BIOL 1711- Cartilage and Bones

Connective tissue— specifically, bone connective tissue (i.e., compact and spongy bone), dense irregular connective tissue (lines the inner cavity and covers most of the outer layer surface of bones) and reticular connective tissue (of red bone marrow, when present)

Nervous tissue— detects and motors sensory stimuli about the status of the bone (e.g., if it is broken, etc.)

What types of tissues are bones primarily composed of?

Support and protection

Levers for movement

Hematopoiesis

Storage of mineral and energy reserves (reserves of calcium and phosphate are stored within bone connective tissue, and chemical energy is stored in adipocytes as yellow bone marrow in the shaft of some adult bones)

What are the functions of bone

Long, short, and flat

How are bones classified?

Bones that are greater in length than width, have elongated cylindrical shafts (diaphysis), is the most common bone shape, and vary in size (e.g., fingers are small long bones)

What are long bones?

Upper limbs (arm, forearm, palm and fingers) and lower limbs (e.g., thigh, leg, sole of the foot, and toes)

Where are long bones found in the body?

Bones whose length is nearly equal to their width.

What are short bones?

Carpals (wrist bone), tarsals (bones in the foot)

Sesamoid bones (sesame shaped bones) are included here as well, such as the patella

Where are examples of short bones found in the body?

Bones with flat, thin surfaces that may be slightly curved. Has extensive surface area for muscle attachment and to protect underlying soft tissue

What are flat bones?

The roof of the skull, the scapulae, the sternum, and the ribs

What are examples of flat bones in the body?

Bones that have elaborate complex shapes which don’t fit into any other category.

What are irregular bones

The vertebra, hip bones (ossa coxae), several bones in the skull such as the ethmoid and sphenoid

What are examples of irregular bones in the body?

The elongated cylindrical shaft, which provides leverage and supports majority of the weight of a long bone.

What is the diaphysis?

Compact bone on the external component, with spongy bone extending internally in the form of spicules (thin needle like structures), as well as the medullary cavity (hollow space), which contains red bone marrow in children and is later replaced by yellow bone marrow in adults

What is the diaphysis composed of?

The expanded knobby region that is at each end of a long bone.

What is the epiphysis?

Thin layer of compact bone on the outside, and an inner more extensive region of spongy bone which helps resist stress that is applied from many directions. Also has a thin layer of hyaline cartilage (articular cartilage) also covers the joint surface of each of this part of the bone, which helps reduce friction and absorbs shock in moveable joints

What is the epiphysis of a bone composed of? Why are they important?

The region of a long bone where the bone widens and transfers force between the diaphysis and epiphysis.

What is the metaphysis?

In a growing bone, it contains a region called the epiphyseal plate (growth plate), which is a thin layer of hyaline cartilage in the long bones of juvenile skeleton that allows for continued growth.

In adults, there the remnant of the epiphyseal plate is a thin defined area of compact bone called the epiphyseal line.

What is the metaphysis composed of?

A tough sheath that covers the outer surface of bone, except for those covered by articular cartilage. Is anchored to the bone by numerous collagen fibres called perforating fibres or Sharpey’s fibres that run perpendicular to the diaphysis.

What is the periosteum?

Two layers:

1) thin fibrous outer layer of dense irregular connective tissue that protects the bone from surrounding structures, anchors blood vessels and nerves, and serves as an attachment site for ligaments and tendons

2) Inner cellular layer that consists of bone stem cells (osteoprogenitor cells) and osteoblasts

What is the periosteum composed of?

A relatively delicate layer that lines all internal surface of the bone within the medullary cavity. It is composed of a very thin layer of connective tissue containing cells (e.g., osteoprogentior cells and osteoblasts) and reticular fibres

What is the endosteum?

No. Instead, osteoblasts will signals cells in the bone marrow to form these cells and migrate to the area of bone remodelling.

Are osteoclasts located in the endosteum and periosteum?

External surface is generally composed of compact bone covered by a periosteum.

The interior is composed entirely of spongy bone, and there is no medullary cavity

Explain the gross anatomy of other bone classes (not long bones).

Via a nutrient foramen (small opening/hole in the bone), allowing only 1 nutrient artery and 1 nutrient vein to enter and exit the bone. Nerves also enter the bone and innervate the periosteum endosteum and marrow cavity.

How does a veins, arteries, and nerves enter the bone?

The soft connective tissue of bone that includes both red bone marrow and yellow bone marrow

What is bone marrow?

Also called myeloid tissue.

Contains reticular connective tissue, developing blood cells, and adipocytes.

Primary function is forming the blood cells of the blood.

What is red bone marrow?

Is more widely distributed and located in the spongy bone of most bones of the body, as well as the medullary cavity of long bones.

Where is red bone marrow located in children?

Only located in select portions of the axial skeleton such as the flat bones of the skull, the vertebrae, the ribs, the sternum, and the hip bones (ossa coxae). As well as the proximal and distal epiphysis of each humerus and femur in the appendicular skeleton.

Where is red bone marrow located in adults?

Trigger the conversion of yellow bone marrow to red bone marrow to facilitate the production of additional erythrocytes.

What happens to the bone marrow during severe anemia?

Bone stem cells derived from mesenchyme.

Their division products another stem cell along with a committed cell that matures to become an osteoblast.

Located in both the periosteum and endosteum.

What are osteoprogenitor cells?

Cells that are positioned side by side on the bone surfaces, and functions to synthesize and secrete the initial semisolid organic form of bone matrix called “osteoid”. As osteoid is later hardened as a result of calcification, these cells produce new bone and later become entrapped in the matrix they product, differentiating into osteocytes.

What are osteoblasts?

Mature bone cells derived from osteoblasts that have lost their bone forming ability when the they becomes embedded within the calcified osteoid.

Maintains some connections via cytoplasmic projections with osteoblasts.

They maintain the bone matrix and detect mechanical stress on a bone, if stress is detected osteoblasts are signaled and new bone matrix may be deposited.

What are osteocytes?

Large, multinuclear cells that are derived from fused bone marrow cells.

Have a ruffled border where they contact the bone, increasing their surface area exposure.

Often located within or adjacent to a depression or pit on the bone called a “resorption lacuna”.

Are phagocytic and are involved in the breakdown of bone called bone resorption.

What are osteoclasts?

Organic component: osteoid which is composed of collagen proteins and semisolid ground substance (contains proteoglycans and glycoproteins). These components give the bone tensile strength by resisting stretching and twisting and contributes to its overall flexibility.

Inorganic component: primarily salt crystals (e.g., hydroxyapatite) that are primarily calcium phosphate. Crystals deposit around collagen fibres, which hardens the matrix. This accounts for the rigidity (relative inflexibility) of bone that provides its compressional strength.

Explain the composition of bone extracellular matrix

Loss of protein (e.g., collagen) = brittle bones

Insufficient calcium = soft bones

Why is a correct proportion of organic and inorganic substances in the extracellular matrix of bone important?

The basic functional and structural unit of mature compact bone. It is a small and cylindrical structure.

What is an osteon?

Contains no osteons

Has an open lattice of narrow rods and plates of bone called trabeculae (which provides resistance by distributing stress throughout the entire bone structure), and bone marrow fills between these structures.

Has parallel lamellae and lacunae between each lamellae, which is then connected by numerous cannaliculi. Osteocytes are also located within the lacunae.

Describe the anatomy of spongy bone

Derived from mesenchymal cells and produce cartilage matrix

What are chondroblasts?

Mature cartilage cells (occurs after these cells become encased in the matrix they produce) that maintain the matrix and occupt small spaces called lacunae.

What are chondrocytes?

Dense irregular connective tissue sheet that covers hyaline cartilage, except articular cartilage, which helps it maintain its shape

What is the perichondrium?

Cartilage has no calcium present in the matrix, whereas bone does. This makes cartilage both resilient and flexible.

Mature cartilage is avascualar, whereas bone has an extensive blood supply. This means that nutrients and oxygen are supplied to the cartilage via diffusion from blood vessels in the perichondrium

What are the main differences between bone and cartilage?

1) Osteoblasts secrete osteoid

2) Calcification/mineralization occurs whereby hydroxyapatite crystals deposit in the bone matrix due to calcium ions and phosphate ions reaching critical levels and precipitating out of solution and deposit in and around collagen fibres.

Explain how bone formation occurs.

Requires vitamin D (to enhance calcium absorption) and vitamin C (for collagen formation), and calcium and phosphate for calcification.

What substances are required for bone formation??

The process by which bone matrix is destroyed proteolytic enzymes and hydrochloric acid (secreted by the lysosome of osteoclasts)

Those substances dissolve the inorganic mineral parts of the bone matrix (calcium and phosphate), resulting in osteolysis.

Liberated calcium and phosphate ions then enter the extracellular fluid of nearby tissues and then the blood.

Explain what bone resorption is and how it occurs.

The release of stored calcium and phosphate from the bone matrix.

What is osteolysis?

During bone remodelling and when blood calcium levels are low

When does bone resorption occur?

Prior to birth, during embryologic development

When does cartilage development and growth begin?

Appositional growth (in width), occurs along the peripheral edge of cartilage

Interstitial growth (in length), occurs within the internal regions of cartilage

What are the two forms of cartilage growth and where do they occur?

Both interstitial and appositional cartilage growth occur simultaneously during early embryonic development

Interstitial growth decreases rapidly as cartilage matures because it becomes semi-rigid and has limited ability to expand, so further growth can only occur at the periphery via appositional growth.

Once cartilage is fully mature, new cartilage growth typically stops and will only occur again due to injury (even so, this growth is limited due to avascularity of mature cartilage)

When does cartilage growth occur? For how long?

1) A chondrocyte within a lacunae begins to exhibit mitotic activity

2) Two cells (now called chondroblasts) are produced by mitosis from one chondrocyte and occupy one lacuna

3) Each cell produces new matrix and begins to separate from its neighbour. Each cell is now called a chondrocyte.

4) Cartilage continues to grow internally and increase in length.

Explain the steps of interstitial growth of cartilage

1) Mitotic activity occurs in stem cells within the internal edge of the perichondrium.

2) New undifferentiated stem cells and committed cells that differentiate into chondroblasts are formed. Chondroblasts produce new matrix at the periphery

3) As a result of matrix formation, the chondroblasts push apart and become chondrocytes. Chondrycytes continue to produce more matrix at the periphery and cartilage grows in width.

Explain the steps of appositional growth of cartilage

Also called ossification

Refers to the formation and development of bone connective tissue.

What is osteogenesis?

Begins in the embryo and continues after birth as the skeleton grows in childhood and adolescence.

By 8-12 weeks of embryonic development, the skeleton begins forming from either a thickened condensations of mesenchyme (intramembranous ossification) or a hyaline cartilage model of bone (endochondral ossification)

When does osteogenesis occur? What are the 2 different ypes?

The flat bones of the skull (e.g., frontal bone), some of the facial bones (zygomatric bone, maxilla), the mandible, and the central part of the clavicle

What bones are formed via intramembranous ossification?

Produces most bones of the skeleton, including the upper and lower limbs, pelvis, the vertebrae, and the ends of the clavicle

What bones are formed via endochondral ossification?

1) Ossification centres form within thickened regions of mesenchyme beginning at the 8th week of development

2) Osteoid undergoes calcification

3) Woven bone (primary none that is immature and not well organized) and its surrounding periosteum form

4) Lamellar bone (secondary bone) replaces woven bone, as compact bone and spongy bone form

Explain the general steps of intramembranous ossification.

1) The fetal hyaline cartilage model of bone develops

2) Cartilage calcifies and a periosteal bone collar forms, initiating ossification process to start.

3) The primary ossification centre forms in the diaphysis and periosteal bud invades and spongy bone develops (which will result in the break down of cartilage and replacement of spongy bone).

4) The diaphysis elongates and the medullary cavity forms as secondary ossification centres from in the epiphyses. At the epiphysis bone displaces cartilage from the inside out (radiating out in a circular pattern).

5) Bone replaces almost all cartilage, except the articular cartilage and epiphyseal plates

6) Lengthwise growth continue until the epiphyseal plates ossify and from the epiphyseal line

Explain the general steps of endochondral ossification

The growth of long bones in length. It is dependent upon the growth of cartilage within the epiphyseal plates.

Superior surfaces increase cartilage volume via chondroblasts, while inferior surfaces remove cartilage (osteoclasts) and replace it with bone (osteoblasts)

The epiphyseal plate maintains constant thickness during childhood as it is pushed away from the centre of the shaft. But then at maturity, the rate of hyaline cartilage production slows down and the rate of osteoblast activity increases.

This causes the epiphyseal plates to narrow until it ultimately disappears (all that remains is the epiphyseal line, which is ossified), signalling the end of interstitial growth (i.e., bone has reached its max length).

What is interstitial growth in bone? How does it work?

The growth in bone width which occurs within the periosteum.

Osteoblasts int the inner cellular layer of the periosteum product and deposit bone matrix within the external circumferential lamellae. As lamellae increase in number, the structure increases in diameter.

As this new bone is laid down, osteoclasts along the medullary cavity resorb bone matrix, causing the medullary cavity to expand.

These combined effects of bone growth at the periphery and bone resorption within the medullary cavity transform an infant bone into a larger version of an adult bone, and this process occurs throughout a person’s lifetime

What is appositional growth in bone? How does it work?

The constant dynamic process in which bone is continuously added (bone deposition) and old bone tissue is removed (bone resorption. It occurs at both the periosteal and endosteal surfaces of a bone.

What is bone remodelling?

It is estimated that about 20% of the adult human skeleton is replaced yearly.

However, it does not occur at the same rate everywhere in the skeleton. E.g., compact bone is replaced at a slower rate than spongy bone, and different components of a bone are replaced at varying rates (e.g., the diaphysis vs the epiphysis).

At what rate does bone remodelling occur in the skeleton?

Explain the changes in bone marrow types that occur during maturity.

During this time there is a progressive decline of red bone marrow, primarily within the medullary cavities of long bones and the inner core of most epiphyses. As this happens there is a decrease in red blood cells, and an increase in adipocytes. This fatty substance is yellow bone marrow.

What is the proximal epiphysis?

The end of the bone closest to the body trunk.

What is the distal epiphysis?

The end of the bone farthest from the trunk.

What is the central (haversian) canal?

The cylindrical channel at the centre of the entire length of the osteon. It contains blood vessels and nerves that supply the bone.

What are concentric lamellae?

Rings of bone connective tissue that surround the central canal and form the bulk of the osteon. It has an alternating pattern of collagen fibres that give strength and resilience to the bone.

What are canaliculi?

Interconnecting channels that extend from each lacunae, through the lamellae, and connect to other lacunae, and the central canal. It houses osteocyte cytoplasmic projections that permit intercellular contact and communication to transport nutrients, minerals, gas, and waste between osteocytes and blood vessels within the central canal.

What are perforating (Volkmann) canals?

Similar to the central canal, but run instead perpendicular to help connect multiple central canals within different osteons.

What are circumfrential lamellae?

Rings of bone immediately internal to the periosteum of the bone (external part of this component) or external to the endosteum (internal part of this component).

What are interstitial lamellae?

Components of compact bone between osteons or leftover parts of osteon that have been partially resorbed. Are incomplete and have no central canal.