Chapter 6: The skeletal system bone tissue Flashcards
Bone is a type of _____ tissue
Bone is a type of connective tissue – it has cells, fibers, & ground substance
Skelton is divided into two skeleton
s
axial and appendicular skeleton
Skull, vertebrate and ribs = axial
Limbs and girdles = appendicular skeleton
Structure of a Long Bone
- Bone that is longer than it is wide
- Includes all limb bones (except ankle/wrist bones)
- Are slightly curved for strength
- Consists of a shaft (diaphysis) and 2 expanded ends (epiphyses)
- The metaphysis is between the diaphysis and epiphysis (contains the epiphyseal plate, in a growing bone).
- Periosteum is a tough connective Tissue layer that covers the shaft of the bone
- Hyaline cartilage covers articular surfaces
- Compact bone on outer edges of shaft & epiphyses
- Spongy bone inside epiphyses
- Marrow (medullary) cavity is in hollow shaft
- Endosteum lines marrow cavity
Shaft of a bone is called
diaphysis
End of a bone is called
epiphyses
Part of bones between the shaft and end of bone
metaphysis
2 Types of bones
Compact Bone - found on outter edges of shaft & epiphyses, the stronger bone
Spongy bone - found inside epiphyses
Dif between then is the size and distributions of the spaces between then
80% of skeleton is conpact, 20% is spongy
Histology of Bone Tissue
• Extracellular matrix surrounds the widely separated cells
• It’s composed of ground substance and fibers
i) 2/3 of it is inorganic matter (crystallized mineral salts)
• Provides the quality of “hardness” to bone
ii) 1/3 of it is organic matter (collagen fibers)
• Provides the qualities of “flexibility” & “tensile strength” to bone
Four types of cells are present in bone tissue:
1) Osteoprogenitor cells
2) Osteoblasts
3) Osteocytes
4) Osteoclasts
Osteoprogenitor cells
- Mitotic stem cells found in periosteum & endosteum
- only bone cells that undergo cell division (mitotic)
- Come from Mesenchyme tissue (embryonic connective tissue in which all other connective tissues arise)
- eventually divide and turn into osteoblasts
Osteoblasts
- Bone-building cells that produce the matrix components (collagen, etc.)
- make the extracellular matric, very metabolically active. Eventually the matrix becomes hardened and the osteoblasts that are trapped in the bone become the 3rd type (osteocytes)
Osteocytes
Mature bone cells (osteoblasts that are trapped in matrix)
Osteoclasts
- Cells that break down bone and are found in the endosteum
- completely different from the other 3
- Breaking down of bone tissues = bone resorption
Compact bone
• Forms the outer layer of all bones & most of diaphysis of long bones
• Arranged into structural units called osteons or Haversian systems
• Each osteon consists of layers (rings) of bone matrix called lamellae
• Lamellae encircle a Haversian (central) canal
• Volkmann (perforating) canals run horizontally thru bone & join adjacent
• Between lamellae are lacunae which contain osteocytes
• Osteocytes extend cytoplasmic processes into canaliculi
Canaliculi connect lacunae to each other & to central canal [Canaliculi = little tunnels, that allow cytoplasmpic process from the osteocytes and connects with central canal with blood supply to get nutrients and wasts and connects to other osteocytes]
Lamellae = layer
Lacunae are little spaces
Spongy Bone
- Does not contain osteons
- Lamellae are arranged in thin columns of bone called trabeculae
- Contains osteocytes in lacunae and canaliculi
- Between trabeculae have bone marrow
- Trabeculae are arranged in various directions to resist stress
Blood Supply of the Bone
• Bone is richly supplied with blood
• In a long bone:
- Nutrient artery enters the compact bone near the middle of the shaft & branches in the marrow cavity
- Other arteries enter the ends of the bones & also branch to supply the marrow & bony tissues
- Periosteal arteries supply the periosteum & outer part of compact bone
Bone Formation name
Termed ossification (or osteogenesis) Ossification - the process by which bone is formed
Bone formation occurs in 3 situations
1) In fetus/embryo to form the bony skeleton
2) Into early adulthood as growth in length of bones
3) Throughout life as remodeling of bone & repair of fractures
Are two main types of ossification:
1) Intramembranous ossification
- Bone arises from embryonic mesenchyme tissue
- Occurs in the formation of flat bones of the skull and mandible and clavicles
2) Endochondral ossification
–Bone develops from existing hyaline cartilage model
–Most bones of the body are formed in this way
– more complex cause first you form a model using cartilage then must replace with bone.
Intramembranous Ossification
4 Steps
i) Ossification centre forms
– Mesenchymal cells differentiate into osteogenic cells; then into osteoblasts
– Osteoblasts secrete the organic extracellular matrix
ii) Calcification occurs
– Trapped osteoblasts become osteocytes
– Mineral salts are deposited & matrix calcifies
iii) Trabeculae form
– Matrix develops into trabeculae that fuse with one another to form spongy bone
– red bone marrow forms in the spaces between trabeculae
iv) Periosteum develops
– Mesenchyme @ periphery of bone condenses to form the periosteum
– Thin layer of compact bone forms under periosteum
Endochondral Ossification
6 Steps
i) Cartilage model develops
– Mesenchymal cells crowd together in shape of future bone
– Become chondroblasts & secrete cartilage extracellular matrix
– Produce hyaline cartilage model
–Perichondrium develops around cartilage model
ii) Growth of cartilage model
– Cartilage model grows in length & thickness
– Trapped chondroblasts become chondrocytes
– Chondrocytes in the mid-region enlarge & matrix becomes calcified
– Chondrocytes die & lacunae form & merge into small cavities
iii) Primary ossification centre
– Nutrient artery penetrates perichondrium
– Osteogenic cells in perichondrium differentiate into osteoblasts
– Perichondrium becomes periosteum
– Capillaries grow into mid-region inducing growth of a primary ossification centre
– Osteoblasts deposit bone extracellular matrix over calcified cartilage remnants
– Bony trabeculae form
– Ossification spreads to ends of cartilage model
iv) Marrow cavity forms
– Osteoclasts breakdown some of the newly formed trabeculae
– Marrow cavity is formed in the shaft
– Wall of the shaft is replaced by compact bone
v) Secondary ossification centre
– Artery grows into epiphysis & secondary ossification centre forms @ time of birth
– Forms spongy bone
vi) Cartilage remains
– Remaining hyaline cartilage at the joint surface becomes articular cartilage
– Hyaline cartilage remains b/t the diaphysis & epiphysis – epiphyseal plate (growth plate)
– Bone grows in length by adding new chondrocytes on the epiphysis side of the plate
– Replace old chondrocytes by bone on the diaphysis side of the plate
– At adulthood, epiphyseal plate becomes ossified
Term for a bone growing in legth
Term for a bone growing in width
Interstitial (growns in legth by more chondrocytes and condroblasts are produce that produces more cartilage)
Appositional (grows in thickness width. New chondroblasts are produced around the peripheral of the model that produces more cartilage so it grows outward)
Bone remodeling
- Bone undergoes constant remodeling, replacing old osseus tissue w/ new osseus tissue
- Bone repairs fractures (any break in a bone)
Calcium Homeostasis
• Bone is the body’s major calcium reservoir
• Calcium must be maintained in a very narrow range that our blood calcium levels must be maintained at (9-11mg/100ml). Our bone tissue acts like a buffer where if calcium is too high the boens can absorb the calcium or relase some if cacium is too low.
• Calcium concentration in the blood is regulated by two hormones:
1) Calcitonin (trigger with high Ca2+ levels)
2) Parathyroid hormone (PTH) (tiggered with low Ca2+ levels)
Calcitonin
– Calcitonin increases when blood [Ca2+] increases
– calcitonin inhibits bone resorption & more calcium salts are deposited in bone matrix
–Calcitonin decreses cacum levels in the blood. (it’s released as Ca levels rise). It tells the osteoclasts to stop breking down the bones and tells osteoblasts ramp it up to make more bone tissue at a faster rate
Parathyroid hormone (PTH)
– PTH increases when blood [Ca2+] decreases
– PTH stimulates the osteoclasts to reabsorb bone, releasing Ca2+ to blood
– PTH acts on the kidneys to decrease loss of Ca2+ in the urine
–Parathyroid hormone (PTH) increases as blood cacium levels decreases. Helps bring cacium levels back up. Tells osteoblasts to stop making new bone tissue. Tells osteoclasts to break bown bone.
–It also acts on kidneys (these get rid of cacium in urine) to retain more cacium.
–They also stimulate the formation of vitamin D which helps us absorb cacium from our food.
