1. Bone Development & Growth

2. Objectives
Distinguish between intramembranous and endochondral bones, and explain how such bones develop and grow
Describe the effects of sunlight, nutrition, hormonal secretion, and exercise on bone development and growth

3. Bone Development
Parts of the skeletal system begin to form during the first few weeks of prenatal development
Bony structures continue to grow and develop into adulthood
The process of bone development is called osteogenesis
Bones form when bone tissue replaces existing connective tissue in one of two ways
Intramembranous
Endochondral

4. Intramembranous Bones
Are bones that originated within sheet-like layers of connective tissue
Include the bones of the skull, clavicles, sternum, mandible, maxillae and zygomatic

5. Endochondral Bones
Are bones that begin as masses of cartilage later replaced by bone tissue
Includes majority of the bones of the skeleton
The process of forming endochondral bones is called endochondral ossification

6. Osteogenesis of Intramembranous Bones
Sheets of unspecialized connective tissues appear at the site of future bones
Partially differentiated connective tissue cells collect around blood vessels in these layers
Connective tissue cells further differentiate into osteoblasts, which deposit spongy bone
Osteoblasts become osteocytes when bony matric completely surrounds them
Connective tissue on the surface of each developing structure forms a periosteum
Osteoblasts on the inside of the periosteum deposit compact bone over the spongy bone

7. Osteogenesis of Endochondral Bones
Masses of hyaline cartilage form models of future bones
Cartilage tissue breaks down and periosteum develops
Blood vessels and differentiating osteoblasts from the periosteum invade the disintegrating tissue
Osteoblasts form spongy bone in the space occupied by cartilage
Osteoblasts beneath the periosteum deposit a thin layer of compact bone
Osteoblasts become osteocytes when bony matrix completely surrounds them

8. Long Bone Development
Bone tissue begins to replace hyaline cartilage in the center of diaphysis
This region is called the primary ossification center, in which bone develops from it toward the ends of the cartilaginous structure
Osteoblasts from the periosteum deposit a thin layer of compact bone around the primary ossification center

9. Long Bone Development cont.
The epiphyses of the developing bone remain cartilaginous and continue to grow
A secondary ossification center appears in the epiphyses and spongy bone forms In all directions
As spongy bone is deposited in the diaphysis and in the epiphysis, a band of cartilage called the epiphyseal plate remains between the two ossification centers

10. Endochondral ossification

11. Growth at the Epiphyseal Plate
The cartilaginous cells of the epiphyseal plate form four layers:
Zone of resting cartilage
Zone of proliferating cartilage
zone of hypertrophic cartilage
Zone of calcified cartilage

13. Zone of resting cartilage
Is closest to the end of the epiphysis
It is composed of resting cells that do not actively participate in growth
this layer anchors the epiphyseal plate to the bony tissue of the epiphysis

14. Zone of Proliferating Cartilage
This is the second layer
It includes rows of many young cells undergoing mitosis
As new cells appear and as extracellular matrix forms around them, the cartilaginous plate thickens

15. Zone of Hypertrophic cartilage
Composed of rows of older cells left behind when new cells form
This layer enlarges and thickens the epiphyseal plate more
The entire bone lengthens as a result
At the same time, invading osteoblasts secrete calcium salts and accumulate in the extracellular matrix adjacent to the oldest cartilaginous cells
The extracellular matric calcifies and the cells begin to die

16. Zone of Calcified Cartilage
This is a thin layer
It is composed of dead cells and calcified extracellular matrix
In time, large multinucleated cells called osteoclasts break down the calcified matrix by secreting an acid
Osteoclasts also phagocytize components of the bony matrix
Afterwards, bone building osteoblasts invade the region and deposit bone tissue

17. Continued Growth Bones continue to develop after you are born
Between early childhood and late adolescence, bone cells gradually replace the cartilage in long bones of the arms and legs
When all the cartilage in the epiphyseal plate is replaced with bone, a person has reached their full height
The epiphyseal plates become epiphyseal lines

18. Ossification Timeline
Age
Occurrence
3rd month of prenatal development
Ossification in long bones begins
4th month of prenatal development
Most primary ossification centers have appeared in the diaphysis of bones
Birth to 5 years old
Secondary ossification centers appear in the epiphyses
5-12 years old (females)
5-14 years old (males)
Ossification rapidly spreads from the ossification centers
15-18 years old (females)
17-20 years old (males)
Bones of upper limbs and scapulae completely ossified
16-21 years old (females)
18-23 years old (males)
Bones of the lower limbs and hip bones completely ossified
21-23 years old (females)
23-25 years old (males)
Bones of the sternum, clavicles, and vertebrae completely ossified
By 23 years old (females)
By 25 years old (males)
Nearly all bones completely ossified

19. Homeostasis of Bone Tissue
After the intramembranous and endochondral bones form, the actions of osteoclasts and osteoblasts continually remodel them
Bone remodeling occurs throughout life as osteoclasts resorb bone tissue and osteoblasts replace the bone
The processes of resorption and deposition occur on the surfaces of the endosteum and periosteum
The rate of bone remodeling is not uniform, spongy bone is faster than compact bone
These processes replace 10% to 20% of the skeleton each year

20. Factors Affecting Bone Development, Growth & Repair
Nutrition
Exposure to sunlight
Hormonal secretions
Physical exercise

21. Nutrition Factors
Vitamin D is necessary for proper absorption of calcium in the small intestine
Without enough of it, the bone matrix softens and thereby deforms the bone
Vitamins A and C are also required
Vitamin A is necessary for osteoblast and osteoclast activity during normal development
A lack of vitamin A may retard (slow) bone development
Vitamin C is required for collagen synthesis
A deficiency prevents osteoblasts from producing enough collagen in the extracellular matrix of the bone tissue, and as a result bones become abnormally slender and fragile.

22. Hormonal Factors
The pituitary gland secretes growth hormone, which stimulates division of cartilage cells in the epiphyseal plates
Without this hormone pituitary dwarfism occurs
If too much growth hormone is produced, then pituitary gigantism occurs
The thyroid hormone thyroxine stimulates replacement of cartilage in the epiphyseal plates of long bones with bone tissue
This hormone also increases cellular metabolism, including stimulating osteoblast activity
The parathyroid hormone stimulates an increase in the number and activity of osteoclasts, which break down bone

23. Hormonal Factors Cont.
Both sex hormones, testosterone for males and Estrogen for Females, promote formation of bone tissue
At puberty, these hormones are abundant, causing the long bones to grow considerably
Sex hormones also stimulate ossification of the epiphyseal plates and thus, stop bone lengthening at a relatively early age
Estrogen has a stronger affect on the epiphyseal plates and, thus females typically reach their maximum heights sooner than males

24. Physical Factors Physical stress stimulate bone growth
For example, when skeletal muscles contract, they pull at their attachments on bones, and the resulting stress stimulates the bone tissue to thicken and strengthen (hypertrophy)
With a lack of exercise, bone tissue wastes becoming thinner and weaker (atrophy)
This is why the bones of athletes are usually stronger and heavier than those of non- athletes