is replaced by an epiphyseal line
Describe the highly organized chondrocytes arrangement in the epiphyseal disc:
- they line up in rows and undergo a series of histologic morphologies that progress as zones from the epiphyseal plate to the diaphysis
- 1) resting zone (stem cells)
- 2) proliferation zone (chondrocytes undergo mitosis and synthesize cartilage matrix)
- 3) hypertrophic zone (chondrocytes enlarge)
- 4) calcification zone (matrix calcifies, chondrocyte die)
- 5) erosion zone (chondroclasts erode dead cartilage)
- 6) ossification zone (endochondral bony trabeculae form)
What are the diagnostic feature of heterogeneous trabeculae?
basophilic (blue) cartilage cores covered with eosinophilic (red) bone tissue
What is the bone in picture B missing?
an epiphyseal plate
What is bone growth like in adult bones versus juvenile bones?
- growth in adult bones is largely intramembraneous-like
- growth in juvenile bones is largely enchondral
How can the shaft of a long bone increase its diameter?
- by apposition of new bone subperiosteally (right underneath the bone's outermost layer) in a manner similar to that of intramembranous ossification
- in response to this subperiosteal deposition, the marrow cavity also enlarges by resorbing bone beneath the endosteum
What are the three stages of bone development?
- 1) cartilage model
- 2) primary center
- 3) secondary center
- first stage of bone development where the bone starts out as all hyaline cartilage
- characterized by interstitial and appositional growth via chondrocyte division
- is the middle of diaphysis (shaft); this is the first site of ossification
- the periosteum & bone sleeve forms, and the chondrocytes hypertrophy (swell) leading up to matrix calcification
- also there's blood vessel invasion
is the epiphysis' of long bones; this is the second site of ossification
What do primary and secondary centers form together?
an epiphyseal plate
What happens after closure of the epiphysis?
growth ceases and the only cartilage is articular
What same process is used during bone development, mature bone remodeling, and the final stage of fracture healing?
- osteon formation
- this involves the erosion of pre-existing bone by osteoclasts to form long cylindrical resorption channels
- osteoblasts subsequently lay down concentric layers of lamellar bone on the inner walls of the channels until the bone completely surrounds the blood vessels trapped in the middle of the newly formed osteon
In a microradiograph, which type of bone shows up dark and which shows up light?
- newest bone (LEAST calcified) = dark
- oldest bone (MOST calcified) = light
What are four methods of analyzing the same section of ground bone?
fractured bone repair steps
- 1) blood vessels torn within the fracture release blood which clots to produce a large fracture hematoma
- 2) it's gradually removed by macrophages & replaced by procallus tissue (a SOFT fibrocartilage-like mass rich in collagen and fibroblasts)
- if broken, the periosteum re-establishes continuity over this tissue
- 3) the procallus is invaded by regrowing blood vessels and osteoblasts and gradually replaced by trabeculae of primary (woven) bonel; a HARD callus forms throughout the original area of fracture
- 4) the primary bone is then remodeled as compact and cancellous bone to match adjacent uninjured areas
- fully functional vasculature is reestablished
The formation of which of the following directly leads to the appearance of the definitive epiphysial disc/growth plate?
secondary center of ossification
What does this picture depict?
intramembranous ossification; mesenchyme straight to bone
All of the following are derived from the periosteal stem cells except:
- osteoclasts: they're derived from monocytes
- osteoblasts, osteocytes, & osteogenic cells are all derived from the mesenchye, which is ALSO where the periosteal stem cells come from
What does this picture depict?
- endochondral ossification - because there's both
- basophilia of hyaline cartilage and acidophilia acidophilia of bone, it must be endochondral:
- mesenchyme -> cartilage -> bone
Growth in girth of a long endochondral bone is determined by which of the following?
- the periosteum
- the outermost layer, it's in charge of appositional growth which is how bone girth would increase
What type of cell is the arrow pointing to?
- you know it's an OSTEOCLAST because it's multi-nucleated
- they assist with bone degradation
True or False: growth in girth of long bones is dependent upon calcification of cartilage?
- growth in bone girth is subperiosteal and appositional; there's no dependence on cartilage calcification
The epiphysial disc exhibits a diagnostic polarization in relation to the epiphysis and diaphysis. In which direction on this histograph would the epiphysis be located?
- all the way on the left
- left: hyaline cartilage --> must be the epiphysial disc
- blue: ~chrondrocyte proliferation (blue/basophilic b/c of protein production)
- middle white: ~chondrocyte hypertrophy
- red: ~subsequent calcification
True or False: The extracellular matrix of intramembranous ossification is characterized by both acidophilia and basophila.
FALSE because I SAY SO
What are the cells the arrows are pointing to?
- they're basophilic = protein production = bone formation
True or False: all forms of osteogenesis imperfecta are due to mutations that arise in the epiphysial growth plate?
FALSE: some forms arise because of a mutation in collagen type I
Are these statements about osteogenesis true or false?
- 1) all compact bone tissue forms first as spongy bone
- 2) all bone tissue forms first as osteoid
- 3) all bone tissue grows by apposition only
- They're all TRUE
All of the following can be identified in this histograph except:
- all the things shown are involved in bone DEVELOPMENT and growth, not breakdown
- aka osteogenic cells, osteoblasts, osteocytes, & osteoid