Mm Histology 3 Bone

Front 1

Bone

Back 1

Hard, whitish tissue making up the skeleton in humans and other vertebrates

Resistant to tensile loading (aka can be compressed but not pulled out)

Front 2

Cartilage

Back 2

A tough, elastic, fibrous connective tissue found in various parts of the body

It makes up the embryonic and young vertebrate skeleton, then is converted to bone with aging

Resist compressive loading (cannot be compressed but can be stretched out)

Weight on tissues does not compress cartilage.

Found in joints, outer ear, and larynx

Front 3

5 Types of bones (shapes)

Back 3

1. Long Bones

2. Short Bones

3. Flat Bones

4. Irregular Bones

5. Sesamoid Bones

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Long Bones

Back 4

Have a body longer than it is wide

Growth plates (epiphysis) at either end, having a hard outer surface of compact bone and a spongy inner known an cancellous bone containing bone marrow.

Both ends of the bone are covered in hyaline cartilage to help protect the bone and aid shock absorbtion

ex. Femur, Humerus, Tibia, Metacarpals, Metatarsals and Phalanges.

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Short Bones

Back 5

Approximately as wide as they are long

Have a primary function of providing support and stability, little movement.

Consist of only a thin layer of compact, hard bone with cancellous bone on the inside along with relatively large amounts of bone marrow.

Ex. Carpals and Tarsals - the wrist and foot bones.

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Flat Bones

Back 6

Strong, flat plates of bone

Main function is protection to the bodies vital organs and being a base for muscular attachment.

Anterior and posterior surfaces are formed of compact bone to provide strength for protection with the centre consisiting of cancellous (spongy) bone and varying amounts of bone marrow.

In adults, the highest number of red blood cells are formed in flat bones.

Ex. Scapula (shoulder blade), Sternum (breast bone), Cranium (skull), os coxae (hip bone), Pelvis, and Ribs

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Sesamoid bones

Back 7

Short or irregular bones, imbedded in a tendon.

Sesamoid bones are usually present in a tendon where it passes over a joint which serves to protect the tendon.

Ex. Patella (knee cap) which sits within the Patella or Quadriceps tendon. Other sesamoid bones are the Pisiform (smallest of the Carpals) and the two small bones at the base of the 1st Metatarsal.

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Irregular Bones

Back 8

Do not fall into any other category, due to their non-uniform shape.

They primarily consist of cancellous bone, with a thin outer layer of compact bone.

Ex. Vertebrae, Sacrum and Mandible (lower jaw)

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Spongy Bone

Back 9

aka Trabecular Bone

aka Cancellous bone

Soft, weak, flexible, not dense

Found at the ends of long bones & next to joints

Highly vascular, contains bone marrow, and is active in the exchange of calcium ions

Has plates that are laid down in a regular pattern in regard to stress on the bone.

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Compact Bone

Back 10

aka Cortical Bone

Dense, stiff bone

Provides protection and support to the body

Note - osteoporosis can affect both types of bone

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Lamellar Bone

Back 11

Normal type of adult mammalian bone (cancellous or compact)

Note - Compact (cortical) and cancellous (trabecular) are gross descriptive terms; Woven (disorganized fetal bone) and lamellar are histological descriptors

"Lamella" = thin layer

Cancellous is composed of parallel lamellae

Compact is composed of concentric lamellae

Interstitial lamellae are randomly oriented

*Note: osteons run parallel to long axis of the bone

Front 12

Woven Bone

Back 12

Fetal bone, removed during maturation

Randomly dispersed, disorganized collagen

Osteoblasts on outside, canaliculi randomly placed

Mechanically weak

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Bone Matrix

Back 13

The intercellular substance of bone tissue consisting of collagen fibers, ground substance, and inorganic bone salts.

Front 14

Ground substance

Back 14

A gel-like substance surrounding the cells of connective tissue

Primarily composed of water, glycosaminoglycans (hyaluronan), proteoglycans, and glycoproteins

Components vary depending on the tissue

aka "extra fibrillar matrix"

Does not include fibers (collagen and elastic fibers), but does include all the other components of the extracellular matrix

Usually it is not visible on slides, because it is removed during the preparation process

Front 15

Hydroxyapatite

Back 15

Main inorganic compound of bone

a mineral that gives stiffness and tensile strength to bones

inserts as granules or sheets between collagen fibrils (in gaps/holes)

Front 16

Fibrous matrix of bone

Back 16

99 % Collagen, 1 % Ground Substance

Bone mineral are placed in holes between staggered collagen models

Front 17

Trabecula

Back 17

Functional unit of trabecular (spongy) bone

A small beam of connective tissue

Provides support and structure

In osteoporosis, there is loss of the trabeculae in the plates, leaving very thin bars of bone.

Front 18

Osteon

(aka "haversian system")

Back 18

Functional unit of compact bone

A central canal containing blood capillaries and the "concentric" lamellae around it

In the center of the osteon is a blood vessel. This space is called the Haversian Canal by some investigators. It is surrounded by concentric lamellar structures. Outside this “solar system” are some interstitial lamellae that lie at the interface between one osteon and another due to remodeling of bone.

Front 19

Lacunae

Back 19

a gap or unfilled space

one of the numerous minute cavities in the substance of bone that contain nucleate cells

Front 20

Canaliculi

Back 20

Microscopic ducts between the lacunae of ossified bone

Materials picked up by osteocytes adjacent to blood vessels are distributed throughout the bone matrix via the canaliculi

Extend out to neighboring cells via gap junctions

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Osteocytes

Back 21

A branched cell embedded in the matrix of bone tissue.

Can live as long as the organism itself, do not divide

Derived from osteoblasts: when osteoblasts become trapped in the matrix that they secrete, they become osteocytes.

Osteocytes are found in spaces called lacunae and their processes are found in canaliculi

Osteocytes are networked to each other via long cytoplasmic extension in canaliculi, which are used for exchange of nutrients and waste through gap junctions. The processes also interact with surrounding matrix by integrin receptors.

Although osteocytes have reduced synthetic activity and (like osteoblasts) are not capable of mitotic division, they are actively involved in the routine turnover of bony matrix, through various mechanosensory mechanisms.

Bone lining cells also communicate with the osteocytes within the bone. These bone lining cells determine where and when new bone is deposited.

The network of osteocytes is critical for overall biomechanical process. If there is drop out of even a few osteocytes, the mechanical characteristics of the bone are degraded, so there must be turnover of the osteocytes to repair damage.

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3 Types of Cartilage

Back 22

1. Hyaline - transparent cartilage, found on joint surfaces, simple structure, firm consistency with a lot of collagen. It contains no nerves or blood vessels.

2. Fibro-cartilage - contains organized fibrous bundles of collagen with lines of cells in between, good for areas under tension (like the intervertebral disks in the spinal cord)

3. Elastic- contains arching elastic fibers, principal protein is elastin but also has collagen fibers (ex. the outer ear and epiglottis)

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Aggrecan

Back 23

Main component of cartilage

A proteoglycan with type II collagen

Major part of the ground substance of cartilage

Attracts water and acts as lubricant

Aggrecan is composed of monomers hyaluronic acid linked along the length of the core protein. The hyaluronic acid are extended GAGs (chondroitin sulfate + keratin sulfate)

Aggrecan has fixed negative charges and hence resistance to compression is due to electrostatic repulsion.

The charges hold water in these feathered domains and it swells.

Front 24

Fibrous matrix of Cartilage

Back 24

50% collagen (with occasional elastic fibers), 50% ground substance

Collagens form (micro)fibrils that are smaller than bone

Front 25

2 Types of Bone/Cartilage Growth

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Appositional (bone and cartilage) -

Growth by forming new layers on the surface of pre-existing layers

Process of increasing in thickness rather than length

Interstitial (cartilage only) -

growth from a number of different centers within an area;

cells in the center grow, divide, and spread out

In contrast with appositional growth, it can occur only when the materials involved are nonrigid, such as cartilage.

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2 essential processes by which bone tissue is created (during fetal development)

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1. Endochondral Ossification

2. Intramembrous Ossification

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Endochondrial Ossification

Back 27

Uses hyaline cartilage “bones” as models for bone construction

Most bones

Requires breakdown of hyaline cartilage prior to ossification & formation begins at the primary ossification center

The perichondrium covering the hyaline cartilage “bone” is infiltrated with blood vessels converting it to vascularized periosteum. This change in nutrition causes the underlying mesenchymal cells to specialize into osteoblasts.

1. Formation of bone collar - Osteoblasts of the converted periosteum secrete osteoid against the hyaline cartilage diaphysis encasing it in a bone collar

2. Cavitation of the hyaline cartilage -

Chondrocytes within the shaft hypertrophy & signal surrounding cartilage matrix (VEGF) to calcify. Chondrocytes die due to lack of nutrients (impermeability of calcified matrix). Matrix deteriorates thus opening up cavities

3. Invasion of internal cavities by the periosteal bud, and spongy bone formation -

The forming cavities are invaded by a collection of elements : Periosteal bud contains a nutrient artery and vein, lymphatics, nerve fibers, red marrow elements, osteoblasts, and osteoclasts. Osteoclasts erode the calcified cartilage matrix & osteoblasts secrete osteoid around the remaining hyaline cartilage forming bone-covered cartilage trabuculae (the formation of spongy bone)

4. Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses -

Osteoclasts open up a medullary cavity by breaking down the newly formed spongy bone. Cartilage is growing, bones being calcified and eroded and then replaced by bony spicules on the epiphyseal surfaces facing the medullary cavity.

5. Ossification of the epiphyses -

Secondary ossification centers appear in one or

both epiphyses. Steps 1-4 occur there except no medullary cavity forms. Finally, hyaline cavity remains at Epiphyseal surface (articular cartilage) & Epiphyseal plates (junction of the diaphysis and the epiphysis)

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Intramembranous growth

Back 28

Cartilage not present

Skull and clavicle

1. Ossification center appears in fibrous connective tissue membrane

2. Bone is secreted within the fibrous membrane

3. Woven bone & periosteum form

4. Bone collar of compact bone and red marrow appear

mesenchymal condensation=>bone tissue directly without any other intermediate structures such as the cartilage model (which defines endochondral development—see above).

This bone sheet forms between two membranes such as for example the dura mater and scalp in skull bones.

The osteocytes are embedded in the matrix.

Growth of the intramembranous bone occurs at suture lines where plates of intramembranous bone meet.

Osteoblasts are on the matrix surface in the so called periosteum.

Front 29

Indian Hedgehog

PTHrP

Back 29

Indian Hedgehog (pro-ossification) - released by herterotrophic chondrocytes, stimulates chondrocytes to proliferate, also periosteal cells to release PTHrP

PTHrP (limits ossification) - inhibits ossification by inhibiting the release of Indian Hedgehog so that too much doesn't happen

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Determination of Bone and Cartilage

Back 30

1) Initial Mesenchyme Condensations: Wnt signaling drives mesenchymal condensation as a result of Sox9/Runx1 expression.

2) In areas of high oxygen and some tensile stress bone forms. BMP 2, 4, 7, then runx2 and osterix=> activates osteocalcin and drives bone formation

3) In areas of low oxygen tension and intermittent compression, Indian hedgehog and FGF activate and expression of Sox9, Sox5, Sox6 drives the progenitor cell to chondrocytes.

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Bone Remodeling

Back 31

Purpose -

To replace old bone tissue with new

To reflect changed biomechanical loading patterns (strain and damage resulting in microcracks)

In response to hormonal changes

In response to pathologic conditions (e.g., inflammation)

Osteoblasts on the bone surface lay down new bone by secreting un- mineralized material “osteoid”. Some become mature periosteal bone lining cells, and some become osteocytes in the middle of the bone.

When the bone tissue is loaded there is pressure on the canniliculi. These osteocytic process senses stress via integrins and via stretch activated Ca2+ channels. The bone cell network within the bone sends information to osteoblasts on the bone surface to direct them to proliferate and to lay down new osteoid.

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Effects of hormones on skeletal metabolism (aka which increase bone resorbtion, which decrease it)

Back 32

• Increase bone resorption

– Parathyroid hormone
– Glucosteroids
– Thyroid hormone
– Vitamin D metabolites in high doses

• Decrease bone resorption

– Calcitonin

– Gonadal steroids

Front 33

Bone Remodeling signals: Sclerostin (Osteocytes - Osteoblasts)

Back 33

Osteocytes - Osteoblasts : The osteocytes make sclerostin under normal conditions, blocking Wnt signaling (which normally stimulates osteoblast proliferation)

Sclerostin is produced when there is no pressure loading, but when osteocytes are fully loaded they stop producing Sclerostin and Wnt induces more osteoblasts to form. Thus, new bone is created under mechanical load. This occurs rapidly.

Front 34

Bone Remodeling Signals : RANKL, RANK, & OPG (Osteoblast - Osteoclasts)

Back 34

RANKL - produced by osteoblasts

RANK - receptor on Osteoclasts for RANKL

OPG (osteoprotegerin) - a soluble inhibitor made by osteoblasts that limits osteoclast maturation.

When RANKL binds to RANK, this induces pre-Osteoclasts to mature to functional Osteoclasts, allowing them to dissolve bone.

OPG binds RANKL and inhibits RANKL availability.

Activated osteoclasts sit on bone in the Howship’s lacuna and use H+ATPase to dissolve hydroxyappitite.

The ruffled boarder is used to absorb and cause transcytosis of degraded collagen fragments that contain hydroxyproline.

Degradation of bone by osteoclasts releases activated TGF beta which, in contrast, stimulates new bone tissue formation (one aspect of balance).

The osteoclast produces a “cement line”, a residual discontinuity of new and old bone.

Note that the osteoclasts are active both on the surface of the bone as well as in the center of the osteons where the blood vessels lie.

During inflammatory conditions, the ratio of Rankl/OPG changes. The inflammatory cells make Rankl and osteoclasts are activated and bone is reabsorbed.

Front 35

Bone

-Composition

-Collagen types

-Special Features

-Lacunae Characteristics

-Developmental Origin

-Location/environment

-Growth Mechanism

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-Composition :

Fibrous matrix is 99.6% of demineralized dry weight

Ground substance is 0.4% of dry weight

-Collagen types :

Collagen I (main fibrils)

Collagen V (embedded, nucleating)

-Lacunae Characteristics :

Interconnected thru canaliculi processes, communication between lacuna thru gap junctions

-Location/environment :

Forms in areas of high pO2 and high tensile loading

-Growth Mechanism :

Appositional only

-Developmental Origin :

Mesenchymal: differentiation due to Runx, etc. TFs

Front 36

Cartilage

-Composition

-Collagen types

-Special Features

-Lacunae Characteristics

-Developmental Origin

-Location/environment

-Growth Mechanism

Back 36

-Composition :

Fibrous matrix is 50% of dry weight

Ground substance is 50% of dry weight

-Collagen types :

Collagen 2 (main fibrils)
Collagen 6 (embedded, nucleating)
Collagen 4 (prevents further collagen growth)

-Lacunae Characteristics :

Isolated, completely enclosed, no communication (except thru the ECM)

-Location/environment :

In areas of low pO2 and high compressive loading

-Growth Mechanism :

Appositional and interstitial

-Developmental Origin :

Mesenchymal: differentiation due to Sox TFs