Cartilage & Bone Flashcards
Differences between cartilage and CT
Cartilage, compared to CT, is harder due to chondromucoid in the matrix, avascular, and cells (chondrocytes) lie in lacuna.
Cartilage Types
Hyaline, elastic, and fibrous
Hyaline
Most common. Skeletal model for most bones.
In adults replaced by bone except for articular surfaces, costal cartilage, larynx, nose, trachea and bronchi.
Does not calcify under normal conditions in the adult.
Chondrocytes are in “cell nests”
Matrix has collagen fibers (Type II) that cannot be seen
- White glassy appearance covered by perichondrium
Hyaline in depth
covered by perichondrium that consists of an outer fibrous layer and an inner cellular layer called chondrogenic that contains stem cells, which, in the absence of blood vessels (in an environment of low oxygen tension), differentiate into chondroblasts. The latter cells synthesize cartilage matrix that is rich in chondromucoid (proteoglycan and glycoprotein), in which are embedded type II collagen fibrils (also synthesized by chondroblasts). The chondroblast entrapped in its own matrix is called chondrocyte, which lies in a space (just like the space surrounding a candle in a cake) called lacuna. Nests of chondrocytes (all originate by division of the same chondrocyte) surrounded by large lacunae and dense matrix called territorial matrix are the hallmarks of hyaline cartilage in H&E-stained section. Type II collagen fibrils are not visible in these sections because they have the same refractive index as that of the matrix in which they are embedded. Examples of hyaline cartilage include most long bones in the fetus and young animals until they are ossified, articular cartilage in adult bones (note, articular cartilage is not covered by perichondrium), and walls of respiratory passages (nose, trachea, larynx, bronchus).
Elastic
Like hyaline cartilage, except the matrix contains elastic fibers that impart flexibility and yellowish color to it; examples: pinna of the ear, auditory tube, epiglottis.
Has elastic fibers, perichondrium highly flexible.
Yellow in color and more resilient
Found in external ear, auditory canal, laryngeal cartilages.
Chondrocytes are arranged in cell nests.
- Elastic fibers are more apparent in the matrix. Identical to hyaline cartilage except for the presence of elastic fibers.
Fibrous cartilage
Essentially like dense connective tissue because it contains type I collagen fibers, which are densely packed and longitudinally arranged as in the tendon, except that chondrocytes lies in lacunae and are arranged in rows; examples: intervertebral disks, pubic symphysis, and insertions of some tendons.
Irregular layers of collagenous fibers and very little matrix.
Acts as an intermediate between tendon and cartilage.
Found in Intervertebral discs acts a mechanical shock absorbers
Other examples: ligaments, tendon bone junctions, cardiac skeleton, menisci.
Growth of cartilage
Interstitial : Results from cell division of preexisting chondrocytes and occurs in early stages of cartilage formation, articular cartilage, and epiphyseal plates of long bones.
Appositional: results from differentiation of chondrogenic cells in the perichondrium to form new chondroblasts, which lay down a new layer of cartilage matrix at the periphery.
Interstitial Growth of Cartilage
New tissue is created within the matrix of existing cartilage
- Cartilage grows from within
Chondrocytes in matrix divide (mitotically), lacunae may possess two, four, eight daughter chondrocytes - isogenous or nest cells.
Increase in matrix volume.
Occurs in existing cartilage and results in growth of cartilage
Appositional Growth of Cartilage
New tissue is laid down on the outside of the existing structure
- Cartilage grows from the outside
Mesenchymal cells are found in special aggregates called chondrogenic centers.
Chondroblasts form from these centers. Chondroblasts differentiate into chondrocytes
Chondrocytes deposit matrix (territorial and inter territorial).
Includes new matrix formation on existing cartilage.
Degeneration of Cartilage
Chondrocytes hypertrophy and matrix becomes calcified; normal process in endochondral ossification (replacement of cartilage by bone), but it can abnormally occur in an environment of hypercalcemia. Regeneration: very poor, except in young animals, results from the activity of chondrogenic cells in the perichondrium.
General Function of Bone
Support, protection, movement, electrolyte balance, and blood formation. Differences between cartilage and bone: Compared to cartilage, bone is harder due to calcification, vascular, canalicular, and grows only by appositional mechanism.
Macroscopic structure of a long bone
Shaft, diaphysis; extremities, proximal and distal epiphyses; epiphyseal plate or growth plate, located between epiphysis and diaphysis; metaphysis, a small actively growing area between epiphysis and diaphysis; marrow cavity, a central space that contains bone marrow cells; articular cartilage, the joint surface between two bones; periosteum, an outer covering that consists of an outer fibrous layer and an inner osteogenic layer of stem cells; and endosteum, a layer of bone cells that lines bone marrow, as well as all marrow spaces within a bone.
Epiphyses
Trabeculae of spongy bone
Macroscopic structure of a cranial bone
Consists of three parts: outer plate( compact), middle diploe (spongy bone), and inner plate (compact)
the outer surfaces of both plates are covered with periosteum, although the periosteum of the inner plate is fused with the dura-mater of the cranial cavity
Inner periosteum is thicker
the inner surfaces of both plates face marrow spaces of the diploe and thus are lined by endosteum.
Microscopic structure of a typical compact bone
Consists of ground substance containing inorganic calcium and phosphate salts;
four cell types: osteoprogenitor, osteoblasts, osteocytes, and osteoclasts;
Type I collagen embedded in the matrix, which is arranged in the form of lamellae, which, based upon their location, are classified as: outer circumferential, under the periosteum; inner circumferential, under the endosteum; concentric, a part of osteons (also called Haversian systems); and interstitial, between osteons.
Structure of an Osteon (Haversian system) in depth
These are cylinders that run parallel to the diaphysis, just like pillars in a house; have a central space called Haversian canal (also called central canal) that contains blood vessels and is surrounded by 4-20 circumferential lamellae, which contains bone cells, osteocytes; adjacent Haversian canals are connected with each other by Volkmann’s canals, which also connect to the periosteum and endosteum and thus bring blood vessels and nerves to the bone. Three types of osteons, depending upon the type of bone cells that line the Haversian canal: mature osteons are lined by osteoprogenitor cells, forming osteons are lined by osteoblasts, and resorption osteons (also called resorption cavities) are lined by osteoclasts.
Bone cells
Osteoprogenitor
Osteoblasts
Osteocytes
Osteoclasts
Osteoprogenitor cells are in the periosteum, endosteum, osteons, and perforating canals
Osteoblasts are on the bone surfaces and synthesize osteoid matrix
Osteocytes are mature osteoblasts, are branched, are in lacunae, and use canaliculi for communication and exchange
Osteocytes maintain homeostasis of bone and blood concentrations of calcium and phosphate
Osteoclasts are multinucleated cells responsible for resorption, remodeling, and bone repair
Osteoclasts belong to the mononuclear macrophage-monocyte cell line and are found in enzyme-eroded depressions (Howship’s lacunae)
Osteoprogenitor
stem cells, found in the endosteum, inner layer of periosteum, and Haversian canal; they multiply and some of them become osteoblasts at sites where new bone must be synthesized; may become chondroblasts in an environment of low oxygen tension.
Osteoblasts
Bone forming cells, found in the same locations where osteoprogenitor cells are; synthesize osteoid (uncalcified matrix made of type I collagen and osseomucoid) that is subsequently calcified; stress and fractures stimulate osteoprogenitor cells to differentiate into osteoblasts, which build new bone.
Produce Bone Matrix
Osteocytes
mature osteoblasts surrounded by their own matrix, therefore, osteocytes do not line the bone surface; like chondrocytes, they also lie in tiny cavities called lacunae, which are interconnected by slender channels called canaliculi; thus neighboring osteocytes in a mature osteon connect with each other via cytoplasmic processes that lie in canaliculi; this canalicular arrangement allows osteocytes to get nutrients from distant vessels (for example, vessels of the central canal), which cannot penetrate the calcified matrix.
Lie in lacunae
Functions of osteocytes:
maintenance of bone density and blood concentration of calcium and phosphate; calcium is continuously renewed by osteocytes (osteolytic homeostasis, do not confuse this with bone resorption that occurs at the level of osteoclasts).
Osteoclasts
bone eating cells, found on the bone surface (periosteum, endosteum, central canal), multinucleated cells that are derived from monocytes, often lie in pits called Howship’s lacunae, the side of osteoclasts facing the bone surface has ruffled border that is composed of finger-like projections of the cell membrane extending into the Howship’s lacunae, thus increasing the surface area of bone resorption.
reabsorb bone
Spongy bone
is composed of interconnected trabeculae, which surround cavities filled with bone marrow. Each trabecula is lined by endosteum containing osteoprogenitor cells, osteoblasts, and osteoclasts (remember all marrow spaces are lined by endosteum); the bony matrix contains osteocytes and may or may not form osteons (note, only thicker trabeculae have osteons).
Immature bone or woven bone
it is the primary bone formed for the first time, regardless of when and where; contains more osteocytes and irregularly arranged collagen bundles; is remodeled and replaced by mature bone with time.
First bone tissue to appear in embryonic development and in fracture repair.
Random disposition of fine collagen fibers ( woven bone)
Temporary, replace secondary by bone(adults) except places; tooth sockets, insertions of some tendons.
Lower mineral content (easily penetrated by x-rays)
Higher proportion of osteocytes than in secondary bones.
Bone Marrow
is a general term for soft tissue present in the marrow cavity of a long bone and in the spaces of a spongy bone; two types: red and yellow; red: marrow cavity of all bones in young animals are filled with red marrow (myeloid tissue, also called hematopoietic tissue); in adults, however, red marrow turns into fatty yellow bone marrow, except in the skull, vertebrae, sternum, pelvic girdle.
Ossification
The bone formation is called ossification, which is two types: intramembranous and endochondral.
