1. Tissues of the Skeletal System
Biology 322 Human Anatomy
Tissues of the Skeletal System

2. Recall:
SYSTEMS are composed of one or more organs, all serving a common function
ORGANS are composed of one or more types of tissues, all serving a common function
TISSUES are composed of one or more types of cells and their products, all serving a common function

3. Organs of skeletal system = Bones
(e.g. femur, ulna, vertebra, mandible)
Like all organs, these contain four types of tissue:
Epithelium,
Connective tissue,
Nervous tissue,
Muscle tissue
However: Two types of specialized
connective tissues predominate:
Cartilage Tissue & Bone Tissue
each of which is surrounded by
dense irregular connective tissue:
Perichondrium and Periosteum

4. Cartilage: Function = Flexible Support
Skeletal System:
Costal cartilage
Intervertebral disk
Pubic symphysis
Meniscus in a joint
Other Systems:
External ear
External nose
Larynx
Trachea & bronchi

5. Cartilage:
Also: Forms embryonic "model" for many bones and:
a) Remains on ends of long bones throughout life
b) Remains at growth plates (epiphyseal plates) of
immature growing bones.

6. Cartilage:
Young cells = chondroblasts
Actively forming new
extracellular matrix
Mature cells = chondrocytes
Maintain and repair
Extracellular Matrix
Ground substance = very
firm gel
Fibers = collagen, elastic
Chondrocytes located in spaces called lacunae

7. Cartilage: Three types:
Hyaline
Elastic
Fibrous
All three types:
Cells are the same
Differences due to fibers

8. Hyaline Cartilage:
Extracellular matrix appears smooth,
No fibers evident by light microscopy
Cells (in lacunae) form isogenous groups or nests
Extracellular matrix often stains more darkly around cells

9. Elastic Cartilage:
Many dark elastic fibers visible in extracellular matrix
Cells (in lacunae) form isogenous groups or nests
Extracellular matrix often stains more darkly around cells

10. Fibrous Cartilage:
Many large collagen fibers visible in extracellular matrix
Cells (in lacunae) usually individual (no isogenous groups)
Extracellular matrix usually stains more darkly around cells

11. Bone Tissue:
Young cells = osteoblasts
Actively forming new
extracellular matrix
Mature cells = osteocytes
Maintain and repair extracellular matrix
Located in lacunae
Third cell type = osteoclast
Reabsorb extracellular
matrix

12. Bone tissue:
Extracellular matrix:
Fibers: Collagen
Ground substance contains
precipitated calcium phosphate
called calcium hydroxyapatite
Bone tissue constantly remodeling itself:
Old extracellular matrix removed by osteclasts
New extracellular matrix produced by osteocytes

13. Two forms of bone tissue:
Trabecular
("woven", "spongy")
Shelves or ridges ("trabeculae") of extracellular bone matrix with osteocytes within their lacunae.
Lamellar
("compact", "Haversian")
Concentric rings ("lamellae") of extracellular matrix and osteocytes, surrounding a central canal containing nerves and capillaries

15. Lamellar Bone:

16. Lamellar Bone:
Osteocytes in lacunae, between lamellae

17. Both trabecular and lamellar bone:
Osteocytes have lost ability to divide so
Growth can occur only by differentiation of new
osteoblasts, which then
form extracellular matrix
around themselves
Osteoblasts at edges
As the osteoblasts become trapped in their matrix, they mature into osteocytes located in lacunae

18. Both trabecular and lamellar bone:
Osteocytes have long cellular extensions to retain connections
with each other.
These extensions of osteocytes run through little canals called canaliculi in the rigid extracellular matrix

19. Two types of bone formation ("osteogenesis") for growth, remodeling, and repair:
- Endochondral osteogenesis:
Model of cartilage formed first.
Cartilage then replaced by bone .
- Intramembranous osteogenesis:
Bone formed within pre-existing
ordinary connective tissue, often
at edge of existing bone.

20. Bones (organs) grow by both intramembranous and endochondral osteogenesis:
Average age (years) at completion of ossification:
Scapula
Clavicle
Os coxa
Vertebrae
Sacrum
Sternum - body
- manubrium
Humerus, radius, ulna
Femur, tibia, fibula

21. Bones repair fractures by both intramembranous and endochondral osteogenesis:

22. Bone tissue is constantly remodeling itself to adjust to different physical and biochemical demands placed upon it.
Old bone is destroyed by osteoclasts
and new bone is produced by
osteoblasts and osteocytes.
Example:
If you have a cast on your arm, the bones will become smaller since they are not being stressed and more bone tissue is removed than is synthesized.
But:
When the cast is removed and the bones are again subjected to stress, they will become larger again.

23. Bone remodeling regulated by many hormones:
Growth hormone stimulated osteoblast activity &
collagen synthesis
Thyroid hormone stimulates osteoblast activity & collagen
synthesis; stimulates formation of ossification centers
Testosterone stimulates osteoblast activity & bone growth
Progesterone stimulates osteoclast activity & bone loss
Estrogen stimulates osteoblast activity & bone growth

24. Two hormones primarily responsible for day-to-day remodeling of bone to regulate concentration of calcium in blood:
Calcitonin Secreted by thyroid gland
Stimulates osteoblasts to produce more matrix
Inhibits osteoclasts from breaking down matrix
Thus: Calcium removed from blood & stored in bone
Parathyroid Hormone Secreted by parathyroid glands
Inhibits osteoblasts from producing more matrix
Stimulates osteoclasts to break down matrix
Thus: Calcium released from bone into blood