A&P - 6.3 Bone Structure (bone cells and compact vs. spongy bone)

bone cells

• osteogenic cell

• osteoblast

• osteocyte

• osteoclast

osteogenic cell

• undifferentiated cell with high mitotic activity as they differentiate and develop into osteoblasts

• the only bone cells that divide

• immature osteogenic cells are found in the deep layers of the periosteum and the marrow

• function: develop into osteoblasts

• location: deep layers of the periosteum and the marrow

osteoblast

• cell responsible for forming new bone

• found in the growing portions of bone including periosteum and endosteum

• do not divide

• synthesize and secrete the collagen matrix and calcium salt

• function: bone formation

• location: growing portions of bone, including periosteum and endosteum

osteocyte

• as the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte

• primary cell in mature bone (most common type of bone cell)

• responsible for maintaining mineral concentration of the matrix via secretion of enzymes

• lack mitotic activity

• function: maintain mineral concentration of matrix

• location: entrapped in matrix

lacunae

• (singular = lacuna) each osteocyte is located in a space called a lacuna and is surrounded by bone tissue

• found at borders of adjacent lamellae

canaliculi

• (singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that is uses to communicate and receive nutrients and wastes to be removed from them

• canaliculi connect with the canaliculi of other lacunae and eventually with the central canal

osteoclast

• cell responsible for resorbing or breaking down old bone

• found on bone surfaces

• multinucleated

• originate from monocytes and macrophages

• function: bone resorption

• location: bon surfaces and at sites of old, injured, or unneeded bone

overview of compact bone

• denser, stronger of the two types of bone tissue

  • dense osseous tissue that can withstand compressive forces

• found under the periosteum and in the diaphysis of long bones

• provides support and protection

• diagram of compact bone: this cross-sectional view of compact bone shows the basic structural unit, the osteon

microscopic anatomy: compact bone

NOTE: OSTEON with central canal (aka Haversian system)

• the microscopic structural unit of compact bone is called an osteon, or Haversian system

  • concentric lamellae

  • circumferential lamellae

  • interstitial lamellae

  • osteocytes in lacunae

  • perforating canal

  • compact bone vs spongy bone

osteon

• (also, Haversian system) basic structural unit of compact bone

• made of concentric layers of calcified matrix called lamellae

lamellae

• concentric rings of calcified matrix

• types:

  • concentric lamellae

  • circumferential lamellae

  • interstitial lamellae

concentric lamellae

form cylindrical osteons

circumferential lamellae

layers that wrap around the entire bone

interstitial lamellae

fill the spaces between osteons, often being remnants of remodeling

central canal or haversian canal

• longitudinal channel in the center of each osteon

• contains blood vessels, nerves, and lymphatic vessels

perforating canal or volkmann’s canal

vessels and nerves from central canal (haversian canal) branch off at right angles through a perforating canal (volkmann’s canal) to extend to the periosteum and endosteum

overview of spongy bone

• (also, cancellous bone) trabeculated osseous tissue that supports shift in weight distribution

• spongy (cancellous) bone has open spaces 

• located where bone are not heavily stressed or stress in many directions (ex: flat bone)

• contains osteocytes housed in lacunae, but not arranged in concentric circles

microscopic anatomy: spongy bone

• lamellae form struts and plates (trabeculae) creating an open network

  • reduces weight of skeleton

  • no blood vessels in matrix

    • nutrients reach osteons through canaliculi open to trabeculae surfaces

  • red bone marrow is found between trabeculae

trabeculae

• lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae

• trabeculae may appear to be a random network, but each trabeculae forms along lines of stress to provide strength to the bone

• the spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily

• spaces in some spongy bones contain red marrow, protected by trabeculae, where hematopoiesis occurs