SKELETAL SYSTEM Unit 3

The skeletal system acts as support, mineral storage, produces blood cells, and protects the body.

Are bones living? Bones have a very unique structure, as they are made of both living and non-living material Bone is a connective tissue that consists of: – Bone Matrix (foundation) – Cells (bone cells, blood cells)

Skeletal system components – Bones (206 total) Divisions 1.Axial skeleton (126 bones) » Bones of skull, thorax, and vertebral column » Form longitudinal axis of body 2.Appendicular skeleton (80 bones) » Bones of the limbs and girdles that attach them to the axial skeleton – Cartilages – Ligaments and other connective tissues An Introduction to Bones

Figure 6 Section 1 1 Axial Skeleton (126 Bones) The axial skeleton consists of the bones of the skull, thorax, and vertebral column. These elements form the longitudinal axis of the body. The adult skeletal system, which can be divided into the axial skeleton and the appendicular skeleton The appendicular skeleton includes the bones of the limbs and the pectoral and pelvic girdles that attach the limbs to the axial skeleton. Appendicular Skeleton (80 Bones)

Functions of the skeletal system – Support (support for body, attachment for soft tissues) – Storage of minerals (calcium and phosphate) Calcium most abundant mineral in body (~2–4 lb) – 98% stored in bones – Blood cell production (all formed elements of blood) – Protection (delicate tissues and organs surrounded by bone) – Leverage (act as levers with skeletal muscles to move body) An Introduction to Bones

Bone classification Six categories based on shape 1.Flat bones Thin, roughly parallel surfaces Examples: cranial bones, sternum 2.Sutural bones Irregular bones formed between cranial bones Number, size, and shape vary 3.Long bones Relatively long and slender Examples: various bones of the limbs

4. Irregular bones Complex shapes Examples: vertebrae, bones of pelvis, facial bones 5.Sesamoid bones Small, flat, and somewhat shaped like sesame seed Develop in tendons of knee, hands, and feet – Individual variation in location and number 6. Short bones Small and boxy Examples: bones of the wrist (carpals) and ankles (tarsals) Bone classification Cont.

Bone Cells Bone tissue is a type of connective tissue, so it must consist of a lot of bone matrix But where does the bone matrix all come from? The 3 Bone cells: – Osteoblasts – Osteocytes – Osteoclasts

Bone Structure 1.Osteoblasts Bone-building cells. – Called ossification… Synthesize and secrete collagen fibers & other organic parts of bone matrix. – Collagen is a protein – It is made by the ER and Golgi – **Remember how the ER/Golgi make proteins…from way back in freshman biology? The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve just secreted.

Bone Structure 2.OSTEOCYTES – Mature bone cells. – Osteoblasts that have become trapped by making too much bone matrix. – No longer secrete matrix, but continue to live… – Responsible for maintaining the inner compact and spongy bone 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 matrix

3.Osteoclasts Huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell). Cells that break down bone  They digest bone matrix – this process is called bone resorption and is part of normal bone growth, maintenance, and repair. The osteoclast secretes digestive enzymes to break down the bone matrix. Why do we want a cell that eats away at bone?

Bone Matrix Bone matrix resembles reinforced concrete rebar. The rebar is like collagen fibers and the cement is like hydroxyapatite – Collagen (1/3) Composed of fibers, and organic protein Collagen is the most abundant organic substance – Hydroxyapatite (2/3) Primarily calcium phosphate (called hydroxyapatite) This gives bone its hardness; resists compression and twisting

Bone Matrix If hydroxyapatite removed, bone is too bendable If collagen removed, bone is too brittle