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.