Lecture Notes: Unit II: Principles of Support and Movement
Chapter Six - The Skeletal System: Bone Tissue
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I. Basic Functions of the Skeletal System
A. Support of soft tissues and existence of attachment sites for muscles, creating a framework for
the body.
B. Protection from injury is afforded to internal organs by bones which overlie and/or surround them.
C. Movement is facilitated since bones provide leverage for muscle contraction.
D. Minerals are stored in bones, particularly calcium and phosphorus; these may be mobilized when
needed elsewhere in the body.
E. Blood cell-producing cells are stored in the red bone marrow (aka myeloid tissue) of certain bones
or in certain regions of specific bones. Blood cell production is known as hemopoiesis or hematopoiesis.
F. Storage of energy occurs in the lipids found in yellow bone marrow (aka adipose c.t.) (Note:
yellow bone marrow may convert back to red bone marrow if the need arises.)
II. Histology of Skeletal Tissue
A. Structurally, the skeletal system includes several types of connective tissue: cartilage, dense connective tissue, and in greatest abundance: osseous connective tissue (bone c.t.)
B. Bone is a connective tissue consisting of sparsely distributed cells (four types) surrounded by
intercellular substance (matrix) containing mineral salts and collagen fibers.
I. The four types of cells found in bone are:
a. Osteoprogenitor (osteogenic) cells - undergo cell division and develop into osteoblasts
b. Osteoblasts - bone-building cells in that they lay down new bone matrix
c. Osteocytes - mature bone cells and the principal cells that maintain bone matrix
d. Osteoclasts - derived from monocytes (white blood cells) which fuse into one large, multinucleated cell and function as phagocytes to break down bone matrix and cells.
2. The intercellular material (matrix) has a framework of collagen fibers around and between
which abundant mineral salts accumulate. The primary mineral salt is a calcium phosphate
called hydroxyapatite (Ca10(PO4-3)6(OH)2). Other inorganic salts are: calcium carbonate,
magnesium sulfate and a few others. These salts crystallize to give bone its characteristic
hardness. (Note: mineralization or calcification refers to the process where inorganic salts are
deposited in a framework of collagen fibers. Remember, it is the collagen fiber arrangement in
the matrix of osseous connective tissue which gives bone its tremendous tensile strength.)
A typical long bone has several 5 regions and 7 major structures:
l. The 5 regions are: (a) proximal and distal epiphyses (singular is epiphysis) which are the
dilated "ends" of the bone; (b) the diaphysis - the tubular shaft of the bone; and (c) proximal
and distal metaphyses - the area between each epiphysis and the diaphysis. It is important to
understand that each metaphysis is called an epiphyseal plate in a child's long bone and it
consists of actively growing hyaline cartilage c.t.. However, in an adult bone, the metaphyses
are called epiphyseal lines and consist of osseous connective tissue.
2. The 10 major structures are:
(a) Periosteum - connective tissue structure covering the entire surface of the bone except
.for overlying the articular cartilages.The periosteum consists of two layers: an outer,fibrous
c.t. layer for protection and for attachment of ligaments and tendons, and an inner, osteogenic layer populated by osteoprogenitor cells which will e stimulated into activity by bone
damage/breakage. The periosteum also serves to nourish the bone by providing sites for
the major blood vessels to pass into and out of the bone.
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(b) Proximal and distal articular cartilages - hyaline cartilage pads at the ends of bone which
serve to absorb shock and reduce friction between adjacent, articulating bones.
(c) Spongy bone (aka cancellous or trabecular bone) fills the epiphyses and occupies a narrow rim around the medullary cavity.
(d) Compact bone - forms the tubular diaphysis. This is true Haversian bone and provides
A rigid "wall" to support not only the bone itself, but the tissues of the body.
.
(e) Medullary cavity - the space within the diaphysis. This cavity is occupied by myeloid
tissue (red bone marrow) in the child; but by yellow bone marrow in the adult.
(f) Endosteum - connective tissue structure lining the medullary cavity. Like its counterpart, the periosteum, it consists of two layers: a protective, fibrous layer and an osteogenic layer housing the osteogenic cells.
(g) Nutrient artery and nutrient foramen - a hole in the diaphysis through which perforates
The major nutrient blood vessel. Surrounding this area was the primary ossification center,
and it has tremendous significance to the ongoing viability of the bone. (h)
Epiphyseal plate (child's bone metaphysis) or Epiphyseal line (adult's bone metaphysis)
C. Histologic arrangement of bone is that all bones have regions of relatively solid osseous c.t.
(compact, or dense, bone) and more porous osseous c.t. (spongy bone). Bone is categorized as
being either compact or spongy
1. Compact bone is a highly structured and complex tissue.
a. The structural unit of compact bone is the osteon (aka Haversian system) characterized by
its "concentric ring structure" consisting of a central (Haversian) canal which is surrounded
by interspersed layers of matrix and osteocytes (within their small spaces called lacunae)
(1) Central (Haversian) canals - run parallel with the diaphysis along the bone's length.
(2) Concentric lamellae - are produced by the secretory action of osteoblasts and consist
of (calcified) bone tissue matrix that surround the central canal.
(3) Osteocytes in their lacunae -lacunae are the small spaces in between the lamellae that
house an osteocyte (mature bone cell)
(4) Canaliculi - tiny, bone matrix canals running throughout the lamallae which are
occupied by osteocytic cellular extensions; osteocytes retain junctions with
one another to share nutrients (since diffusion through a solid milieu is nonexistent)
(5) Perforating (Volkmann's) canals - these run perpendicular to the length of the diaphysis
and interconnect the central (Haversian) canals to each other so that the
entire blood supply of the bone becomes one, huge network of vertically
and horizontally
running
blood with
vessels.
(6) Interstitial lamllae
- areas between
osteons
canaliculi and osteocytes; these are
fragments of old osteons that have been partially obliterated during bone
growth and remodeling
,
(7) Circumferential lamellae - lamellae that encircle the bone just beneath the periosteum
are called outer circumferential lamellae; those that encircle the medullary cavity are called inner circumferential lamellae.
2. Spongy bone is not as complex as compact bone
a. Trabeculae ("little beams") are the structural unit of cancellous, or spongy, bone. These
form an irregular latticework of thin, spicule-like columns of bone matrix, NOT osteons.
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b. Spongy bone forms most of the structure of short, flat, and irregular bones as well as the epi
physes of long bones.
c. Spongy bone tissue is light-weight, supports and protects the myeloid tissue.
IV. Methods of Bone Formation (Ossification or Osteogenesis)
A. Intramembranous ossification
1. Forms flat bones (sternum, ribs, certain skull bones) and the mandible and clavicles.
2. Ossification forms from mesenchymal cells as they differentiate into osteoblasts and lay down
osteoid matrix in response to a primary ossification center ( penetrating blood vessel).
3. Matrix surrounds the cells (which are now called osteocytes) and then calcifies.
4. Calcifying matrix centers form bridges ("beams") of trabeculae that constitute spongy bone with
myeloid in between.
5. Periosteum first forms a collar of spongy bone that is subsequently replaced by several layers
(lamellae) of compact bone.
B. Endochondral ossification
1. Replacement of cartilage connective tissue by bone connective tissue
2. Forms all long bones (except the clavicles) and most other bones in the body.
3. Process of endochondral ossification
a. A cartilage model is initially formed.
b. Growth of the cartilage model occurs
c. A primary ossification center develops in the diaphysis
d. Development of secondary ossification centers in the epiphyses (usually not
until after birth)
e. Formation of articular cartilages and the epiphyseal plate
V. Bone Growth (two distinct types)
A. Lengthwise growth - also known as interstitial growth
1. The epiphyseal plate has four zones
a. Zone of resting cartilage
b. Zone of proliferating cartilage
c. Zone of hypertrophic cartilage
d. Zone of calcified cartilage
2. Activity of the epiphyseal plate is the only means by which the diaphysis can increase in length
3. Lengthwise growth of the bone ceases when the epiphyseal plate is completely replaced by bone.
B. Growth in thickness - also known as appositional growth
1. Addition of new bone tissue by osteoblasts around the outer surface of the bone
2. Medullary cavities get wider and wider as old bone is removed from the inner perimeter, and
new bone deposits along the outer surface of the bone.
VI. Factors which affect bone growth
A. Adequate dietary intake of minerals and vitamins is essential for growth and maintenance of
healthy, robust bone connective tissue
1. Calcium and phosphorus are needed in large concentrations; other minerals in smaller amts.
2. Vitamins A, B12, C, D (for normal calcium absorption) and K
3. Hormones
a. Insulinlike growth factors (IGF's) are stimulated by human growth hormone and is the
most important bone growth hormone during childhood
b. Thyroid hormones (T3 and T4)
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c. Insulin
d. Gender hormones (aka the sex steroids): estrogen and testosterone (Low-to-medium levels
of both estrogen and testosterone have a STIMULATING effect on epiphyseal plate
chondrocytes; thus girls and boys grow taller during early puberty. A high level of estrogen is one of the most powerful messages to "shut down" epiphyseal plate activity – thus
promoting epiphyseal line formationl; which is why, generally, interstitial bone growth
ends earlier in females and they are not as tall as their male counterparts.
VII. Homeostasis of bone connective tissue
A.Bone remodeling
1. Remodeling is the ongoing replacement of old bone conn. tissue by new bone c.t.
2. Old bone is constantly destroyed by osteoclasts, whereas new bone is constructed by
osteoblasts
B. Fracture and repair of bone
1. A fracture is any break in a bone
2. Types of fractures (fx)
a. Open (compound) fx - the broken ends of the bone protrude through the skin
b. Closed (simple) fx - broken ends of the bone do not break the skin
c. Comminuted fx - bone was splintered upon impact with many fragments resulting
d. Greenstick fx - partial fracture where one side of the bone breaks but other side bends
(Seen in children only whose long bones have not yet fully ossified)
e. Impacted fx - one end of the fractured bone is driven into the interior of another bone
f. Pott's fx - a fracture at the distal end of the fibula with resultant disruption of the distal
tibial articulation with the talus.
g. Colle's fx - a fracture at the distal end of the radius with distal displacement posteriorly.
3. Steps involved in repairs of fractures
a. Formation of a fracture hematoma - a clot is formed where blood vessels are broken
b. Fibrocartilaginous callus formation - conversion of the clot into granulation tissue due
to invasion of capillaries into the area creating a structure called a procallus. Now, fibro
blasts and osteogenic cells invade the procallus giving rise to a fibrocartilaginous callus.
c. Bony callus formation - osteogenic cells differentiate into osteoblasts producing the
trabeculae of spongy bone. Fibrocartilage is thus converted into bone: the bony callus.
d. Bone remodeling - compact bone replaces spongy bone.
4. Calcium homeostasis
a. Parathyroid hormone (PTH) - the "anti-bone" hormone
1. Produced by the Parathyroid glands
2. Promotes reabsorption of bone matrix ..... by
3. Increasing the number and stimulating the activity of osteoclasts.
b. Calcitonin (CT) – "pro-bone" hormone
1. Secreted by the Thyroid gland (its parafollicular cells)
2. Functions in several ways simultaneously:
(a) inhibits activity of osteoclasts
(b) speeds blood calcium uptake by bone
(c) accelerates calcium deposition into bones