The Structure of Bone and Cartilage:
Objectives:
Classification of Bones:
Bones contain dense connective tissue: cartilage, adipose tissue, bone marrow that forms blood cells, nerves, blood vessels.
Osteology: the study of bone structure and bone disorders.
- Identify bones as either long, short, flat, irregular, or sesamoid.
- Describe the gross structure of a long bone both interiorly and exteriorly.
- Describe the chemical composition of bone.
- Lab Manual
- Carolina Biological Distance Learning Lab Kit: Musculoskeletal System (and Skeleton)
- Optional for furthering your learning, but not required: 3 chicken bones:
- One baked at 250 degrees F for 2 hours or more
- One soaked in an acidic solution: vinegar for 1 week OR cola for 3-4 days OR nitric acid for 3-4 days
- Untreated chicken leg bone
Classification of Bones:
- Long bones
- Short bones
- Flat bones
- Irregular bones
- Sesamoid bones (sesamoid = sesame seed)
Bones contain dense connective tissue: cartilage, adipose tissue, bone marrow that forms blood cells, nerves, blood vessels.
Osteology: the study of bone structure and bone disorders.
- Osteo = bone
- Ology = study of
- Epiphysis - enlarged proximal and distal ends of a long bone
- Epi = over/above
- Physis = growing
- Diaphysis - middle shaft area composed of compact bone
- Dia = through
- Epiphyseal plate - growth plate; area where the bone grows in length in a growing bone
- Epiphyseal line - in a bone that is no longer growing
- Red marrow - found in the epiphyseal cavities of the spongy bone, and produces blood cells
- Articular cartilage - composed of hyaline cartilage that covers both epiphyses
- Articul = joint
- Periosteum - tough, connective tissue membrane that covers the bone exterior
- Peri = around
- Osteo = bone
- Endosteum - tough, connective tissue membrane that lines the bone interior filled with fatty yellow marrow
- Endo = within, inside
- Medullary cavity - hollow center of the bony diaphysis containing yellow marrow, or adipose tissue
- Medulla = marrow, pith
- Osteoblasts - bone-building cells for bone formation
- Blast = builder
- Osteoclasts - bone-breaking-down cells for bone tissue repair
- Klasis = breaking
Chemical Composition of Bones:
- 25% water
- 25% protein fibers, including collagen, which provides tensile strength and flexibility so the bone won't break with normal stress
- Collagen decreases with age
- Bones become brittle with age and more prone to breaks
- 50% mineral salts: calcium phosphate and calcium carbonate (hardness to bone)
- Decreased with rickets, causing bones to be soft and bend due to body weight
- Osteoporosis: loss of bone minerals, causing bones to become more spongy and lose trabeculae
Mesenchymal Stem Cells: The Hematopoietic Stem Cells From Which Bone Derives (These Are Also Referred to as Chondroblasts or Perichondrial Cells, From Which Chondrocytes Derive)
Osteoblasts:
- Bone-forming cells
- Secrete osteoid, a protein that hardens and calcifies/mineralizes to become bone
- Secrete hormones, such as prostaglandin
- Secrete alkaline phosphatase, an enzyme needed for the mineralization of bone
- Secrete matrix proteins
Osteocytes:
- Inactive osteoblasts
- Trapped and surrounded by bone matrix
- Originate from osteoblasts
- Occupy spaces (lacunae) in the bone
- Surrounded by processes that reach out to other osteocytes and osteoblasts
Osteoclasts:
- Break down bone
- Bone resorption
- Bone remodelling
- Phagocytic-like
- Large cells with multiple nuclei
- Secrete enzymes
- Maintain calcium homeostasis
Bone Growth:
Cartilage Forms First From Mesenchymal Stem Cells Called Chondroblasts:
These Differentiate Into Chrondrocytes in Response to Specific Signals, Such as Growth Factors:
Chondrocytes Get Trapped in a Matrix and Begin to Harden/Ossify and Degenerate:
Endochondrial Ossification is occurring here by chondrocytes getting caught/trapped in the bone matrix. This occurs at the epiphyseal plate at the end of long bones. At the top are healthy chondrocytes, which become degenerated over time (bottom). This is the formation of a hardened, calcified matrix (extracellular). This is the secondary center of ossification.
Hyaline Cartilage is Formed, Which is the Model for Bone Formation:
Chondrocytes continue to multiply and divide, and the hyaline bone model thickens and lengthens. Further secretion of extracellular matrix (interstitial growth) occurs, along with continued formation of chrondroblasts from the perichondrium, a thick layer of connective tissue surrounding the cartilage of the newly developing bone. This double-layered connective tissue consists of a fibrous layer on the outer side and a chondrogenic layer on the inner side. The fibrous layer is made up of fibroblasts and these produce collagenous fibers.
Undifferentiated, the chondrogenic layer is able to form chondroblasts or chondrocytes. The perichondrium is a protective layer that also surrounds elastic and hyaline cartilage, and it aids in the repair and growth of the cartilage should it become damaged or injured.
Primary Ossification Center:
The primary or initial ossification center occurs in the diaphysis (long, longitudinal portion) of developing long bones. Here, the periosteum is formed from the perichondrium. In fact, once the perichondrium becomes vascularized with blood supply, it becomes the periosteum. The periosteum is the outer layer of bone, or the the covering. It contains osteoprogenitor cells (mesenchymal stem cells), which become osteoblasts for the formation of new bone material.
Bone Collar Formation:
Around the hyaline cartilage model of the developing bone, a bone collar forms around the diaphysis of the periosteum. It is there to support the growing bone and help maintain the shape of the bone and provide stability. Here, the osteoblasts secrete a substance called osteoid (specific proteins, fiber Type-1 Collagen, osteocalcin and chondroitin sulfate) on the side of the hyaline cartilage model, which serves as an adhesive or matrix to support the newly developing bone material. This material is unmineralized and forms before the maturation process of the bone material. Over time, this material hardens and does become mineralized with calcium and other substances, and the osteoid and bone cells become new bone tissue. Osteoid accounts for about half of the bone's volume and close to that in weight, so it is a hefty, solid substance.
The Matrix Becomes Calcified:
As the chondrocytes continue to divide, multiply and grow, they stop secreting collagenous fibers. Instead, they begin to secrete alkaline phosphatase (an enzyme) for mineral deposition for the hardening and calcification of bone to occur. The calcified matrix becomes a scaffold or backbone for the formation of the bone trabecula, which is formed from the secretion of osteoid by mesenchymal progenitor cells. At this point, special macrophages called osteoclasts secrete enzymes that break down spongy bone in order to form the medullary cavity, where bone marrow is found.
Secondary Center of Ossification:
Once infants are born, secondary ossification centers appear in the ends of long bones (at each end, called the epiphysis). The process that occurs here is very similar to what occurs in the primary ossification center. A plate of cartilage remains in the epiphysis called the epiphyseal plate, from which cartilage continues to form, however, it is replaced by bone. This enables the bones to grow and lengthen through approximately age 20, or until the cartilage is completely replaced with new bone. An epiphyseal line remains that distinguishes between the primary and secondary centers of ossification. The thickness of the bone diameter occurs by bone deposition beneath the periosteum. This process is honed by osteoclasts, which degrade the bone until it reaches the perfect thickness. From this point forward, the degradation from the inside and formation from the outside remain balanced and constant unless there is injury to the bone.
The Five Zones of Endochondrial Ossification:
- Zone of Resting Cartilage (Normal, Hyaline Cartilage)
- Zone of Proliferation (Rapid mitosis of chondrocytes, giving them a column-shaped appearance)
- Zone of Maturation/Hypertrophy (Chondrocyte enlargement and secretion of alkaline phosphatase enzyme)
- Zone of Calcification (Formation of matrix)
- Zone of Ossification (Calcified matrix)
The Mineralized Bone Matrix:
I. Organic (30%)
A. Collagen (90%)
-Tensile strength
-Type 1 Collagen
-Strands of repeating units
-Woven Bone (mechanically weak)
-Lamellar Bone (mechanically strong)
B. Ground Substance: Proteoglycans
-Hyaluronic Acid
-Chondroitin Sulfate
II. Inorganic (70%)-Bone Mineral
A. Hydroxyapatite Crystals, Major Salt
-Compressional strength
B. Calcium Salts
C. Phosphate Salts
D. Trace Minerals
-Magnesium
-Sodium
-Potassium
-Carbonate
Mineralized Bone:
-Stores minerals
-Stores growth factors, including insulin-like growth factors, transforming growth factors, bone proteins
-Stores fatty acids (yellow marrow)
-Acid-base balance due to presence of alkaline salts
-Storage of heavy metals/detoxification
-Calcium balance
A. Collagen (90%)
-Tensile strength
-Type 1 Collagen
-Strands of repeating units
-Woven Bone (mechanically weak)
-Lamellar Bone (mechanically strong)
B. Ground Substance: Proteoglycans
-Hyaluronic Acid
-Chondroitin Sulfate
II. Inorganic (70%)-Bone Mineral
A. Hydroxyapatite Crystals, Major Salt
-Compressional strength
B. Calcium Salts
C. Phosphate Salts
D. Trace Minerals
-Magnesium
-Sodium
-Potassium
-Carbonate
Mineralized Bone:
-Stores minerals
-Stores growth factors, including insulin-like growth factors, transforming growth factors, bone proteins
-Stores fatty acids (yellow marrow)
-Acid-base balance due to presence of alkaline salts
-Storage of heavy metals/detoxification
-Calcium balance
Bone Marrow:
https://youtu.be/zWo9-3GJpr8