CHAPTER 6-2
Bone Development, Growth and
Remodeling
Objectives
1.
2.
3.
4.
5.
6.
7.
8.
Functions of the skeletal system
Classification of bones based on shape
General features of bone
Bone cells and matrix of bone
Compact and spongy bone
Bone marrows
Bone development
Overview of bone growth and remodeling
Bone development

Ossification or osteogenesis
- is the process of forming new bone

Two methods of ossification:
1. Endochondral ossification
2. Intramembranous ossification
Classification based on formation
1.
Endochondral ossification = The process by
which bone is formed from hyaline cartilage.
2.
Intramembranous ossification = Bone formed
by replacing a fibrous membrane and not
from cartilage. (ex. Clavicle and skull bones).
Endochondral ossification

The process by which bone is formed from hyaline
cartilage

Most bones in the body are formed by this method
(including the vertebrae, pelvic bones and limb
bones).

Consists of 3 sites of ossification.
1. The primary ossification center
2. The metaphysis
3. The secondary ossification center
Endochondral ossification
During the first 8 weeks of fetal development,
hyaline cartilage forms a model of future bone
formation. The center (diaphysis) of the
cartilage is “primary ossification site” which
contains numerous chondrocytes in lacunae.
Surrounding the cartilage model is an outer
layer chondrocytes called the “perichondrium”
which differentiate into “osteoblast cells” and
begin to lay down a bony collar around the site.
 Once the collar is formed the perichondrium
becomes periosteum.

Endochondral ossification

Buds of connective tissue grow from the periosteum
into the caritlage and transform the primary
ossification site into the primary marrow space. This
space is lined with spongy bone.

The metaphysis forms between the marrow space and
the cartilaginous epiphyseal end.
-It is considered as a transitional zone where cartilage
is formed into bone at each end of the diaphysis. The
metaphysis is just beneath the epiphyseal plate where
bone growth continues until after adulthood.
The secondary ossification center
Begins at the time of birth.
 Forms in the epiphysis and develops similarly
to the primary ossification center.
 The bone formed in the secondary ossification
site persists as spongy bone and growth occurs
beneath the outer covering of hyaline cartilage
which persists as articular cartilage within the
joint cavity on each end of the epiphysis.

Endochondral ossification
Endochondral ossification
Intramembranous ossification

Bone formed by replacing a fibrous
membrane and not from cartilage. Ex. Skull and
clavicle

Basic Overview of Process
– During the first 8 wks of embryonic development,
fibrous connective tissue membranes form in the
areas of future flat bones.
– Beginning around 8 wks, an “ossification center”
forms in the membrane. This center is composed of
osteoblasts.
– The osteoblasts begin to secrete hydroxyapatite.
– The internal spongy bone forms.
– The external compact bone forms.
Intramembranous ossification
Intramembranous ossification
Bone Growth

Once the cartilage models of embryonic
development are replaced by bone, they must
continue to grow through infancy, childhood
and adolescence.
– Increased length: bones continue to lengthen
because hyaline cartilage remaining in the
epiphyseal plates continues to grow. As
adulthood approaches, this cartilage becomes less
active and is eventually replaced by bone.
Epiphyseal plate growth zones

1.
2.
3.
4.
5.
Consists of five distinct zones:
Zone of resting cartilage
Zone of cartilage proliferation
Zone of cell hypertrophic cartilage
Zone of calcified cartilage
Zone of ossification
Epiphyseal plate growth zones
Bone growth
Appositional growth - Increased Width: bones
continue to widen as osteoblasts form more
layers of bone around the outside and
osteoclasts break down some of the bony
matrix inside.

Why would bone need to be broken down
inside as it grows outside?
Appositional growth
Control of Bone Growth
Bones increase in length and width because of
the influence of minerals, vitamins, and
hormones in the body.
 Calcium and phosphate are necessary for
calcification
 Vitamins A, C and D promote bone growth.
 The specific hormones which affect growth are

growth hormone (GH), thyroid hormone (T3
and T4), and the sex steroids (estrogen and
testosterone).
Chemicals important in bone
homeostasis
Bone remodeling

Even though the bones in an adult do not
continue to grow as described above, they are
constantly being remodeled. This means that
bone is always being broken down by
osteoclasts and reformed by osteoblasts (really
no different from remodeling ones home).

Each week we turn over about 5% of our bone
mass.
Control of Remodeling


Two major factors influence remodeling.
Calcium levels - our bodies need a
homeostatic level of calcium in the blood for
all cells to function properly.
Mechanical stress - the varied activities of life
put different stresses on each bone as we
age, which requires slight adjustments to
compensate for these stresses.
Bone blood supply

Bone is very vascular and well nourished by
blood.
Bone fractures
Bone fractures

Types of bone fractures
Simple fractures do not
break the skin.
 Compound fractures
puncture the skin and
are at great risk of
infection.

Bone fracture repair

There are four primary stages for bone
repair after a fracture.
Bone fractures