parts of the skeletal system ◦ bones (skeleton) ◦ joints ◦ cartilages ◦ ligaments two...
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Parts of the skeletal system◦ Bones (skeleton)◦ Joints◦ Cartilages◦ Ligaments
Two subdivisions of the skeleton◦ Axial skeleton◦ Appendicular skeleton
Support the body Protect soft organs Allow movement due to attached skeletal
muscles Store minerals and fats Blood cell formation
The adult skeleton has 206 bones Two basic types of bone tissue
◦ Compact bone Homogeneous, dense, smoothe
◦ Spongy bone Small needle-like
pieces of bone Many open spaces
Figure 5.2b
Figure 5.1
Long bones◦ Typically longer than they are wide◦ Have a shaft with heads at both ends◦ Contain mostly compact bone- not much spongy
bone◦ Example:
Femur Humerus Phalanx
Short bones◦ Generally cube-shape◦ Contain mostly spongy bone◦ Example:
Carpals Tarsals
Flat bones◦ Thin, flattened, and usually curved◦ Two thin layers of compact bone surround a layer
of spongy bone◦ Example:
Skull (parietal) Ribs Sternum
Irregular bones◦ Irregular shape◦ Do not fit into other bone classification categories◦ Example:
Vertebrae Hip bones
Diaphysis◦ Shaft◦ Composed of compact bone
Epiphysis ◦ Ends of the bone◦ Composed mostly of spongy bone
Periosteum◦ Outside covering of the diaphysis◦ Fibrous connective tissue membrane
Sharpey’s fibers◦ Secure periosteum to underlyingbone
Arteries◦ Supply bone cells with nutrients
Articular cartilage Covers the external surface of the epiphyses Made of hyaline cartilage Decreases friction at joint surfaces
Epiphyseal plate Flat plate of hyaline cartilage seen in young, growing bone
Epiphyseal line Remnant of the epiphyseal plate (growth plate) Seen in adult bones
Medullary cavity ◦ Cavity inside of the shaft◦ Contains yellow marrow (mostly fat) in adults◦ Contains red marrow (for blood cell formation) in
infants
Surface features of bones◦ Sites of attachments for muscles, tendons, and
ligaments◦ Passages for nerves and blood vessels
Categories of bone markings◦ Projections or processes—grow out from the bone
surface◦ Depressions or cavities—indentations
Table 5.1 (1 of 2)
Table 5.1 (2 of 2)
Osteon (Haversian system)◦ A unit of bone containing central canal and matrix
rings Central (Haversian) canal
◦ Opening in the center of an osteon◦ Carries blood vessels and nerves
Perforating (Volkman’s) canal◦ Canal perpendicular to the central canal◦ Carries blood vessels and nerves
Figure 5.3a
Lacunae◦ Cavities containing bone cells (osteocytes)◦ Arranged in concentric rings
Lamellae◦ Rings around the central canal◦ Sites of lacunae
Figure 5.3b–c
Canaliculi ◦ Tiny canals◦ Radiate from the central canal to lacunae◦ Form a transport system connecting all bone cells
to a nutrient supply
Figure 5.3b
In embryos, the skeleton is primarily hyaline cartilage
During development, much of this cartilage is replaced by bone
Cartilage remains in isolated areas◦ Bridge of the nose◦ Parts of ribs◦ Joints
Epiphyseal plates allow for lengthwise growth of long bones during childhood◦ New cartilage is continuously formed◦ Older cartilage becomes ossified
Cartilage is broken down Enclosed cartilage is digested away, opening up a
medullary cavity Bone replaces cartilage through the action of
osteoblasts
Bones are remodeled and lengthened until growth stops◦ Bones are remodeled in response to two factors
Blood calcium levels Pull of gravity and muscles on the skeleton
◦ Bones grow in width (called appositional growth)
Figure 5.4a
Bone startingto replacecartilage
Epiphysealplatecartilage
Articularcartilage
Spongybone
In a childIn a fetusIn an embryo
New boneforming
Growthin bonewidth
Growthin bonelength
Epiphysealplate cartilage
New boneforming
Bloodvessels
Hyalinecartilage
New center ofbone growth
Medullarycavity
Bone collar
Hyalinecartilagemodel
(a)
1. Perichondrium becomes vascularized to a greater degree and becomes a periosteum
2. Bone collar is laid down around the hyaline cartilage model just beneath the periosteum
3. Periosteal bud invades the marrow cavity4. Cavity formation occurs within the hyaline cartilage5. Osteoblasts lay down bone around cartilage spicules in
the bone’s interior
6. Osteoclasts remove cancellous bone from the shaft interior, leaving a marrow cavity that then houses fat.
Figure 5.4a, step 1
Bone startingto replacecartilage
In an embryo
Bone collar
Hyalinecartilagemodel
(a)
1. Perichondrium becomes vascularized to a greater degree and becomes a periosteum
2. Bone collar is laid down around the hyaline cartilage model just beneath the periosteum
Figure 5.4a, step 2
Bone startingto replacecartilage
In a fetusIn an embryo
Growthin bonelength
Bloodvessels
Hyalinecartilage
New center ofbone growth
Medullarycavity
Bone collar
Hyalinecartilagemodel
(a)
3. Periosteal bud invades the marrow cavity4. Cavity formation occurs within the
hyaline cartilage5. Osteoblasts lay down bone around
cartilage spicules in the bone’s interior
6. Osteoclasts remove cancellous bone from the shaft interior, leaving a marrow cavity that then houses fat.
Figure 5.4a, step 3
Bone startingto replacecartilage
Epiphysealplatecartilage
Articularcartilage
Spongybone
In a childIn a fetusIn an embryo
New boneforming
Growthin bonewidth
Growthin bonelength
Epiphysealplate cartilage
New boneforming
Bloodvessels
Hyalinecartilage
New center ofbone growth
Medullarycavity
Bone collar
Hyalinecartilagemodel
(a)
Figure 5.4b
Osteocytes—mature bone cells Osteoblasts—bone-forming cells Osteoclasts—bone-destroying cells
◦ Break down bone matrix for remodeling and release of calcium in response to parathyroid hormone
Bone remodeling is performed by both osteoblasts and osteoclasts
Bone cells:1. Osteoblasts
Bone-building cells. Synthesize and secrete collagen
fibers and other organic components of bone matrix.
Initiate the process of calcification. Found in both the periosteum and
the endosteum
The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve just secreted.
2. Osteocytes Mature bone cells. Osteoblasts that
have become trapped by the secretion of matrix.
No longer secrete matrix.
Responsible for maintaining the bone tissue.
Yellow arrows indicate osteocytes – notice how they are surrounded by the pinkish bone matrix.
Blue arrow shows an osteoblast in the process of becoming an osteocyte.
On the right, notice how the osteocyte is “trapped” within the pink matrix
3. OsteoclastsCells that digest bone matrix – this process is called bone
resorption and is part of normal bone growth, development, maintenance, and repair.
•Here, we see a cartoon showing all 3 cell types. Osteoblasts and osteoclasts are indicated.
Osteocyte
Calcitonin causes decreased osteoclast activity which results in decreased break down of bone matrix and decreased calcium being released into the blood.
Calcitonin also stimulates osteoblast activity which means calcium will be taken from the blood and deposited as bone matrix.
Notice the thyroid follicles on the right. The arrow indicates a C cell
Calcium is released by the bones
• Released by the cells of the parathyroid gland in response to low blood [Ca2+].
• PTH will bind to osteoblasts and this will cause 2 things to occur:
• The osteoblasts will decrease their activity and they will release a chemical known as osteoclast-stimulating factor.
• Osteoclast-stimulating factor will increase osteoclast activity.