6 Osteogenesis

  1. What are the 2 types of bones in the body?
    • 1. Endochondral
    • 2. Membrane (Dermal)
    • most of the bones in the body are Endochonral: long bones of the extremities, ribs, other bones of the thorax, vertebrae, & pelvis
    • most of the flat bones of the skull are Membrane bones & form completely differently than the endochondral bones
  2. What process forms Endochondral bones?
    • Endochondral Ossification
    • start with a cartilage template that resembles a small version of the adult bone & use it to deposit bone on
    • bone initially deposited is called Woven bone
    • after remodeling it BECOMES lamellar (compact or spongy)
  3. Steps of Endochondral Ossification
    • start with a cartilage template that somewhat resembles the adult bone that’s going to be formed
    • 1. chondrocytes in the center of the cartilage (mini-bone template) begin to ENLARGE (hypertrophy); they will eventually die, & the cartilage in between them will become calcified
    • that will make the region porous so blood vessels can enter, bringing with them osteogenic cells (blasts + clasts)
    • 2. osteoblasts will start laying new bone on top of the calcified cartilage
    • this forms the Primary Center of Ossification
    • 3. at about the same time all that’s going on, the Periosteal Collar is being made; this is bone that’s forming around the shaft/diaphysis
    • the collar is DE NOVO bone synthesis - new bone is made on the edge of hyaline cartilage (no template)
    • perichondrium → periosteal collar → bony collar
    • 4. the same events that occurred to form the Primary Center of Ossification now happen at the 2 Epiphyseal ends: hyaline cartilage chondrocytes hypertrophy, intervening cartilage calcifies, chondrocytes die, ingrowth of blood vessels + osteoprogenitor cells, new bone is laid down on top of calcified cartilage
    • these are called the Secondary Centers of Ossification
    • 5. a portion of hyaline cartilage called the Epiphyseal Plate is maintained between both Secondary Centers of Ossification & the central Primary Center of Ossification
    • the Epiphyseal Plate is what allows long bones to grow in length
    • Image Upload 1
  4. Image Upload 2
    • can see hyaline cartilage on the two ends
    • however central hyaline cartilage contains much larger cells (hypertrophied)
    • cartilage in between cells is going to become calcified (stains blue/purple instead of pink)
    • chrondrocytes will die
    • area becomes porous, allowing BVs to migrate in & bring with them osteogenic cells to build bone on the calcified cartilage
  5. Epiphyseal Growth Plate
    • Image Upload 3
    • Resting: standard hyaline cartilage (see double Ds, there’s some division)
    • Proliferating: when cells proliferate in earnest they do so along the long axis, parallel to the bone - resembles a “stack of coins” as opposed to just a clump of cells
    • Hypertrophic: chondrocytes are much LARGER & the in between cartilage is darker than in the resting & proliferating zones - this is because it’s undergoing CALCIFICATION
    • Calcified: calcified cartilage is the template upon which osteoblasts will lay down new bone; chondrocytes are dead in this type of bone
    • the calcified end is closest to the Primary Center of Ossification (diaphysis)
    • Image Upload 4
  6. What is diagnostic of Endochondral Ossification?
    • osteoblasts laying down new bone on top of calcified cartilage - MIXED SPICULES
    • Image Upload 5
    • blue: CC (calcified cartilage) - blue because it has lots of sulfated proteoglycans, binds the eiosin
    • pink: bone, more rich in cartilage so it binds the hemotoxalin
    • you know this is new bone because of the osteoid being synthesized by osteoblasts
  7. Epiphyseal Plate Thickness
    • stays the same thickness, just shifts ‘up’ toward the epiphysis
    • there’s erosion of hyaline cartilage at the diaphyseal end, & addition of hyaline cartilage at the epiphyseal end
  8. What happens when the Epiphyseal Growth Plate closes?
    • there’s no longer proliferation of chondrocytes at the ‘epiphyseal’ (upper) end, but there’s STILL erosion of them at the diaphyseal end
    • eventually the plate gets thinner & thinner → eventually disappears
    • in adult bone there’s no evidence of it whatsoever
  9. Bone Remodeling
    • bones remodel throughout life as well as to repair fractures & especially during development
    • the woven bone initially deposited is very disorganized - doesn’t have the strength of lamellar bone
    • both spongy & compact bone need to be remodeled after initial placement
  10. What bone initially makes up the bone collar?
    • the bone collar is initially formed as SPONGY bone - have lots of marrow spaces
    • these will eventually be filled in by osteoblasts until what’s left is compact (Haversian) bone
    • but Haversian systems don’t come about UNTIL remodeling
  11. Compact Bone Remodeling
    • osteoclasts excavate a channel
    • once there’s exposed bone, osteoblasts are recruited to the wall of the channel, line up & begin synthesizing bone
    • some will get entrapped → osteocyte, while the rest continue to form the Osteon
  12. Image Upload 6
    • image shows Osteons (Haversian systems) in development
    • faint blue/purple line shows the extent to which osteoclasts excavated the bone (initial channel that was created)
    • osteoblasts lined up on that surface & laid down new bone
    • some osteoblasts became trapped → osteocyte
    • the rest can be seen synthesizing new bone (Osteoid)
    • Image Upload 7
  13. Spongy Bone Remodeling
    • during development, the spicules are small & not robust
    • they need to enlarge & become thicker to form adult bone
    • Image Upload 8
    • there will be removal of woven bone by osteoclasts, & new lamellar bone will get deposited by osteoblasts
    • space for red bone marrow still exists even in adult spongy bone
  14. Intramembranous Ossification
    • builds the flat bones in the cranium & face
    • bone formation will begin DE NOVO, from condensation of mesenchyme
    • there’s NO cartilage template
    • bone will be initially deposited woven, then remodeled into compact & spongy bone as needed
    • have important difference in terms of bone building, but once Intramembranous bone is BUILT, it’s remodeled in the exact same way as Endochondral bone
    • Image Upload 9
  15. Image Upload 10
    • see condensation of mesenchyme & cells differentiating into osteoblasts
    • bone is initially made as woven spongy bone
    • if the bone is supposed to become compact, the white spaces will be filled in, followed by remodeling to Haversian systems
    • if the bone is supposed to stay spongey, then the marrow spaces would persist
  16. periosteal vs. endosteal lamellae
    periosteal (outer) lamellae lie beneath the periosteum while endosteal (inner) lamellae lie beneath the endosteum
  17. 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)
  18. periosteum
    • 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
  19. 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
  20. 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
  21. 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)
  22. 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
Card Set
6 Osteogenesis
Histology Exam 2