Bone Tissue

Bone tissue classification

Connective tissue

Hallmark of all connective tissue

An extensive extracellular matrix

What makes bone matrix unique among connective tissues

The matrix is calcified (bony matrix) making it rigid and hard

Type of matrix found in bone

Calcified bony matrix

Tissue types found within a single bone

Cartilage (articular surfaces) fat (yellow bone marrow) dense regular connective tissue (periosteum) and smooth muscle (in arterial walls)

Why a single bone qualifies as an organ

It contains multiple tissue types working together toward a common function

Classification of bone as an organ (visceral or somatic)

Somatic organ — it is NOT found in a body cavity

Why bone is NOT a visceral organ

It is not found in a body cavity; it is a somatic organ

Diaphysis

The long cylindrical mid-portion (shaft) of a long bone

Proximal epiphysis

The upper expanded end of a long bone; the end closer to the trunk

Distal epiphysis

The lower expanded end of a long bone; the end farther from the trunk

Metaphysis

The "neck" region that connects the epiphysis to the diaphysis

Medullary cavity — also called

The medulla

Medullary cavity — location

Runs through the interior of the diaphysis

Medullary cavity — contents

Yellow bone marrow (fat storage)

Nutrient foramina — definition

Small holes on the bone surface through which blood vessels arteries veins and nerves enter and exit

Why bones are more vascular than cartilage

Bone contains nutrient foramina allowing extensive blood vessel supply; cartilage lacks this

Two consequences of bone being highly vascular

Bones bleed when broken AND they heal faster than cartilage

Cross-sectional (transverse) cut of a bone

A cut made perpendicular to the long axis at approximately mid-diaphysis

Frontal (coronal) cut of a bone

A cut made through the proximal component dividing the bone front to back

Longitudinal cut of a bone

A cut made along the long axis of the bone

Compact (dense) bone — primary location in a long bone

Found primarily in the diaphysis (shaft)

Compact bone — general appearance

Dense hard and solid

Structural unit of compact bone

The osteon (also called the Haversian system)

Spongy bone — alternative name

Cancellous bone

Spongy (cancellous) bone — primary location

Found primarily in the metaphysis and epiphysis

Trabeculae — definition

Thin struts or plates of bone that form the framework of spongy (cancellous) bone; also called "walls of bone"

Does spongy bone contain osteons with central canals?

No — spongy bone does NOT contain osteons with central canals

Microscopic structures spongy bone DOES contain

Osteocytes in lacunae canaliculi and lamellae — but these are NOT arranged concentrically around a central canal

What fills the spaces between trabeculae in spongy bone

Red bone marrow

Yellow bone marrow — location

Medullary cavity of the diaphysis

Yellow bone marrow — function

Fat storage

Red bone marrow — location

Cavities within spongy bone in the epiphysis and metaphysis

Red bone marrow — function

Blood cell production (hematopoiesis)

Hematopoiesis — definition

Blood cell production; occurs in red bone marrow

Periosteum — definition

Connective tissue covering that covers the outer surface of bone; has two layers

Periosteum — outer (fibrous) layer composition

Dense regular connective tissue

Periosteum — inner (osteogenic) layer — what it contains

Cells capable of developing into osteoblasts (bone-forming cells)

Endosteum — definition

Connective tissue membrane that lines the inner surface of the medullary cavity

Endosteum — tissue classification

Connective tissue membrane

Order of layers when a pin is pushed straight through a bone

Periosteum → Compact Bone → Endosteum → Yellow Bone Marrow → Endosteum → Compact Bone → Periosteum

Articular cartilage — location on a long bone

Covers the epiphyses (articular surfaces)

Articular cartilage — type of cartilage

Hyaline cartilage

Epiphyseal plate — location

Between the epiphysis and diaphysis (at the metaphysis)

Epiphyseal plate — type of cartilage

Hyaline cartilage

Epiphyseal plate — function

The site where bones grow longer via ossification

Epiphyseal line — definition

The ossified remnant of the epiphyseal plate after bone growth stops; appears as a visible line of bone

What the presence of an epiphyseal line indicates

The person has stopped growing (growth plate has fully ossified)

What the presence of an epiphyseal plate indicates

The person is still growing

Bone cell lineage in order

Osteogenic Cells → Osteoblasts → Osteocytes

Osteogenic cells — classification

Stem cell precursors

Osteogenic cells — function

Differentiate into osteoblasts

Osteogenic cells — where they are found

Inner (osteogenic) layer of the periosteum

Osteoblasts — origin

Derived from osteogenic cells

Osteoblasts — function

Build and deposit bone matrix

Osteocytes — origin

Mature osteoblasts that have become embedded in the bone matrix

Osteocytes — function

Maintain bone matrix; embedded in lacunae

Osteocytes — where they reside

Inside lacunae (small cavities in the bone matrix)

Osteoclasts — lineage

A completely DIFFERENT lineage from osteoblasts and osteocytes; NOT derived from osteoblasts

Osteoclasts — function

Break down (resorb) bone and release calcium into the bloodstream

Osteoclasts — key structural characteristic

Multinucleate (contain multiple nuclei)

Two physiological roles of calcium released by osteoclasts

Required for nerve conduction and muscle contraction

Hypercalcemia — definition

Elevated calcium levels in the blood; a result of increased osteoclast (bone resorption) activity

Hypercalcemia — etymology

Hyper = high; calc = calcium; emia = in blood

Lacunae (singular: lacuna) — definition

Small cavities in the bone matrix where osteocytes reside

Canaliculi — definition

Microscopic tunnels branching out from lacunae that carry cytoplasmic extensions of osteocytes

Canaliculi — function

Allow osteocytes to receive nutrients and remove waste despite being embedded in hard matrix; connect osteocytes to the blood supply

How osteocytes appear on a stained microscope slide

Dark spots with hair-like projections representing canaliculi

Why canaliculi are essential

Bone matrix is hard and impermeable; osteocytes need a physical pathway to reach blood vessels for nutrient and waste exchange

The osteon — definition

The cylindrical structural unit of compact bone

Osteon — shape and orientation

Cylinder shaped; runs parallel to the length of the bone

Osteon — also called

Haversian system

Concentric lamellae — definition

Concentric rings of bone matrix that encircle the central canal within an osteon

Lamellae — what the term means

Layers or rings (same term used for rings in the Pacinian corpuscle)

Central (Haversian) canal — contents

Blood vessels lymphatics and nerves

Central (Haversian) canal — orientation

Runs parallel to the long axis of the bone

Central (Haversian) canal — also called

Haversian canal or Osteon canal

Central canal — cross-section appearance

Round

Central canal — longitudinal section appearance

Oval or elongated

Interstitial lamellae — location

Between osteons (filling the gaps)

Interstitial lamellae — what they represent

Remnants of old osteons that were partially resorbed

Do interstitial lamellae surround a central canal?

No — interstitial lamellae do NOT surround a central canal

Circumferential lamellae — location

Outer circumference of the entire bone

Circumferential lamellae — description

Encircle the entire bone (not just one osteon)

Do circumferential lamellae surround a central canal?

No — circumferential lamellae do NOT surround a central canal

Which type of lamellae DO surround a central canal

Only concentric lamellae (within osteons)

Which types of lamellae do NOT surround a central canal

Interstitial lamellae and circumferential lamellae

Three types of lamellae in compact bone

Concentric lamellae (within osteons) interstitial lamellae (between osteons) circumferential lamellae (outer circumference)

Perforating (Volkmann's) canal — orientation

Perpendicular to the long axis of the bone

Perforating (Volkmann's) canal — also called

Volkmann's canal

Perforating (Volkmann's) canal — contents

Blood vessels and lymphatics (NOT nerves)

Volkmann's canal — cross-section appearance

Oval

Volkmann's canal — longitudinal section appearance

Round

What determines whether a canal appears round or oval under a microscope

The plane of section (how the bone was cut)

Why does Volkmann's canal appear oval in cross-section

It runs perpendicular to the long axis so a transverse cut catches it at an angle producing an oval shape

How to identify a cross-section of compact bone under a microscope

Concentric lamellae arranged in rings around a central canal are visible

How to identify a longitudinal section of compact bone under a microscope

Concentric lamellae are NOT visible; canals run parallel to the long axis

Why soft tissue is absent on a stained bone microscope slide

Acid used during preparation destroys soft tissue; only the bony matrix remains

What dark stain fills on a bone microscope slide

Lacunae and canaliculi

What the holes in a stained bone slide represent

Spaces where cells and blood vessels once resided