Skeletal System

• Study of system began in 2nd century

– Galen

•By 18th century system was completely described

Q: How many bones are there in an adult human?

• Skeletal system develops from the middle germ layer: Mesoderm [all but facial bones, which are derived from ectoderm]

• Development begins in the first few weeks after fertilization by 3rd month there are 600 distinct bones

• At birth there are 450 distinct bones• By age 25 all skeletal growth is completed

A: 206

As bones grow & form their proportions change

• At birth: the head is as big around as the chest and ¼ of the body’s length

• Adult: the head is ½ as big around as the chest and 1/7 of the body’s length

Functions of the Skeletal System

• Support• Protection• Movement• Storage: minerals

fat• Hemopoiesis [hematopoiesis]

2 Major Divisions of Skeleton

• 80 bones - form vertical axis of body

• Skull -28 • Vertebral column- 26• Ribs/sternum- 25• Hyoid bone -1

• 126 bones – free appendages and their attachments to axial skeleton

• Upper Extremities- 60• Lower Extremities –

60• Pectoral Girdles – 4• Pelvic Girdle - 2

Axial Skeleton Appendicular Skeleton

Bones come in different shapes• Long = longer than wide • Short = just as wide as long• Flat = thin, flattened shape – usually curved• Irregular = don’t fit into any of the above

categories• Sesamoid = small, round – grow in certain

tendons where there is considerable pressure• Wormian/sutural = small bones at joints of

random cranial bones

Bone is Connective Tissue

Cells in a MatrixMatrix = collagen & proteoglycans &

calcium salts –primarily calcium phosphate

Compact/dense Cancellous/spongyCells = osteocytes, osteoblasts &

osteoclasts*highly vascularized

Bone Cells are responsible for the integrity of bone tissue = very dynamic tissue

Osteocytes = maintain bone tissue – live in spaces called lacunae. Connected by canaliculi

Osteoblasts = form new bone tissue [ossification/osteogenesis]

Osteoclasts = break down bone tissue

REMODELING of bone = constant activity.

Blood Vessels

• Within compact bone there is an osteon which contains the Haversian or central canal – runs on long axis of bone

• Running perpendicular are Volkmann’s or perforating canals

Spongy Bone does not have osteons. It has trabeculae [ plates of bone tissue in an irregular pattern]

Short and Flat bones are primarily made of spongy bone, with coverings of compact bone

Spongy bone has openings in trabeculae filled with red bone marrow

General features of bonesLong bone = composite bone

Diaphysis = shaft

Epiphysis = expanded portion at end of diaphysis. Covering the end is Articular Cartilage. Where the two meet is called the Epiphyseal Plate [growth plate]. Becomes the Epiphyseal Line.

Within diaphysis is a hollow space called: Medullary cavity

Outer surface of all bones have Periosteum: rich with nerve fibers, lymphatic vessels, blood vessels and osteoblasts

Inner surface of all bones have Endosteum: osteclasts

Blood vessels enter bone tissue via Nutrient Foramen [foramen =hole or opening]

In addition to foramen, bones have many surface markings and characteristics that make each bone unique. [i.e. holes, depressions, smooth facets, projections]