A&P The Skeletal System: Bone Tissue

Functions of Bone and the Skeletal System

3.Mineral Homeostasis
4.Blood Cell Production
5.Triglyceride Storage


The skeletal system is the structural framework for the body by supporting soft tissues and providing attachments for tendons


Most skeletal muscles attach to bones- when the muscles contract they pull the bones to produce movement

Mineral Homeostasis

Bone tissue stores several minerals including calcium and phosphorus, which contribute to bone strength
Bone tissue stores 99% of body's calcium
Bone releases minerals into the bloodstream to maintain mineral balance in body

Blood Cell Production

Within certain bones, red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis
Red bone marrow consists of: developing blood cells, adipocytes, fibroblasts and macrophages within a network of reticular fiber

Triglyceride Storage

Yellow bone marrow consists mainly of adipose cells, which store triglycerides
A potential chemical energy reserve

Osteogenic cells

Or Osteoprogenitor cells
Unspecialized bone stem cells derived from mesenchyme
The only bone cells to undergo cell division
Resulting cells are osteoblasts
Found along inner portion of periosteum, in the endosteum, and in the canals within bone that conta


Bone-building cells
Synthesize and secrete collagen fibers and other organic compounds needed to build the extracellular matrix of bone tissue
Initiate calcification
As osteoblasts surround themselves with extracellular matrix, they become trapped in thei


Mature bone cells, the main cells in bone tissue
Maintains the daily metabolism of bone through the exchange of nutrients and wastes with blood


Huge cells derived from the fusion of as many as 50 monocytes (type of white blood cell)
Are concentrated in the endosteum
The cell releases powerful lysosomal enzymes and acids, from the ruffled border of its cell membrane, which digest the protein and m

Bone Matrix

Composed of water, collagen fibers and crystallized mineral salts (calcium phosphate

Compact Bone Tissue

Strongest bone tissue
Found beneath periosteum of all bones and makes up the bulk of the diaphysis of long bones
Provides protection and support and resists the stresses produced by weight and movement

Spongy Bone

*Also referred to as trabecular or cancellous bone tissue
*Does not contain osteons
*Always located in the interior of the bone protected by a covering of compact bone

Periosteal Arteries

Small arteries accompanied by nerves, enter the diaphysis through many perforating canals and supply the periosteum and outer part of the compact bone

Nutrient Artery

*Near the center of the diaphysis, a large nutrient artery passes through a hole in the compact bone called the nutrient foramen.
*In the medullary cavity, it divides into proximal and distal branches which supply both the inner part of compact tissue of

Metaphyseal Arteries

The metaphyseal arteries enter the metaphyses of long bones and supply the red bone marrow and bone tissue of the metaphyses

Epiphyseal Arteries

The epiphyseal arteries enter the epiphyses of a long bone and supply the red bone marrow and bone tissue of the epiphyses.

Veins that carry blood away from long bones are found in three places

1. One or two nutrient veins accompany the nutrient artery and exit through the diaphysis
2. Numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through the epiphyses and metaphyses
3. Many small periosteal veins a

Ossification or Osteogenesis

The process by which bone is formed

Bone formation occurs in 4 principal situations

1. The initial formation of bones in an embryo and fetus
2. The growth of bones during infancy, childhood and adolesense until their adult size is reached
3. The remodeling of bone (the replacement of old bone by new bone tissue throughout life)
4. The re

The initial formation of bone in an embryo or fetus follows one of two patterns

1. Intramembranous Ossification
Bone forms directly within mesenchyme, which is arranged in sheet like layers that resemble membranes
2. Endochondral Ossification
Bone forms within hyaline cartilage that develops from mesenchyme

Intramembranous Ossification

Development of the Ossification Center
Formation of Trabeculae
Development of the periosteum

Intramembranous Ossification
Development of the Ossification Center

-At the site where the bone will develop, specific chemical messages cause mesenchymal cells to cluster
-These cells differentiate into osteoprogenitor cells, then into osteoblasts
-The cluster site is called the ossification center
-Osteoblasts secrete e

Intramembranous Ossification

-The secretion of the extracellular matrix stops
-The now osteocytes lie in lacunae and extend their cytoplasmic processes into canaliculi that radiate in all directions
-Calcium and other mineral salts are deposited and the extracellular matrix calcifies

Intramembranous Ossification
Formation of Trabeculae

-As the extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue
-The connective tissue associated with blood vessels in the trabeculae turn into red bone marr

Intramembranous Ossification
Development of the periosteum

-The mesenchyme condenses at the periphery of the bone and develops into periosteum
-Eventually, compact bone replaces the surface layers of spongy bone

Endochondral Ossification

Development of the Cartilage Model
Growth of the Cartilage Model
Development of the Primary Ossification Center
Development of the Medullary (Marrow) Cavity
Development of the Secondary Ossification Centers
Formation of Articular Cartilage and the Epiphys

Endochondral Ossification
Development of the Cartilage Model

At the site where the bone is to be formed, chemical messengers cause mesenchyme to clump together into the shape of a bone
Mesenchyme turns into chondroblasts
Chondroblasts secrete cartilage extracellular matrix, producing a cartilage model made of hyali

Endochondral Ossification
Growth of the Cartilage Model

Chondroblasts become Chondrocytes
Interstitial Growth occurs
Growth in length from within as chondrocytes multiply and more extracellular matrix is produced
Appositional Growth occurs
Growth in thickness due to increase of extracellular matrix on the oute

Endochondral Ossification
Development of the Primary Ossification Center

Primary ossification proceeds inward from the external surface of the bone
A nutrient artery penetrates through perichondrium and goes through nutrient foramen
Stimulates osteoprogenitor cells in the perichondrium to differentiate into osteoblasts
Once th

Endochondral Ossification
Development of the Medullary (Marrow) Cavity

Osteoclasts break down some of the newly formed spongy bone trabeculae
This results in the medullary cavity in the diaphysis

Endochondral Ossification
Development of the Secondary Ossification Centers

When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop
Usually around the time of birth
Ossification centers spongy bone remains in the interior of the epiphyses
No medullary cavity is formed here
Secondary ossi

Endochondral Ossification
Formation of Articular Cartilage and the Epiphyseal (Growth) Plate

The hyaline cartilage that covers the epiphyses becomes the articular cartilage
Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal growth plate
The region responsible for the lengthwise growth of bone

Growth in Length
Involves Two Major Events

Interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate
Replacement of cartilage on the epiphyseal side of the epiphyseal plate with bone by endochondral ossification

The epiphyseal (growth) plate is a layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones

Zone of Resting Cartilage
Zone of Proliferating Cartilage
Zone of Hypertrophic Cartilage
Zone of Calcified Cartilage

Zone of Resting Cartilage

Nearest to the epiphysis
Consists of small, scattered chondrocytes
Cells do not function in bone growth
Anchor epiphyseal plate to the epiphysis of the bone

Zone of Proliferating Cartilage

Slightly larger chondrocytes are arranged like stacks of coins
Undergo interstitial growth as they divide and secrete extracellular matrix
Divide to replace those that die at the diaphyseal side of the epiphyseal plate

Zone of Hypertrophic Cartilage

Consists of large, maturing chondrocytes arranged in columns

Zone of Calcified Cartilage

Only a few cells thick and consists mostly of dead chondrocytes because the extracellular matrix surrounding them had calcified
Osteoclasts dissolve the calcified cartilage and osteoblasts and capillaries from the diaphysis invade the area
Osteoblasts lay

Growth in Length

At the bone surface, periosteal cells differentiate into osteoblasts, which secrete bone extracellular matrix
The osteoblasts become surrounded and turn into osteocytes
This forms ridges on either side of a periosteal blood vessel
The ridges enlarge and c

Remodeling of Bone

Bone remodeling is the ongoing replacement of old bone tissue with new bone tissue
Involves bone resorption- the removal of minerals and collagen fibers from bone by osteoclasts
Involves bone deposition- the addition of minerals and collagen fibers to bon

Factors Affecting Bone Growth



Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride and manganese
Necessary during bone remodeling


Vitamin A stimulates activity of osteoblasts
Vitamin C is needed for synthesis of collagen, the main bone protein
Vitamin D helps with the absorption of calcium from foods in the gastrointestinal tract into the blood
Vitamin K and B12 also needed for synt


Insulin growth factors (IGFs) are the most important for bone growth
Produced by the liver and bone tissue
Stimulate osteoblasts, promote cell division at the epiphyseal plate and in the periosteum, and enhance synthesis of the proteins needed to build ne

Phases of repair of a bone fracture

Reactive Phase
Reparative Phase: Fibrocartilaginous callus formation
Reparative Phase: Bony callus formation
Bone Remodeling Phase

Reactive Phase

An early inflammatory phase
Blood vessels crossing the fracture line are broken
A clot forms around the site of a fracture
Called a fracture hematoma
Forms 6-8 hrs after injury
Nearby bone cells die
Swelling and inflammation occur in response
Phagocytes a

Reparative Phase: Fibrocartilaginous callus formation

Blood vessels grow into the fracture hematoma and phagocytes begin to clean up dead bone cells
Fibroblasts from the periosteum invade the fracture site and produce collagen fibers
Cells from the periosteum develop into chondroblasts and produce fibrocarti

Reparative Phase: Bony callus formation

In areas closer to the healthy bone tissue, osteoprogenitor cells develop into osteoblasts
Begin to produce spongy bone trabeculae
The trabeculae join living and dead portions of the origional boe fragments
Fibrocartilage is converted into spongy bone

Bone Remodeling Phase

Dead portions of the original fragments of bone are gradually reabsorbed by osteoclasts
Compact bone replaces spongy bone around the periphery of the fracture
A thickened area on the surface of the bone remains as evidence of a healed fracture

Open (Compound) Fracture

The broken ends of the bone protrude through the skin


The one is splintered, crushed, or broken into pieces at the site of the impact, and smaller bone fragments lie between the two main fragments

Greenstick Fracture

A partial fracture in which one side of the bone is broken and the other side bends
Occurs only in children whose bones are not fully ossified and contain more organic material than inorganic material

Stress Fracture

A series of microscopic fractures in bone that forms without evidence of injury to other tissues
Result from repeated strenuous activity

Parathyroid hormone

Secreted by the parathyroid galnds
Increases blood Ca2+ level
Operates via a negative feedback system
Also acts on the kidneys to decrease the loss of Ca2+ from urine
Stimulates the formation of Vitamin D
Promotes absorption of calcium from foods


Works to decrease blood Ca2+ levels
Secreted from the thyroid
Inhibits activity of osteoclasts, speeds blood Ca2+ uptake by bone and accelerates Ca2+ deposition on bones


A condition of porous bone
Bone mass becomes so depleted that bones easily fracture
Can occur
If levels of a parathyroid hormones remain elevated fro a long period of time
If osteoclast activity increases, while osteoblasts activity decreases
Following me


Result from inadequate calcification of the extracellular bone matrix,
Usually caused by a Vitamin D deficiency
Is a disease in children where the growing bone becomes soft or rubbery are are easily deformed