Name _____________________________________ Hr. _____ Date ____________________
Chapter 7 Notes: Bone Development & Homeostasis
The Early Embryonic Skeleton
Embryonic skeleton – composed of fibrous connective tissue membranes and hyaline cartilage
Bone Formation and Growth
Ossification/osteogenesis-- Begins at week
of development
There are two different types of bone formation
1.
2.
Intramembranous Ossification
Results in the formation of
As the matrix forms,
and
.
form, joining together, forming the lattice of spongy bone
Outside vascularized connective tissue develops into the
Bone collar of compact bone forms, and
marrow appears
Most of this bone will be remodeled into compact bone over time
.
Woven Bone and
Periosteum Form
An ossification center
appears in the fibrous
membrane
Bone collar of compact
bone forms and red marrow
appears
Bone matrix (osteoid) is
secreted within the fibrous
membrane
Endochondral Ossification
Begins in the
Forms
month of development
bones below the
.
Uses hyaline cartilage “bones” as models for bone construction
Requires breakdown of hyaline cartilage prior to ossification
Stages of Endochondral Ossification
1. Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal
plates
2. Formation of the medullary cavity; appearance of secondary ossification centers in the
epiphyses
3. Invasion of internal cavities by the periosteal bud, and spongy bone formation
4. Cavitation of the hyaline cartilage
5. Formation of bone collar
Vascularized connective tissue develops into the
outside and the
inside
Most of this bone will be remodeled repeatedly over time
Postnatal Bone Growth
Growth in length of long bones
Most bone growth stops during adolescence
Continued growth of
and
.
Is accompanied by remodeling in order to maintain the proper shape of the epiphysis and
diaphysis
Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different
zones: growth, transformation, and osteogenic
Postnatal Bone Growth
Regulated by hGH and the sex hormones
In children, cartilage production continues on the epiphyseal (distal) side
cells are destroyed & replaced to increase the
of bone
Growth in length of long bones
Cartilage on the side of the epiphyseal plate closest to the
is relatively inactive
Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth
Postnatal Long Bone Growth
Cells in the growth zone divide quickly, pushing the epiphysis away from the
Cells in hypertrophic zone hypertrophy causing lacunae to erode and enlarge
Cartilage matrix
and the chondrocytes die
This leaves long
of calcified cartilage at the epiphysis-diaphysis junction
.
The spicules become the osteogenic zone and are invaded by
from the medullary cavity
The cartilage is eroded by osteoclasts and osteoblasts secrete matrix to form spongy bone
The spicule tips are removed by osteoclasts
Long Bone Growth
At the end of adolescence, epiphyseal plates divide less often and plates are replaced by bone
tissue
Longitudal growth ceases and the epiphysis/diaphysis fuse.
Called epiphyseal plate closure
Females at
years;
Males at
years
Appositional Bone Growth
Growth in width
From the inside out
Compact bone lining the medullary cavity is destroyed
Osteoblasts from
continue to add more
bone to the outer surface
Bone diameter can still increase (appositional)
Bone Homeostasis - Remodeling
Remodeling - replacement of old bone by new
Bone is a very
active tissue
Spongy transforms to compact or vice versa; old to new
Bone is remodeled along the lines of mechanical stress.
Different rates in different regions
Distal head of the femur is replaced ~ every
Bone is replaced every
months
to
Delicate balance between
years
and
.
Too much bone tissue, bones become thick and heavy
Too much mineral causes bumps or spurs which interfere with joint function
Too much Ca2+ loss or crystallization makes bones brittle, breakable
Bone Growth and Remodeling
Two control mechanisms
1.
2.
Response to Mechanical Stress
law – a bone grows or remodels in response to the forces or demands placed upon it
Observations supporting Wolff’s law include:
Long bones are thickest
along the shaft (where bending stress is greatest)
Curved bones are thickest where they are most likely to
.
Hormonal Mechanism
Rising blood Ca2+ levels trigger the thyroid to release
Calcitonin stimulates
deposit in bone
Falling blood Ca2+ levels signal the
PTH signals
.
to release
to degrade bone matrix and release Ca2+ into the
.
.
Hormonal Regulation of Bone Growth During Youth
During infancy and childhood, epiphyseal plate activity is stimulated by
.
During puberty, testosterone and estrogens:
1. Initially promote adolescent
2. Cause
.
and
of specific parts of the skeleton
3. Later induce epiphyseal plate closure, ending
bone growth
Bone Homeostasis - Regulation
Hormonal regulation of bone growth and remodeling
hGH (
)
-responsible for general growth of all body tissues
-becoming
or
depends on hGH levels
-works with the sex hormones
-aids in the growth of new bone
-causes degeneration of
cells in epiphyseal plates
Hormonal regulation of bone growth and remodeling
Sex hormones – androgens and
Insulin and
- important for normal bone growth & development
- important for bone and connective tissue growth & metabolism
Calcium Homeostasis
Bones are important for
homeostasis
Bone tissue is the main reservoir for Ca2+ ions in the body (500-1000 times more calcium is in
bone than in the rest of the tissues)
Blood levels are regulated very tightly by the endocrine system
Bone serves as a “buffer” to prevent sudden changes in blood Ca2+ level
Calcium Homeostasis - Regulation
2 hormones are primarily involved in Ca2+ homeostasis
Parathyroid Hormone (PTH, parathormone) from the parathyroid glands increases blood calcium
levels
Calcitonin from the thyroid gland decreases blood calcium levels
B. Control of Remodeling: Hormonal Regulation of Calcium