2.  Parts of the skeletal system ◦ Bones (skeleton) ◦ Joints ◦ Cartilages ◦ Ligaments  Two subdivisions of the skeleton ◦ Axial skeleton ◦ Appendicular skeleton

3.  Support the body  Protect soft organs  Allow movement due to attached skeletal muscles  Store minerals and fats  Blood cell formation

4.  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

5. Figure 5.1

6.  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

7.  Short bones ◦ Generally cube-shape ◦ Contain mostly spongy bone ◦ Example :  Carpals  Tarsals

8.  Flat bones ◦ Thin, flattened, and usually curved ◦ Two thin layers of compact bone surround a layer of spongy bone ◦ Example :  Skull (parietal)  Ribs  Sternum

9.  Irregular bones ◦ Irregular shape ◦ Do not fit into other bone classification categories ◦ Example :  Vertebrae  Hip bones

10.  Diaphysis ◦ Shaft ◦ Composed of compact bone  Epiphysis ◦ Ends of the bone ◦ Composed mostly of spongy bone

11.  Periosteum ◦ Outside covering of the diaphysis ◦ Fibrous connective tissue membrane  Sharpey’s fibers ◦ Secure periosteum to underlying bone  Arteries ◦ Supply bone cells with nutrients

12.  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

13.  Medullary cavity ◦ Cavity inside of the shaft ◦ Contains yellow marrow (mostly fat) in adults ◦ Contains red marrow (for blood cell formation) in infants

14.  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

15. Table 5.1 (1 of 2)

16. Table 5.1 (2 of 2)

17.  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

18. Figure 5.3a

19.  Lacunae ◦ Cavities containing bone cells (osteocytes) ◦ Arranged in concentric rings  Lamellae ◦ Rings around the central canal ◦ Sites of lacunae

20. Figure 5.3b–c

21.  Canaliculi ◦ Tiny canals ◦ Radiate from the central canal to lacunae ◦ Form a transport system connecting all bone cells to a nutrient supply

22. Figure 5.3b

23.  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

24.  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

25.  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)

26. Figure 5.4a Bone starting to replace cartilage Epiphyseal plate cartilage Articular cartilage Spongy bone In a childIn a fetusIn an embryo New bone forming Growth in bone width Growth in bone length Epiphyseal plate cartilage New bone forming Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (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 cavity 4.Cavity formation occurs within the hyaline cartilage 5.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.

27. Figure 5.4a, step 1 Bone starting to replace cartilage In an embryo Bone collar Hyaline cartilage model (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

28. Figure 5.4a, step 2 Bone starting to replace cartilage In a fetusIn an embryo Growth in bone length Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (a) 3.Periosteal bud invades the marrow cavity 4.Cavity formation occurs within the hyaline cartilage 5.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.

29. Figure 5.4a, step 3 Bone starting to replace cartilage Epiphyseal plate cartilage Articular cartilage Spongy bone In a childIn a fetusIn an embryo New bone forming Growth in bone width Growth in bone length Epiphyseal plate cartilage New bone forming Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (a)

30. Figure 5.4b

31.  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

32.  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.

33. 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

34. 3. Osteoclasts Cells that digest bone matrix – this process is called bone resorption and is part of normal bone growth, development, maintenance, and repair.

35. Here, we see a cartoon showing all 3 cell types. Osteoblasts and osteoclasts are indicated. Osteocyte

36.  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

37.  Calcium is released by the bones Released by the cells of the parathyroid gland in response to low blood [Ca 2+ ]. 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.