BIOL-4000 LECTURE NOTES #5B
BONE and BLOOD
CARTILAGE AND BONE LECTURE TEXT - POWERPOINT
BLOOD LECTURE TEXT - POWERPOINT
Cartilage is a resilient connective
tissue composed of cells embedded in an extracellular matrix that is gel-like
and has a rigid consistency.
support to softer tissues
formation and growth of long bones
extracellular matrix containing mainly,
collagen and/or elastin fibers
Collagen provides tensile strength and
durability, however, proteoglycans are also important. For example, if you
inject papain (an enzyme that digests the protein cores of proteoglycans) into
the ears of a rabbit, after a few hours the ears will loose their stiffness and
Three types of cartilage -
extracellular matrix differs in terms of concentration of collagen and elastin fibers.
R. Nims & S.C. Kempf 12/2000
1. Hyaline cartilage
a. dominant component of extracellular
b. matrix is collagen.
c. Bluish-white in life
e. important in formation and growth of
f. In adult, mainly found lining outer
wall of respiratory system and on surfaces of bone joints where it is called
g. Undergoes calcification in bone
formation and also as part of aging process.
2. Elastic cartilage
a. high concentration of elastin fibers
b. in extracellular matrix. (Example -
c. does not calcify
3. Fibrous cartilage (fibrocartilage)
a. found at connection of tendons to bone.
b. contains very large bundles of collagen
c. resists compression and shear forces.
d. also found in intervertebral discs.
STRUCTURE AND FORMATION OF
This tissue acts to support softer
tissues and also is important in the formation and healing of endochondral
bines such as the long bones of the arm and leg. The qualities of the different
types of cartilage depend on differences in the concentration of collagen and
elastin fibers in the extracellular matrix and on the proteoglycan molecules
that these fibers are associated with.
Cartilage is devoid of blood vessels.
Thus the nutrition of cells within the cartilage matrix is dependent on the
diffusion of nutrients from blood capillaries in the perchondrium and/or
adjacent tissues through the matrix.
Hyaline and elastic cartilage are
surrounded by a connective tissue capsule called the PERICHONDRIUM that
contains the capillaries from which the nutrients diffuse into the cartilage
matrix. Articular hyaline cartilage and fibrocartilage do not have a
More information on cartilage:
R. Nims & S.C. Kempf 12/2000
Hyaline cartilage is the most common cartilage
in the body. It is bluish-white and translucent. Important in the formation of
long bones of the body in embryo and during growth.
In adult, mainly found lining the respiratory
passages such as trachea. Also at ventral ends of ribs and as ARTICULAR
CARTILAGE on the bone surfaces
Dominant component of extracellular matrix is
collagen fibers. Other components are sulfated proteoglycans and hyaluronic
Main tissue components
1. perichondrium - vascularized connective tissue sheath
surrounding cartilage (except in case of articular cartilage). Rich in
collagen. Contains fibroblasts that secrete the materials for the collagen
fibers. Inner layer (next to cartilage matrix) contains cells that are thought
by some to be fibroblasts and by others to be undifferentiated mesenchyme
cells. In either case, these cells can differentiate to form chondroblasts.
2. chondroblasts - immature cartilage cells. Secrete
extracellular matrix, but are not yet rigidly embedded in that matrix.
3. chondrocytes - mature cartilage cells that are embedded in
rigid extracellular matrix. These cells reside in small spaces within the
matrix that are called lacunae. May be more than one cell in a lacuna. Living
chondrocytes have an eliptic shape. Organelle systems in cytoplasm are typical
of cells that secrete. Chondrocytes in hyaline cartilage that are grouped
together are called isogenic groups.
Similar to hyaline cartilage except,
1. matrix impregnated with elastic fibers
2. yellow in color
3. chondrocytes are more closely packed
and only one chondrocyte per lacuna. No isogenic groups
4. does not calcify under normal
conditions and does not show ossification in old age.
5. exhibits less accumulation of glycogen
and lipids than hyaline cartilage.
An irregular, dense, fibrous tissue with thinly
dispersed, encapsulated chondrocytes. No perichondrium, so it blends with
adjacent connective tissue. Most easily seen in articular disks such as the
intervertebral disks. Also found where tendon connects to bone. Shows
resistance to compression, durability and high tensile strength.
HISTOGENESIS OF CARTILAGE
As the embryo develops, mesenchymal
cells will aggregate into closely knit clusters and differentiate into
chondroblasts. These cells will begin to secrete collagen and
mucopolysaccharide matrix containing chondroitin sulfate. The matrix secretion
will cause the chondroblasts to be pushed apart.
As this occurs, the cartilage cells
will undergo divisions. This will result in small clusters of chondroblasts
within the developing matrix which will also start to secrete matrix and be
pushed away from each other. This sort of growth of cartilage is termed interstitial
growth due to the fact that the
extracellular matrix is secreted into spaces between the cells.
Growth of cartilage can also be appositional, that is a layer of chondroblasts can lay down
matrix at the outer edge of a mass of cartilage.
As the cartilage continues to grow, the
central regions become more rigid due to various secretory products and the
cells in this region become embedded in rigid matrix and take on the
characteristics of mature chondrocytes. The outer edge of the cartilage mass
becomes invested with additional mesenchymal cells that differentiate into
fibroblasts to form a specialized connective tissue covering for the cartilage
known as perichondrium. Chondroblasts that differentiate from mesenchyme cells
at the inner edge of the perichondirum also secrete matrix causing appositional
growth of the cartilage mass.
Similar histogenesis can result in
elastic (external ear) or fibrous cartilage (intervertebral discs) in other
parts of the body.
Bone is one of the hardest substances
in the body. You might look at it and think of it as dead, mineralized
material. It's important to realize that bone is a living tissue composed of
cells and their associated extracellular matrix. IT IS A CONNECTIVE TISSUE!
1. periosteum - a layer of connective tissue that covers the
bone containing a high concentration of collagen fibers. Two distinct layers.
External layer very fibrous, while internal layer is more cellular and
vascularized. Some of the collagen fibers penetrate the calcified bone matrix
and bind the periosteum to the bone. These fibers are called Sharpey’s Fibers.
Cells of this connective tissue play important roles in bone histogenesis and
in the healing of fractures.
2. endosteum – similar to the periosteum, but only one cell
thick. Lines the internal surfaces of bone.
3. Important roles of the periosteum and
endosteum are nutrition of bone cells and provision of osteoblasts for bone
histogenesis and repair.
There are 3 different bone cell types.
1. osteoblasts - immature bone cells that synthesize and
secrete the osteoid matrix that will calcify to form the bones extracellular
matrix. This matrix is composed of glycoproteins and collagen. In areas where
these cells occur, they are located on the surfaces of forming bone and are not
yet embedded in the extracellular matrix. These cells have cytoplasmic processes
that bring them into contact with neighboring osteoblasts, as well as nearby
osteocytes. Ultrastructure shows organelle systems typical of secretory cells.
2. osteocytes - mature bone cells. These cells are
osteoblasts that have become embedded in calcified bone matrix. They reside in
lacunae within the matrix and are in contact with neighboring osteocytes via
cytoplasmic processes that extend through small tunnels called canaliculi.
Contacting cytoplasmic processes form gap junctions. This communication between
osteocytes is important in the tranfer of nutrients to these cells and wastes
out of them since they may be far removed from blood capillaries. The cells are
flattened and their internal organelles exhibit the characteristics of cells
that have reduced synthetic activity.
3. osteoclasts - these are large, multinucleate cells that act
to reabsorb bone during specific stages in bone formation and healing, and
during the continual reworking of internal bone architecture that occurs
Bone matrix - 50% inorganic components
composed mainly of various calcium salts. The organic matter composing the
other 50% of the matrix is made-up of 95% collagen and a mixture of various
glycosaminoglycans associated with proteins.
HISTOGENESIS OF SKELETAL STRUCTURE
Two modes of bone formation
1. Endochondral - cartilage template formed that is replaced by
bone (e.g. vertebral column, long bones of limbs – most bones in body)
2. Intramembranous - direct formation of bone structure with no
cartilagenous template (e.g. flat bones of skull)
In the embryo, osteoblasts are derived from mesenchymal cells. These cells
either aggregate where bones are to form (intramembranous bone formation) and lay down the matrix that will later become
calcified, or they migrate into pre-existing cartilage "models)" of
the presumptive bone and replace the cartilage with a calcareous matrix (endochondral
bone formation). Long bone
growth is also endochondral in nature. Osteoblasts are different than the
chondroblasts that begin the histogenesis of cartilage and should not be
confused with them.
As the primordial bone matrix is layed
down, the osteoblasts become entrapped in lacunae within the matrix and are
then known as mature osteocytes. As bone is being formed, there is also
localized removal of the bone matrix by another set of connective tissue cells
known as osteoclasts. These
cells are thought to differentiate from monocytes and are responsible, in part,
for the internal architecture of bones in that they excavate localized portions
of the forming bone and make passageways for such things as blood vessels
nerves. Osteoclasts will continue to remodel the bone throughout a person’s
A third population of cells involved in
bone formation are the cells of the marrow. These are the stem cells for blood
cells and all their progeny (see Blood below).
INTRAMEMBRANOUS BONE FORMATION
Occurs mainly in bones of the skull.
Mesenchymal cells aggregate and begin to
secrete matrix that is characterized by bundles of collagenous fibers. The
secreted osteoid matrix has a high affinity for calcium salts, that are brought
into the area of bone formation by the circulatory system. These deposit within
and on the matrix to form calcified bone. As this calcification takes place,
the mesenchymal cells undergo morphological changes. They loose the appearance
of mesenchymal cells and round up becoming true osteoblasts. The osteoblasts
become oriented in epithelial-like layers along the forming bone. The
osteoblasts and the collagen and other components of the intercellular matrix
form the organic osteoid framework of the bone.
As a strand of matrix is invested with
inorganic salts it forms a spicule of bone. The spicules will merge to form
larger calcified structures called trabeculae. These will thicken with the
deposition of more osteoid matrix and inorganic salts as the osteoblasts
continue their secretion in an appositional manner. This secretion by the
osteoblasts is cyclic and results in layers of bone material called lamellae.
The deposition of lamellae traps some of the osteoblasts within the osteoid
matrix. Once trapped they are considered mature osteocytes. Osteocytes are
characterized by cytoplasmic processes that contact similar processes of
adjacent osteocytes. Gap juctions form at points of contact allowing transfer
of small molecules between cells. This transfer is important in co-ordinating
bone growth and in the nourishment of osteocytes which may be separated from
blood vessels by a considerable amount of calcified bone. Channals through
which cytoplasmic processes of osteocytes extend are called canaliculi.
Growing adjacent trabeculae will
contact and fuse forming the structure of the mature bone. As intramembranous
bones grow, selective reabsorption of bone material is also occurring due to
the activities of osteoclast cells. This results in the formation of much of
the internal architecture of the bones, providing spaces for blood vessels and
ENDOCHONDRAL BONE FORMATION
Most of the bones in the mammalian body
are initially formed by endochondral means. That is to say, a template of
hyaline cartilage that is in the shape of a miniature of the bone is layed down
prior to the bone’s formation. This deposition of cartilage occurs as
previously discussed and is accomplished by the action of chondroblasts
functioning in both interstitial and appositional growth capacities.
The typical examples that are used to
describe endochondral bone formation are the long bones of the limbs. Perhaps
it will be easier to understand their histogenesis if we first consider the
general structure of bones. We will consider this structure as it exists in
long bones, however, it should be kept in mind that girdle bones, such as the
pelvic girdle, and the intramembranous flat bones of the skull are made up of
the same basic components, though they may be arranged somewhat differently.
Bones of the body, including the long
bones may be considered a rigid form of connective tissue. The cells of this
tissue are embedded within a matrix that consists of organic and inorganic
components. The organic matrix, or ground substance, consists of collagen
fibers for the most part. The inorganic component mostly consists of calcium salts,
calcium phosphate (85%), calcium carbonate (10%), and small amounts of calcium
and magnesium flouride. While we tend to think of the inorganic components as
being the contributing factors in a bone's structural integrity, you should
realize that the collagen fibers also contribute significantly to the bones
strength and resilience.
Two types of bone tissue can be
distingished. These are cancellous, or spongy, bone that lies centrally within
the shaft of long bones, and compact or dense bone that lies more peripherally.
You should realize that the actual mineralized matrix of these two types of
bone is the same. It contains embedded osteocytes that are in communication via
gap junctions at their contacting cytoplasmic processes. The difference between
spongy and compact bone lies simply in the size of open spaces within the
The spongy bone consists of slender,
irregular trabeculae with large spaces between them where blood vessels,
nerves, and marrow cells are located.
Compact bone appears solid, no large
cavities within it.
Since the actual mineralized matrix of
both types of bone is the same, there is no distinct boundary between spongy
and compact bone.
The shaft of a long bone consists of a
medullary or central volume of spongy bone surrounded by a thick cortical layer
of compact bone. The compact layer can be subdivided into an outer series of
sub-layers called periosteal lamellae that were secreted by the periosteal
cells during its development and growth, and an inner component consisting of
multiple concentric sub-layers surrounding the halversian canals. Radial
cavities called Volkmann’s canals also extend through the compact bone. These
radial cavities and halversian canals form a network within the compact bone
that is continuous with the cavities of the spongy bone. Blood vessels and
nerves extend through the channels of this network.
So how is this structure established?
The first step in endochondral bone formation is
the histogenesis of a cartilage miniature of the bone. This takes place as
discussed above via the action of chondroblasts that have migrated to the area.
The chondroblasts secrete a cartilagenous matrix that is laid down both
interstitially and appossitionally. The end result is a cartilage template of
the bone in miniature that contains chondrocytes embedded within the cartilage
Actual osteogenesis (bone ossification) begins
with the establishment of a periosteum on the shaft (or diaphysis) of the
cartilage template and the laying down of an intramembranous collar of bone on
the circumference of the cartilage diaphysis. This is followed by hypertrophy
(they get bigger) and eventual death of the chondrocytes within the cartilage
matrix. As the chondrocytes degenerate they reabsorb some of the surrounding cartilage
matrix causing enlargement of the lacunae in which they reside. This process is
known as hypertrophication.
As this occurs, the chondrocytes loose their ability to maintain the remaining
cartilage matrix and it becomes partially calcified. The end result is an area
of porous calcified cartilage within the central regions of the diaphysis. As
this is occurring, osteoclasts that have arrived in the area via the
circulatory system, begin excavating passageways or tunnels through the
intramembranous collar surrounding the diaphysis. These passageways provide a
means through which blood vessels, nerves and undifferentiated mesenchymes
cells can enter into the lacunae (spaces) in the remnants of the cartilage
matrix that have been left by the degenerating chondrocytes. The mesenchyme
cells will differentiate into osteoblasts and hematopoietic stem cells that are
distributed within the bone.
The osteoblasts, blood vessels, and nerves form
the osteogenic bud that comes to lie more or less centrally within the diaphysis
of the forming bone.
As the invading cells spread out within the
diaphysis of the cartilage template and ossification begins, this central
volume of active bone deposition is called a primary ossification center.
The osteoblasts begin to secrete osteoid matrix
on the remnants of calcified cartilage. The osteoid matrix becomes mineralized
forming cancellous bone in the shaft of the diaphysis. Some of the osteoblasts
become trapped within the mineralized bone and become mature bone cells,
osteocytes. These cells maintain contact with other osteocytes and/or with
osteoblasts via contacting cytoplasmic processes that extend through canaliculi
the mineralized matrix.
As the cancellous bone is layed down,
chondroclasts (which are the cartilagenous equivalent of osteoclasts) reabsorb
the calcified cartilage as it is replaced by osteoid matrix (i.e., the
calcified chondroid matrix does not form bone!). At this point, it is important
to note that this means the actual bone tissue, matrix and mineralization, is
the result of the action of a new group of cells, the osteoblasts. NOTE THAT
THE CARTILAGE IS NOT TRANSFORMED INTO BONE TISSUE!
The primary ossification center rapidly extends
longitudinally within the diaphysis as the shaft of the cartilage template is completely
replaced by cancellous bone tissue. As the ossification center extends
longitudinally, so does the calcified outer collar of bone layed down by the
As ossification proceeds in the diaphysis,
secondary ossification centers form in the cartilage of the bulbuous ends, or
epiphyses, at either end of the long bone shaft. Osteogenic tissues in these
regions also act to form mineralized bone. This process is similar to the
primary ossification we've just discussed with one difference. Since there is
no periosteum on the surface of the epiphyses, there is no periostial external
collar of bone.
What we have just discussed is endochondral bone
formation. This involved the deposition of cancellous, or spongy bone, within a
cartilage matrix. This is not the final step in bone formation. In fact, there
really is no such thing as a final step in this process.
During and after endochondral bone formation,
there is considerable internal remodeling of the architecture of the bone. This
is accomplished by the efforts of osteoblasts, osteocytes, and osteoclasts.
Osteoclasts act to reabsorb much of the
cancellous bone that has been layed down during endochondral bone formation. As
this occurs, channels are hollowed out within the spongy bone structure. These
are in addition to the cavities already formed in spongy bone. In more
peripheral regions where compact bone will be present, these channels will give
rise to the halversian systems as compact or dense bone is laid down within
As these peripheral channels are hollowed out,
osteoblasts from the marrow invade the channels and form an epithelium on the
channel's inner wall. These osteoblasts lay down cyclical layers of osteoid
matrix which becomes mineralized and decrease the diameter of the channels. As
this occurs, some of the osteoblasts are trapped within the matrix and become
osteocytes with the characteristic long cytoplasmic processes that extend
through canaliculi in the mineralized bone and contact each other. As this
ossification takes place, large cavities like those present in spongy bone are
not formed, thus, this kind of bone is called compact or dense bone. Since
there is no cartilage precursor to the compact bone, it may be considered
intramembranous as far as its mode of formation is concerned.
The final result of this ossification process is
the replacement of much of the spongy bone within the shaft of the diaphysis
with compact bone which has many halversian canals running through it.
Osteoclasts hollow out Volkmann’s canals that extend radially between haversian
The reabsorption and redeposition of compact and
spongy bone continues throughout life. Thus, the bones of your body are living,
FINALLY, we have to consider how bones grow. Obviously,
bones don't remain the length that they are at birth.
Let's go back to endochondral bone formation and
recall that it was proceeded by the deposition of cartilage. This cartilage was
eroded and replaced as the primary and secondary ossification centers did their
work. In an area just below the base of each epiphysis, where the tissues of
the primary and secondary ossification centers could meet, a plate of activily
growing cartilage remains. These epiphyseal plates are responsible for the
increase in length of bones during adolescent growth. Cartilage is layed down
as in early endochondral bone formation. This cartilage is subsequently eroded
and replaced by bone tissue in a process that is essentially the same as what
we have just discussed. As adult bone lengths are acheived, the epiphyseal
plates cease growth and are completely ossified. See your book and the text
Powerpoints for more information on long bone growth.
If blood is prevented from clotting, it can be
separated into its two major components by centrifugation (called hematocrit)
you get cellular and plasma fractions (52 - 57% plasma , 43 - 48% cells).
Plasma - aqueous solution, large and small molecules other than water
form 10% of weight, the rest is water.
Plasma proteins - 7%
alpha, beta, and gamma globulins
gamma globulins are antibodies (immunoglobulins)
Inorganic salts - 0.9%
Other organic molecules - 2.1% (vitamins, amino
acids, lipids, hormones, etc.)
Blood cells - two basic types - erythrocytes (red blood cells, hemoglobulin),
leukocytes (white blood cells, no hemoglobin). Also platelets which are
important in clotting, but these are actually fragments of a type of leukocyte.
Erythrocytes - red blood cells. No nucleus, biconcave disks,
about 7.2 um in diameter. Responsible for carrying oxygen bound to hemoglobin
to the tissues of the body.
Leukocytes, or white blood cells, can be divided into a number of major
sub-types. At light level, these sub-types are distinguished by there structure
as characterized by specific staining patterns.
Stains for this purpose were first developed by
Dimitri Romanovsky in 1891. These initial stain mixtures were later modified by
other investigators, so we have modified Romanovsky type stains called
Leishman's or Wright's stains for example.
Components of Romanovsky stain are methylene
blue and eosin. Blood cells are classified by the type of stain that binds to
them or their components.
1. basophilia - affinity for methylene blue which is a basic
2. azurophilia - affinity for azure dyes (purples) which
result from the oxidation of methylene blue in the mixture.
3. acidophilia or eosinophilia - affinity for eosin which is an acid satin
4. neutrophilia - affinity for complex dyes that are formed in
the mixture that have a salmon-pink to lilac color. The term neutrophilia comes
from the early misconception that these dyes were neither acid nor base and
TYPES OF BLOOD CELLS
erythrocytes - no nucleus, biconcave disk, high concentration of hemoglobin
that makes them acidophilic, about 7.2 um in diameter.
reticulocytes - Young erythrocytes recently released into
blood often contain ribosomal RNA that precipitates and stains within the cell.
leukocytes - white blood cells
Stem cells that give rise to the different types
of blood cells are located in the bone marrow.
Two major classifications are used for
1. One is based on the presence or absence of
the stained granules seen with the light microscope.
a. granulocytes - cells with specific granules that are quite
evident by virtue of the fact that they have affinity for specific stains.
b. agranulocytes - blood cells that don't have the specific
cytoplasmic granules of granulocytes.
2. The second is based on the morphology of the
a. mononuclear - nucleus is not composed of identifiable
b. polymorphonuclear - nucleus is composed of two or more distinct
BASIC LEUKOCYTE CELL TYPES
neutrophils - Compose 60-70% of the leukocytes. First line of cellular defense
against microorganisms, especially bacteria. Phagocytose small particles and
microorganisms. Cytoplasm contains granules surrounded by a membrane. Nucleus is
polymorphonuclear. It has 2-5 lobes linked together by fine threads of nuclear
material. Cytoplasm contains both many specific and also some azurophilic
granules (though they are often not particularly evident). These contain
enzymes that can function in digestion of phagocytosed particles. Neutrophils
are capable of amoeboid movement.
eosinophils - Compose 1-4% of the leukocytes, so they are less numerous than
neutrophils. Increase in number in allergic reactions. Recognize and
phagocytose antigen-antibody complexes and particles that are associated with
these complexes that are formed during an immune response. Can migrate using
amoeboid movement. May be involved in preventing blood clotting at times when
it is best that it not occur. Specific granules are lysosomes that contain
enzymes that can degrade phagocytosed particles. The nucleus is usually
bilobate (two lobes). Ovoid, eosinophilic (acidophilic) granules are present in
cytoplasm. These are much larger than the granules of a neutrophil.
basophils - Compose 0-1% of leukocytes. These cells secrete histamine.
Histamine causes dialation of blood vessels and in effect makes them leaky.
This allows serum proteins such as antibodies to infiltrate into tissues.
Basophils also have limited capacity for amoeboid movement and phagocytosis.
They have a large irregular nucleus that is generally S-shaped. The cytoplasm
is filled with specific granules larger than those in other granulocytes.
Granules are membrane bound and contain histomine and heparin.
lymphocytes - involved in humoral and cell mediated immune responses. Two
major catagories, T-lymphocytes that are involved in receptor mediated
responses of the immune system and B-lymphocytes that respond to antigens as
mediated by T- lymphocytes and produce antibodies against these antigens. In
blood smears, lymphocytes form a heterogeneous population that show up as
large, medium, and mostly small lymphocytes. Whether these are T- or B-
lymphocytes is not apparent in standard stained blood smears. These cells have
a spherical nucleus. Their chromatin is condensed and appears as coarse clumps
often having a clockface like appearance in tissues, but not in blood smears.
Small lymphocytes have very little cytoplasm surrounding their nucleus, while large
lymphocytes have a greater amount of cytoplasm. The cytoplasm may contain tiny
purple, azurophilic granules, but they are still considered to be
monocytes - circulating blood cells that can cross plasma
membranes and differentiate into macrophages. The nucleus is generally kidney
shaped or horseshoe shaped, but it may be oval. It is often positioned off
center. The chromatin is not as condensed as in lymphocytes and is often
fibrillar in appearance. The nucleus is more lightly stained than in large
lymphocytes and may have 2-3 nucleoli. The cytoplasm is basophilic and
frequently contains tiny azurophilic granules that may cause the cytoplasm to
be a blue-gray color. These cells may look like a band neutrophil in some
megakaryocytes - Large cell with irregularly lobed nucleus.
Platelets form as the result of fragmentation of the cytoplasm/plasma membrane
of megakaryocytes. Platelets are small, enucleated cell fragments that exist in
high concentrations in the blood. Important in healing of cuts and abrasions.
These cells are generally only found in the bone marrow.
Other "cellular" blood components
platelets - enucleated disk-like cell fragments that are 2-5 um diameter
often appear in clumps in blood smears. Function in clotting.
Blood cells have short life in circulatory
system and so they must be continually renewed.
erythrocytes and granulocytes are derived from
stem cells in the red bone marrow of healthy mammals.
lymphocytes are derived from stem cells in the
bone marrow and also in the lymphatic organs.
HEMATOPOIESIS OF ERYTHROCYTES
Involves what could be called a series
of stem cell stages:
Hemocytoblast – multipotent stem cell that gives rise to all types
of blood cells. In the case of erythrocyte hematopoiesis, it will divide and
one of the two cells formed will become a proerythroblast.
proerythroblast - probably can be considered stem cell for
erythrocyte line. Synthesizes proteins to form cytoplasmic components since it
divides rapidly. Also very small amounts of hemoglobin. 14-17 um diam.
Pinocytosis occurs. Metabolically very active. Proerythroblasts divide many
times and form cells that will become basophilic erythroblasts.
basophilic erythroblast - smaller, 13-16 um. undergoes mitotic
division. Some hemoglobin synthesized. Many mitochondria. These cells divide
many times and form cells that will become polychromatic erythroblasts.
polychromatic erythroblast - smaller, 12-15 um diam. size decrease due to
less cytoplasm and smaller nucleus. More hemoglobin. These cells divide many
times and form cells that will become normoblasts.
Normoblast - smaller, 8-10 um. nucleus eccentric. Abundance of hemoglobin.
Mitochondria and golgi apparatus begin to degenerate. After 3 divisions nucleus
is extruded and the cells become reticulocytes.
No further division after this point.
Reticulocyte - young, enucleate erythrocyte. Hemoglobin
synthesis continues for short period. Since RNA cannot be renewed from nucleus,
synthesis eventually ceases. Remaining organelles are autophagocytosed or
exocytosed. Cell becomes a mature erythrocyte.