1. Skeletal System: Day Three
Bone Formation, Growth, and Remodeling, Bone Diseases and Disorders

2. Bone Development and Growth
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bone Development and Growth
Bones form by replacing connective tissues in the fetus.
Some form within sheetlike layers of connective tissue (intramembranous bones), while others replace masses of cartilage (endochondral bones).
Ossification is another word for bone building.

4. Intramembranous Bones
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Intramembranous Bones
The flat bones of the skull form as intramembranous bones that develop from layers of connective tissue.
Osteoblasts deposit bony tissue around themselves.
Once osteoblasts deposit bone and are located in lacunae, they are called osteocytes.
Cells of the membranous connective tissue that lie outside the developing bone give rise to the periosteum.

5. Intramembranous Bone

6. Ossification: formation of bone
2 methods:
1. Intramembranous
2. Endochondral

7. The Major Players Osteoblasts – build bone
Osteoclasts – break down bone tissue
Osteogenic cells – “highly mitotic” bone stem cells in membranes
Osteocytes – mature
bone cells in lacunae

8. I. Intramembranous Ossification
End goal: Production of flat bones of skull and most of the clavicle.

9. Formation of the Bony Skeleton
Week 8 (of pregnancy)
fibrous membranes and hyaline cartilage begin to ossify.
Intramembranous ossification: development from fibrous membrane
Endochondral ossification: development from hyaline cartilage

10. Intramembranous Ossification
Forms most bones of the skull and clavicles
All bones formed via this route ARE FLAT BONES
Fibrous CT from mesenchymal cells is framework on which ossification begins

11. STEP ONE Four Major Steps:
An ossification center appears in the fibrous
CT membrane
Mesenchymal cells cluster and become
OSTEOBLASTS forming ossification
center

12. Step Two Bone matrix (osteoid) is secreted within the fibrous membrane
-osteoblasts secrete osteoid (fibers, GAGs,
-trapped osteoblasts become osteocytes

14. Woven bone and periosteum form
STEP THREE
Woven bone and periosteum form
-accumulating osteoid is laid down between embryonic blood, forming random network
result is a network of trabeculae.
-vascularized mesenchyme condenses on the external surface of the bone and forms periosteum

15. Step Four Bone collar of compact bone forms and red marrow appears
-Trabeculae just deep to the periosteum thicken, forming a bone collar that will later be replaced by mature bone
-Spongy bone persist internally and its vascular tissue becomes red marrow

16. Endochondral Bone Growth

17. CopyrightThe McGraw-Hill Companies, Inc
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Endochondral Bones
Most of the bones of the skeleton fall into this category.
They first develop as hyaline cartilage models and are then replaced with bone.
Cartilage is broken down in the diaphysis and progressively replaced with bone while the periosteum develops on the outside.
Cartilage tissue is invaded by blood vessels and osteoblasts that first form spongy bone at the primary ossification center in the diaphysis.

18. 6. Secondary ossification centers appear later in the epiphyses.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5. Osteoblasts beneath the periosteum lay down compact bone outside the spongy bone.
6. Secondary ossification centers appear later in the epiphyses.
7. A band of hyaline cartilage, the epiphyseal plate, forms between the two ossification centers.
8. Layers of cartilage cells undergoing mitosis make up the epiphyseal plate.
9. Osteoclasts break down the calcified matrix and are replaced with bone-building osteoblasts that deposit bone in place of calcified cartilage.

19. CopyrightThe McGraw-Hill Companies, Inc
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10. Epiphyseal plates are responsible for lengthening bones while increases in thickness are due to intramembranous ossification underneath the periosteum.
11. A medullary cavity forms in the region of the diaphysis due to the activity of osteoclasts.

21. Bone Formation: Ossification
By birth, most hyaline cartilage have been converted to bone except for two regions:
Articular cartilages
Cover long bone ends (epiphysis)
Persist for lifetime
Reduce friction at the joint surfaces
Epiphyseal Plate
Allows for longitudinal growth of long bones during childhood
Will eventually disappear leaving only the epiphyseal line

22. Bone Growth New cartilage is continuously formed
Old cartilage is broken down and replaced by bony matrix
Process of long-bone growth
controlled by hormones: growth hormone and sex hormone
Ends during adolescence when epiphyseal plate is converted to bone

23. New cartilage added on the external part of the articular cartilage and on the epiphyseal plate surface away from medullary cavity
Old cartilage in broken down and replaced by bone on the internal part of the articular cartilage and on the epiphyseal plate near the medullary cavity

24. Homeostasis of Bone Tissue
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Homeostasis of Bone Tissue
Osteoclasts tear down and osteoblasts build bone throughout the lifespan with the processes of resorption and deposition, with an average of 3% to 5% of bone calcium exchanged annually.

25. Bone Remodeling Bone is very active tissue
Bones are remodeled continuously in response to two factors:
Calcium levels in the blood
Pull of gravity and muscles on the skeleton

26. Bone Remodeling: Calcium levels in blood
Blood calcium low
Parathyroid glands release parathyroid hormone (PTH)
PTH activate osteoclasts (destroy bone cells)
Osteoclasts break down bone and release calcium into the blood
Blood calcium high
Thyroid gland releases calcitonin
Calcitonin stimulates Calcium to be deposited from the blood into the bone

28. Bone Remodeling: Pull of gravity and muscles on the skeleton
Bones become thicker to increase strength where bulky muscles attached
Bones of physically inactive people tend to lose mass

29. Rickets Disease of children where bones fail to calcify
Bones soften and a bowing of the weight-bearing bones of the legs occur
Due to lack of calcium in the diet or a lack of Vitamin D (necessary to absorb calcium into the blood)
Not seen much in U.S., but is a problem in other parts of the world.

30. Osteoporosis A loss in bone mass leading to thin, fragile bones
Common consequence of aging, esp. in women
Estrogen helps to maintain skeleton in women
Lack of estrogen during menopause in women as well as a diet poor in calcium and protein, lack of Vitamin D, smoking, insufficient weight- bearing exercise can contribute to osteoporosis.