Parts of the skeletal systemBones (skeleton)JointsCartilagesLigamentsTwo subdivisions of the skeletonAxial skeletonAppendicular skeleton

Support the bodyProtect soft organsAllow movement due to attached skeletal musclesStore minerals and fatsBlood cell formation

The adult skeleton has 206 bonesTwo basic types of bone tissueCompact boneHomogeneous, dense, smootheSpongy boneSmall needle-like pieces of boneMany open spacesFigure 5.2b

Figure 5.1

Long bonesTypically longer than they are wideHave a shaft with heads at both endsContain mostly compact bone- not much spongy boneExample:FemurHumerusPhalanx

Short bonesGenerally cube-shapeContain mostly spongy boneExample:CarpalsTarsals

Flat bonesThin, flattened, and usually curvedTwo thin layers of compact bone surround a layer of spongy boneExample: Skull (parietal)RibsSternum

Irregular bonesIrregular shapeDo not fit into other bone classification categoriesExample: Vertebrae Hip bones

DiaphysisShaftComposed of compact boneEpiphysis Ends of the boneComposed mostly of spongy bone

PeriosteumOutside covering of the diaphysisFibrous connective tissue membraneSharpeys fibersSecure periosteum to underlyingboneArteriesSupply bone cells with nutrients

Articular cartilageCovers the external surface of the epiphysesMade of hyaline cartilageDecreases friction at joint surfacesEpiphyseal plateFlat plate of hyaline cartilage seen in young, growing boneEpiphyseal lineRemnant of the epiphyseal plate (growth plate)Seen in adult bones

Medullary cavity Cavity inside of the shaftContains yellow marrow (mostly fat) in adultsContains red marrow (for blood cell formation) in infants

Surface features of bonesSites of attachments for muscles, tendons, and ligamentsPassages for nerves and blood vesselsCategories of bone markingsProjections or processesgrow out from the bone surfaceDepressions or cavitiesindentations

Table 5.1 (1 of 2)

Table 5.1 (2 of 2)

Osteon (Haversian system)A unit of bone containing central canal and matrix ringsCentral (Haversian) canalOpening in the center of an osteonCarries blood vessels and nervesPerforating (Volkmans) canalCanal perpendicular to the central canalCarries blood vessels and nerves

Figure 5.3a

LacunaeCavities containing bone cells (osteocytes)Arranged in concentric ringsLamellaeRings around the central canalSites of lacunae

Figure 5.3bc

Canaliculi Tiny canalsRadiate from the central canal to lacunaeForm a transport system connecting all bone cells to a nutrient supply

Figure 5.3b

In embryos, the skeleton is primarily hyaline cartilageDuring development, much of this cartilage is replaced by boneCartilage remains in isolated areasBridge of the noseParts of ribsJoints

Epiphyseal plates allow for lengthwise growth of long bones during childhoodNew cartilage is continuously formedOlder cartilage becomes ossifiedCartilage is broken downEnclosed cartilage is digested away, opening up a medullary cavityBone replaces cartilage through the action of osteoblasts

Bones are remodeled and lengthened until growth stopsBones are remodeled in response to two factorsBlood calcium levelsPull of gravity and muscles on the skeletonBones grow in width (called appositional growth)

Figure 5.4a1. Perichondrium becomes vascularized to a greater degree and becomes a periosteum2. Bone collar is laid down around the hyaline cartilage model just beneath the periosteumPeriosteal bud invades the marrow cavityCavity formation occurs within the hyaline cartilageOsteoblasts lay down bone around cartilage spicules in the bones interior6. Osteoclasts remove cancellous bone from the shaft interior, leaving a marrow cavity that then houses fat.

Figure 5.4a, step 11. Perichondrium becomes vascularized to a greater degree and becomes a periosteum2. Bone collar is laid down around the hyaline cartilage model just beneath the periosteum

Figure 5.4a, step 2Periosteal bud invades the marrow cavityCavity formation occurs within the hyaline cartilageOsteoblasts lay down bone around cartilage spicules in the bones interior6. Osteoclasts remove cancellous bone from the shaft interior, leaving a marrow cavity that then houses fat.

Figure 5.4a, step 3

Figure 5.4b

Osteocytesmature bone cellsOsteoblastsbone-forming cellsOsteoclastsbone-destroying cellsBreak down bone matrix for remodeling and release of calcium in response to parathyroid hormoneBone remodeling is performed by both osteoblasts and osteoclasts

Bone cells:OsteoblastsBone-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 theyve just secreted.

2.OsteocytesMature 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

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 bonesReleased 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.