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HDTD/ Bone- Lecture I
Lecturer Dr. Zahraa Qasim Ali
Dear students:
By the end of this first lecture of the “Bone”, I hope each one of you will be able
to:
1. Define bone histologically (cells & extracellular structure)
2. List the functions of bones.
3. List types of bones, their histological features & its location.
Introduction:
Over 200 bones in the body provide mechanical strength for movement and body
structure. The major function of mineralized bone, which contains mostly collagen
type I fibers with calcium hydroxyapatite and calcium carbonate crystals, is to give
form to the body. These bones also attach and support the skeletal muscles to allow
movement. Bone has compact and cancellous regions and has a role in maintaining
serum and tissue calcium and phosphorus levels. Bone marrow has a hematopoietic
function, in that the marrow contains immature blood cells and stem cells to
constantly replenish (replace) them. The major pool of hematopoietic cells is located
in the pelvis and sternum.
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Histology & function of the Bone:
Bone is a specialized form of connective tissue that, like other connective tissues,
consists of cells and extracellular matrix. The feature that distinguishes bone from
other connective tissues is the mineralization of its matrix, which produces an
extremely hard tissue capable of providing support and protection.
By virtue of its mineral content, bone also serves as a storage site for calcium and
phosphate. Both calcium and phosphate can be mobilized from the bone matrix and
taken up by the blood as needed to maintain appropriate levels throughout the body.
Thus, in addition to support and protection, bone plays an important secondary role
in the homeostatic regulation of blood calcium levels.
Bone matrix contains mainly type I collagen along with other matrix (non-
collagenous) proteins.
It is good to know
, three forms of non- collagenous proteins, which are
[
(1)
Bone-
specific, vitamin K–dependent proteins
, which include
osteocalcin
(which
captures calcium from the circulation and attracts and stimulates osteoclasts in bone
remodeling), (2)
protein S
(which assists in the removal of cells undergoing
apoptosis), and
matrix Gla-protein (MGP)
(which participates in the development
of vascular calcifications). (3)
Growth factors
and
cytokines
, which are small
regulatory proteins including insulin-like growth factors (IGFs), tumor necrosis
factor _ (TNF), transforming growth factor _ (TGF), platelet-derived growth factors
(PDGFs), bone morphogenic proteins (BMPs), and interleukins (IL-1, IL-6). The
most unique members of this group are BMPs, because they induce the
differentiation] of mesenchymal cells into osteoblasts, the bone- producing cells.
Recombinant human BMP-7, also known as osteogenic protein-1 (OP-1), is now
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used clinically to induce bone growth after bone surgery involving large bone
defects, spinal fusions, or implantation of graft materials
.
]
Bone matrix contains lacunae connected by a network of canaliculi.
Within the bone matrix are spaces called
lacunae
(sing., lacuna), each of which
contains a bone cell, or
osteocyte
. The osteocyte extends numerous processes into
small tunnels called
canaliculi
. Canaliculi course through the mineralized matrix,
connecting adjacent lacunae and allowing contact between the cell processes of
neighboring osteocytes. In this manner, a continuous network of canaliculi and
lacunae-containing cells and their processes is formed throughout the entire mass of
mineralized tissue. Bone tissue depends on the osteocytes to maintain viability.
Key:
CL- circumferential lamellae IL- interstitial lamellae
HC- Haversian canal L- lacuna
VC- Volkmann’s canal Arrow- lamellar boundary
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In addition to osteocytes, four other cell types are associated with bone:
1.
Osteoprogenitor cells
are cells derived from mesenchymal stem cells; they
give rise to osteoblasts.
2.
Osteoblasts
are cells that secrete the extracellular matrix of bone; once the
cell is surrounded with its secreted matrix, it is referred to as an
osteocyte
.
3.
Bone-lining cells
are cells that remain on the bone surface when there is no
active growth. They are derived from those osteoblasts that remain after bone
deposition ceases.
4.
Osteoclasts
are bone-resorbing cells present on bone surfaces where bone is
being removed or remodeled (re-organized) or where bone has been damaged.
Osteoprogenitor cells and osteoblasts are developmental precursors of the
osteocyte. Osteoclasts are phagocytotic cells derived from fusion of
hemopoietic progenitor cells in bone marrow that give rise to the neutrophilic
granulocyte and monocyte lineages.
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Clinical points:
A benign tumor arising from osteoblasts is called osteoma.
A malignant tumor arising from the same cells is called osteosarcoma.
Osteosarcoma are most commonly seen in bones adjoining (attaching) the
knee. They can spread to distant sites in the body through the blood stream.
BONES AND BONE TISSUE
Bones are the organs of the skeletal system; bone tissue is the structural
component of bones.
Typically, a bone consists of bone tissue and other connective tissues, including
hemopoietic tissue, fat tissue, blood vessels, and nerves. If the bone forms a freely
movable or synovial joint, hyaline cartilage is present. The ability of the bone to
perform its skeletal function is attributable to the bone tissue and, where present, the
hyaline or articular cartilage.
Bone tissue is classified as either compact (dense) or spongy (cancellous).
If a bone is cut, two distinct structural arrangements of bone tissue can be
recognized. A compact, dense layer forms the outside of the bone (compact bone);
a sponge-like meshwork consisting of trabeculae
(thin, anastomosing spicules of
bone tissue) forms the interior of the bone (spongy bone). The spaces within the
meshwork are continuous and, in a living bone, are occupied by marrow and blood
vessels.
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Bones are classified according to shape; the location of spongy and compact
bone varies with bone shape.
Spongy and compact bone tissues are located in specific parts of bones. It is useful,
then, to outline briefly the kinds of bones and survey where the two kinds of bone
tissue are located. On the basis of shape, bones can be classified into four groups:
1. Long bones
are longer in one dimension than other bones and consist of a
shaft and two ends (e.g., the tibia and the metacarpals).
2. Short bones
are nearly equal in length and diameter (e.g., the carpal bones of
the hand).
3. Flat bones
are thin and plate-like (e.g., the bones of the calvaria [skullcap]
and the sternum). They consist of two layers of relatively thick compact bone
with an intervening layer of spongy bone.
4.
Irregular bones
have a shape that does not fit into any one of the three groups
just described; the shape may be complex (e.g., a vertebra), or the bone may
contain air spaces or sinuses (e.g., the ethmoid bone).
Types of Bone
Based on histology: Compact bone & Spongy bone
Based on maturity: Mature/ lamellar bone & Immature/woven bone
Based on manner of development: Cartilage bone & Membrane bone.
General structure of bones:
Bones are covered by a periosteum
except in areas where they articulate with
another bone. In the latter case, the articulating surface is covered by cartilage.
The periosteum that covers an actively growing bone consists of an outer
fibrous layer that resembles other dense connective tissues and an inner, more
cellular layer that contains the osteoprogenitor cells.
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The bone Haversian system is surrounded by osteocyte lacunae and
canaliculi around the marrow cavity with blood vessels and nerve fibers
adjacent to bone cells.
Where a bone articulates with a neighboring bone, as in synovial joints, the
contact areas of the two bones are referred
to as articular surfaces. The
articular surfaces are covered
by hyaline cartilage, also called articular
cartilage.
The lining tissue of both the compact bone facing the marrow cavity and the
trabeculae of spongy bone within the cavity is referred to as endosteum. The
endosteum is often only one cell layer thick and consists of osteoprogenitor
cells that can differentiate into bone matrix–secreting cells, the osteoblasts,
and bone-lining cells.
The marrow cavity and the spaces in spongy bone contain bone marrow.
Red bone marrow consists of blood cells in different stages of development
and a network of reticular cells and fibers that serve as a supporting
framework for the developing blood cells and vessels. As an individual grows,
the amount of red marrow does not increase proportionately with bone
growth. In later stages of growth and in adults, when the rate of blood cell
formation has diminished, the tissue in the marrow cavity consists mostly of
fat cells; it is then called
yellow marrow.
It is good to know that [
In response to appropriate stimuli, such as extreme
blood loss, yellow marrow can revert to red marrow. In the adult, red marrow
is normally restricted to the spaces of spongy bone in a few locations such as
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the sternum and the iliac crest. Diagnostic bone marrow samples and marrow
for transplantation are obtained from these sites
]
.
Mature spongy bone is similar in structure to mature compact bone except
that the tissue is arranged as trabeculae
or spicules; numerous
interconnecting marrow spaces of various sizes are present between the bone
tissues. The matrix of the bone is lamellated.
Nutrient foramina are openings in the bone through which blood vessels pass
to reach the marrow. The greatest numbers of nutrient foramina are found in
the diaphysis and epiphysis.