Connective tissue provides
a matrix that connects and binds the cells and organs and ultimately gives
support to the body. (Hence called support tissue also.)
The main constituent of
connective tissue is the extracellular matrix. Due to this abundance of
extracellular matrix, the cells in connective tissues are widely placed.
Other tissues (like epithelium, muscular and nervous) are formed mainly by
cells.
The function of connective
tissues:
Mechanical function i.e.
supporting function as described below:
- The Loose areolar tissue holds together structures like skin, muscles, blood vessels etc. and binds together the various layers of hollow viscera (stomach, intestine, urinary bladder, uterus).
- Reticular tissue forms a framework that supports the cellular elements of various organs like spleen, lymph nodes and glands.
- Enables the movement of skin over underlying structure.
- Allows mobility and stretching in hollow organs.
- Hold the bone at joints (in the form of ligament).
- Provide attachment for origin and insertions of many muscles (in the form of deep fascia, intermuscular septa, aponurosis and tendon). Tendon also transmits the pull of muscles to their insertion.
- Hold the tendons of muscles at wrist and ankle (in the form of retinacula- thickened deep fascia).
- Provide planes along which blood vessels, lymphatics and nerves travel (through areolar tissue and fascial membrane).
- Provides support and protection to the brain and spinal cord (in the form of dura mater).
Other functions:
- The matrix serves as medium through which nutrients and metabolic wastes are exchanged between cells and their blood supply.
- Provides immunity: due to presence of cells of immune system - macrophages and plasma cells.
- Wound repair: Fibroblast produces the collagen fibres necessary for wound repair.
- Adipose tissue stores nutrition as well as provides insulation.
- Regeneration of tissues (like cartilage and bone) due to the presence of undifferentiated mesenchymal cells. (Undifferentiated mesenchymal cells are capable of differentiating into specialized cells like chondroblast-cartilage forming cells and osteoblast- bone forming cells)
Connective Tissues are broadly
classified into:
General
Connective tissue
Specialized
connective tissue (Bone, Blood, cartilage)
General Connective Tissue
The constituents for general
connective tissues are as follow:
Cells
Fibres
Ground substance
Cells
Fibroblast- these are the
most numerous cells of connective tissue. They are so named because they
produce collagen fibers (also reticular and elastic fibers).In tissue sections,
these cells appear to be spindle shaped and nucleus appears to be flattened.
Fibroblast become very active when there is need to lay down the collagen
fibres e. g. during wound repair. Inactive forms are known as fibrocytes.
Mesenchymal cell-
Embryonic connective tissue is known as mesenchyme which is made up of small
cells with slender branching processes that join to form a network. Various
components of mature connective tissue are derived from mesenchyme. Mesenchymal
cells are capable of differentiating into any specialized cells. It is believed
that some undifferentiated mesenchymal cells persist as such and these are the
cells from which other types can be formed when required.
Pigment cells- they are
easily distinguished as they contain brown pigment (melanin) in their
cytoplasm. They are most abundant in the connective tissue of skin, of choroid
and iris of eyeball. Of the many cells that contain pigment in their cytoplasm
only a few are actually capable of synthesizing melanin. Such cells are called
melanocytes, remaining cells are those that engulf pigment released by other
cells. Such cells are called chromatophores or melanophore and are probably
modified fibroblasts.
Fat cells or adipocytes-
some cells store fat in large amounts and become distended with it. These are
called fat cells, adipocytes or lipocytes.
Mast cells- these are
small round or oval cells. The nucleus is small and centrally placed. The
distinguish feature of these cells is the presence of numerous granules in the
cytoplasm. They release various substances when appropriately stimulated e.g.
release of histamine is associated with allergic reaction when a tissue is
exposed to an antigen to which it is sensitive. They are most frequently seen
around blood vessels and nerves.
Macrophages- these cells
are part of mononuclear phagocyte system. Macrophage cells of connective tissue
are also called histiocytes or clasmatocytes. They have ability to phagocytose
unwanted material like bacteria invading the tissue and damaged tissues. Fixed
macrophages resemble fibroblast but free or motile macrophages are round. The
nuclei of macrophages are small and stain intensely than those off fibroblasts.
Lyhmphocytes-lymphocytes
represent one variety of leukocytes and are in aggregation in lymphoid tissues.
They reach connective tissue from these sources and are numerous when tissue
undergoes inflammation. They have the ability to recognize the substances that
are foreign to host body and destroy them by producing antibodies against them.
They are of two types B lymphocytes and T lymphocytes.
Plasma cells- very few
plasma cells can be seen in normal connective tissue. Their number increases in
the presence of certain types of inflammation. These are mature B lymphocyte
that have lost their power of further division. Plasma cells is seen to be
small and rounded with nucleus having car wheel resemblance. The cytoplasm is
basophilic.
Fibers
Collagen –Collagen fibers
are most abundant. With light microscopy they are seen in bundles. the bundles
are made up of collections of individual collagen fibers which are 1-12
micrometer in diameter. In turn collagen fibers are made up of fibrils which are
20-200 nm in diameter. Each fibril consists of a number of microfibrils, 3.5 nm
in diameter.
Bundle of collagen fibers appear
white with naked eye. With H & E, they are stained light pink.
Collagen fibers can resist
considerable tensile forces without significance increase in their length. They
are also pliable and can bend easily.
Chemically collagen fibers are
made up of protein called collagen.
Types of Collagen fibers
Type I- they are found in
connective tissue, tendons, ligaments, fasciae, aponeurosis, bone, dermis,
meninges etc.
Type II- found in hyaline
cartilage, vitreous body.
Type III is the reticular fibres
Type IV- in the basal laminae of
basement membrane.
Various other types are also
recognized.
Elastic fiber- Elastic fibers
are made up of a protein called elastin. They run singly (not in bundles),
branch and anastomose with other fibers. Elastic fibers are thinner than those
of collagen (0.1-0.2 micrometer). In some situation they are thick e.g.
ligamentum flava and in other they are fenestrated as in walls of arteries.
Elastic fibers can be stretched
(like a rubber band) and returns to their original length when tension is
released. They are seen as shining line in unstained preparations.
Reticular fiber- these fibers
are variety of collagen fiber (Type III). They are much finer and uneven in
thickness. They form a network (or reticulum) by branching and anastomosing
with each other. They do not run in bundles. Reticular fibers provide a
supporting network in many situations e.g. spleen, lymph nodes and bone marrow;
most glands including liver; and the kidneys. They are the essential component
of all basement membrane.
Ground substance
The intercellular ground
substance is a highly hydrated, complex mixture of glycosaminoglycans, proteoglycans
and multiadhesive glycoproteins. The complex molecular mixture of the ground
substance is colorless and transparent, it fills the space between cells and fibers
of the connective tissue and because it is viscous acts as both a lubricant and
a barrier to the invaders.
Glycosaminoglycans (GAGs) are
linear polysaccharides formed by repeating disaccharide units usually composed
of uronic acid and a hexosamine. The hexosamine can be glucuronic or iduronic
acid. With exception of hyaluronic acid, these linear chains are bounded
covalently to a protein core, forming a proteoglycan molecule. Because of
abundance of hydroxyl, carboxyl and sulfate groups in the carbohydrate moiety
of most GAGs, the GAGs are intensely hydrophilic and acts as polyanions. With
the exception of hyaluronic acid, all other GAGs are sulfated to some degree in
adult state. The carbohydtrate portion of proteoglycans constitutes 80-90% of
this molecule.
Types of Connective tissue
Loose areolar- e.g. superficial
fascia
Dense irregular – e.g. dermis of
the skin
Dense regular- e.g. tendon
Elastic Tissue- e.g. ligamentum flava
Adipose Tissue
REFERENCES:
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Wheater's Functional Histology: A Text and Color Atlas
Junqueira's Basic Histology: Text and Atlas