Skeletal System

Human skeleton begins as

Cartilage and fibrous membranes

Most abundant type of cartilage

Hyaline cartilage

What age does skeleton growth stop?

25

How many bones make up the adult skeleton?

206 bones

How many bones make up the axial skeleton?

80 bones

How many bones make up the appendicular skeleton?

126 bones

Functions of bone

1. Support - for body and soft organs

2. Protection - protect brain, spinal cord and vital organs

3. Movement - levers for muscle action

4. Mineral and growth factor storage - calcium, phosphorus and growth factors reservoir.

5. Blood cell formation - hematopoiesis (occurs in red marrow cavities of certain bones)

6. Triglyceride (fat) storage - fat, used for an energy source, is stored in bone cavities. 

7. Hormone production - osteocalcin secreted by bones helps to regulate insulin secretion, glucose levels and metabolism. 

Diaphysis

Shaft/body of long bone

Epiphyses

Distal and proximal ends of a long bone.

Metaphases

Areas where the epiphyses and diaphysis join.

Epiphyseal growth plate

In adolescents through the end of active growth, the metaphysis of the long bones contains hyaline cartilage, actively dividing allow the bone to grow in length.  

What the epiphyseal growth plate becomes…

Epiphyseal line

Bone

Highly vascularized connective tissue with a hard, mineralized extracellular matrix. Is found in two different arrangements. 

Compact bone

Provides protection and support, forms diaphysis of long bones and external layer of all bones.

Spongy bone

Less organized tissue, lightweight and provides tissue support, forms epiphysis and internal cavity of long bones. 

• Lacks osteons, lamellae are arranged in a lattice of thin columns - trabeculae.

• Hematopoiesis (blood cell production) occurs in spongy bone.

Osteons or Haversian systems

Units of compact bone, formed from concentric lamellae. 

• Withstands stress and resist twisting

• Bone salts are found between collagen fibers (rings of calcified matrix)

• Arranged around a central canal

Lacunae

Small spaces between the lamellae which house osteocytes.

Canaliculi

Small channels filled with extracellular fluid connecting the lacunae.

What is found in an osteons central canal?

Blood and lymphatic vessels.

Perforating (Volkmann’s) canals

Allow transmit of these vessels to the outer cortex of the bone.

Trabeculae

Lamellae arranged in a lattice of thin columns.

• Protect red bone marrow

• Oriented along lines of stress to resist stress

Cartilage

Poorly vascularized connective tissue with a matrix composed of chondroitin sulfate and various fibers.

Fiber types that distinguish cartilage (3)

Hyaline, fibrocartilage and elastic.

Articular cartilage

Thin layer of hyaline cartilage covering the epiphysis of long bones.

• Found where the bone forms a joint surface, where a bone moves against another.

• Reduces friction

• No blood vessels, limited in repairing

Periosteum

Tough sheath of dense, irregular connective tissue on the outside of the bone.

• Growth in thickness

• Assists in fracture repair

• Attachment points for tendons and ligaments. 

• Osteoblasts

Medullary cavity

Space within the diaphysis of long bones that contains fatty yellow bone marrow in adults.

Endosteum

Membrane that lines the medullary cavity.

• Osteoclasts, osteoblasts and connective tissue.

Perichondrium

Dense irregular connective tissue membrane that surrounds cartilage.

Chondrocytes

Cells that form cartilage

Osteoblasts

Bone building cells - synthesize and secrete collagen fibers and other organic compounds. 

• secrete unmineralized bone matrix - osteoid

• Actively mitotic

Osteocytes

Mature osteoblasts (maintenance).

• Mature bone cells in lacunae that no longer divide.

• Maintain bone matrix and act as stress/strain sensors. 

• Respond to mechanical stimuli

• Communication information to osteoblasts and osteoclasts so bone remodeling can occur. 

Osteoclasts

Large bone breakdown (resorption) cells, a type of white blood cell, fixed macrophages in bone substance. 

• When active they are located in resorption bays

• Cells have ruffled borders to increase surface area for enzyme degradation of bone. 

Epithelium

Form the capillary walls

Red bone marrow

Hematopoiesis

Yellow bone marrow

Fat storage

Bone is richly supplied with…

Blood, periosteal arteries and veins supply the periosteum and compact bone.

• Nerves accompany the blood vessels making the periosteum rich in sensory nerves sensitive to tearing or tension. 

Ossification/osteogenesis

Process of forming new bone.

Four situations in which bone formation occurs

1. Formation of bone in embryo

2. Growth of bones until adulthood

3. Remodeling of bone

4. Repair of fractures

Intra-membranous ossification

Produces spongy bone (skull bones), may subsequently be remodeled to form compact bone.

• Simpler of two methods

• Used in forming flat bones of skull

• Bone forms from mesenchymal cells that develop within a membrane, without going through a cartilage stage

• Many ossification centers

Endochondral ossification

Process whereby hyaline cartilage is replaced by bone, forms both compact and spongy bone.

• Used for most bones especially long bones

• Primary ossification center - diaphysis of long bone

• Secondary ossification center - epiphysis of long bone

Deficiency of vitamin A

Retards bone development

Deficiency of vitamin C

Results in fragile bones

Deficiency of vitamin D

Rickets, osteomalacia

Insufficient growth hormone

Dwarfism

Excessive growth hormone

Gigantism, acromegaly

Insufficient thyroid hormone

Delays bone growth

Sex hormones

Promote bone formation, stimulate ossification of epiphyseal plates

Physical stress…

Stimulates bone growth