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Connective Tissue
Previously, we talked about the organization of cells into tissues and the characteristics and
functions of epithelial tissue, which is one of four major types of tissue found in animals.
Another major tissue type is connective tissue, which is defined as tissue where cells are
distributed within an extracellular matrix. An extracellular matrix is defined as any part of an
organism's tissue that is located outside of the cells.
Extracellular matrix in the body includes bones, cartilage, and blood plasma
So, in connective tissues, the cells are surrounded by non-cellular material, which is considered
part of the tissue along with the cells. In fact, many connective tissues are mostly composed of
extracellular material.
In the human body, extracellular matrix can be anything from bone, or cartilage, or blood
plasma, or even a mix of extracellular proteins and fibers. Connective tissues are a diverse group
of tissues that serve several different functions. Many connective tissues are structural and
supportive tissues that help maintain the shape of the body, its organs, and its tissues. Some
connective tissues literally connect other tissues together, and some connective tissues, like
blood, aren't structural and don't connect other tissues together in a literal sense, but are still
classified as connective tissue because the cells are suspended in an extracellular fluid.
Bone
Let's go through the different types of connective tissue and talk a little bit about their functions.
First, let's start with bone tissue. I think everyone is probably somewhat familiar with bones. We
all know that bone tissue is hard and stiff, and that it forms the internal framework of our body.
Bones pretty much define our general form and shape.
Bone tissue itself is mostly extracellular material composed of a hard, mineralized form of
calcium phosphate, but that's not all there is to bone tissue. Interspersed within the hard
extracellular component of bone are osteocytes which are mature bone cells. Osteocytes are the
cells which secrete the calcium phosphate that hardens and forms the bones. In this picture you
can see some osteocytes scattered throughout the bone.
Fibrous Connective Tissue
A second type of connective tissue is fibrous connective tissue which is composed mainly of
dense fibers made of parallel bundles of collagen, which is an extracellular protein that forms
rope-like filaments with great tensile strength. In fibrous connective tissue, single collagen fibers
combine in groups of three and twist around each other in a very similar manner to the way that
traditional ropes are made.
Tendons and ligaments are examples of fibrous connective tissue
This triple fiber is called a tropocollagen helix and it is bundled together in parallel with
thousands of other tropocollagen helices to make a collagenous fiber. In turn, hundreds of
collagenous fibers combine to make fibrous connective tissues like the tendons, which connect
muscles to bones, and ligaments, which connect bones together at the joints.
In this MRI image, the white line here is a ligament, which is connecting two bones. And this
white line here is a tendon, connecting a muscle to the bone. The parallel bundles of
tropocollagen helices create filaments that are bendable but have great tensile strength and are
perfectly suited to connecting muscles and bones.
Loose Connective Tissue
The most abundant type of connective tissue in the body is loose connective tissue. Loose
connective tissue is used as a base for epithelial tissues and as filler between tissues and around
organs to maintain their structure, keep them connected, and keep them in their proper places. In
this section of the prostate gland, all of the light blue stained area is loose connective tissue.
Scattered throughout loose connective tissue are cells called fibroblasts which are cells found in
the extracellular matrix that secrete matrix proteins. These darker blue spots in the connective
tissue here are fibroblast nuclei. Fibroblasts are the cells that create and maintain the
extracellular matrix, but they do more than just that, they are also necessary for wound repair,
tissue remodeling, and tissue expansion.
Loose connective tissue gets its name from the loose connection of fibers that it contains
including elastic fibers made of elastin which is an extracellular protein with elastic properties.
Loose connective tissue also contains lots of collagen fibers. Because collagen is such a key
component of connective tissue, it has been widely used by cosmetic surgeons and others to fillin facial areas to reduce wrinkles and give skin a fuller, tighter appearance. One very specific
type of loose connective tissue is the basement membrane, which is the well-defined layer of
extracellular matrix that serves as a base for epithelial tissues. In this picture of ciliated
epithelium, you can see the underlying basement membrane, which is this pink layer here, that
doesn't have the purple cell nuclei in it.
Fat
Another very specialized type of loose connective tissue is fat, which biologists call adipose
tissue. Each fat cell or adipose cell, stores fat in a large internal droplet and is surrounded by
loose connective tissue. Fat serves many purposes, including energy storage, insulation of the
body against heat loss, and padding for various body parts and organs. Here is a picture of what
fat tissue looks like under a microscope. Most of the pink material you see is extracellular
matrix, and these clear, bubble-like things are fat droplets that are contained within fat cells.
Cartilage
Another type of connective tissue is cartilage which is a tough yet flexible solid extracellular
tissue that is part of the skeletal system. Cartilage is used as padding between bones at the joints
and is also used as flexible support for structures like the windpipe, nose, and ears. In sharks, the
entire skeleton is composed of cartilage. Mature cartilage-producing cells called chondrocytes
can be found embedded throughout the cartilage. In this picture, cartilage is stained pink, and
you can see several chondrocytes scattered throughout the tissue.
The major classes of blood cells include red blood cells, platelets, and white blood cells
Blood
And finally, the last type of connective tissue is blood, which is categorized as connective tissue
because the blood cells are suspended in the extracellular fluid of the blood which is called
plasma. There are three major classes of cells circulating in the blood: red blood cells that
transport oxygen and carbon dioxide throughout the body, white blood cells, which are various
types of immune cells that fight against infection, and platelets which are extremely small pieces
of cells that form blood clots. We'll talk more about the different types of blood cells and their
functions when we talk about the circulatory and immune systems.
Lesson Summary
So let's review. Connective tissue is tissue where cells are distributed within an extracellular
matrix. An extracellular matrix is defined as any part of an organism's tissue that is located
outside of the cells. Connective tissues have many different functions. Some connective tissues
like bone, cartilage, and loose connective tissue function mainly to support and provide structure
for the body, organs, and tissues. Other connective tissues, like fibrous connective tissue,
function mainly to connect other tissues together. And some connective tissues, like fat and
blood, have lots of different functions which are critical to several different organ systems.