Bone Tissue

Bones

• These are the rigid organs that form the framework of the body

Joints

• Points where bones articulate and allow movement

Cartilage

• Provides cushioning and reduces friction between bones

Ligaments

• Connect bones to cach other and provide stability to joints

Bond Structure and Composition

• Bones are complex structures composed of various tissues, cells, and minerals.

• Bones are considered organs because they consist of different types of tissues.

• In bones we find bone tissue, adipose tissue, and blood vessels.

• Bone tissue consists of four types of cells: osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts.

Osteoprogenitor

• Origin: Mesenchymal skeleton cells.

• Function: Osteoprogenitor cells differentiate into cells that build bone (osteoblasts)

Osteoblasts

• Origin: Osteoprogenitor cells.

• Function: Osteoblasts produce extracellular matrix and deposit minerals like calcium and phosphate. As the deposition of minerals happens, the extracellular matrix becomes harder and harder, trapping the osteoblast inside. The trapped osteoblast matures into an osteocyte!

Osteocytes

• Origin: Osteoblasts.

• Function: Osteocytes are mature bone cells and the main cells in bone tissue.

Osteoclasts

Origin: Monocytes. Osteoclasts are multinucleated cells derived from the junction of several monocytes.

Function: Osteoclasts resorb bone tissue by breaking down bone and releasing calcium and other minerals into the bloodstream.

Formation of Bone Tissue Cells

• Osteoprogenitor cells divide and differentiate into osteoblasts, and then osteoblasts become osteocytes.

• Osteoprogenitor cells come from the embryonic skeleton, specifically from the mesenchymal skeleton.

• Mesenchymal skeleton is present at week 6 of embryonic development.

Mesenchyme

• Originates from the mesoderm, one of the three embryonic layers.

•  Gives rise to the majority of the body's connective tissue, including bones.

• Mesenchymal cells secrete extracellular matrix, leading to the formation of spaces between the cells.

• Blood does not originate

• Blood originates from hematopoietic stem cells in the bone marrow.

• Blood is an exception to the rule that most connective tissues derive from mesenchyme.

Bone Formation (Ossification)

• Development: Initial bone formation during embryonic development.

• Growth: Growth of existing bones during childhood and adolescence.

• Remodeling throughout life: Ongoing process of replacing old bone tissue with new bone tissue throughout life.

• Fracture Repair: Repair of bone fractures through the deposition of new bone tissue at the fracture site.

Intramembranous Ossification

• Mesenchymal skeleton cells arrange in sheets (membrane-like structures) with a space in between.

• Step 1. Development of the ossification center

• Step 2. Calcification

• Step 3. Formation of trabeculae

• Step 4. Development of the Periosteum

Step 1. Development of the ossification center

• Mesenchymal skeleton cells receive chemical messages to divide and differentiate into osteoprogenitor cells.

• Osteoprogenitor cells then differentiate into osteoblasts, cluster in the center.

Step 2. Calcification

• Extracellular matrix secreted by the osteoblasts undergoes calcification - calcium and other mineral salts are deposited forming the mineralized bone matrix (hard).

• Osteoblasts become trapped in the calcified extracellular matrix and transform into osteocytes.

Step 3. Formation of trabeculae

• Calcified extracellular matrix develops into bone trabeculae, which then fuse to form the spongy bone.

Step 4. Development of the Periosteum

• Mesenchymal skeleton cells at the periphery condense to form the fibrous layer and the osteogenic layer, which together form the Periosteum.

• The periosteum plays a vital role in bone growth, repair, and nutrient supply.

• It serves as an attachment site for tendons and ligaments, facilitating movement and stability.

Periosteum Layers

• Fibrous layer: Closer to the bone surface side; Contains cells that give rise to fibroblasts, which produce fibers on the outside of the bone.

• Osteogenic layer (also called Osteoprogenitor Layer): Closer to the spongy bone area side; Contains osteoprogenitor cells, capable of differentiating into osteoblasts; Osteoblasts secrete the extracellular matrix that gets calcified and forms the compact bone (with no spaces in the extracellular matrix) underneath the periosteum.

By the end of the intramembranous ossification, we have:

• Fibrous Layer

• Osteogenic layer

• Compact bone

• Sponge bone with Red Bone Marrow

• Compact bone

• Osteogenic layer

• Fibrous Layer

Intramembranous ossification gives rise to:

• All flat bones in our skull

• Most of the skull bones as well as the "flat parts" of the temporal and sphenoid bones are formed via intramembranous ossification

• The clavicle is a long bone formed by both intramembranous and endochondral ossification.

• The lateral end of the clavicle is formed by intramembranous ossification, the medial end by endochondral ossification.

Endochondral Ossification

• Endochondral ossification takes longer than intramembranous ossification.

• Occurs in all bones below our skull, with the exception of the clavicle, which undergoes both intramembranous (lateral end) and endochondral (medial end) ossifications.

• Bones develop by replacing hyaline cartilage present in a cartilage model that was produced by the mesenchymal cells.

Steps of Endochondral Ossification

• Step 1. Development of Cartilage Model

• Step 2. Growth of Cartilage Model

• Step 3. Development of Primary Ossification Center

• Step 4. Development of the Medullary Cavity

• Step 5. Development of Secondary Ossification Centers

• Step 6. Formation of Articular Cartilage and Epiphyseal Plate

Step 1. Development of Cartilage Model

• Mesenchymal cells receive chemical signals to organize themselves into a bone shape and, while doing that, to differentiate into chondroblasts

• Chondroblasts develop a cartilage model made of hyaline cartilage

• Some mesenchymal cells stay closer to the periphery of the cartilage model and form the perichondrium

• The perichondrium is at the periphery of the cartilage model.

Step 2. Growth of Cartilage Model

• Cartilage model grows in

• Length: Chondroblasts within the hyaline cartilage model secrete extracellular matrix.

• As more and more extracellular matrix is produced, the cartilage model grows in length.

• With calcification of the extracellular matrix, the chondroblasts become trapped and transform into chondrocytes.

• Width: Mesenchymal cells present in the perichondrium differentiate into new chondroblasts.

• Since the perichondrium is at the periphery of the cartilage model, more and more chondroblasts are deposited at the periphery of the cartilage model.

• These chondroblasts secrete extracellular matrix that is deposited at the periphery of the cartilage model, consequently making the cartilage model grow in thickness, in width.

Step 3. Development of Primary Ossification Center

• Chondrocytes in the center of the cartilage model die, leaving empty spaces. (beginning of primary ossification center)

• Blood vessels penetrate through the cartilage model, and invade the empty spaces left behind by the dead chondrocytes.

• The invasion of blood vessels signals the mesenchymal cells at the perichondrium to differentiate into osteoblasts and follow the pathway of the blood vessels to reach the center of the cartilage model.

• As expected, the osteoblasts, will begin to build bone from the center, justifying the name of this step as "Development of Primary Ossification Center"!

• As soon as the mesenchymal cells of the perichondrium start to differentiate into osteoblasts, the perichondrium transitions into what is called periosteum

• Osteoblasts at the center of the cartilage model start to deposit extracellular matrix on top of the already present calcified extracellular matrix, leading to the formation of bone with empty spaces - spongy bone!

• The Primary Ossification Center keeps increasing in size, spreading through what will become the shaft of the future bone.

• Keep in mind that even though all these changes are happening in the shaft (diaphysis) of the future bone, cartilage continues to grow at the ends (epiphyses) of the future bone. Hence, the cartilage model keeps growing in length as spongy bone is being formed in the "future shaft".

Step 4. Development of the Medullary Cavity

• Osteoclasts, the cells that crush up the bone.

• When reaching the Primary Ossification Center, osteoclasts crush up the spongy bone matrix, enlarging the spaces that eventually combine to become the medullary cavity.

• The great majority of the spongy bone in the "shaff" is destroyed and becomes the medullary cavity.

• Just a little spongy bone is left at the edge, right underneath of what will be compact bone.

Step 5. Development of Secondary Ossification Centers

• Blood vessels pierce through each future bone epiphyses, leading to the formation of ossification centers.

• Since these ossifications happened after the primary center, they were named Secondary Ossification Centers.

• Osteoblasts derived from periosteum follow the pathway of the blood vessels, reach the center of the secondary ossification centers within the future bone epiphyses, and form spongy bone.

• This spongy bone remains intact, it is not destroyed by osteoclasts.

Step 6. Formation of Articular Cartilage and Epiphyseal Plate

• By the time the fetal bone is fully formed, cartilage remains between the diaphysis and each epiphysis as the growth plate, and also at the joint surface of the epiphyses as articular cartilage.

• Growth plate, also known as the epiphyseal plate, consists of hyaline cartilage and is located at the metaphysis

• The growth plate, as the name implies, is the place where the bone will grow in length.

Abnormalities in endochondral ossification can cause:

• skeletal growth disorders such as dwarfism or gigantism.

• Disruptions in the growth plate can result in uneven limb length.

Bone Structure and Growth

• Each bone has two epiphyses, two metaphyses and a single diaphysis.

• Epiphyses are the most proximal and the most distal parts of a bone.

• Diaphysis is the shaft of a bone.

• Metaphysis is the area between an epiphysis and the diaphysis.

• Metaphysis is the place where we find the growth plate, also known as epiphyscal plate.

Growth Plate

• The growth plate allows long bones to grow in length:

• Growth plate consists of hyaline cartilage.

• It is a plate found in the metaphysis, the region between the epiphysis and diaphysis.

• This hyaline cartilage plate is made of several layers of chondrocytes.

• Chondrocytes close to the epiphysis, form more cartilage.

• Chondrocytes close to the diaphysis (shaft) will eventually give rise to bone as the extracellular matrix produced by those chondrocytes gets calcified; chondrocytes die, disintegrate and osteoblasts take over, deposit more extracellular matrix that gets calcified, they get trapped and transform into osteocytes.

• When chondrocytes closer to the epiphysis stop multiplying, the growth plate becomes thinner, thinner, and thinner, until it becomes a line named epiphyseal line.

• The presence of epiphyseal line is evidence of the ending of bone growth.

Bone Tissue (Osseous Tissue) Histology

• Bone tissue is a type of connective fissue with a hard extracellular matrix containing mineral salts, collagen fibers, and water.

• Approximately 50% of bone tissue consists of crystallized mineral salts, 30% of collagen fibers, and 20% of water.

• There are two main types of bone tissue: compact bone and spongy bone.

Compact Bone

• Dense bone tissue found underneath the periosteum, forming the outer layer of bones

• Compact bone is not solid and consists of several osteons.

• Osteons are only present in compact bone.

• Osteons are the structural units of compact bone, consisting of concentric lamellac (layers of calcified extracellular matrix containing collagen fibers) surrounding a central canal.

• The Central canal (also known as Haversian Canal) is at the center of the osteon and allows the passage of blood vessels and nerves.

• These blood vessels deliver nutrients and remove waste from each osteon.

• Lacunae is a "little lake" that houses an osteocyte. (plural: lacunac)

• Canaliculi are "little canals" that connect lacunae with each other and with the central canal.

• Canaliculi allow communication and nutrient exchange between osteocytes.

• Osteocytes within lacunae extend into the canaliculi and communicate with other osteocytes via gap junctions.

Spongy Bone

• Spongy bone contains empty spaces, making it lighter than compact bone.

• Trabeculae are only present in spongy bone.

• Trabeculac contain trabecular lamellac (similar to concentric lamellae in osteon).

• Lacunae (similar to lacunae in osteon) are "little lakes" that house osteocytes.

• Canaliculi provide passageways for nutrients and oxygen between osteocytes.

• Passing through the empty spaces between the thin columns of bone (trabeculae), we find blood vessels. These blood vessels are the famous red bone marrow.

• Red bone marrow is the site of red and white blood cell production (hematopoiesis) and platelet formation in adults.

• Red bone marrow is found in flat bones, and within the medullary cavity in the shaft of long bones, as well as the epiphysis of long bones.

• The red bone marrow found within the medullary cavity is later substituted by yellow bone marrow (mostly adipose cells).

• Yellow bone marrow functions as a site for fat storage.

Bone Remodeling

• Ongoing "remodeling", renewal of old bone with new bone tissue.

• Two main processes: Bone Resorption and Bone Deposition.

• Compact bone: old osteons are replaced by new osteons.

• Spongy bone: old trabeculae are replaced by new trabeculae.

• Bone remodeling occurs throughout lifetime.

Bone Resorption

• Old bone tissue is removed.

• Osteoclasts crush old bone tissue, causing bone REShaping (RESorption).

• Resorption is essential for removing damaged or old bone and facilitating bone turnover

Bone Deposition

• New bone tissue is deposited.

• Osteoblasts, the cells born to build bone, perform bone Deposition (they deposit new bone tissue.

• Deposition restores bone integrity, repairs microdamage, and maintains bone strength.

Function of Bones

• Support

• Protection

• Movement

• Mineral Storage

• Blood Cell Production

• Storage

Support

• Skeleton provides structural framework for the body.

• Supports softer body tissues.

Protection

• Bones protect important organs.

• Cranial bones protect the brain, vertebral column protects spinal cord, ribs protect heart, lungs, and major blood vessels.

Movement

• Skeletal muscles attach to the skeletal system and cause movement when they contract and relax.

• Muscles attach to bones via tendon.

Mineral Storage

• Bones contain crystallized mineral salts, mainly calcium and phosphate.

• Osteoclasts crush the bone and release stored minerals back into the bloodstream.

• Minerals stored in bones play vital roles in metabolic processes and maintaining mineral balance in the body.

Blood Cell Production

• Red bone marrow in spongy bone is the site of blood cell production (hematopoiesis).

• In adulthood, red bone marrow is replaced by yellow bone marrow in most long bones.

Storage

• Yellow bone marrow, found in the medullary cavity, serves as a storage site for fat.

Bone Classification

• Bones exhibit diverse shapes and structures, reflecting their varied functions and locations in the body

• Flat Bones

• Irregular Bones

• Long Bones

• Short Bones

• Sesamoid Bones

• Pneumatized Bones

• Sutural Bones

Flat Bones

• Thin and flat bones with a broad surface area.

• Examples: frontal bone, parietal bone, occipital bone, os coxa, scapula, sternum, ribs

Irregular Bones

• Have irregular, asymmetrical shapes.

• Examples: temporal bone, sphenoid bone, vertebrac (cervical, thoracic, lumbar), os coxa, calcaneus.

Long Bones

Longer than wider

- Examples: humerus, radius, ulna, femur, fibula, tibia, clavicle, metatarsals, metacarpals, phalanges.

Short Bones

• Similar length, width, and depth, giving them a "cuboidal" shape.

• Examples: all carpal bones, all tarsal bones.

Sesamoid Bones

• Develop within tendons.

• The pisiform, a carpal bone, develops within the tendon of the flexor carpi ulnaris muscle.

• Hence, besides belonging to the short bone category like all carpal bones, the pisiform is also classified as a sesamoid bone.

• The patella is the largest sesamoid bone in our body. The patella develops within the quadriceps femoris tendon.

Pneumatized Bones

• Bones with empty spaces filled with air.

• Example: Ethmoid bone (contains ethmoid air cells), frontal bone (contains frontal sinus).

Structural Bones

Small bones found within sutures, the joints between cranial bones.

- Located between specific bones, such as the frontal and parietal bones.

Bone Surface Markings (Bone Features)

• Depressions and Openings

• As the names imply, they are either depressions or openings in bones.

  • Foramen: a hole in the bone. E.g. supraorbital foramen.

  • Fissure: an elongated hole E.g. Superior orbital fissure.

  • Fossa: an indentation or shallow depression. E.g. Mandibular fossa.

  • Sulcus: a small groove. E.g. Intertubercular sulcus.

  • Meatus: a canal or tube-like passage. E.g. External acoustic meatus.

• Processes

• Processes are bony protrusions, parts of a bone that stick out.

  • Spinous process: a sharp, slender projection. E.g. Spinous process of a vertebra.

  • Tubercle: a small bump. E.g. Greater tubercle of the humerus.

  • Trochanter: a larger bump. E.g. Greater trochanter of the femur.

  • Tuberosity: a rough surface, serving as a muscle attachment site. E.g. Tibial tuberosity.

  • Facet: a flat, smooth surface used in articulation. E.g. Superior articular facet of a vertebra.

  • Condvle: a rounded, smooth surface used in articulation. E.g. Occipital condyles of the occipital bone.

  • Epicondvle: a projection above a condyle, serving as a muscle attachment site. E.g Medial epicondyle of the humerus.

  • Head: a rounded projection with a neck underneath it. E.g. Head of the femur.

  • Crest: a narrow, prominent ridge. E.g. Iliac crest of the llium.

  • Line: a narrow, elongated ridge. E.g. Linea aspera of the femur.