Skeletal Anatomy

What is the skeleton

• Skeleton: A structural framework providing support and integrity to the bodies of some animals

• Can be made of different materials based on the type of skeleton

Exoskeleton

Exoskeleton: An external skeleton

• Utilized by many invertebrate animals and all Arthropods

• Composed of two protein layers

Endoskeleton

An internal skeleton

• Utilized by vertebrate animals

• Composed of bone and cartilage

Endoskeleton parts

The endoskeleton can be broken up into the axial and appendicular skeletons

Axial skeleton

The “central bones” of the endoskeleton

• Includes the skull, vertebral column, and rib cage

Appendicular Skeleton

The bones of the appendages

Long Bones

• Long bones: Cylindrical bones of the appendicular skeleton

• Longer than wide

• Important roles include support, movement, and hematopoiesis

Hematopoiesis

Blood cell production

Epiphysis

The end of a long bone that forms joints with other bones

• Contains red bone marrow

Diaphysis

Long, hollow shaft in the center of the bone

Medullary Cavity

Located within the diaphysis

• Contains red and yellow bone marrow

Red Bone Marrow

Produces stem cells capable of generating red and white blood cells

Yellow Bone Marrow

Produces stem cells capable of generating fat, bone, cartilage, and muscle. Fat can be stored in addition to its production

Metaphysis

Found between the medullary cavity and epiphyseal plate

● Similar to the epiphysis

Epiphyseal Plate

Epiphyseal plate:

• Capable of lengthening the diaphysis through ossification

What are long bones made of?

Pockets of cancellous bone

• google def: “honeycombed tissue found at ends of long bones”

Cortical bone

Dense, outer layer of bone that supports the body weight

A.K.A: “Compact bone”

Osteon

The functional unit of cortical bone, composed of thinly layered cylinders

• “Haversian systems”

Google def: “the fundamental functional and structural unit of compact bone. It acts as a microscopic transport and support network, giving dense bone its incredible strength and ability to heal.”

Lamellae

Cylindrical layers of the osteon

Haversian Canals

Tubes at the center of an osteon unit that contain blood vessels for nutrient supply

Volkmann’s Canals

Channels containing blood vessels that connect the Haversian systems to the periosteum.

Periosteum

An external sheath surrounding the cortical bone that provides nutrients in order to promote growth and healing

Endosteum

An internal sheath lining the internal surface of cortical bone

Cancellous Bone

The spongy, inner layer of bone

• Soaks up red bone marrow via a web of trabeculae

• “Spongy bone”

Trabeculae

Branching structures made of connective tissue

• Support the cancellous bone

Bone Remodeling

● Bone remodelling: The process of alternating between ossification (bone formation) and resorption (bone loss)

● Old bone can be replaced with new bone

Osteoblasts

Bone building cells

• Secrete proteins and utilize calcium from the blood

• Mature into osteocytes

Osteocytes

Mature osteoblasts that have been trapped within the bony matrix

● Live within osteons

● Maintain bone

Osteoclasts

Bone Degrading cells

Eat and resorb bone by releasing enzymes and reducing pH (increasing acidity)

• Release calcium and phosphate back into the blood

Bone Composition

What is bone made of?

• Osteoid

• Hydroxyapatite

Osteoid

Osteoid: Bone’s organic component

• Contains many proteins, including collagen fibers

• Provides tensile strength to bone (pulling or stretching without deforming)

Hydroxyapatite

Bone’s inorganic component

• Mineralized material embedded within the osteoid

• Contributes to bone density and strength (durability)

Bone remodelling and calcium homeostasis

• Bone remodeling is involved in maintaining the homeostatic balance of the molecules used to generate bone

  • Key molecule is calcium

  • Increased bone production requires calcium usage, decreasing systemic blood calcium

  • Increased bone resorption frees up calcium from the bone, increasing systemic blood calcium

Calcium Homeostasis Hormones

• Hormones can manipulate the homeostatic balance of calcium

PTH (parathyroid hormone)

• Parathyroid hormone (PTH): Increases calcium in the bloodstream

• Stimulates osteoclasts, increasing bone resorption and releasing calcium

• Increase reabsorption of calcium in the kidney, decreasing calcium excretion in the urine

Calcitonin

• Calcitonin: Decreases calcium in the bloodstream

• Inhibits osteoclast activity, decreasing bone resorption, allowing calcium to remain in bone

• Decreases reabsorption of calcium in the kidney, increasing calcium excretion in the urine

• Mnemonic: Calcitonin “tones down” the calcium in the blood

Embryonic Ossification

• Embryonic ossification: Bone formation during fetal development

  • Models of embryonic ossification:

  • Intramembranous ossification

  • Endochondral ossification

Intramembranous Ossification

● Intramembranous ossification: Direct bone formation within sheets of embryonic connective tissue

• Osteoblasts deposit osteoid

• Osteoid calcifies and forms cortical bone

• Occurs mostly in flat bone

• Include bones of the skull, etc.

Endochondral Ossification

• Endochondral ossification: Indirect bone formation requiring cartilage replacement with bone

• An initial cartilage model forms first

• Osteoid is deposited, gradually replacing the cartilage

• The osteoid calcifies and forms cortical bone

• Most bones in the body use this model, including long bones and the ribs

Connective tissue (CT)

A supportive matrix that can be composed of cells, fibers, and a gel-like filler substance

Fibroblasts

• Fibroblasts: CT resident cells which secrete the constituents of the fibrous connective tissue, maintaining and remodeling the matrix

Fibrous Connective Tissue

CT that contains a high concentration of fibers

Fibrous Connective Tissue Functions

• Fibrous connective tissue offers different function throughout the body

Tendons

Tendons: Attach muscle to bone

Two types (Muscle to Bone)

Ligaments

Ligaments: Attach bones to other bones

(Like to Like)

Periosteum

External sheath surrounding cortical bone

• Provides nutrients and innervation to the bone

Endosteum

Internal sheath that exists between the cortical and cancellous bone

Cartilage

• Cartilage: Fibrous connective tissue that does not possess blood vasculature or nerve innervation

• In comparison, bone is highly vascularized and innervated

Joints

Meeting points between two or more bones

• Vascularized, innervated, and contains connective tissue

• Muscles can generate movement at joints

• Muscles originate on one side of a joint, insert onto the bone(s) on the opposite side of the joint, and contract to generate movement

What is in charge of secreting calcitonin?

Thyroid Gland

When does the epiphyseal plate ossify?

It ossifies at puberty, turning into bone and preventing further growth.

What is the epiphyseal plate made of?

Hyaline cartilage

What does the epiphseal growth plate lengthen?

The diaphysis

Where does hematopoeisis occur in the bone?

The epiphysis

What do osteoclasts get derived from?

Monocytes, with their primary function being to resorb bone.