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Cartilage - Biology and Growth

Notes

Cartilage - Biology and Growth

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Cartilage Biology

Key points

  • Cartilage is connective tissue with a semi-solid extracellular matrix that comprises collagen fibers and ground substance, which provides both support and protection for other body tissues.
    • Three types covered here: hyaline, fibrous, and elastic.

Hyaline cartilage

Hyaline Cartilage

  • Most abundant type of cartilage in the body.
  • Hyaline cartilage forms most of the fetal skeleton and is important in endochondral bone growth until the end of adolescence.
  • In the adult, it persists in the nose, trachea, and larynx, thorax, and also covers the articular surfaces of long bones (where it has no perichondrium).
  • Degeneration and calcification of hyaline cartilage may either be physiologic or pathologic. It is physiologic in the case of endochondral bone formation but it is pathologic in the case of osteoarthritis, which leads to pain and restricted joint movement. There are other forms of arthritis, such as rheumatoid arthritis, which is an inflammatory form of arthritis in which the immune system aggressively attacks the cartilage, bone, and synovial membranes of the joints
  • Calcification (the processes by which cartilage is replaced by osseous tissue) is common in hyaline cartilage, but very rare in elastic or fibrous cartilages.

Hyaline Cartilage Layers

  • Perichondrium, outer surface
  • Matrix

Perichondrium

  • Comprises inner and outer layers (although in slow-growing or inactive perichondrium, it is not always possible to visually distinguish two separate layers).
  • Inner layer is the chondrogenic (aka cellular) layer; it comprises chondrogenic cells.
  • Outer layer it he fibrous layer; it comprises Type I collagen fibers, blood vessels, which supply nutrients to the cartilage below, and, fibroblasts, which are thought to produce collagen fibers and/or chondroblasts (although intertextual variation exists regarding this point).

Matrix

  • Appears glass-like under the microscope.
  • Is basophilic and stains purple on H & E section
  • Contains territorial matrixes that comprise the proteoglycan-rich, dark staining area around the lacunae.
  • Ground substance of the matrix, which is gel-like, comprises the following components:
    • Proteoglycan aggregates, which are bound to collagen fibrils, and which provide a semi-solid structure to cartilage,
    • Chondronectin, which adheres collagen fibers to chondrocytes, and
    • Intercellular Water, which is 60-80% of net weight of the cartilage (varies by location).
  • Matrix comprises type II collagen fibers, which are invisible in standard histologic preparations.
  • Houses lacunae, which are spaces in the matrix.
  • Inside the lacunae are the chondrocytes, which are mature cartilage cells that generate and maintain the matrix.
  • Chondroblasts are derived from chondrogenic cells, and eventually mature into chondrocytes.
  • Clusters of chondrocytes are called isogenous groups.
  • Interterritorial matrix outside of the isogenic groups; it stains lightly.

Elastic Cartilage

Elastic Cartilage

  • Also known as yellow fibrocartilage.
  • Shares many similarities to hyaline cartilage (for instance, it also has perichondrium and contains invisible type II collagen fibers within the matrix).
  • Helpful distinguishing feature of elastic cartilage are its interwoven elastic fibers, which provide flexibility.
  • Elastic cartilage provides flexibility and resistance to permanent deformation.
  • Present in the external ear, the auditory (Eustachian) tube, and the epiglottis.

Fibrous Cartilage

Fibrous Cartilage

  • Also shares many similarities with hyaline cartilage
  • Two helpful distinguishing features are its lack of perichondrium and its rows of chondrocytes that have type I and type II fibers in between them.
  • The bundles of type 1 and type II collagen fibers provide strength and durability for shock absorption.
  • The matrix is dense and contains relatively little ground substance.
  • The type of joint and person's age determine the relative proportions of these two collagen fiber types within the fibrous cartilage.
  • NO perichondrium is present in fibrous cartilage.
  • Fibrous cartilage is found, most notably, in intervertebral discs, pubic symphysis, menisci of the knee, and tendons where properties of stress resistance are particularly important.

Cartilage Formation and Growth

Two Types

  • Interstitial
  • Appositional

Both types occur simultaneously during early development; the relative rate of each type of growth determines the shape and structure of the cartilage.

Interstitial growth

  • Interstitial growth occurs during early stages of cartilage formation, and in the growth platesnd articular cartilages of growing long bones.
  • Cells divide within matrix to produce daughter cells that secrete their own matrix.

Appositional growth

  • New matrix forms at the periphery of the cartilage.
  • Chondrogenic cells in perichondrium differentiate to become chondroblasts
  • Chondroblasts secrete new matrix, eventually become chondrocytes.
  • Older chondrocytes remain active and maintain matrix.

Clincal Correlation

Hyaline cartilage that has been only nominally damaged can slowly repair itself via appositional growth, but severely damaged cartilage is often replaced by tougher, more fibrous connective tissue.

Hormones that impact the growth of hyaline cartilage

  • Stimulators: thyroxine, testosterone, and somatotropin.
  • Inhibitors: cortisone, hydrocortisone, and estradiol.

Nutritional states that affect bone growth

  • Hypovitaminosis A diminishes the thickness of epiphyseal plates; decrease in growth rate.
  • Hypervitaminosis A accelerates ossification of epiphyseal plates; short stature (dwarfism).
  • Hypovitaminosis C inhibits matrix production and distorts cartilage columns in epiphyseal plates (bones are weak, repair of fractures obstructed; scurvy develops)
  • Hypovitaminosis D inhibits calcification of matrix, causing softening of the bones (osteomalacia); in children, growing bones become bowed (rickets)

Full-Length Text

  • Here we will draw the key histologic features of cartilage and its forms of growth.
  • To begin, start a table.
  • Denote that cartilage is connective tissue with a semi-solid extracellular matrix that comprises collagen fibers and ground substance, which provides both support and protection for other body tissues.
    • We will discuss similarities and differences between the three cartilage types: hyaline, elastic, and fibrous.
  • First, we'll draw hyaline cartilage, which is the most abundant type of cartilage in the body.
  • To begin, draw the following hyaline cartilage layers:
    • The matrix, which appears glass-like under the microscope.
    • Then, draw the perichondrium layer.
  • Indicate that the matrix comprises type II collagen fibers, which are invisible in standard histologic preparations.
  • Next, within the matrix, draw a few lacunae.
  • Inside of them, draw chondrocytes, which are mature cartilage cells that generate and maintain the matrix.
  • Next, label the cluster of chondrocytes as an isogenous group.
  • Denote the following:
    • Hyaline cartilage forms most of the fetal skeleton and is important in endochondral bone growth until the end of adolescence.
    • In the adult, it persists in the nose, trachea, and larynx, thorax, and also covers the articular surfaces of long bones (where it has no perichondrium).
  • Degeneration and calcification of hyaline cartilage may either be physiologic or pathologic.
    • It is physiologic in the case of endochondral bone formation but it is pathologic in the case of osteoarthritis, which leads to pain and restricted joint movement.
    • There are other forms of arthritis, such as rheumatoid arthritis, which is an inflammatory form of arthritis in which the immune system aggressively attacks the cartilage, bone, and synovial membranes of the joints
  • Calcification (the processes by which cartilage is replaced by osseous tissue) is common in hyaline cartilage, but very rare in elastic or fibrous cartilages.
  • Next, draw elastic cartilage (aka yellow fibrocartilage), which shares many similarities to hyaline cartilage (for instance, it also contains invisible type II collagen fibers within the matrix) but denote that a helpful distinguishing feature of elastic cartilage are its interwoven elastic fibers.
  • To illustrate these fibers, draw the matrix, a lacuna, and a chondrocyte.
  • Then, draw the elastic fibers, themselves, as squiggly fibers of elastic proteins, which provide flexibility.
  • Draw the perichondrium overlying the cartilage.
  • Denote that elastic cartilage provides flexibility and resistance to permanent deformation.

Bend and release your ear — feel it pop back into place. Elastic cartilage provides this resistance to deformation.

  • Denote that elastic cartilage is present in the external ear, the auditory (Eustachian) tube, and the epiglottis.
  • Next, draw fibrous cartilage, which also shares many similarities with hyaline cartilage, but denote that two helpful distinguishing features are its lack of perichondrium and its rows of chondrocytes that have type I and type II fibers in between them.
  • The bundles of type 1 and type II collagen fibers provide strength and durability for shock absorption.
    • The matrix is dense and contains relatively little ground substance.
    • The type of joint and person's age determine the relative proportions of these two collagen fiber types within the fibrous cartilage.
  • To illustrate these rows of chondrocytes and collagen fibers, draw a matrix.
    • NO perichondrium is present in fibrous cartilage.
  • Draw a row of lacunae and chondrocytes.
  • On each side of the chondrocytes, draw collagen fiber bundles.
  • Fibrous cartilage is found, most notably, in intervertebral discs, pubic symphysis, menisci of the knee, and tendons where properties of stress resistance are particularly important.

END OF ESSENTIAL/START OF ADVANCED

  • Details about cartilage matrix:

Now, let's return to the hyaline cartilage diagram to draw a few more histologic details of cartilage.

  • The matrix, which is basophilic and stains purple on H & E section, contains territorial matrixes that comprise the proteoglycan-rich, dark staining area around the lacunae.
  • Then, indicate interterritorial matrix outside of the isogenic groups; it stains lightly.
  • Denote that the ground substance of the matrix, which is gel-like, comprises the following components:
    • Proteoglycan aggregates, which are bound to collagen fibrils, and which provide a semi-solid structure to cartilage.
    • Chondronectin, which adheres collagen fibers to chondrocytes.
    • Intercellular Water, which is 60-80% of net weight of the cartilage (varies by location).
  • Now, divide the perichondrium into inner and outer layers (although in slow-growing or inactive perichondrium, it is not always possible to visually distinguish two separate layers).
  • Label the inner layer as the chondrogenic (aka cellular) layer.
  • Indicate that it comprises chondrogenic cells.
  • Draw a chondroblast within the matrix and show that chondrogenic cells become chondroblasts.
  • Chondroblasts produce the matrix and collagen fibers and eventually mature into chondrocytes.
    • They comprise a large Golgi apparatus, an abundance of rough endoplasmic reticulum (rER), lipids, and glycogen.
  • Next, label the outer layer as the fibrous layer.
  • Indicate that it comprises:
    • Type I collagen fibers.
    • Blood vessels, which supply nutrients to the cartilage below.
    • Fibroblasts, which are thought to produce collagen fibers and/or chondroblasts (although intertextual variation exists regarding this point).

Now, let's address the formation and growth of cartilage, which occurs via two separate processes: interstitial and appositional growth. Both types occur simultaneously during early development; the relative rate of each type of growth determines the shape and structure of the cartilage.

Begin with interstitial growth, which occurs within cartilage. Let's use a simple hyaline cartilage example of interstitial growth:

  • Draw a matrix, single lacuna, and chondrocyte (for simplicity, omit the perichondrium).
  • Next, draw another matrix and lacuna, but in this lacuna show two dividing cells.
  • Now, show another matrix but in it show that each of the dividing cells has developed its own lacuna and show that the chondrocytes secrete new matrix.
  • Lastly, show a matrix with two lacunae that are further apart to indicate that the daughter chondrocytes move away from each other in their lacunae as they secrete new matrix.
  • Interstitial growth occurs during early stages of cartilage formation, and in the growth plates and articular cartilages of growing long bones.

Now let's show appositional growth in which new matrix forms at the periphery of the cartilage. Again, let's use a simple hyaline example.

  • Draw the following layers from superior to inferior:
    • Perichondrium layer, which divides into both fibrous and chondrogenic layers.
    • New matrix layer, which is being created via appositional growth to expand the cartilage.
    • Older matrix layer
  • Now, within the chondrogenic layer of the perichondrium, draw the flat chondrogenic cells; these cells will differentiate to become chondroblasts. Note that intertextual variation exists regarding early cell differentiation.
  • Within the new matrix, draw a chondroblast.
  • Indicate that it secretes the new matrix.
  • Inferior to the chondroblast, draw a chondrocyte and its lacuna within the new matrix; when chondroblasts are surrounded by matrix, they become chondrocytes.
  • Within the older matrix, draw a lacuna and mature chondrocyte, which remains active and maintains the matrix.
  • Hyaline cartilage that has been only nominally damaged can slowly repair itself via appositional growth, but severely damaged cartilage is often replaced by tougher, more fibrous connective tissue.

Lastly, let's address specific hormones that impact the growth of hyaline cartilage.

  • Denote that the following hormonal substances stimulate cartilage histogenesis: thyroxine, testosterone, and somatotropin.
  • Denote that the following hormonal substances inhibit cartilage histogenesis: cortisone, hydrocortisone, and estradiol.

For reference, consider how the following nutritional states affect bone growth:

  • Hypovitaminosis A diminishes the thickness of epiphyseal plates; decrease in growth rate.
    • Hypervitaminosis A accelerates ossification of epiphyseal plates; short stature (dwarfism).
  • Hypovitaminosis C inhibits matrix production and distorts cartilage columns in epiphyseal plates (bones are weak, repair of fractures obstructed; scurvy develops)
  • Hypovitaminosis D inhibits calcification of matrix, causing softening of the bones (osteomalacia); in children, growing bones become bowed (rickets).