Anatomy and physiology of the skeletal system

An intro lecture to bone physiology and health

1. Skeletal System
BME 45
Professor Lauren D. Black III

2. Outline of Today’s Class
•Review Slide for Nerve/ NMJ
•Skeletal System Overview
•Types of Bones
•Bone Composition
•Bone Growth and Development
•Bone Health
•Bones’ role in Calcium Regulation and Homeostasis
•Homework intro/ sample problems.

3. Skeletal System
Total of 206 bones:
•Serves as attachment points for
organs and softer body tissues
•Protect vital organs
•Movement is carried out by the
interaction of the skeletal and
muscular systems
•Supports the body
•Houses bone marrow tissue
(used for blood cell formation)

4. Divisions of Skeletal System
•Consists of 80 bones
•Includes skull, rib cage, vertebral column
•Supports and protects organs
•Provides framework for muscles that:
–adjust positions of neck, head, and trunk
–perform respiratory movements
–anchor and stabilize the appendicular bones
Axial Skeleton

5. Divisions of Skeletal System
•Consists of 126 bones
•Makes up the
appendages and girdles
that attach appendages
to the body
•pectoral girdle consists of
the clavicle and scapula.
•pelvic girdle is formed by
the two coxal bones
which are fused
anteriorly
Appendicular Skeleton

6. Types of Bones
Long Bones
•Includes any bone that is longer than it is wide
•They (generally) have the following features:
–growth plates (epiphysis) at either end
–a hard outer surface of compact bone
–a spongy inner layer known as cancellous bone
which contains the bone marrow
–The ends of the bone covered in hyaline cartilage to
help protect the bone and aid shock absorption
Humerus Tibia
Femur

7. Types Of Bones
Short Bones
•approximately as wide as they
are long
•primary function of providing
support and stability with
little movement
•consist of only a thin layer of
compact, hard bone with
cancellous bone on the inside
along with relatively large
amounts of bone marrow
Carpals
Tarsals

8. Types Of Bones
Flat Bones
•strong, flat plates of bone
•Function is protection of the bodies vital
organs and being a base for muscular
attachment
•Anterior and posterior surfaces are
formed of compact bone to provide
strength for protection with the center
consisting of cancellous (spongy) bone
and varying amounts of bone marrow
•In adults, the highest number of red
blood cells are formed in flat bones
Ribs and
Sternum
Cranium
Scapula

9. Types of Bones
Irregular Bones
•bones which do not fall into
any other category, due to
their non- uniform shape
•primarily consist of cancellous
bone, with a thin outer layer
of compact bone
•Protect nervous tissue and
serve as an anchor for skeletal
muscle attachments
Sacrum
Vertebrae
Mandible

10. Types of Bones
Sesamoid Bones
•usually short or irregular
bones
•usually present in a tendon
where it passes over a joint
which serves to protect the
tendon
•modify pressure, to diminish
friction, and occasionally to
alter the direction of a muscle
pull.
Patella
Pisiform

11. Bones are Layered
•The outer layer of a bone is compact bone,
which is hard and dense
•The next layer is spongy bone, which is like
honeycomb, lighter and slightly flexible
•The inner layer of some bones, but not all is
bone marrow, which is jelly like
•Bone marrow is used for blood cell
formation

12. Structures of Bone
•Lamellar Bone
–Collagen fibers arranged in parallel layers
–Normal adult bone
•Woven Bone (non-lamellar)
–Randomly oriented collagen fibers
–In adults, seen at sites of fracture healing,
tendon or ligament attachment and in
pathological conditions

13. Lamellar Bone
•Cortical bone
–Comprised of osteons
(Haversian systems)
–Osteons communicate
with medullary cavity
by Volkmann’s canals

14. Lamellar Bone
•Cancellous bone (trabecular or
spongy bone)
–Bony struts (trabeculae) that are
oriented in direction of the
greatest stress

15. Woven Bone
•Coarse with random
orientation
•Weaker than lamellar
bone
•Normally remodeled to
lamellar bone
Figure from Rockwood and Green’s: Fractures
in Adults, 4
th
ed

16. Bone Composition
•Cells
–Osteocytes
–Osteoblasts
–Osteoclasts
•Extracellular Matrix
–Organic (35%)
•Collagen (type I) 90%
•Osteocalcin, osteonectin, proteoglycans, glycosaminoglycans, lipids (ground substance)
–Inorganic (65%)
•Primarily hydroxyapatite Ca
5(PO
4)
3(OH)
2

17. Osteoblasts
•Derived from
mesenchymal stem cells
•Line the surface of the
bone and produce
osteoid
•Immediate precursor is
fibroblast-like
preosteoblasts
Picture courtesy Gwen Childs, PhD

18. Osteocytes
•Osteoblasts surrounded by
bone matrix
–trapped in lacunae
•Function poorly
understood
–regulating bone metabolism
in response to stress and
strain
Picture courtesy Gwen Childs, PhD

19. Osteocyte Network
•Osteocyte lacunae are connected by canaliculi
•Osteocytes are interconnected by long cell processes that project
through the canaliculi
•Preosteoblastsalso have connections via canaliculi with the
osteocytes
•Network probably facilitates response of bone to mechanical and
chemical factors

20. Osteoclasts
•Derived from hematopoietic
stem cells (monocyte
precursor cells)
•Multinucleated cells whose
function is bone resorption
•Reside in bone resorption
pits
•
Parathyroid hormone stimulates
receptors on
osteoblaststhat activate
osteoclastic bone resorption
Picture courtesy Gwen Childs, PhD

21. Components of Bone Formation
•Cortex
•Periosteum
•Bone marrow
•Soft tissue

22. Mechanisms of Bone Formation
•Cutting Cones
•Intramembranous Bone Formation
•Endochondral Bone Formation

23. Cutting Cones
•Primarily a mechanism to remodel
bone
•Osteoclasts at the front of the
cutting cone remove bone
•Trailing osteoblasts lay down new
bone
Courtesy Drs. Charles Schwab and Bruce Martin

24. Intramembranous (Periosteal) Bone Formation
•Mechanism by which a long
bone grows in width
•Osteoblasts differentiate directly from preosteoblasts and lay down seams of osteoid
•Does NOT involve cartilage anlage
Picture courtesy Gwen Childs, PhD

25. Endochondral Bone Formation
•Mechanism by which a long bone
grows in length
•Osteoblasts line a cartilage precursor
•The chondrocytes hypertrophy, degenerate and calcify (area of low oxygen tension)
•Vascular invasion of the cartilage occurs followed by ossification (increasing oxygen tension)
Picture courtesy Gwen Childs, PhD

26. Crucial Roles of Calcium and Phosphate
•Calcium is an essential dietary element –diet, bones
•Circulating Ca
++
is under direct hormonal control and normally maintained in
a relatively narrow range.
•P
iis also an essential dietary element, and it is stored in large quantities in
bone complexed with Ca
++

27. Regulation of Ca and Pi by PTH
•Parathyroid hormone (PTH) and 1,25 – DihydroxyvitaminD
(Calcitriol) are the two physiologically most important hormones
that are dedicated to maintenance of normal blood [Ca
++
] and [P
i]
in humans.
•PTH is an 84 amino acid peptide secreted by four small parathyroid
glands that lie on the dorsal surface of the thyroid gland.
•Calcitriol is made from Vitamin D obtained through diet or in the
skin by the action of sunlight on precursors made from Acetyl-CoA.
•A third hormone, Calcitonin, has actions opposite those of PTH and
is made by the C cells in the Thyroid gland – plays a minor role in
Calcium homeostasis.

28. Calcium balance in the
body
Small Intestine Bone Kidney
Parathyroi
d Gland
PTH No direct action
1.Promotes
osteoblastic
growth and
survival
2.Regulates M-
CSF, RANKL, and
OPG production
by osteoblasts
3.Chronic high
levels promote
net Ca
++
and
P
irelease from
bone
1.Stimulates 1α-
hydroxylase activity
2.Stimulates
Ca
++
reabsorption by
the thick ascending
limb of Henle’s loop
and the distal tubule
3.Inhibits
P
ireabsorption by
proximal nephrons
(represses NPT2a
expression)
No direct
action
1,25-
Dihydroxyvitam
in D
1.Increases
Ca
++
absorption
by increasing
TRPV channels,
calbindin-D and
PMCA
expression
2.Marginally
increases
P
iabsorption
1.Sensitizes
osteoblasts to
PTH
2.Regulates
osteoid
production and
calcification
1.Minimal actions on
Ca++ reabsorption
2.Promotes
P
ireabsorption by
proximal nephrons
(stimulates NPT2a
expression)
1.Directly
inhibits PTH
gene
expression
2.Directly
stimulates
CaSRgene
expression

29. Bone Physiology
•In adults, bone remodeling involves
1.destruction of preformed bone with the release
of Ca
++
, P
i, and hydrolyzed fragments of the
proteinaceous matrix (calledosteoid) into blood
and
2.new synthesis of osteoid at the site of resorption
and subsequent calcification of the osteoid,
primarily with Ca
++
and P
ifrom blood.
•Bone remodeling occurs continually in about 2
million discrete sites involving subpopulations of
bone cells.
–Cells that promote the formation of bone
(osteoblasts)
–Cells that promote the resorption of bone
(osteoclasts)

30. Integrated Physiological Regulation of Ca++ and Pi
Metabolism
•Integrated response of PTH
and calcitriol to a
hypocalcemicchallenge
•Gonadal and adrenal steroid
hormones also have
profound effects on C++ and
Pi metabolism and on bone.

31. General Bone Health
•It is important to eat the right foods to keep healthy bones
•Foods that contain calcium such as dairy products can help
bone growth and strength
•Throughout your lifetime it is important to exercise
•Even with age you must exercise: it can reduce probability
for fractures and chances of getting diseases such as
osteoporosis

32. Bone Review
•Different Divisions of the Skeletal System
•Different Types of Bones and examples
•Osteoblasts vs. Osteoclasts
•Different methods of Bone formation and repair
•The role of Bone in calcium homeostasis