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Skeletal System

bodys framework of bones and their associated cartilage and ligaments

Functions of the Skeletal System

1. Support
2. Protection-surround vital organs (eg. brain, heart and lungs)
3. Levers for movement-muscles attached
4. Mineral Homeostasis-stores and releases minerals as needed (eg. calcium and phosphorus)
5. Blood Cell production-in red marrow

Bone Histology

1. Matrix
2. Fixed Cells

What is bone?

A solid connective tissue


1. 1/3 fibers (organic matrix)-collagen
2. 2/3 ground substance (inorganic matrix)-mineral salts (hydroxyapatite) mainly calaium salts
-hardness depends on amount of mineral salts
-resistance to deformation depends on collagen fibers


deposition of mineral salt crystals in bone

Fixed Cells

1. osteoblasts
2. osteocytes
3. osteoclasts


-for new bone by secreting matrix around themselves


-mature bone cells trapped within lacunae
-maintain daily metabolic processes of bone cells
-produce osteobalsts


-huge cells form fusion of many phagocytic cells
-contain enzymes and acids to digest bone matrix (resorption)
-involved in bone repair and remodelling
-remove Ca+2 from bone when it is needed in blood
-produce acids

What are the two types of bone?

1. Compact
2. Spongy

Compact bone

-bone arranged in cylindrical structures called osteons or
Haversian systems
--very dense and strong, no spaces in between

Structure of an osteon

1. Central (Haversian) canal
2. Lamellae
3. Lacunae
4. Canaliculi


-bone arranged in cylindrical structures
-a.k.a Haversian systems
-the cylinder of bone
-"little splints"
-align along lines of stress to resist bending of bone
-compact bone forms from the shafts of long bones and the
outer shell of all bones


layers of calcified matrix, around central canal

Central (Haversian) Canals

-runs longitudionally thorough the center
-contains blood vessels and nerves supplyinbg the bone


holes between lamellae which contain osteocytes


-tiny tunnels connecting lacunae
-pathways for exchange of nutrients and wastes with blood
-vessels in central canal

Spongy (cancellous) bone

-arrangement in an irregular network of small spicules with
spaces between
-arrangement gives strength but lightness
-red marrow
-found in the ends of long bone and center of other bones

What are spicules?

-composed of layers of matrix with lacunae containing
osteocytes, connected by canaliculi
-nutrients and wastes are exchanged with the red marrow

Red Marrow

-fills spaces between spicules
-site of red blood cell formation (hematopoiesis)

What are the two parts of spongy bone?

1. Periosteum
2. Endosteum


-membrane covering outer surface of bone (except at joints)
-some collagen fibers (Sharpey's fibers) penetrate bone to anchor the periosteum
-rich in sensory nerves which detect pain
1. outer fibrous layer
2. inner cellular layer

Outer fibrous layer

-dense irregular connective tissue
-site of tendon, ligament and joint capsule attachment

Inner Cellular layer

-contains osteoblast cells for growth, remodeling and fracture repair


-thin cellular layer lingin inner surfaces of bone
-contains osteoblasts for remodeling and repair of bone

Blood and Nerve supply of Bone

-has a rich blood supply
-bones are very metabolically active and usually heal rapidly
-veins and nerves accompany arteries
1. Periosteal arteries
2. Nutrient arteries

Periosteal arteries

-many small vessels that penetrate the periosteum and bone matrix through Volkman's canals(perforating canal)
-supply vessels in central canals of compact bone
-there is a rich blood supply

Nutrient arteries

-large vessels that enter bones through large holes (nutrient formina) and supply mainly the marrow of spongy bone

Bone Shapes

1. Long Bones
2. Short Bones
3. Flat Bones
4. Irregular Bones
5. Sesamoid Bones

Long Bones

-greater in length than width
-levers for body motion
-(most bones of limbs and paws)

Structure of Long Bones

1. diaphysis
2. epiphyses
3. metaphyses
-atricular cartilage
-medullary cartilage


-shaft, mainly compact bone


-proximal to distal ends
-mainly spongy bone with an outer shell of compact bone
-spaces filled with red marrow


-narrrow zone between diaphysis and epiphysis
1. epiphyseal plate(growth plate)-in immature bones
-plate of hyaline cartilage at metaphysis
-site where bones grow in length
2. epiphyseal line-in mature bones
-line of bone whre epiphyseal plate has ossified (closed)

Articular cartilage

-hyaline cartilage covering joint surfaces of epiphyses
-reduces friction and absorbs shock at joints
-on the end of bones

Medullary cavity

-(marrow cavity)
-space in central diaphysis which contains yellow (fatty) marrow

Short bones

-nearly equal length and width
-spongy bone except a surface
-(carpals and tarsals of paws)

Flat bones

-2 thin, flat layers of compact bone enclosing spongy bone
-surround and protect organs
-provide large surface area for muscle attachment
-(most skull bones, rib cage, shoulder blades)

Irregular bones

-don't fit into other categories
-complex shapes and variable composition
-(vertebrae, come facial bones)

Sesamoid bones

-shaped like a sesame seed
-develop in tendons
-provide strength to areas of unusual mechanical stress
-(patella is the largest)-in horse hooves

Bone Formation (Ossification)

-process of bone formation
-embryonic "skeleton" is composed of loose fibrous connective tissue membranes and hyaline cartilage whcih serve as a template (model for the skeleton)
-ossification is replacement of these tissues with bone tissue
-(remeber all connective tissues come from embryonic mesoderm)
-don't initally have boneis fetus'

2 Methods of Ossification

1. Intramembranous ossification
2. Endochondrial ossification

Intramembranous ossification

-bone forms within loose fibrous connective tissue membranes
-forms flat bones of skull
-ossification center develops in fibrous membrane
-embryonic mesenchyme cells differentiate into osteoblasts
secrete bone matrix around themselves
-osteocytes become trapped in lacunae
-matrix around adjacent osteocytes fuses to form spicules of spongy bone connective tissue between spicules differentiates into red marrow
-osteoblast cells and connective tissue around spongy bone condense to form periosteum and endosteum
-osteoblasts of periosteum form a thin layer of compact bone on surface of bone

Endochondrial ossification

-bone forms within a hyaline cartilage model
-most bones form this way
1. cartilage model develops
2. cartilage model grows
3. primary ossification center
4. osteoclasts hollow out medullary cavity
5. secondary ossification center

Cartilage Model develops

-embryonic mesenchyme cells cluster in shape of future bone
-differentiate into chondroblasts, which secrete hyaline cartilage matrix
-outer layer of chondroblasts and connective tissue condense to form perichondrium

Cartilage model grows

1. interstitial growth-chondrocytes of model divide and secrete matrix between themselves -model grows in length
2. appositional growth-chondroblasts in perchondrium divide and secrete matrix beneath perichondrium
-model grows in thickness

Primary Ossification center develop

-chondrocytes in center of diaphysis die and are replaced by osteoblasts
-osteoblasts secrete bone matrix, forming spongy bone
-ossification center grows outward towards metaphyses of bone

Osteoclasts hollow out medullary cavity

-remodeling repairs spongy bone of diaphysis with compact bone

Secondary Ossification centers develop

-begin in center of each epiphysis and spread outwards, filling epiphyses with spongy bone
-articular cartilage-hyaline cartilage remains on joint surface
-epiphyseal plate-(growth plate) remaining hyaline cartilage between epiphysis and diaphysis
-weak spot in bone

Bone Growth

-regulated by hormones
1. Growth Hormone (GH)
-from pituitary gland
-promotes both of bone
2. sex steroids (estrogens and androgrens)-promote growth
-anabolic steriods-stimulate tissue growth, including bone
-high levels at maturity eventually close growth plates
-makes bones larger and heavier
-Growth in Length-interstitial growth only at epiphyseal plates
-injury to the plate can cause early closure
-Growth in thickness-appositional growth
-osteoblasts in periosteum divide and secrete matrix beneath periosteum

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