The document discusses the skeletal system and bone tissue. It covers the functions of the skeletal system including support, movement, protection, mineral storage, and blood cell production. It describes the structure of bone and the types of cells involved in bone formation and resorption. The document discusses how bone growth occurs through childhood, the role of bone in calcium homeostasis, factors affecting bone health, and changes that occur in bone with aging.

2. The Skeletal System
 Topics to be covered
 Functions of Bone and Skeletal System
 Structure of Bone
 Histology of Bone Tissue
 Blood and Nerve Supply of Bone
 Bone Formation
 Bone’s Role in Calcium Homeostasis
 Aging and Bone Tissue

4. Functions of Bone and Skeletal System
4. Mineral Homeostasis:
Calcium balance: contribute to bone strength; stores
99% of body’s calcium;
Phosphorus balance: stored in bones and releases it on
demand of the body

10. Histology of Bone Tissue
 Four types of cells are present in bone tissue Osteogenic cell
 Osteogenic cells
 Undergo cell division; the resulting cells
develop into osteoblasts
 Osteoblasts
 Bone-building cells Osteoblast
 Synthesize extracellular matrix of bone tissue
 Osteocytes
 Mature bone cells, maintains bone tissues
 Exchange nutrients and wastes with the blood
 Osteoclasts
Osteocytes
 Release enzymes that digest the mineral
components of bone matrix (resorption)
 Regulate blood calcium level
Osteoclast

12. Bone Formation
 The process by which bone forms is called
ossification or osteogenesis
 Bone formation occurs in four situations:
1. Formation of bone in an embryo
2. Growth of bones until adulthood
3. Remodeling of bone
4. Repair of fractures

13. 1. Formation bone
1. Development of the ossification center.
 At site where bone develops specific chemical
messages cause mesenchymal cells to aggregate and
differentiate, first into osteogenic cells then
osteoblasts.
Blood capillary
Ossification center
Mesenchymal cell
Osteoblast
Collagen fiber
1 Development of ossification center

14. 1. Formation bone
2. Calcification:
 Within a few days calcium and other mineral salts
are deposited and extracellular matrix becomes
calcified.
Osteocyte in lacuna
Canaliculus
Osteoblast
Newly calcified bone
matrix
2 Calcification

15. 1. Formation bone
3. Formation of trabeculae:
 Extracellular matrix develops into trabeculae that
fuse with each other to form spongy bone
 Blood vessels grow into the spaces
 Connective tissue associated with blood vessels
differentiates into red marrow
Mesenchyme
condenses
Blood vessel
Spongy bone
trabeculae
Osteoblast
3 Formation of trabeculae

16. 1. Intramembranous ossification
4. Development of the periosteum:
 In conjunction with formation of trabeculae, the
mesenchyme condenses at the periphery
 Eventually a thin layer of compact bone replaces the
spongy bone, but spongy bone remains in the center
 Much of the new bone is remodeled as the bone is
transformed into adult shape and size
Periosteum
Spongy bone tissue
Compact bone tissue
4 Development of the periosteum

17. Bone Growth During Infancy, Childhood
and Adolescence
 Growth in Length
 The growth in length of long bones
involves four major events:
1. Growth of cartilage on the
epiphyseal plate
2. Replacement of cartilage by bone
tissue in the epiphyseal plate
3. The activity of the epiphyseal plate
is the way bone can increase in
length
4. At adulthood, the epiphyseal plates
close and bone replaces all the
cartilage leaving a bony structure
called the epiphyseal line

18. Bone Growth During Infancy, Childhood and
Adolescence
 Growth in Thickness
 Bones grow in thickness at the outer surface
 Remodeling of Bone
 Bone forms before birth and continually renews
itself
 The ongoing replacement of old bone tissue by new
bone tissue
 Old bone is continually destroyed and new bone is
formed in its place throughout an individual’s life

20. Factors Affecting Bone Growth and Bone
Remodeling
 Normal bone metabolism depends on several factors
 Minerals
 Large amounts of calcium and phosphorus and smaller
amounts of magnesium, fluoride, and manganese are
required for bone growth and remodeling
 Vitamins
 Vitamin D helps build bone by increasing the absorption
of calcium from foods in the gastrointestinal tract into
the blood
 Vitamins K are also needed for synthesis of bone
proteins

21. Bone’s Role in Calcium Homeostasis
 Bone is the body’s major calcium reservoir
 Levels of calcium in the blood are maintained by
controlling the rates of calcium resorption from
bone into blood and of calcium deposition from
blood into bone
 Both nerve and muscle cells depend on calcium ions
(Ca2+) to function properly
 Blood clotting also requires Ca2+
 Many enzymes require Ca2+ as a cofactor

22. Bone’s Role in Calcium Homeostasis
 Actions that help elevate blood Ca2+ level
 Parathyroid hormone (PTH) regulates Ca2+
exchange between blood and bone tissue
 PTH increases the number and activity of osteoclasts
 PTH acts on the kidneys to decrease loss of Ca2+ in the
urine
 PTH stimulates formation of calcitriol a hormone
that promotes absorption of calcium from foods in
the gastrointestinal tract

24. Bone’s Role in Calcium Homeostasis
 Actions that work to decrease blood Ca2+ level
 The thyroid gland secretes calcitonin (CT) which
inhibits activity of osteoclasts
 The result is that CT promotes bone formation and
decreases blood Ca2+ level
 Accelerates uptake of calcium and phosphate into bone
matrix

25. Exercise and Bone Tissue
 Bone tissue alters its strength in response to
changes in mechanical stress
 Under stress, bone tissue becomes stronger through
deposition of mineral salts and production of collagen
fibers by osteoblasts
 Unstressed bones diminishes because of the loss of bone
minerals and decreased numbers of collagen fibers
 The main mechanical stresses on bone are those
that result from the pull of skeletal muscles and
the pull of gravity
 Weight-bearing activities help build and retain
bone mass

26. Aging and Bone Tissue
 The level of sex hormones diminishes during
middle age, especially in women after menopause
 A decrease in bone mass occurs
 Bone resorption by osteoclasts outpaces bone
deposition by osteoblasts

Female bones generally are smaller and less
massive than males
 Loss of bone mass in old age has a greater adverse effect
in females

27. Aging and Bone Tissue
 There are two principal effects of aging on bone tissue:
1) Loss of bone mass
 Results from the loss of calcium from bone matrix
 The loss of calcium from bones is one of the symptoms in osteoporosis
2) Brittleness
 Results from a decreased rate of protein synthesis
 Collagen fibers gives bone its tensile strength
 The loss of tensile strength causes the bones to become very brittle and
susceptible to fracture