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Chapter 4 Connective Tissue Proper.
Connective Tissue Proper is a broad category that includes tissues like bone, fat, and blood,
as well as tissues that connect the top layer of the skin, or epithelium,
to the rest of the body. These tissues provide structure,
store energy reserves, and transport materials throughout the body.
Each of these different connective tissues vary widely in both their appearance
and function. Yet they do have some common components.
Three common components are: specialized cells, extracellular
protein fibers, and a fluid called ground substance.
Together the extracellular fibers and the ground substance are called
matrix and this matrix surrounds the specialized cells.
We saw in epithelial tissue that cells made up the bulk of the tissue
but with connective tissue, the matrix accounts for most of this
connective tissue. Connective tissues occur throughout the body but are
beneath the surface of the epithelium and are never exposed to the outside environment.
Many connective tissues contain blood vessels and many
connective tissues contain sensory receptors that detect pain, pressure, and
temperature changes. There are three general categories of connective tissues:
connective tissue proper, fluid connective tissue, and
and supporting connective tissue. In this segment, we will concentrate
on connective tissue proper.
Connective tissue proper contains a syrupy, viscous
ground substance with up to three different types of extracellular proteins
are two cell types that are permanent residents of the tissue.
Other cells such as macrophages, mast cells, lymphocytes,
plasma cells, and microphages are not permanent and will
migrate to tissue that has been damaged and will aggregate or cluster there
to help with repair. The number of cells and cell types at any given
moment depend on the condition of these tissues.
Fibroblasts are one of the two most abundant permanent cells of connective tissue proper.
They produce the proteins that form the extracellular fibers.
Fibrocytes are the second most abundant cell in connective tissue proper.
And they will differentiate into fibroblasts
They help to maintain the fibers of this tissue.
Adipocytes are commonly known as fat cells. Each cell
has one large lipid droplet that takes almost the entire cell.
This lipid droplet can be used as an energy source.
Note the appearance of the adipocyte looks like a class ring
as the nucleus is pushed to the side of the cell to make room for the lipid droplet.
The mesenchymal cells are stem cells that are found in many
connective tissues. These cells respond to local injury
or infection by dividing to producing daughter cells that differentiate
into fibroblasts, macrophages,
or other connective cells.
Macrophages are large cells that are scattered throughout the matrix.
These cells are scavengers that engulf damaged cells or pathogens that enter
the tissue. The name macrophage means “big eater”.
When macrophages engulf damaged cells or pathogens they become
stimulated and release chemicals that attract large numbers of additional
macrophages and other cells involved in tissue defense. These macrophages
can be either fixed or free. Fixed macrophages
stay in a tissue for long periods of time whereas free macrophages
migrate rapidly through the tissue.
Macrophages provide a front line defense until other cells can be
recruited to help against the attack of pathogens or in the repair
of tissue.
Mast cells are small and mobile cells that are most common near
blood vessels. Inside the cytoplasm of the mast cell contains
granules. These granules contain chemicals called histamine
and heparin. Histamine and heparin are released after injury or
infection and stimulate inflammation. You’ve heard of
anti-histamines that are medicines to combat the affect of histamines
and are used to reduce the inflammatory effects of the common cold.
Lymphocytes are cells that travel through the connective tissues
as they migrate throughout the body. They are found in increased numbers
wherever there is tissue damage. Some lymphocytes develop wherever there is tissue damage. Some lymphocytes develop into
plasma cells that will produce antibodies that will defend
against disease. The microphages are phagocytes that
move through connective tissues in small numbers. When an infection
or injury occurs, the chemicals released by the macrophages
and the mast cells attract numerous microphages
to the site where they will engulf debris and pathogens.
Melanocytes synthesize and store the pigment melanin.
Melanin gives the tissue a brown color and it helps to protect
the nuclei of the tissues against radiation.
They are most abundant in the skin where they help to determine skin color.
They are also abundant in the connective tissues of the eye and
the deeper layer of the skin. In addition to these specialized
cells, connective tissues contain three types of protein fibers:
collagen, reticular, and elastic. Fibroblasts
produce all three of the fibers by secreting the protein subunits that
make up these proteins. Fibrocytes are responsible for maintaining
these fibers. Collagen fibers are long
straight and unbranched. They are the most common fibers in connective
tissue proper. Like a rope, collagen fibers contain a
bundle of proteins that wind together. This fiber is flexible
but stronger when pulled from either end. Tendons connect
muscles to bone and ligaments connecting bone to bone
consist almost entirely of collagen fibers. They can withstand
tremendous forces. Uncontrolled skeletal movements are
more likely to break a bone than to snap a tendon or ligament.
Reticular fibers also contain protein subunits but the
fibers are arranged differently from the collagen fibers.
Reticular fibers are thinner than collagen fibers. They are arranged
into a branching, interwoven framework that is tough, yet flexible.
Like a network of fibers in a cloth, they are able to
withstand forces applied from many directions.
This woven network is called a stroma. This stroma is able to
stabilize the functional cells (called parenchyma) of organs such as the liver.
Reticular fibers also stabilize the positions of nerves
and blood vessels within organs. Elastic
fibers contain the protein elastin. These fibers are
branched and wavy. They stretch and then return to their original
length. Elastic ligaments of the spine contain numerous
elastic fibers. These ligaments connect the vertebrae of the spine
together and allow you to bend and return
to standing without damaging these ligaments.
Lastly, the ground substance of connective tissue is a syrupy fluid
that fills the spaces between the cells and surrounds the tissue fibers.
In connective tissue proper, the ground substance is a clear,
colorless, and viscous fluid. The fluid is syrupy because
of the proteoglycans and the glycoproteins.
Ground substance is dense enough that bacteria has trouble moving through it
like swimming through molasses. This makes it easier
for the phagocytes to catch up to the bacteria and destroy them.
In the next segment, we will discuss the different types of connective tissue proper
that include loose connective tissues and dense connective tissues.