Bone Development: Formation and Growth
• Osteogeneis (ossification) – process of
bone formation
• Cranial bones develop by a method called
intramembranous ossification whereas…
• Other bones form by endochondral
ossification.
Intramembranous Ossification
“within membrane”
•
Forms directly from sheet like layers of
fibrous embryonic C.T.
(mesenchymal).
•
Step 1 – mesechymal cells differentiate
into osteoblasts that secrete organic
matrix of bone
Step 2 – bone forms between blood
vessels randomly. Trabeculae forms
to create spongy bone. Mesenchymal
cells condense on outer surface to
form periosteum.
Step 3 – Trabeculae thicken at
periphery to form plates of compact
bone.
•
•
All bones of the skull and clavicle form
by this method.
Endochondral Ossification
“within cartilage”
• Hyaline cartilage model
– Cartilage built up in roughly the same shape of
bone, then broken down, reorganized and
hardened (mineralized) to form bone.
• Step 1 – hyaline cartilage formation via
chondroblasts, which are surrounded by
perichondrium, which becomes bone forming
periosteum.
• Step 2 – cartilage calcifies in the center of
diaphysis. Chondrocytes die due to calcification
and nutrition cut off. Ends of bone still have
healthy chondrocytes so the bone model
enlarges.
Endochondral Ossification
“within cartilage”
• Step 3 – Primary ossification center expands and
periosteal bud containing nutrients artery and vein, bone
marrow cells, osteoblasts and osteoclasts invades
diaphysis.
• Osteoblasts begin to deposit bone matrix forming spongy
trabeculae.
• Spongy bone is broken down by osteoclasts and the
medullary cavity is formed.
Endochondral Ossification
“within cartilage”
• Step 4 – Blood vessels enter epiphysis and secondary
ossification centers develop here about time of birth.
• No cavity formation here.
• Hyaline cartilage remains covering the epiphysis articular
surfaces and cartilage becomes “trapped” between the
two ossification centers creating the epiphyseal plate.
• Bone lengthens by addition of bone material at this plate.
• Most bones inferior to the skull and long bones form in
this manner.
Bone Diseases
• Achondroplasia
– Chondrocytes fail to multiply and long
bones grow slowly and stop growing
early.
– Head and trunk are normal proportions.
• Osteoporosis
– Bone reabsorption outpaces bone
deposition.
• Osteomyelitis
– Bacterial infection of bone marrow.
Fractures
• Complete fracture
– Break across bone
is complete
– Simple fracture
• Bone breaks cleanly
into 2 pieces w/ no
penetration of skin
– Compound fracture
• Broken bone
protrudes through
soft tissue and skin
Blood clots to form
a hematoma
Blood vessels grow
into clots
Inflammation is
apparent
Fibroblasts and
osteoprogenitor
cells invade the clot
Cell debris removed
by phagocytes and Collagen,
osteoclasts
fibrocartilage and
hyaline produced to
connect bone
3 weeks
Osteoblasts
produce spongy
bone trabeculae
joining living and
dead fragements
3-4 months
Bony callus
remodeled to
remove excess
bony material to
reshape bone and
medullary cavity.
sagittal suture
coronal suture
squamosal suture
sutural bones
lambdoidal suture
Fontanels
“soft spots”
• Allows skull to be
compressed during birth and
allows for brain growth.
• Anterior (frontal)
– diamond shaped
• Posterior (occipital)
– diamond shaped
• Sphenoid (anterior lateral)
– small and irregular shaped
• Mastoid (posterior lateral)
– irregularly shaped
Division of the Skeleton
Axial Skeleton – skull, vertebral
column, and bony thorax
Skull – encloses and protects
brain. Provides attachment for
head and neck muscles. Mostly
interlocking flat bones.
Vertebrae – individual bones of
the vertebral column act as a
supporting rod. Helps transfer
trunk weight to lower limbs.
Division of the Skeleton
Axial Skeleton – skull,
vertebral column, and bony
thorax
Ribs – Protects heart, lungs,
and other thoracic organs.
Provides attachment for
many muscles for breathing
and support, 12 pair.
Sternum – breast bone
Division of the Skeleton
Appendicular Skeleton – the
limbs and their girdles which
append to the axial skeleton
Each limb is composed of a
girdle and 3 segments
connected by moveable joints.
Upper limbs – arm, forearm,
hand
Shoulder girdle (pectoral) –
clavicle, scapula. Attaches
the upper limb to the trunk
(axial skeleton)
Division of the Skeleton
Appendicular Skeleton – the
limbs and their girdles which
append to the axial skeleton
Lower limbs – thigh, leg, foot.
Carries the entire weight of the
erect body.
Pelvic girdle – consists of paired
hip bones. Attaches the lower
limbs to the spine and supports
visceral organs of pelvis. Full
weight of body passes through
this girdle to lower limbs.
Bones of the Skeleton
• Bone markings
– Projections that are sites of muscle and
ligament attachment.
– Projections that help form joints
– Depressions and openings allowing blood
vessels and nerves to pass