Connective tissues. The four types of connective tissues include connective tissue proper,
cartilage, bone, and blood. There are three characteristics that are shared by all of
the different types of connective tissue. All connective tissue contains relatively few cells
with large amounts of space between them. All connective tissue contains a large amount of
extracellular matrix. Extracellular matrix is ground substance and protein fibers together.
All connective tissues have the same embryonic origin, which is the mesenchyme. Mesenchyme comes
from the mesoderm germ layer of the embryo. All connective tissues have specialized cells. These
specialized cells secrete extracellular matrix proteins. Fibroblasts are found in areolar tissue
and reticular connective tissue. Adipocytes are present in adipose tissue. Chondroblasts
are present in cartilage tissue. Osteoblasts are found in bone tissue, and hematopoietic
stem cells mature into the many different types of blood cells. It may be hard to believe that
these very different tissue types are actually related, but it is true. The cells that make
up the different connective tissue types all have the same embryonic origin. During the early stages
of development, the embryo forms three distinct layers; the ectoderm which is the outer layer,
the mesoderm which is the middle layer, and the endoderm which is the inner layer. The cells of
the mesoderm have mesenchymal stem cells that can differentiate into many different types of cells.
The mesenchymal stem cells give rise to the different cell types needed to form all of the
different types of connective tissue. Mesenchymal stem cells give rise to the osteoblasts which form
bones, the chondrocytes that form cartilage, the fibrocytes that form connective tissue proper,
and the adipose sites that form the adipose or fat cells. Connective tissues are composed of not only
specific cell types, but also the protein fibers and ground substance that makes up the surrounding
extracellular matrix connective tissue cells are interspersed within the unique extracellular
matrix of the tissue. The composition and density of proteins and molecules that make up the matrix
varies with the different types of connective tissues. It is the specific composition of
the cells, ground substance, and protein fibers that make up the connective tissue,
that gives it the ability to provide its specific function within the body. This is the
complementarity between structure and function. In connective tissues the ground substance refers to
the substance that surrounds the cells and fibers of the extracellular matrix. The ground substance
is a thick or jelly-like substance, except for in bone in which the ground substance is calcified.
The matrix has a scaffolding made up of fibrous proteins that provide support for the connective
tissue. There are three types of protein fibers that can be found in connective tissue;
collagen fibers, reticular fibers, and elastic fibers. Connective tissue proper.
The main function of connective tissue proper is to bind tissues and to resist stress and tearing
due to stretching and tension placed on the tissue. There are many types of immune cells that
work in your connective tissue proper, working to screen and destroy invaders. Your immune cells
include phagocytes which are the neutrophils, monocytes and macrophages, and inflammatory
mediators, which are the basophils, mast cells and eosinophils, and natural killer cells or nk
cells. The primary cell type in connective tissue proper is the fibroblast. The fibroblast produces
components for the extracellular matrix of connective tissue proper which includes
collagen and elastin. The word fibroblast means fiber formers and they live up to their name.
Fibroblasts manufacture proteins and secrete them into the extracellular matrix where they
act as building blocks., The tissue types of connective tissue proper.
Connective tissue proper contains tissues that fall into two different categories; loose
connective tissues and dense connective tissues. There are three types of loose connective tissues;
areolar connective tissue, adipose or fat tissue, and reticular tissue. The three types of dense
connective tissues are dense regular connective tissue, dense irregular connective tissue,
and elastic connective tissue. Areolar connective tissue is a type of loose connective tissue. It is
composed of a lot of matrix and relatively few cells. Areolar connective tissue is the most
abundant type of connective tissue found in the body. This tissue surrounds organs and connects
skin to underlying muscles. Iin a practical exam, you can identify areolar connective
tissues by looking for black fine elastic fibers, pink thick collagen fibers,
and purple nuclei that are made up of mostly fibroblasts. Areolar tissue allows a high degree
of movement between adjacent body parts. The key functions of areolar tissue can be summarized
as providing support, strength, and elasticity. Areolar tissue appears as a disorganized network
of fibers with lots of space between them and a small number of spread out cells.
Adipose tissue. The function of adipose tissue is to store energy and produce heat. Adipose tissue
is considered a loose connective tissue type and belongs to the category connective tissue proper.
There are actually two different types of fat tissues; white fat and brown fat. Your white fat
stores energy in the form of a lipid droplet, while brown fat functions to generate body heat.
Infants have a lot more brown fat than adults do. This is one of the reasons that infants have a
higher normal temperature than adults. Reticular tissue. Reticular connective tissue contains a
network of reticular fibers. The word "reticulum" means net or network. Reticular fibers are very
noticeable in these tissue types. These reticular fibers are synthesized by the fibroblasts.
Reticular connective tissue is found in bone marrow and surrounding the kidneys, the spleen,
and the lymph nodes. Reticular connective tissue forms a flexible skeleton or internal framework
that can support many free blood cells. These blood cells, mostly lymphocytes,
are located in the lymph nodes, the spleen, and red bone marrow. Dense connective tissues. Dense
connective tissue contains more collagen fibers than loose connective tissue. As a consequence,
it displays a greater resistance to stretching. There are two types of dense connective tissue;
regular dense connective tissue, and irregular dense connective tissue. Ligaments and tendons are
made up of dense regular connective tissue. Dense regular connective tissue contains elastin fibers
in addition to collagen fibers, which allows the ligament to return to its original length after
stretching. Dense irregular connective tissue. In dense irregular connective tissue, the direction
of fibers is random. This arrangement gives the tissue greater strength in all directions
and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh.
In other tissues, stretching in several directions is achieved by alternating layers
and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example
of dense irregular connective tissue. This tissue type is rich in collagen fibers.
Elastic connective tissue. Elastic connective tissue contains a lot of elastin fibers. These are
the black wavy lines seen here in the image. When an organ containing these fibers is stretched,
the elastin fibers recoil or go back to their original length and pull the organ
back into shape. True elastic connective tissue is very rare, but they are present in relatively
high concentrations in some organs including the largest arteries in the body. Cartilage. Cartilage
is a supportive connective tissue that functions in the body to provide structural support,
protection of soft tissues, and increased strength. You are born with a lot more cartilage
than you have as an adult. As you grow, much of this cartilage is used to guide the growth of
bones. For example, the human skull is composed of separated bones at birth and then these bones fuse
during development to form the skull. Once the bones have fused, the cartilage is broken down
by the body and not replaced. This is why infants are born with what's known as a 'soft spot' at the
top of their heads. Cartilage tissue consists of a dense collection of collagen and elastin fibers.
The primary cell and cartilage are the chondrocytes. The fibers and the ground substance
that make up the matrix of cartilage, is made from chondroblasts which are immature chondrocytes.
Cartilaginous tissue is avascular and not innervated. The cartilage gets its nutrients
from the dense irregular connective tissue of the perichondrium via diffusion through the
matrix. Whenever there is an injury that occurs in an area that contains a lot of cartilage,
such as the achilles heel, the amount of time necessary for healing is greatly increased.
The fact that cartilage has no nerves, means that cartilage is not sensitive to pain or pressure.
Cartilage is closely related to bone. Whenever cartilage gets calcified, the chondrocytes of
the cartilage die and the cartilage will be replaced by bone tissue. Cartilage has
more flexibility than bone due to a flexible material called chondroitin, which exists in
the matrix. Chondroitin is sometimes taken as an over-the-counter supplement for joint pain.
There are three types of cartilage; hyaline cartilage, fibrocartilage, and elastic cartilage.
Hyaline cartilage is the most abundant type of cartilage found in the body. Your entire skeleton
actually started out as hyaline cartilage. As an adult, you have hyaline cartilage covering
the surfaces of your bones at each of your joints. Your ribs also contain hyaline
cartilage at the anterior ends. You have hyaline cartilage in your bronchi, your bronchial tubes,
and trachea. Hyaline cartilage forms the nose and the vocal cords. Hyaline cartilage functions in
the body to allow for gliding motion of adjacent structures by providing a smooth gliding surface.
It also provides flexibility and support. Hyaline cartilage means "glass". It gets its name from
its clear glass-like appearance when observed with the naked eye. However, this quality is not
visible on stained slides when viewed under the light microscope. Hyaline cartilage has
clearly defined cells, the chondrocytes, that seem round. The cells are surrounded by a blank space
called the lacunae. In histology, a lacuna is a small space containing an osteocyte in bone
or a chondrocyte in cartilage. The fiber portion of the matrix of hyaline cartilage contains only
thin fibrils of collagen that are too thin to be seen with the light microscope. The ground
substance has a jelly-like consistency and holds large amounts of water. The structural properties
of hyaline cartilage give it the ability to resist compression and provide strength and flexibility.
Fibrocartilage. Fibrocartilage is a tough form of cartilage that consists of chondrocytes, scattered
among clearly visible dense bundles of collagen fibers within the matrix. Fibrocartilage is found
in portions of your joints, your intervertebral discs, your knees, as in the meniscus,
and the pubic symphysis which joins the anterior portions of the hip bones together. Fibrocartilage
provides structure and support. It is also the strongest type of cartilage. Fibrocartilage has
the ability to resist strong compression and strong tension. The areas of the body that have
this type of cartilage are subjected to a lot of push and pull. Fibrocartilage has thick collagen
fibers, which makes it somewhat similar to dense regular connective tissue. Elastic cartilage.
Elastic cartilage contains a lot of elastic fibers with thin collagen fibrils in its matrix. As you
would expect, this cartilage is more elastic than hyaline cartilage. This extra elasticity allows
for better tolerance of repeated bending. The body has elastic cartilage tissue in the eustachian
tube which regulates inner ear pressure. It also forms the structure of the outer ear,
called the oracle, and it forms the epiglottis. The epiglottis bends down to cover the glottis,
or opening of the larynx, each time we swallow. This structure is made up of elastic cartilage
and is highly bendable. Bone or asseous tissue. Bone is a tissue, also known as osseous tissue.
Bone is actually alive and filled with blood vessels. It has incredible strength due to
inorganic calcium salts that form a rigid ground substance in the matrix. Bone functions in the
body to provide structural support and protection to internal body structures.
Bone is able to withstand a great deal of compression and tension. The primary cell of the
bone tissue is the osteoblast. Osteoblasts secrete the collagen fibers and the materials needed for
the ground substance. These materials are then calcified by the addition of calcium salts, which
form the rigid ground substance of bone. When the bone is mature, the matrix consists of collagen
fibers surrounded by a calcified rigid ground substance. The osteoblasts mature into osteocytes.
The osteocytes reside in the lacunae filling up the space. Blood cells. Fluid connective tissue
includes the blood and the lymph. Various specialized cells circulate within a watery
fluid containing salts, nutrients, and dissolved proteins. Mature red blood cells lack a nucleus.
Notice that we don't see any purple in the red blood cell slides, because there is no nucleus to
take up the purple nuclear stain. Blood is pretty different from the other connective tissue types.
It is liquid and does not bind things together or provide any mechanical or structural support.
However, it develops from the same mesenchyme, like the other connective tissue types.
Blood consists of blood cells surrounded by a liquid non-living matrix called blood plasma.
Its cells and matrix are very different from those in other connective tissues.
Blood functions as transport vehicles for the cardiovascular system, carrying defense cells,
nutrients, wastes, respiratory gases, and many other substances throughout the body.
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