Bone

Bone

Skeletal System - Cells and Intercellular substance ICS

Functions

1.	Skeleton supports the body
2.	Attachment of muscle and Leverqge, changing the magnitude and direction of forces
3.	Protection of vital organ
	Skull - brain
	Thorax - heart and lungs
4.	Storage of minerals, Calcium (98%) and lipids, yellow bone marrow.
5.	House red bone marrow, production of formed elements of the blood.

Find with use - hypertrophy
        Without use atrophy

Types of bone

	Spongy bone, cancellous bone - bony spicules/bone marrow
	Compact bone, dense - compact mass, spaces seen with microscope

Development - constant change through out life

1.	Intramembranous bone - dermal substrate
2.	Endochondral bone - cartilaginous substrate
3.	Heterotrophic - ectopic bone formation

Intramembranous
	Skull cap
	Facial bones
	Sides of skull

Embryonic cells - osteoblasts - osteocytes
Osteoid - semi-gel with calcium
Always starts to form in proximity of blood vessels
Surrounded by periosteum

(Constant exchange of Calcium between blood and bone - regulated by the parathyroid glands, four located on posterior surface of thyroid.

Endochondral

Cartilage laid down by the 7th week of development - formation of hyalin cartilage template
Embryonic cells - Chondrocytes - matrix surrounded by perichondrium
Note: no BVs present, diffusion responsible for transport

Long (Dog) bone formation -
	Primary ossification center
	Secondary ossification centers

Cartilage hypertrophies, osteoblasts grow displacing ICS.  BV's migrate in and bring along osteoblasts, which lay down bone adjacent to cartilage remnants, constantly forming spongy bone.

Process occurs on ends and center, but not in metaphysis, epiphyseal discs, which is constantly growing and pushing the ends of the bones apart.




Osteoclasts - gangs of osteoblasts which fuse to form large bone eating cells, constantly remodeling as stresses and strains change.

Periosteum - remnants of CT on outside (around) the bone.  Lots of free nerve endings, very sensitive -relatively  inactive except when fractures occur and new bone needed.

Ricket's - childhood disease where the osteoid has insufficient calcium, generally due to lack of Vitamin D, calcium not absorbed for the gut.

Osteomalacia - adult condition when osteoid lacks sufficient calcium

Osteoporosis - adult condition with loss of bone mass with age, hormonal and genetic factors?





Haversian systems (Osteons) - Functional unit of compact bone; contains BV's, nerves, and lymphatics.



Classification
1.	Long bones - arms and legs
2.	Short or irregular bones - carpals and tarsals.
3.	Flat bones - skullcap, scapula
4.	Irregular bones - facial bones and vertebrae
5.	Pneumatic bones - air cells & sinuses
6.	Sesamoid bones - patella
7.	Accessory (supernumerary) bones - additional non-fusing ossification center, e.g., foot

All have Periosteum
	   Compact layer
	   Spongy layer

Strength
	Leg bones support 3600 lbs
	10 % give in bony structure
	Mechanical and hydrostatic strength

Hydroxyapatite

	Ca10(PO4)6(OH)2

90Sr - Beta emitter, replaces Calcium and destroys, irradiates, hemopoetic tissue
t1/2 = 28.5 years!

Hormones
	Parathormone
		Parathyroid
	Increases 
		Number of osteoclasts
		Activity of osteoclasts
	Results
		Increase resorption of bone
		Releases Calcium ions
		Raise blood Calcium levels

	Calcitonin
		Thyroid gland
	Opposite effect
		Decreases resorption of bone by
		decreasing activity of osteoclasts
		Tends to stimulate osteoblasts to build bone
	Therefore lowers blood calcium levels
	Also lowers blood phosphate levels



		Terminology

Crest - sharp prominent bony ridge
Condyle - rounded prominence that articulates with another bone
Epicondyle - small projection located on or above a condyle
Facet - smooth, flat articular surface
Fissure - narrow  cleft like passage
Foramen - a hole
Fossa - a depression
Fovea - a pit (usually for attachment)
Head - larger end of a long bone
Line - a slight bony ridge
Meatus - a canal
Neck - region below the head
Process - a prominence or projection
Spine - a slender pointed projection
Sulcus - a groove
Trochanter - a large somewhat blunt process
Tubercle - a nodule or small rounded process
Tuberosity - a broad process, larger than a tubercle

Skeleton

Axial				80
	Skull				29
	Vertebral column		26
	Thorax, ribs and sternum	25

Appendicular			126
	Pectoral girdle		4
	Upper limbs			60
	Pelvic girdle			2
	Lower limbs			60

Cranium			8
Face				14
Auditory ossicles		6
Hyoid				1
Need for cells to cooperate
Similar in 
     Structure
     Function
     Embryonic origin
     Bound together with
          intercellular materials

Types of Tissue
	Epithelium
	Connective
	Muscular
	Nervous