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Bone Growth

Notes

Bone Growth

Sections




Overview

Here, we learn about bone growth that occurs after birth (postnatal) and essentially ends during adolescence. We will focus on endochondral growth at the epiphyseal plate (aka the growth plate).

Types of Bone Growth

Denote that we will examine longitudinal growth via endochondral ossification and bone widening via appositional bone growth.

Endochondral (aka Interstitial) Growth - Cartilage

Endochondral bone growth refers to bone lengthening (linear growth) at the cartilaginous growth plate (the epiphyseal plate).

  • Note that "endochondral" means "within cartilage". This is the same fundamental process of endochondral ossification that we learned in Bone Development but it is restricted to the growth plates and the articular cartilage that covers the epiphyses.

Appositional (aka Periosteal) Growth - Bone

Appositional bone growth refers to bone widening (thickening) along the bone shaft (the metaphyses and diaphysis).

Bone Histology: Low Magnification

Let's orient ourselves to the basic sections of a bone.

From top to bottom, indicate the:

Epiphysis

The articulating end of bone.

Growth Plate

The area of cartilaginous ("endochondral") growth that produces bone lengthening.

Metaphysis

The region in between the growth plate and the bone shaft.

Diaphysis

The long bone shaft.

If bone is a the shape of a clarinet with a ball sitting in the mouth of the instrument, then:

  • The epiphysis is the ball.
  • The growth plate separates the epiphysis from the metaphysis.
  • The metaphysis is the mouth (the bell) of the instrument.
  • The diaphysis is the long cylindrical pipe.

Appositional Growth

The focus of the tutorial will be on endochondral growth, but let's first compare endochondral (lengthening) and appositional (thickening) bone growth.

Draw a representative bone.

Initial Phase

  • Include articular cartilage at the epiphyseal ends.
  • Include a growth plate separating the epiphysis from the metaphysis: this is the site of endochondral growth.
  • Draw a marrow cavity, for reference.

Growth

  • Now, show bone growth -- draw a longer, wider bone next to the first.

Endochondral Growth: Lengthening

  • Show that endochondral growth at the growth plates produces bone lengthening (linear growth).

Appositional Growth: Widening

Show that appositional growth at the periosteum produces bone widening (thickening).

  • Osteoblasts from the periosteum (external bone) secrete osteoid while osteoclasts breakdown bone on the endosteal surface of the bone matrix.

Note that from outside to inside the bone layers are:

  • Periosteum
    • The outer layer is collagenous.
    • The inner layer is replete with osteoblasts that build bone.
  • Bone collar (aka Periosteal collar)
    • This is the compact bone layer that surrounds the marrow cavity.
  • Endosteum
    • Endosteum lines the bone collar
  • Marrow cavity
    • This central bone region houses spongy bone and bone marrow (stem cells).

Endochondral Bone Growth

Now, let's establish the key zones of activity in endochondral growth at the growth plate.

We'll start with a magnified view of the growth plate and the metaphysis beneath it.

Reserve (aka Resting) Zone

Show that the reserve zone comprises dormant (inactive) chondrocytes arranged in clusters lying in a cartilage matrix.

These cells are quiescent but capable of being activated.

Proliferative Zone

Show that the proliferation zone comprises actively-replicating (mitotic) chondrocytes arranged in columns (scaffolds). This zone is responsible for lengthening the bone by pushing the epiphysis away from the bone shaft.

These chondrocytes proliferate but do NOT hypertrophy. Note that Indian hedgehog is responsible for keeping these cells from maturing and enlarging.

Hypertrophic Zone

The hypertrophic zone is subdivided in a variety of ways based on histological changes. Show the following features of this zone:

  • Maturation: chondrocyte cell growth.
  • Hypertrophy: chondrocytes enlarge (hypertrophy) and vacuolate from lipid accumulation. Their lacunae erode (degenerate).
  • Calcification: cell matrix calcification. Note that alkaline phosphatase is a key enzyme in matrix calcification: it increases phosphate availability.

Ossification

Next, let's show the transition from endochondral growth at the growth plate to ossification (new bone growth) in the metaphysis. This new bone is called primary spongiosa.

Cartilage Degeneration

First, let's focus on the zone of cartilage degeneration (which is directly adjacent to the hypertrophic zone).

  • Draw an osteoclast (we see that they are multinucleated) that is surrounded by cartilage breakdown and calcification.
  • Show chondrocyte apoptosis with cell membrane erosion.
  • Show vascularization ("the zone of vascular invasion").
    • Here, VEGF (vascular endothelial growth factor) promotes vascularization, which recruits chondroclasts to degrade the cartilage. The vascular loops that invade the cartilage in this zone also bring osteoprogenitor cells that differentiate into osteoblasts for bone formation, as shown next.

Osteogenesis

Lastly, let's show the development of new bone (primary spongiosa).

  • Show a row of osteoblasts secreting osteoid. The osteoid ultimately mineralizes.
  • Show osteocytes within their lacunae within the spongy bone tissue.
  • Finally, show a blood vessel: vascularization is key to bone growth.

Osteoid Biological Makeup

  • The principal inorganic substance (mineral) is hydroxyapatite, which is hydroxylated calcium/phosphate.
    • Other constituents include magnesium, potassium, sodium, bicarbonate, and citrate.
  • The principal organic substances are collagen type 1 fibers and ground substance.