Connective tissue is one of the basic tissues which gives structural and metabolic
support to other tissues and organs of the body.
G
ENERAL
F
EATURES
_ Connective tissue is made of (a) cells, (b) fibers, and(c) ground substance.
_ The major constituent is extracellular matrix , strength and same basic
structure
,
C
LASSIFICATION OF
C
ONNECTIVE
T
ISSUE
(B
ASED ON
S
TRUCTURE AND
F
UNCTION
)
Connective tissue can broadly be classified into following four categories:
A. Ordinary connective tissue
1. Loose areolar connective tissue
_ It is a vascular, delicate, flexible .
_ It serves as a packing material by filling spaces between various tissue, e.g. sub-
peritoneal tissue.
2. Dense collagenous connective tissue
_ It is a tough tensile:
(a) Regular e.g. tendon, ligament, aponeurosis.
(b) Irregular , e.g. dermis of skin.
3. Connective tissue with special properties
(a) Elastic tissue , is a specialized dense elastic fibers , e.g. ligamentum nuchae.
(b) Mucoid tissue, is the embryonic connective tissue e.g. Wharton’s jelly.
(c) Reticular tissue, is a modified form of loose connective tissue made of reticular
fibers and cells. e.g. stroma of lymphoid organ.
(d) Adipose tissue , e.g. hypodermis (panniculus adiposus).
B. Scleral connective tissue
_ It is a stiff connective tissue where the matrix is solidified.
_ It provides the general framework of the body.
1. Cartilage . 2. Bone .
د. أدﯾب ﻋﺑد اﻟﻌﺎﻟﻲ اﻻزﺑﺟﻲ
Histology
Lecture :- 4
Connective Tissue
C. Lymphoid tissue . D. Haemopoietic tissue.
ORDINARY CONNECTIVE TISSUE
Composition of Connective Tissue
Cells: Various types of cells are present in the connective tissue. These are grouped
into fixed and free cells, each group performing a special function.
Fixed cells (intrinsic cells)
1. Fibroblasts and fibrocytes
2. Undifferentiated mesenchymal cells
3. Fat cells (adipocytes)
4. Fixed macrophages (histiocytes)
v
Fixed cells are responsible for production and maintenance of extracellular matrix.
Free cells (extrinsic cells/wandering cells)
5. Free macrophages
6. Plasma cells
7. Mast cells
8. Leucocytes—migrated from blood
v
Free cells are responsible for tissue reaction to injury or invasion of
microorganisms.
Fibers
1. Collagen 2. Elastic 3. Reticular
Ground Substance: This refers to the gel-like material in which cells and fires of
connective tissue are embedded.
Cells
1. Fibroblasts
_ flat and fusiform in shape , contain large oval nucleus with prominent nucleolus.
Responsible for the formation of fibers and ground substance.
__ Old inactive fibroblasts are called fibrocytes.
_ Specialized contractile fibroblasts are called myofibroblasts .
2. Undifferentiated mesenchymal cells
_ Stellate in shape . _ Pluripotent cells. _ Resemble fibroblasts.
_ Found along the periphery of blood vessels.
3. Fat cells (adipocytes)
_ large cells. _ Store energy (lipid).
_ Aggregate to form adipose tissue and supported by reticular fibers
.
4. Fixed macrophages or histiocytes
_ irregular in shape , dark eccentrically placed indented nucleus, acidophilic cytoplasm
containing many lysosomes.
_ Are derived from blood monocytes.
_ Are involved in phagocytosis and activate B lymphocytes to form antibodies.
5. Free macrophages
_ During antigenic stimulation or inflammation, the fixed macrophages withdraw their
processes and become free macrophages.
6. Plasma cells
_ Are oval cells with basophilic cytoplasm , eccentrically placed nucleus and derived
from B lymphocytes .Their life span is about 2 weeks.
_ Are involved in the defense of the body by producing antibodies (immunoglobulins).
7. Mast cells
_ Are round or fusiform cells with centrally placed round nucleus.
_ Are found along small blood vessels.
_ contain histamine, a vasodilator and heparin, an anticoagulant.
_ Are involved in inflammatory reactions, allergies and hypersensitive states.
8. Leucocytes
_ Nucleated white blood cells.
_ Perform phagocytosis or mediate immune response .
_ Are classified into two main groups based on nuclear shape and cytoplasmic
granules:
1. granular leucocytes (neutrophils, eosinophils, basophils—named according to their
staining properties), and
2. mononuclear leucocytes (lymphocytes and monocytes).
– Neutrophils :- form the first line of cellular defense against bacteria by engulfing
and destroying them. They increase in number during acute inflammation. Dead
neutrophils are called pus cells.
Eosinophils are involved in selective phagocytosis of antigen antibody complex.
They increase in number in allergic condition and in parasitic infection.
Basophils are containing histamine and heparin granules. In response to antigen,
histamine is liberated inducing an inflammatory response.
Lymphocytes are the smallest cells of the connective tissue with dark spherical
nuclei and a thin rim of basophilic cytoplasm. They mediate immune response to
antigen. They increase in number during chronic inflammatory conditions.
Fibres
1. Collagen fibers
_ Are composed of a protein called collagen.
_ Occur singly or in bundles, e.g. tendon, aponeurosis, etc.
The most common types of collage are:
Type I – found in bones, tendons, dermis, etc.
Type II – found in cartilage
Type III – found in reticular fi bres
Type IV – found in basement membranes
Type V – found in blood vessels and foetal membranes
2. Elastic fibers
_ Are composed of protein called elastin.
_ Elastin is synthesized by fibroblasts and smooth muscle cells (in blood vessels).
_ Fibres occur singly and not in bundles.
_ Are found in ligamentum flava, large arteries, etc.
3. Reticular fibers
_ Are very thin immature collagen fibers and structurally similar to collagen fibers.
_ Form supportive framework of lymphoid organs and glands.
_ Are composed of collagen type III.
Ground Substance
_ It is a transparent, homogeneous viscous solution.
_ Fills the space between cells and fibers
_ facilitating diffusion of metabolites between blood and tissues.
Functions
The following are the main functions of connective tissue:
_ Support. _ Packing . _ Storage. _ Transport. _ Repair. _ Defense.
Adipose Tissue
General Features and Functions
_ Aggregation of fat cells (adipocytes).
_ Is a reservoir of energy.
_ Gives shape to the body and keeps some organs in position.
_ Gives thermal insulation to the body because it is a bad conductor of heat.
Types
Adipose tissue is of following two types:
1. Yellow (white) or unilocular adipose tissue (adult type).
2. Brown or multilocular adipose tissue (embryonic type).
Cartilage
is a firm and flexible type of scleral connective tissue in which the extracellular matrix
has a firm consistency.
GENERAL FEATURES
_ Cartilage supports regions of the body that require varying degrees of flexibility.
_ Avascular structure nourished by diffusion .
_ No nerves are present and regeneration of cartilage is poor.
_ Growth of the cartilage depends mainly on the growth hormone.
COMPONENTS
Cartilage is made of (a) cells – chondrocytes (derived from mesenchymal cells), (b)
fibers – collagen and elastic, and
(c) ground substance – acid mucopolysaccharide (chondroitin sulphate).
TYPES
1. Hyaline cartilage : It is characterised by the presence of highly basophilic
homogeneous matrix. e.g. costal cartilage, tracheal rings, thyroid and cricoid cartilages.
2. Elastic cartilage: It is characterised by the presence of elastic fibers in the
matrix. The elastic fibers are thinner at the periphery and thicker and
branching in the interior of the cartilage, e.g. ear pinna, external auditory meatus,
auditory tube, epiglottis.
3. Fibrocartilage: It is characterised by the presence of dense bundles of
collagen fibers oriented in the direction of functional stress with rows of
chondrocytes between the bundles. e.g. intervertebral disc.
FUNCTIONS
_ Supports soft tissues.
_ Facilitating movements (only of hyaline cartilage).
_ Essential for growth of long bones (only of hyaline cartilage).
Bone
is a rigid form of scleral connective tissue in which the extracellular matrix is
impregnated with inorganic salts, mainly calcium phosphate and carbonate, providing
hardness.
GENERAL FEATURES
• rigid and hard .
• Muscular action and attachment.
• Bears body weight.
• Protects vital organs like brain, heart and lungs.
• Bone stores calcium, phosphate and other ions.
• It contains bone marrow.
TYPES OF BONE
Morphologically, bone consists of:
1. Externally, a solid shell of cortical bone called compact bone (found in shell of
short bones, shaft of long bones and tables of flat bones).
2. Internally, a framework of trabeculae separated by marrow spaces called spongy
or cancellous bone (found in short bones ends of long bones and flat bones).
Microscopically, compact bone consists of:
1. Primary/immature/woven bone—newly formed bone during growth and repair
with low mineral content, e.g. callus.
2. Secondary/mature/lamellar bone—definite adult type after remodelling.
BONE MEMBRANES
The external and internal surfaces of _ bones are covered by membranes called
periosteum and endosteum respectively.
1. Periosteum
_ It is a dense connective tissue membrane covering the external surface of bone,
except on articular surfaces and has two layers :
(a) Outer vascular fibrous layer. (b) Inner cellular osteogenic layer.
_ Very sensitive(rich nerve supply) .
_ It is involved in bone growth and repair .
2. Endosteum
_ It is a thin membrane of vascular loose connective tissue lining the medullary cavity
in the long bones and marrow spaces in the cancellous bones.
_ Osteogenic potential .
BONE COMPOSITION
1. Cells
(a) Osteoprogenitor cells: They are young pluripotential cells derived from
mesenchymal cells. They differentiate into osteoblasts and found along the blood
vessels in the periosteal buds and in the endosteum.
(c) Osteocytes (b) Osteoblasts (d) Osteoclasts
2. Fibres
_ These are constituted of collagen fibrils which are composed of type I collagen.
3. Ground substance
_ This is made of chondroitin sulphate, dermatan sulphate and a specific glycoprotein.
4. Inorganic components (Bone salts/Hydroxyapatite)
_ These include: Calcium phosphate (85%) , Calcium carbonate (10%) and Other
salts (5%).
STRUCTURE OF COMPACT BONE
_ Compact bone consists of three systems (sets) of bony lamellae arranged in an
orderly manner:
1. Circumferential system
– Outer (periosteal)
– Inner (endosteal)
2. Haversian system or osteon
3. Interstitial system
STRUCTURE OF SPONGY OR CANCELLOUS BONE
_ Spongy bone is made of slender bony trabeculae that branch and anastomose with
one another enclosing irregular marrow spaces between them which contain bone
marrow.
_ These trabeculae exhibit parallel lamellae of bony matrix and osteocytes in lacunae
in between.
_ The trabeculae are covered externally by vascular endosteum containing
osteoprogenitor cells, osteoblasts and osteoclasts .
BONE FORMATION/OSSIFICATION
_ Bones are formed by two methods, namely,
1. Intramembranous ossification, i.e. bone formation from condensed
mesenchyme—membrane model. e.g. Flat bones
in the vault of skull, clavicle.
2. Endochondral ossification, i.e. bone formation from cartilage—cartilage model.
e.g. Long bones except clavicle.
Intramembranous Ossification
Stage I: Condensation of mesenchyme
Stage II: Formation of membrane
Stage III: Differentiation of osteoblasts and formation of osteoid
Stage IV: Formation of calcified matrix and establishment of centre of ossification
Stage V: Formation of Periosteum and endosteum
Endochondral Ossification
_ The long and short bones of the body, except clavicle, are formed by endochondral
ossification.
Stage I: Formation of mesenchymal model
Stage II: Formation of cartilage model
Stage III: Appearance of primary centre of ossifi cation and formation of diaphysis
Stage IV: Appearance of secondary centres of ossifi cation and formation of epiphysis
Stage V: Fusion of epiphysis with diaphysis
Role of Vitamins in Bone Formation
_ The following vitamins are essential for normal bone growth:
1. Vitamin D
_ Necessary for absorption of calcium from small intestine.
2. Vitamin C
_ Necessary for collagen synthesis.
3. Vitamin A
_ Necessary for ossification.
Role of Hormones in Bone Formation
_ Balanced endocrine activities are essential for normal bone growth.
_ Any disturbance in the activities may lead to bone abnormalities.
1. Parathyroid hormone—activates osteoclasts to resorb bone
→ ↑ calcium in blood.
2. Calcitonin—inhibits bone resorption by osteoclasts
→ ↓ calcium in blood.
3. Growth hormone—stimulates the growth of epiphyseal plate.
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