Bone & Cartilage Histology
osteocytes
-Active cells enclosed in bone matrix. -Communicate with each other through gap junctions in canaliculi: nutrient transport, mechanosensory. -maintain bone tissue
osteoblasts
-BUILD BONE -Proliferate and secrete osteoid (Type I Collagen). Initiate ECM mineralization by secreting alkaline phosphatase. Become embedded in osteoid as osteocytes, or transform into inactive "bone-lining cells" (in peri/endosteum).
interstitial lamellae
-Bone is constantly remodeled throughout life -between the osteons are remnants of previous concentric lamellae
spongy bone
-Cancellous or trabecular bone -Located internal to compact bone -Appears porous
mature lamellar bone
-Immature woven bone is remodeled into mature lamellar bone: collagen fibers are assembled into parallel sheets ("lamellae") making it mechanically strong -classified as either spongy bone and compact bone.
Elastic Cartilage
-Type II collagen, Elastin ECM -isogenous grouped chondrocyte arrangement -main locations: epiglottis, larynx, pinna of ear, auditory canal and tube -function: shape and elasticity -Appearance: Basophilic matrix, many elastic fibers (weak H&E stain, special dyes used) isogenous groups. Surrounded by a perichondrium
why does blood calcium decreases in bone formation?
-calcium is removed from blood and used in the bone building process thus decreasing it in blood flow
inner chondrogenic perichondrium
-chondrogenic stem cells at edges form this layer
outer Fibrous perichondrium
-contains blood vessels: nutrients diffuse to nearby core of chondrocytes -surrounding mesenchyme stem cells differentiate into fibroblasts and form this outer layer
compact bone
-cortical or lamellar bone -osteon (Haversian system): made of bone lamellae (concentric layers) surrounding a Haversian canal. runs lengthwise in the cortex of a long bone -Volkmann's Canal connecting Osteons, running perpendicular to Haversian Canal, connect to periosteum
endosteum
-covers all inner surfaces of bone and consists of a single layer of osteoprogenitor cells
Fibrocartilage is mainly composed of which of the following types of tissues?
-dense connective tissue and hyaline cartilage
3 types of cartilage
-hyaline (TYPE II ECM) -elastic cartilage (TYPE II, ELASTIN ECM) -fibrocartilage (TYPE I, TYPE II ECM)
why and what causes constant bone remodeling?
-it is a reservoir for calcium -mechanical forces -parathyroid hormone activates osteoclasts -calcitonin activates osteoblasts and inhibits osteoclasts (less sig than PTH)
epiphyseal line
-made of compact bone between the epiphysis and diaphysis -made after individual achieves maximal growth
Immature/Woven Bone Structure:
-product of intramembranous ossification -bony spicules merge to form network of trabeculae -cavity spaces between trabeculae will become occupied by bone marrow cells that arrive with blood vessels -Found predominantly in fetal tissues and arises in bone repair (although it is quickly remodeled).
osteoclasts activity mechanism
-release of hydrolytic enzymes which digests matrix -HCl, low pH decalcifies matrix
epiphyseal growth plate (cartilage)
-replaced by bone except at the articular surface. Cartilage of the epiphyseal growth plate is responsible for maintaining the longitudinal growth of long bones.
function of bone
-shape and support for organs and muscles -allows for locomotion and movement -houses bone marrow -storage for minerals in matric (Ca, Phosphate) -dynamic vascular tissue and capable of repairing self
articular cartilage
-specialized hyaline cartilage -Forms contact surface between bones, shock absorber. -Has NO PERICHONDRIUM (no capillaries for nutrients)~ Is maintained by synovial fluid (providing it with nutrients) -Collagen fibers run perpendicular to the surface to form a wear-resistant surface
what happens to cartilage in endochondral ossification?
-the oxygen/nutrient supply is blocked at the diaphysis leading to hypoxia of chondrocytes --> hypertrophy --> calcification of cartilage --> death and replacement by new bone tissue
Fibrocartilage
-type I, Type II collagen ECM -parallel row chondrocyte arrangement -Appearance: Chondrocytes are arranged in columnar rows between alternating layers of hyaline cartilage and dense type I collagen fibers oriented in the direction of stress, elongated fibroblasts. NOT surrounded by a perichondrium -main locations: articular disks, intervertebral disks, pubic symphysis, insertion of tendons -function: resistance to compression, cushioning, tensile strength
Hyaline cartilage
-type II ECM -isogenous grouped chondrocyte arrangement -main locations: Trachea, bronchi, nose, ventral ends of ribs, articular ends, and epiphyseal plates of long bones -function: Shape and flexibility, models for bone growth in fetus, smooth surface for joint movement -appearance: basophilic (sulphated GAGs)
Remodelling of compact bone
1. osteoclasts build tunnel in existing bone tissue 2. osteoblasts follow in and start secreting osteoid 3. osteoblasts surrounded become osteocytes being arranged in a concentric lamellae around the Haversian canal 4. new osteon is formed
Identify cell labelled by the letter A in the image.
A. osteoclasts
Function of Cartilage
Both tough and flexible tissue: Shock-absorber in joints -Provides flexibility to certain parts -Imparts strength at sites of high stress -No blood vessels or nerves, nutrients and waste are managed by diffusion through highly specialized extracellular matrix -Limited ability to repair damaged tissue. -Involved in bone formation
True or false, cartilage tissue does contain blood vessels:
FALSE
what bones undergo intramembranous ossification?
Flat Bones: skull, maxilla, mandible, pelvis, clavicle. Subperiosteal surface of long bones.
osteoprogenitor cells
Flat-shaped stem cells derived from mesenchyme, located in endosteum and osteogenic layer of periosteum (outer surfaces of bone). Proliferate and differentiate into osteoblasts.
osteoclasts
Formed by fusion of blood monocytes. Large, multinucleated and eosinophilic "macrophage-like" cell (migratory cell). Resorbs bone and mineralized cartilage. -very large cell with various nuclei -"chews away" bone (resorbs)
bone fracture repair
Fracture repair in the adult bears close resemblance to fetal skeletal tissue development: Involves the formation of a cartilaginous callus (Soft Callus) that is replaced by bone (Hard Callus)
Which of the following components of cartilage tissue is primarily responsible for its intense ECM basophilic appearance?
Glycosaminoglycans
What is at the center of an osteon?
Haversian canal
what bones are formed through endochondral ossification?
Length of long bones, vertebrae, pelvis, and base of skull
periosteum
Periosteum covers the outer bone surface: •Outer fibrous periosteum consisting of dense irregular CT, houses blood vessels •Inner osteogenic periosteum consisting of osteoprogenitor (stem) cells, osteoblasts, and osteoclasts
primary and secondary centers of ossification
Primary is in the center, secondary is at the ends.
endochondral ossification
Process of transforming cartilage into bone
True or false, cartilage tissue does not contain nerves:
TRUE
bone lamellae
The osteocytes are arranged in concentric rings of bone matrix (little plates), and their processes run in interconnecting canaliculi. The central Haversian canal, and horizontal canals (perforating/Volkmann's) canals contain blood vessels and nerves from the periosteum.
isogenous groups of chondrocytes
Up to 8 cells in lacuna, all originate from one chondrocyte undergoing mitosis -sit in lacunae
canaliculus
Very small tube or channel, such as is found between lacunae (connecting them together) in compact bone.
During which period of life does the cartilage at the epiphyseal growth plate become fully replaced by bone?
adolescence
Which of the following structures are located in the fibrous perichondrium?
blood vessels that supply nutrients to the cartilage
What are spongy bone cavities filled with?
bone marrow
What process signals osteoclasts to begin removal of cartilage?
calcification of cartilage matrix
Which of the following statements accurately describes endochondral ossification?
cartilage dies and is replaced by bone
Which of the following cells secretes the largest amounts of ECM components in cartilage tissue?
chondroblasts
When chondroblasts become surrounded by the matrix they secreted they are called:
chondrocytes
Which of the following regions contains differentiated chondrocytes that undergo mitosis during interstitial growth of cartilage?
chondrocytes surrounded by cartilage matrix
Which type of bone is mechanically strongest?
compact bone
Which of the following best describes the net effect of PTH on bone density?
decreases bone calcium levels
Which of the following best describes the approximate location of the primary center of ossification?
diaphysis
anatomy of a bone
diaphysis, epiphyses, metaphysis, articular cartilage, periosteum, endosteum
The inner surface of haversian canals within osteons are lined by:
endosteum
The surfaces of spongy bone facing the bone marrow are lined by:
endosteum
Where are osteoprogenitors located in mature bone tissue?
endosteum
Which of the following best describes the approximate location of the secondary centers of ossification?
epiphysis
Spongy bone can only be formed via intramembranous ossification, true or false?
false
Spongy bone does not have lamellae, true or false?
false
True or false, as long bones elongate the epiphyseal growth plate gets progressively thicker and longer?
false
Which of the following cells are found predominantly in the outer fibrous layer of the perichondrium?
fibroblasts
Which of the following types of cartilage lacks a perichondrium?
fibrocartilage
Which of the following best represents the mechanism used by osteocytes to communicate?
gap junctions between cells
Which of the following components of cartilage tissue is directly responsible for attracting high amounts of water to the ECM?
glycosaminoglycans
Articular cartilage is a specialized type of which of the following cartilage types?
hyaline cartilage
Which of the following substances represents the main inorganic component in the ECM of bone tissue? Note: Inorganic means it is NOT secreted by bone cells.
hydroxyapatite
What triggers chondrocytes in the cartilage template to become hypertrophic?
hypoxia due to blocked nutrient supply
Which of the following best describes the effect of PTH on blood calcium levels?
increases blood calcium levels
osteoarthritis
inflammation of the bone and joint
Which of the following regions contains stem cells that differentiate into chondroblasts during appositional growth of cartilage?
inner chondrogenic perichondrium
Which of the following regions contains chondroprogenitors?
inner perichondrium
Which type of cartilage growth is directly responsible for the formation of isogenous groups of chondrocytes?
interstitial growth
Once chondrocytes are surrounded by cartilage matrix they occupy cavities known as:
lacunae
What types of bones are formed via endochondral ossification?
long bones such as femur
Which of the following cells is the embryological precursor of osteoclasts?
monocytes
Which of the following structures is found travelling in haversian canals?
nerves and blood vessels
Does intramembranous ossification use a cartilage template to guide the formation of new bone tissue?
no
Can you differentiate elastic cartilage from hyaline cartilage using only H&E stained slides?
no (elastin doesn't stain H&E)
Identify the cell labelled A in the image.
osteoblasts
Which of the following cells secrete alkaline phosphatase?
osteoblasts
Which of the following cells secrete osteoid?
osteoblasts
Which cells secrete hydrolytic enzymes?
osteoclasts
Which of the following structures is found within the lacunae of osteons, indicated by the blue arrow in the image?
osteocyte
Which of the following is a common name used to describe type I collagen (organic ECM) before it is mineralized?
osteoid
A haversian system is also known as?
osteon
Which of the following best represents the lineage of bone cells, from the earliest stem cell to the most differentiated cell?
osteoprogenitors --> osteoblasts --> osteocytes
Low levels of calcium would trigger release of which of the following hormones?
parathyroid hormone
What forms the bony collar?
perichondrium is replaced by periosteum
The outer surface of compact bone are lined by:
periosteum
Which of the following best describes the histological appearance of osteoblasts?
simple cuboidal basophilic cells
Which of the following best describes the overall effect of PTH on osteoclast activity?
stimulates osteoclast activity
What is the primary source of nutrients for chondrocytes in articular cartilage?
synovial fluid
True or false, bone tissue is highly innervated:
true
true or false, bone tissue is highly vascularized:
true
Which of the following types of fibers is predominantly found in the ECM of bone tissue?
type I collagen
Which of the following types of fibers is predominantly found in the ECM of hyaline cartilage?
type II collagen
Which of the following types of fibers is predominantly found in the ECM of all cartilage tissue?
type II collagen fibers
Can you differentiate fibrocartilage from hyaline cartilage using only H&E stained slides?
yes
Does the bony collar extend from the diaphysis (surrounding the primary center of ossification) towards the epiphysis?
yes
Which zone contains living cartilage cells experiencing hypoxia due to their nutrient supply being blocked by the bony collar?
zone of hypertrophy
Which zone consist of cartilage being resorbed by osteoclasts?
zone of ossification
Which zone contains cells undergoing high rates of mitosis?
zone of proliferation
Which zone contains chondrocytes aligned into rows?
zone of proliferation
Which zone contains chondrocytes arranged into typical isogenous groups?
zone of reserve
Which zone of the epiphyseal growth plate is further away from the primary center of ossification?
zone of reserve
steps of endochondral ossification
1. Hyaline cartilage template surrounded by perichondrium (except at articular surfaces) 2. Perichondrium of diaphysis becomes periosteum, and a bone collar is formed impeding nutrient diffusion to cartilage. 3. Chondrocytes become hypertrophic, which mineralize (calcify) their matrix and then undergo apoptosis. 4. Angiogenic factors released by chondrocytes due to localized hypoxia recruit blood vessels. 5. Blood vessel buds through the bone collar and delivers osteoprogenitors, osteoclasts and hemopoietic stem cells. Osteoclasts remove the calcified cartilage matrix. When the osteoprogenitor cells come in apposition to the remaining calcified cartilage, they become osteoblasts and begin to lay down osteoid. 6. Primary center of ossification is formed in the diaphysis. 7. Bone collar extends, and cartilage at both ends of diaphysis continues to calcify. 8. Osteoclasts continue removing the newly calcified cartilage matrix as osteoblasts continue releasing osteoid: woven bone is formed and bone remodeling begins. 9. Bone collar extends to both ends and secondary ossification occurs in either epiphysis.
intramembranous ossification steps
1. Mesenchymal cells aggregate in the area where a bone will form. 2. Differentiate into osteoprogenitors and osteoblasts. Osteoblasts begin to secrete osteoid (non-mineralized ECM), and become separated, but remain attached through thin cytoplasmic processes. 3. Osteoid becomes mineralized, and entrapped osteoblasts become osteocytes. Tissue becomes vascularized bringing osteoclasts to remodel this primary bone.
cartilage growth layers
1. Outer Fibrous layer of Perichondrium 2. Inner Chondrogenic layer of Perichondrium 3. Appositional Growth: Chondroprogenitors from [2] differentiate into chondroblasts which divide and add cells, and ECM, to the edges of the cartilage 4. Interstitial growth: Chondrocytes in the core divide (mitosis) but are prevented from separating as the ECM solidifies forming isogenous groups (cell nests); important during bone growth
Cartilage Formation
1. embryonic stem cells (mesenchyme) 2. migration of mesenchymal cells and commitment to the chondrocyte lineage: chondroprogenitors 3. Condensation and proliferation 4. Differentiation of cells in the center into chondroblasts which secrete ECM--> cells surrounded by a matrix --> cells separate and become chondrocytes
Composition of Collagen ECM
1. hydrated proteoglycans (GAGs): allows diffusion, resilient, shock absorbing. Negatively charged = BASOPHILIC (PURPLE HEMATOXYLIN) 2. Type II Collagen: makes cartilage solid and firm (some specialized cartilages have type I and elastin~function) 3. cells 4. water (which is attracted to negative GAG)
describe the relationship between calcium and PTH
1. low blood calcium 2. PTH released 3. osteoclasts activated 4. calcium released as bone matrix is "chewed" down 5. blood calcium increases
osteoporosis
A condition in which the body's bones become weak and break easily.
