Bones develop through two main pathways: intramembranous ossification and endochondral ossification. Intramembranous ossification involves mesenchymal cells differentiating directly into bone cells within membranes to form flat bones like the skull. Endochondral ossification uses cartilage as a template, with cartilage replacing by bone except at joints, and involves growth plates that allow long bones to lengthen.

2. • The surface features of bones vary considerably,
depending on the function and location in the
body.
• There are three general classes of bone
markings:
(1) articulations,
(2) projections,
(3) holes.

3. Articulation:
• An articulation is where two bone surfaces
come together (articulus =“joint”).
• These surfaces tend to conform to one
another, such as
1. One being rounded and the
2. Other cupped,
• To facilitate the function of the articulation.

4. Projection:
• is an area of a bone that projects above the
surface of the bone.
• These are the attachment points for tendons
and ligaments.
• In general, their size and shape is an indication
of the forces exerted through the attachment
to the bone.

5. Hole:
• A is an opening or groove in the bone that
allows blood vessels and nerves to enter the
bone.
• As with the other markings, their size and
shape reflect the size of the vessels and
nerves that penetrate the bone at these
points.

8. Bone Formation and Development:
• In the early stages of embryonic development,
the embryo’s skeleton consists of fibrous
membranes and hyaline cartilage.
• By the sixth or seventh week of embryonic life,
the actual process of bone development,
ossification (osteogenesis), begins.
• There are two osteogenic pathways
1. Intramembranous ossification.
2. Endochondral ossification.

10. 1- Intramembranous Ossification:
• Intramembranous ossification is the simpler of
the two methods of bone formation.
• The flat bones of the skull, most of the facial
bones, mandible (lower jawbone), and the
medial part of the clavicle (collar bone) are
formed in this way.
• Compact and spongy bone develops directly
from sheets of mesenchymal (undifferentiated)
connective tissue.

11. • The process begins when mesenchymal cells
in the embryonic skeleton gather together and
begin to differentiate into specialized cells.
• Some of these cells will differentiate into
capillaries, while others will become
osteogenic cells and then osteoblasts.
• Although they will ultimately be spread out by
the formation of bone tissue, early osteoblasts
appear in a cluster called an ossification
center.

12. • The osteoblasts secrete osteoid, uncalcified
matrix, which calcifies (hardens) within a few
days as mineral salts are deposited on it, thereby
entrapping the osteoblasts within.
• Once entrapped, the osteoblasts become
osteocytes.
• As osteoblasts transform into osteocytes,
osteogenic cells in the surrounding connective
tissue differentiate into new osteoblasts.

13. • Osteoid (unmineralized bone matrix) secreted
around the capillaries results in a trabecular
matrix, while osteoblasts on the surface of the
spongy bone become the periosteum.
• The periosteum then creates a protective layer
of compact bone superficial to the trabecular
bone.
• The trabecular bone crowds nearby blood
vessels, which eventually condense into red
marrow.

14. • Intramembranous ossification begins in utero
during fetal development and continues on into
adolescence.
• At birth, the skull and clavicles are not fully
ossified nor are the sutures of the skull closed.
• This allows the skull and shoulders to deform
during passage through the birth canal.
• The last bones to ossify via intramembranous
ossification are the flat bones of the face, which
reach their adult size at the end of the adolescent
growth spurt.

16. 2- Endochondral Ossification:
• In endochondral ossification, bone develops
by replacing hyaline cartilage.
• Cartilage does not become bone.
• Instead, cartilage serves as a template to be
completely replaced by new bone.
• Endochondral ossification takes much longer
than intramembranous ossification.
• Bones at the base of the skull and long bones
form via endochondral ossification.

17. 1- Development of the cartilage
model:
• At the site where the bone is going to form,
specific chemical messages cause the
mesenchymal cells to crowd together in the
general shape of the future bone, and then
develop into chondroblasts.
• The chondroblasts secrete cartilage extracellular
matrix, producing a cartilage model consisting of
hyaline cartilage.
• A covering called the perichondrium (per-i-KON-
dre¯-um) develops around the cartilage model.

19. 2- Growth of the cartilage model:
• Once chondroblasts become deeply buried in the cartilage
extracellular matrix, they are called chondrocytes.
• The cartilage model grows in length by continual cell
division of chondrocytes, accompanied by further
secretion of the cartilage extracellular matrix.
• This type of cartilaginous growth, called interstitial
(endogenous) growth (growth from within), results in an
increase in length.
• In contrast, growth of the cartilage in thickness is due
mainly to the deposition of extracellular matrix material
on the cartilage surface of the model by new
chondroblasts that develop from the perichondrium.
• This process is called appositional (exogenous) growth,
meaning growth at the outer surface (increase in
diameter).

20. • As the cartilage model continues to grow,
chondrocytes in its mid-region hypertrophy
(increase in size) and the surrounding cartilage
extracellular matrix begins to calcify.
• Other chondrocytes within the calcifying
cartilage die because nutrients can no longer
diffuse quickly enough through the extracellular
matrix.
• As these chondrocytes die, the spaces left behind
by dead chondrocytes merge into small cavities
called lacunae.

22. 3- Development of the primary
ossification center:
• A nutrient artery penetrates the perichondrium
stimulating osteogenic cells in the perichondrium to
differentiate into osteoblasts.
• Once the perichondrium starts to form bone, it is known
as the periosteum. Near the middle of the model,
periosteal capillaries grow into the disintegrating calcified
cartilage, inducing growth of a primary ossification center,
a region where bone tissue will replace most of the
cartilage.
• Osteoblasts then begin to deposit bone extracellular
matrix over the remnants of calcified cartilage, forming
spongy bone trabeculae.
• Primary ossification spreads from this central location
toward both ends of the cartilage model.

24. 4- Development of the medullary
(marrow) cavity:
• As the primary ossification center grows
toward the ends of the bone, osteoclasts
break down some of the newly formed spongy
bone trabeculae.
• This activity leaves a cavity, the medullary
(marrow) cavity, in the diaphysis (shaft).
• Eventually, most of the wall of the diaphysis is
replaced by compact bone.

26. 5- Development of the secondary
ossification centers:
• When branches of the epiphyseal artery enter the
epiphyses, secondary ossification centers develop,
usually around the time of birth.
• Bone formation is similar to what occurs in primary
ossification centers.
• However, in the secondary ossification centers spongy
bone remains in the interior of the epiphyses (no
medullary cavities are formed here).
• In contrast to primary ossification, secondary
ossification proceeds outward from the center of the
epiphysis toward the outer surface of the bone.

28. 6- Formation of articular cartilage
and the epiphyseal (growth) plate:
• The hyaline cartilage that covers the epiphyses
becomes the articular cartilage.
• Prior to adulthood, hyaline cartilage remains
between the diaphysis and epiphysis as the
epiphyseal (growth) plate, the region
responsible for the lengthwise growth of long
bones that you will learn about next.