Connective tissues
Connective tissue forms a framework upon which epithelial tissue rests and within
which nerve tissue and muscle tissue are embedded. Blood vessels and nerves travel
through connective tissue . Functions of connective tissue include: transport,
immunological defense, {mechanical support , growth and repair, energy reserve,
haemopoiesis, and inflammation.
All forms of connective tissue share some common structural features and a common
embryonic origin.
 Connective tissue consists of individual cells scattered within a matrix.
 Cells of connective tissue are not directly attached to one another .

Connective tissue is derived from mesoderm (unlike most epithelial tissue which is I
derived from ectoderm and endoderm).
Components of Connective Tissue:
Connective tissue consists of cells embedded in an extracellular matrix .The matrix,
in turn, consists of fibers and ground substance.
Cells of the connective tissue include:
1. Fibroblasts: are the most common cells in proper connective tissue. Fibroblasts
are responsible for secreting collagen and other elements of the extracellular matrix
of connective tissue. Resting fibroblasts typically have so little pale cytoplasm with
branched processes that the cells appear, by light microscopy, as "naked" unclei.
Fibroblast nuclei appear dense (heterochromatic) and are usually flattened or spindleshaped, with one or two nucleoli. Closely related to fibroblasts are the chondroblasts
which produce the matrix of cartilage and the osteoblasts which produce the matrix of
bone .
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2. Mast
cells are secretory cells . Upon the slightest disturbance, they release
chemical signals which diffuse through the surrounding ground substance and trigger
the process of inflammation . Mast cells occur as small individual cells, scattered
rather widely in proper connective tissue. The cytoplasm of mast cells is packed with
secretory vesicles, which can be fairly obvious in high-quality light microscope
preparations. The granules contain histamine, heparin, and various other chemical
mediators whose release signals for a number of physiological defense responses.
3. Macrophages (histocytes): remove and digest the by-products of both bacteria
and normal growth and degeneration. They are irregular cells larger than fibroblasts,
with more cytoplasm, the nucleus is smaller and darker than fibroblast nucleus, the
nucleus eccentric in position. Macrophages contain numerous lysosomes which are
used for .breaking down ingested material. These lysosomes are usually not easily
seen by light microscopy but easily visible by electron microscopy. In macrophages
which have been active and have accumulated indigestible residue, the lysosomes
may be visible by light microscopy as dark intracellular granules, as in the lung
contain carbon particles
4. Plasma cells: small round cells with large eccentric nucleus, the nucleus
characterized by the big dark masses of chromatin which arranged radically.
5. Fat cells: round cell with packed nucleus in the side of the cell and large droplet
of fat, this droplet looks as large cavity occupied the cell .
6. Pigment cells (or melanocytes): star shaped cells filled with dark granules.
7. Mesenchymal cells: star shaped cells with branched processes, like the fibroblast
but smaller than it.
8. Reticular cells: the cell has large nucleus and processes attached with the
processes of the adjacent cells.
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9. Leucocytes: including the eosinophils, neutrophils, and lymphocytes, this found
in the connective tissue of the intestine.
Connective tissue fibers:
1- White or collagenous fibers: they form wave bundles of fibers, also they may
found as branched bundles, Densely packed collagen fibers (in dense connective
tissues such as dermis and tendon) provide main strength with resistance to
tearing and stretching. Loosely packed collagen fibers (in loose connective
tissues, such as hypodermis or the submucosa of internal organs) allow free
movement within definite limits.
2- Yellow or elastic fibers: they are single branched fibers may be thick as in the
aorta, or thin as in the areolar connective tissue, this type of fibers confer
elasticity.
3- Reticular fibers: they found as a network of dark branched fibers. Reticular
fibers do not show up in routine H&E stained specimens, but they can be
demonstrated with silver salts.
Ground substance is the background material within which all other connective
tissue elements are embedded. In proper connective tissue, the ground substance
consists mainly of water whose major role is to provide a route for communication
and transport (by diffusion) between tissues. This water is stabilized by a complex of
glycosaminoglycans (GAGs), proteoglycans, and glycoproteins, all of which
comprise only a small fraction of the weight of the ground substance.
Ground substance may be highly modified in the special forms of connective tissue.
 In blood, the ground substance lacks stabilizing macromolecules. We call this
free- flowing ground substance plasma.
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 In skeletal tissue, the ground substance may become mineralized by
deposition of calcium salts. We call this rigid ground substance bone .
 In cartilage, the ground substance is much more solid than in proper
connective tissue but still retains more resiliency than bone.
Classification of connnective tissues:
Connective tissues classified to proper and specialized connective tissues.
Proper connective tissues include loose and dense connective tissues.
Loose connective tissues include:
1. Areolar con. T. hypodermis
2. Mucoid con. T. (umbilical cord)
3. Reticular con T. (lymph node)
4. Adipose con. T. (under the skin)
5. Mesenchymal con. T. (emberyo)
Dense connective tissue
A. Irregular con. T. (dermis)
B. Regular con. T. include:
1. White fibrous con. T. (tendon)
2. Elastic con. T. (legament of the vertebral column).
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Skeletal Connective Tissues
Bone and cartilage, like all other connective tissues, consist of cells and
extracellular matrix. It is the ground substance of the matrix which is most
responsible for the conspicuous differences between bone and cartilage. The ground
substance of bone is mineralized, making the bone rigid and strong, but brittle. The
ground substance of cartilage is not mineralized but is more like very firm Jell,
making cartilage stiff and incompressible but more flexible and resilient than bone.
In spite of their solidity, both bone and cartilage are capable of growth. In the case
of bone, internal remodeling (essentially, ongoing destruction and renewal is an
active process throughout life.
The ground substance of cartilage is characterized by a firm solid gel-like matrix
reinforced by collagen. Cartilage is relatively stiff and incompressible. However,
since cartilage is not mineralized, it is more flexible and resilient than bone.
The characteristic microscopic texture of cartilage, with cells enclosed in individual
lacunae (small chambers), reflects the nature of cartilage matrix and its mode of
formation. While cartilage is growing, new cartilage matrix is secreted by
chondroblasts, which become isolated from one another as the matrix is formed. As
more matrix is secreted, these chondroblasts are pushed farther apart. Chondroblasts
divide within their lacunae to form clusters of cells called cell nest, which then form
more matrix and become separated from one another. Once growth ceases, the
resting cells are called chondrocytes. Since cartilage is avascular, as the cartilage
increases in size, its cells also grow farther from their source nutrition. As a direct
result, mature cartilage is a relatively inactive tissue with minimal ability to respond
to injury
There are three types of cartilages:
1. Mature Hyaline cartilage found the Slides of "trachea" display the C- shaped
rings of cartilage that reinforce the trachea, in this tissue you can see the single
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chondrocytes and the aggregations of cells (cell nests) surrounded by capsule
found in lacunae in the matrix. the matrix looks homogenous, the hyaline
cartilage surrounded by vesicular fibrous . membrane contain bundles of white
fibers and fibroblasts and blood vessels, this membrane called the
perichondrium, The cells under this membrane are flattened lie in a parallel level
to the surface called chondroblasts. Function = flexible, provides support,
allows movement at joints The fine collagen fibers throughout the matrix of
hyaline cartilage do not stain with routine H&E
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Fetal Hyaline Cartilage: illustrates a cartilage model of a bone in an early stage of
development. Most of the model consists of young chondroblasts that still resemble
mesenchymal cells, having spherical nuclei and cytoplasmic processes. Lacunae
have not developed at this stage. The chondroblasts are numerous, and randomly
distributed in the cartilage without forming isogenous groups. At this stage of
development, cartilage matrix is secreted. On the periphery of the cartilage model
(left side), mesenchymal cells and concentrated and exhibit a parallel arrangement.
The nuclei of these cells are elongated flattened, and the cell membranes are not
distinct. This peripheral area of the cartilage develops into perichondrium, a sheath
of dense connective tissue that surrounds hyaline and elastic cartilage.
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Elastic cartilage : Elastic cartilage differs from hyaline cartilage principally by the
presence of elastic fibers in its matrix After staining the cartilage with orcein, these
are visible as deep purple fibers. this cartilage surrounded by perichondrium. The
fibers enter the cartilagenous matrix from the perichondrium . found in the external
ear and the auditory tubes, epiglottis. Function gives support, maintains shape,
allows flexibility .
2. White fibro cartilage found in the intervertebral disc, pubic symphysis . and
menisci of the knee . the matrix contain bundles of white fibers parallel to each
other with little ground substance, this type of cartilage is not surrounded by
perichondrium. Function = support and fusion, and absorbs shocks.
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Most mature bones are more-or-less hollow. The outer bone is called cortical or
compact bone, while the inner marrow cavity may be crisscrossed by thin
strands of bone called trabeculae or spongy bone.
 All mature bone is formed in layers called lamellae .
 Within the lamellae are small s or lacunae, in which osteocytes reside.
 Neighboring lacunae are interconnected by thin channels, or canaliculi.
 In compact bone, the lamellae are organized into sets of concentric rings
called osteons or Haversian systems.
 Each osteon includes a central channel, the Haversian canal, which
contains a blood vessel .
 Volkmann's
canal is a cross canal connects between the haversian
canals, Volkmann's canal not surrounded by lamellae .
In spongy bone the bone matter found as irregular trabeculae not contain the
osteons, but they branched and meet forming spaces filled with the bone
marrow. These trabeculae surrounded by osteoblasts, the matrix contain oste and
osteoclasts.
osteocytes, osteoblasts and osteoclasts can be identified not only by their
appearance as cells but also by their position in relation to the adjacent bone
lamellae.
 Osteocytes are isolated within lacunae, separated from other cells by bone
matrix.
 Osteoblasts (bone-forming cells) are small cuboidal cells, usually found
lying adjacent to one another upon lamellae they have just secreted.
 Osteoclasts (bone removing cells) are large cells with multiple nuclei,
occur in small. spaces (called Howship's lacunae) which they have eroded
into the surface. Osteoclasts remove preexisting bone. They are active in
bone development and also in bone remodeling .
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BONE
Bone is the main constituent ofthe adult skeleton. It is a skeletal connective
tissue specialized for support and protection . Bone tissue is rich in blood
supply, i.e. highly vascular tissue. Moreover, bone is a type of connective tissue
but with calcified matrix .
Types of Bone Tissue:
Regular (long) or compact bone.
Irregular (flat) or spongy bone.
Structure of bone:
As other connective tissues, bone is formed of :- Cells: steogeneic cells, Osteoblasts, osteocytes and osteoclasts.
- Matrix: of organic and non-organic parts.
-
Fibers: collagen type I fibers.
Bone cells
1- Osteogenic (osteoprogenitor) cells
They are mesenchymal stem cells found in the periosteum and endosteum .
They are small spindle-shaped cells with pale cytoplasm and ovoid nuclei .
Two types are distinguishable by the electron microscope: one can differentiate
into osteoblasts, and the other into osteoclasts .
Osteoblast precursors derive from embryonic mesenchyme and have sparse
RER and Golgi complexes.
Osteoclast precursors derive from blood monoctyes and have abundant free
ribosomes and mitochondria.
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2- osteoblasts:
- They are bone forming cells, found in the growing surface of the bone (in the
periosteum and endosteum) they form one-cell-thick sheets .
- They are dividing cells that synthesize the organic components of bone
matrix .
- They are large rounded or cuboidal cells, with deep basophilic cytoplasm,
well- developed RER and Golgi and eccentric nucleus
- They synthesize and secrete all the organic components of bone matrix and
may be involved in bone mineralization .
- Once surrounded by matrix, osteoblasts are considered mature and called
osteocytes .
3- Osteocytes
-
The cells are branched, smaller than osteoblasts, and not divide.
-
They are terminally differentiated bone (mature) cells found in cavities in
the bone matrix called lacunae, and their processes (branches) extend into
canaliculi in the classified matrix.
-
Osteocytes are isolated from one another by an impermeable bone matrix
and contact one another at the tips of their filopodia, often through gap
junctions.
-
Osteocytes recently derived from osteoblasts are located near bone surfaces
in rounded lacunae; older cells are found farther from the surface in flattened
lacunae.
- They maintain bone matrix.
-
The death of osteocytes results in bone breakdown, or resorption.
4- Osteoclasts
- Osteoclasts are bone-resorbing cells that lie on bony surfaces in shallow
depressions called Hawship's lacunae.
- Osteoclasts are large multinucleated cells (~50 nuclei), with acidophilic
cytoplasm containing abundant lysosomes and mitochondria and well
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developed Golgi complex; and brush border (Ruffled border) of plasmamembrane facing the bone marrow.
-
They are derive from the fusion of blood monocyte derivatives and are
considered components of the mononuclear phagocyte system.
- The cells release acid, collagenase, and other lytic enzymes into the
compartments; these break down bone matrix and release minerals, a process
called bone resorption .
Histology
Bone matrix:
Bone matrix contains organic components (osteoid), and inorganic components
(bone mineral). The organic components constitutes about 50% of bone volume
and 25% of bone weight. It is composed of type I collagen fibers and
unmineralized ground substance, which is composed of proteins. Carbohydrates,
and small amounts of proteoglycans and lipids. The inorganic components (bone
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mineral) makes up about 50% of bone volume and 75% of bone weight . It is
composed of calcium and phosphate, with some bicarbonate, citrate, magnesium
and potassium and trace amounts of other metals.
Compact Bone
is solid in appearance more complex arrangement of blood vessels cells and
matrix surround blood vessels in long bones
1- Periosteum: covering the long bone, and formed of two layers:-
Outer fibrous layer of collagen fibers.
- Inner cellular layer of osteogenic cells and cells and osteoblasts .
2- Endosteum: a cellular layer lining the bone cavities, and formed of
osteogenic cells and osteoblasts.
3- Haversian system (osteon):- Bone lamellae are arranged concentrically
around the blood vessels. The bone lamellae are formed of osteocytes inside
lacunae and canaliculi embedded in calcified matrix.
4- Volkmann's Canals :- They are transverse canals connecting blood vessels
in the Haversian canals to each other and to those in the periosteum and in
marrow cavities.
5- External circumferential lamellae.
6- Internal circumferential lamellae .
7- Interstitial lamellae.
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Spongy or cancellous bone
bone looks spongy in appearance no complex arrangement for blood vessels
fills heads of long bones called marrow always covered by compact bone.
-
Spongy bone forms a fine 3-dimensional lattice with many bony cavities,
with branching and anastomosing slips of bone between the cavities, called
trabeculae.
- Spongy bone is found at the core of the epiphyses of mature long bones, at
the core of short bones, and between the thick plates, or tables, of the flat
bones of the skull, where it is called the diploii.
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Cartilage
Cartilage differs from the "connective tissues proper" which we studied in the
last lab in that the matrix is a gel-like substance which gives the cartilage shape
along with flexibility. Unlike other CT, cartilage has NO blood vessels or nerves
except in the perichondrium .Combining various matrix components including
one or more type of fibers produces a variety of cartilage types.
Type of cartilage.
1-hyaline
2-fibrocartilage
3-elastic
Hyaline
The most common type of cartilage is hyaline. Hyaline cartilage has both elastic
and inelastic fibers, but they are so finely divided that they cannot be seen under
the light microscope. In cartilage the cells wall themselves off from the matrix
inside lacunae or "lakes". Cartilage will usually have a fibrous covering known
as the perichondrium.
Hyaline Cartilage
Elastic cartilage
Ls very rare, found only in the eoiglottis and aura of the ear . The elastic fibers
are found in dense bundles .
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Vesicular Connective Tissues
The Blood
Blood is classified as a specialized form of connective tissue. All connective
tissue consists of cells embedded in a matrix which consists of ground substance
and fibers. Blood may thus be described as connective tissue whose matrix
consists of free-flowing ground substance (plasma) with no fibers.
Red blood cells (RBCs, also called erythrocytes) are continually produced in
bone marrow and recycled in spleen. In mature form the lack nuclei and most
cytoplasmic structures they are flexible bags of hemoglobin, have biconcave
disc shape.
White blood cells (WBCs, also called leukocytes ) include several distinct cell
types, neutrophils, eosinophil, basophils, lymphocytes and monocytes. Certain
developmental and morphological similarities permit the first three these cells to
grouped together as granulocytes. The latter two types are then categorized as a
granulocytes or mononuclear leukocytes.
Granulocytes,
also
called
Dolymorphonuclear
leukocytes,
have
grainy,cytoplasm and elongated or lobed nuclei. They arise in bone marrow
from cells called myeloblasts and pass through stages called myelocytes and
metamyelocytes. The granulocytes include neutrophils, eosinophils, and
basophils .
Neutrophils are the most numerous of the leukocytes, about 60% of the white
blood cell count. Neutrophils take their name from the staining properties of
their cytoplasmic lysosomal granules, these granules are neutrophilic, meaning
they show no special affinity for either acidic or basic stains but are stained
mildly by both. (This is in contrast to the specific granules of eosinophils, which
stain red with acidic stains such as eosin, and those of basophils, which stain
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with basic stains), the nuclei of mature neutrophils are elongated and inched into
several distinct lobes. (Neutrophils are anti-bacterial cells which lyse (break
down) bacterial cells).
Eosinophils or acidophils (eosinophilic granulocytes) normally comprise less
than 2-4 of the per era Their specific granules are intense eosinophilic (stained
by eosin) Eosinophils are about the same size as neutrophils. Their nuclei are
typically band shape (elongated) or two-lobed. The function of eosinophils
remains unclear, although they are known to proliferate in association with
allergies and parasites .
Basophils (basophilic granulocytes) normally comprise less than 1% of the
peripheral leukocytes. Their specific granules are intense basophilic. Like
eosinophils, Basophils are.
similar in size to neutrophils. Their nuclei may be band shaped or segmented.
Basophils seem to be functionally similar to tissue mast cells, involved in
triggering inflammation.
Mononuclear leukocytes comprise both lymphocytes and Monocytes . Both
cell types work together in immune responses.
Monocytes are the largest of the leukocytes, and constitute about 5-10 of the
WBC population in peripheral blood. In blood smears, their nuclei have kidney
or bean shape. Monocytes belong to the same functional population as tissue
macrophages
Monocytes/macrophages
engulf
and
digest
foreign
microorganisms, dead or worn-out cells and other tissue debris. They interact
closely with lymphocytes to recognize and destroy foreign substances.
Lymphocytes are small cells, and they are the second most common white
blood cell type, comprising about 30% of the leukocyte population in peripheral
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blood. Lymphocytes have a round heterochromatic (deeply staining) nucleus
surrounded by a relatively thin rim of cytoplasm .
Lymphocytes also emigrate from blood in response to inflammation, but they
accumulate somewhat later during the inflammatory process than neutrophils.
Their presence in large numbers indicates the continuing presence of antigen.
Lymphocytes produce the antibody molecules (one specific type of antibody per
lymphocyte) which provide the mechanism for chemical recognition of foreign
materials .
Lymphocytes are most easily recognized in histological sections as small
"naked" nuclei (the cytoplasm is usually inconspicuous) which occur here and
there in most connective tissues, especially commonly near mucous membranes.
Lymphocytes are found densely packed in lymphoid tissue-spleen, lymph nodes,
and lymph nodules in mucous membranes (e.g. tonsils, appendix), where they
proliferate.
Platelets are small fragments of cytoplasm derived from bone marrow cells
called Megakaryocytes Megakaryocytes are easy to recognize by their huge size
and large lobulated nuclei. Small bits of megakaryocyte cytoplasm are
continually pinched off
to enter circulation as platelets, where they are
important for blood clotting.
Hemopoietic Bone Marrow:
Blood cells are made in the bone marrow. The bone marrow is the spongy
material in the center of the bones that produces about 95 percent of the body's
blood cells.
There are other organs and systems in our bodies help regulate blood cells. The
lymph spleen, and liver help the production, destruction, and differentiation .
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(developing a specific function) of cells. The production and development of
new cells is a process called hematopoiesis.
Blood cells formed in the bone marrow start out as a stem cell. A "stem cell" (or
hematopoietic cell) is the initial phase of all blood cells. As the stem cell
matures, several distinct cells evolve such as the red blood cells, white blood
cells, and platelets. Immature blood cells are also called blasts. Some blasts stay
in the marrow to mature and others travel to other parts of the body to develop
into mature, functioning blood cells.
Blood is a specialized body fluid. It has four main components: plasma, red
blood cells, white blood cells, and platelets. Blood has many different functions,
including:
 transporting. oxygen and nutrients to the lungs and tissues.

forming blood clots to prevent excess blood loss.

carrying cells and antibodies that fight infection .

bringing waste products to the kidneys and liver, which filter and clean the
blood .

regulating body temperature .
 Regulation of body pH .
The blood that runs through the veins, arteries, and capillaries is known as
whole blood, a mixture of about 5 percent plasma and 45 percent blood cells.
About 7 to 8 percent of total body weight is blood.
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