W3 - Bone Tissue Flashcards
LO1: Name the primary germ layer that bone tissue is derived from
The primary germ layer that connective tissue is derived from, and therefore that bone tissue is derived from, is the mesoderm.
LO2: Revise the functions of bone tissue and the bones of the skeleton (also called osseous tissue)
LO3: Describe the basic components and cell types in bone tissue
cells, protein fibres, ground substance (latter two making up the extracellular matrix)
These components make up the organic part, which comprises 1/3 bone mass. Protein fibres inside this are collagen fibres, ground substance is solid due to presence of calcium phosphate and other inorganic elements.
Other 2/3 comprised of inorganic part, which is mostly made of calcium phosphate.
4 types of cells in bone tissue: osteoprogenitor, osteoblasts, osteocytes, osteoclasts
LO3: Describe the basic components and cell types in bone tissue (osteoprogenitor)
Osteoprogenitor cells are bone stem cells that divide to produce new cells which will become osteoblasts. They are located on the external and internal surfaces of bone.
LO3: Describe the basic components and cell types in bone tissue (osteoblast)
Osteoblasts are immature bone cells (‘blast’ = immature cell) derived from osteoprogenitor cells. They are responsible for producing new bone matrix. Once they become entrapped in the matrix they produce, they become osteocytes.
LO3: Describe the basic components and cell types in bone tissue (osteocyte)
Osteocytes are mature bone cells (‘cyte’ = cell) derived from osteoblasts. They occupy small spaces within the bone matrix called lacunae (singular = lacuna; ‘lacuna’ = lake) and are responsible for maintaining the bone matrix.
LO3: Describe the basic components and cell types in bone tissue (osteoclast)
Osteoclasts are large, multinuclear cells derived from cells similar to those that produce monocytes, which are a type of white blood cell. As such, osteoclasts are phagocytic cells and are responsible for breaking down the bone matrix to aid in the development, growth, maintenance and repair of bone.
The breakdown of bone matrix is referred to as bone resorption and is balanced with the deposition of new bone matrix by osteoblasts. Osteoclasts are located within depressions on the internal surface of bone called resorption lacunae.
LO4: Compare and contrast the structure of compact and spongy bone (compact)
Compact bone is also referred to as dense or cortical bone and forms the external part of the bones of the skeleton. It is solid, hence its name, and has a highly organised structure, with the basic unit of mature compact bone being a cylindrical structure called an osteon or Haversian system. An osteon has several parts including the central canal, concentric lamellae, osteocytes and canaliculi.
As well as osteons, compact bone includes other structures including perforating canals, circumferential lamellae and interstitial lamellae.
LO4: Compare and contrast the structure of compact and spongy bone (structures in compact)
Concentric Lamellae
These are rings of bone tissue that surround the central canal (singular = lamella; ‘lamella’ = plate). They form the majority of the osteon.
Canaliculi
These are small canals that connect the osteocytes within their lacunae and the central canal (singular = canaliculus). They contain cytoplasmic projections from the osteocytes that allow communication between the osteocytes.
Central Canal
This is also referred to as the Haversian canal. It is a channel in the centre of the osteon that contains the blood vessels and nerves that supply the bone.
Osteocytes
The osteocytes are located within lacunae between adjacent concentric lamellae.
Circumferential Lamellae
These are rings of bone tissue running around the entire external and internal circumference of bones.
Interstitial Lamellae
These are the incomplete rings of bone tissue in between the osteons (‘inter’ = between)
Perforating Canals
These are also referred to as Volkmann canals. They are channels that run perpendicular to the central canals and connect several central canals with each other, therefore allowing for communication between osteons. Like central canals, perforating canals contain blood vessels and nerves.
LO4: Compare and contrast the structure of compact and spongy bone (spongy)
Spongy bone is also referred to as cancellous or trabecular bone and forms the internal part of the bones of the skeleton.
Unlike compact bone, it appears porous or spongy, hence its name, and does not have the same highly organised structure. Rather than being composed of osteons, spongy bone is composed of a lattice of bony shelves or beams called trabeculae (singular = trabecula; ‘trabecula’ = beam).
When viewed in cross-section, a trabecula looks similar to an osteon in that it is composed of plates of bone tissue called lamellae with osteocytes within lacunae in between adjacent lamellae and canaliculi connecting the osteocytes. However, a trabecula lacks a central canal.
Instead, blood vessels and nerves are found in spongy bone within the spaces between trabeculae. These spaces also contain red bone marrow, which is the connective tissue containing the stem cells that give rise to all blood cells.
LO5: Describe the parts of a long bone
Epiphysis
This is the expanded region at the end of a long bone (‘epi’ = above or upon, ‘physis’ = growth). A long bone has two epiphyses (one at each end). The epiphysis is composed of an outer layer of compact bone and an inner layer of spongy bone. Where a long bone forms a joint with another bone, the epiphysis is covered by a thin layer of cartilage.
Metaphysis
This is the region between the diaphysis and each epiphysis of a long bone. A long bone has two metaphyses (one at each end). In a growing long bone, the metaphysis contains the epiphyseal or growth plate, which is where the bone grows in length from. In a long bone that has stopped growing, as found in an adult, the metaphysis contains the epiphyseal line, which is the remnant of the epiphyseal plate.
Diaphysis
This is the shaft of a long bone, spanning between the two epiphyses. It is composed of an outer layer of compact bone with a small amount of spongy bone internally. Within the centre of the diaphysis is a hollow space called the medullary cavity, which contains yellow bone marrow. This is the fatty connective tissue that most red bone marrow degenerates into in adults.
LO6: Describe the external and internal tissue coverings of bone
The external and internal surfaces of bone are covered by sheets of tissue called periosteum and endosteum, respectively.
Periosteum
This is the tough sheet covering all external surfaces of bone that are not covered by cartilage (‘peri’ = around, ‘osteum’= bone). In a long bone, the diaphysis is covered by periosteum. Periosteum consists of an outer fibrous layer made of dense irregular connective tissue and an inner cellular layer containing osteoprogenitor cells and osteoblasts. The function of the periosteum is to protect the bone, anchor tendons, ligaments, blood vessels and nerves to the surface of the bone and provide cells for bone growth and repair.
Endosteum
This is the thin membrane covering all internal surfaces of bone (‘endo’ = inside, ‘osteum’ = bone). In a long bone, the medullary cavity is lined by endosteum. Endosteum is composed of a single layer of cells including osteoprogenitor cells, osteoblasts and osteoclasts.
LO7: Compare and contrast the processes of intramembranous and endochondral ossification and provide examples of bones formed by each (intramembrous)
This is the less common type of ossification. It involves the formation of bone directly from mesenchyme, which is the embryonic connective tissue derived from the mesoderm primary germ layer of the embryo that ultimately gives rise to all types of connective tissue. Examples of bones which are formed by intramembranous ossification include the flat bones of the skull (e.g. parietal bones), some facial bones (e.g. zygomatic bones, maxillae, mandible) and the clavicles.
LO7: Compare and contrast the processes of intramembranous and endochondral ossification and provide examples of bones formed by each (endochondral)
This is the more common type of ossification. It involves the formation of bone from a hyaline cartilage model that originates from mesenchyme. The cartilage model grows and is slowly replaced by bone until the bone is fully formed. Most of the bones in the body are formed by endochondral ossification, including the bones of the limbs (e.g. humerus, femur), the vertebrae, the ribs and the pelvis.
LO8: Describe the process of bone growth and bone remodelling (define and provide term for length and width)
Growth in length is called interstitial growth and growth in width is called appositional growth.
Bones do not become dormant once they have reached their full size. Rather, they are continuously remodelling themselves throughout life. The balance of bone formation by osteoblasts and bone resorption by osteoclasts is referred to as bone remodelling.
This process is controlled by hormones and mechanical stress placed on bone. When there is an imbalance between bone formation and bone resorption, with bone resorption occurring at a faster rate than bone formation, this can lead to the weakening of bones seen in osteoporosis. This commonly occurs in older adults, particularly older females due to hormonal changes from menopause.
LO8: Describe the process of bone growth and bone remodelling (interstitial)
In a long bone, interstitial growth occurs from the epiphysial plates in each metaphysis, where cartilage is organised in different zones showing the transition from cartilage to bone.
LO8: Describe the process of bone growth and bone remodelling (appositional)
In a long bone, appositional growth occurs from the periosteum. Osteoblasts in the inner cellular layer of the periosteum deposit new bone matrix to increase the width of the bone. At the same time, osteoclasts in the endosteum resorb old bone matrix to increase the size of the medullary cavity. This ensures that the medullary cavity remains in proportion to the width of the bone.
LO1: Name the primary germ layer that cartilage is derived from
The primary germ layer that connective tissue is derived from, and therefore that cartilage is derived from, is the mesoderm.
LO2: Describe the functions of cartilage
Cartilage is a semirigid connective tissue that is weaker but more flexible than bone.
Support
Cartilage is found in locations such as the trachea and external ear, where it provides structural support.
Providing Smooth Joint Surfaces
Cartilage covers the surfaces of bones where they form joints. These surfaces are called articular surfaces, as a joint is also referred to as an articulation. The cartilage that covers the articular surfaces of bones is a type of cartilage called hyaline cartilage, specifically a type of hyaline cartilage called articular cartilage. This provides a smooth surface to reduce friction between the bones forming the joint.
Providing a Model for Bone Formation
As we saw in Part 1 of this module, most bones in the body are formed by endochondral ossification, where a hyaline cartilage model is slowly replaced by bone.
LO3: Describe the basic components and cell types in cartilage
Like all connective tissues, cartilage has the three basic components of cells, protein fibres and ground substance, with the latter two making up the extracellular matrix. The ground substance is semisolid and there is a combination of different protein fibres depending on the type of cartilage. All types of cartilage contain two main types of cells, which are chondroblasts and chondrocytes (‘chondro’ = cartilage). These are similar to their equivalents in bone tissue.
LO3: Describe the basic components and cell types in cartilage (chondroblast and chondrocyte)
Chondroblast
Like osteoblasts in bone tissue, chondroblasts are immature cartilage cells (‘blast’ = immature cell) derived from stem cells at the surface of the cartilage. They are responsible for producing new cartilage matrix. Once they become entrapped in the matrix they produce, they become chondrocytes.
Chondrocyte
Like osteocytes in bone tissue, chondrocytes are mature cartilage cells (‘cyte’ = cell) derived from chondroblasts. They occupy small spaces within the cartilage matrix called lacunae (singular = lacuna; ‘lacuna’ = lake) and are responsible for maintaining the cartilage matrix.
LO4: Describe the structure, function and location of the three types of cartilage (hyaline)
Most common type
Glass-like appearance
Chondrocytes within their lacunae are scattered throughout the cartilage matrix and the collagen fibres are not readily visible on histological slides
Function: support other structures and provide a model for bone formation
Forms the cartilage precursor in endochondral ossification
Examples: found in parts of respiratory tract (larynx and trachea) and costal cartilage of ribs
Type of hyaline = articular cartilage (covers articular surfaces of bones) reduces friction between bones
LO4: Describe the structure, function and location of the three types of cartilage (fibrocartilage)
Contains interwoven collagen fibres
Collagen fibres are readily visible on histological slides
Chondrocytes in lacunae are arranged in parallel rows between collage fibres and there is little ground substance
Function: provide strength and acts as a shock absorber
Found in: intervertebral discs between adjacent vertebrae, in the pubic symphysis (between 2 pubic bones) and in the wedge-shaped pads of cartilage in the menisci (knee joint)
LO4: Describe the structure, function and location of the three types of cartilage (elastic)
Interwoven elastic fibres
Chondrocytes in lacunae look similar to hyaline - but are more closely compact and surrounded by meshwork of elastic fibres
Function: support other structures while allowing for flexibility
Found in: external ear, nose, epiglottis
