?Connective Tissue Essay
As the name implies, connective tissue serves a “connecting” function. It supports and binds other tissues. Unlike epithelial tissue, connective tissue typically has cells scattered throughout an extracellular matrix. Loose Connective Tissue
In vertebrates, the most common type of connective tissue is loose connective tissue. It holds organs in place and attaches epithelial tissue to other underlying tissues.
Loose connective tissue is named based on the “weave” and type of its constituent fibers. There are three main types: Collagenous Fibers
Collagenous fibers are made of collagen and consist of bundles of fibrils that are coils of collagen molecules. Elastic Fibers
Elastic fibers are made of elastin and are stretchable.
Reticular fibers join connective tissues to other tissues.
Fibrous Connective Tissue
Another type of connective tissue is fibrous connective tissue which is found in tendons and ligaments. Fibrous connective tissue is composed of large amounts of closely packed collagenous fibers.
Specialized Connective Tissues
Adipose tissue is a form of loose connective tissue that stores fat. Cartilage
Cartilage is a form of fibrous connective tissue that is composed of closely packed collagenous fibers in a rubbery gelatinous substance called chondrin. The skeletons of sharks and human embryos are composed of cartilage. Cartilage also provides flexible support for certain structures in adult humans including the nose, trachea and ears. Bone
Bone is a type of mineralized connective tissue that contains collagen and calcium phosphate, a mineral crystal. Calcium phosphate gives bone its firmness. Blood
Interestingly enough, blood is considered to be a type of connective tissue.
Even though it has a different function in comparison to other connective tissues it does have an extracellular matrix. The matrix is the plasma and erythrocytes, leukocytes and platelets are suspended in the plasma.
Classification of connective tissues:
Connective tissues are classified on the basis of types and relative abundance of cells, fibres and ground substance, and on the organization of fibres. The images below will introduce you to the diversity of connective tissues. You will study a number of these connective tissues in greater detail in some of the Blocks.
Loose (or areolar) connective tissue
This is a cellular type of connective tissue, with abundant ground substance and thin and relatively sparse fibres. It has a viscous gel-like consistency and is important for the diffusion of oxygen and nutrients from small vessels, and the diffusion of metabolites back to the vessels. The primary location of loose connective tissue is beneath epithelia that line the internal surfaces of the body, in association with the epithelia of glands and around small vessels. It is the initial site at which antigens, bacteria and other agents that have breached an epithelial surface can be destroyed.
Dense irregular connective tissue
In dense irregular CT, collagenous fibres make up the bulk of the tissue. Fibroblasts are scarce and usually the only cell type present. Little ground substance is present. The abundance of collagenous fibres gives the tissue strength. The fibres are typically arranged in bundles in various directions (hence irregular), which enables the tissue to withstand various stresses. Dense irregular CT is found on the outside of many organs, in the dermis of the skin and as a distinct layer, called the submucosa, within various organs.
Dense regular connective tissue
In dense regular CT, collagenous fibres are packed in dense regular arrays, between which lie rows of cells. Dense regular CT is found in tendons (which connect muscles to bones), ligaments (which connect bones to bones, some
also contain large amounts of elastic fibres and are called elastic ligaments), and aponeuroses (broad flattened tendons whose fibres are arranged in multiple layers, within each layer the fibres are regularly arranged).
Specialized Connective Tissues
Cartilage is a connective tissue whose cells, called chondrocytes, secrete a very specialized matrix. The basophilia of the matrix is due to the GAGs in its ground substance, specifically hyaluronic acid, chondroitin sulfate and keratan sulfate. Collagen (type II) fibrils are also present in the matrix, but are not distinguishable as their refractive index is almost identical to that of the ground substance. The chondrocytes sit in spaces called lacunae, which they fill during life. However, during tissue preparation, chondrocytes shrink and frequently fall out, and lacuna appear only partially filled or empty. Cartilage is described in much greater detail in the MSK Block.
Bone is a connective tissue that is characterized by a mineralized extracellular matrix. The matrix is secreted by cells called osteocytes. It consists mostly of mineralized collagen fibres arranged in lamellae. What little ground substance is present is also mineralized. Bone is described in greater detail in the MSK Block.
Blood is a fluid connective tissue that circulates throught the body. It functions in bringing nutrients and oxygen to tissues, removing waste products, transporting a large number of products including hormones and immunogenic agents, and maintaining homeostasis. The white cells found in blood are shown in the images below.
Muscle tissue is made of “excitable” cells that are capable of contraction. Of all of the different tissue types, muscle tissue is the most abundant in
most animals. Muscle Tissue Types
Muscle tissue contains numerous microfilaments composed of actin and myosin, which are contractile proteins.
There are three major types of muscle tissue:
Cardiac muscle is so named because it is found in the heart. Cells are joined to one another by intercalated discs which allow the synchronization of the heart beat. Cardiac muscle is branched, striated muscle. Skeletal Muscle
Skeletal muscle, which is attached to bones by tendons, is associated with the body’s voluntary movements. Skeletal muscle is striated muscle. Unlike cardiac muscle, the cells are not branched. Visceral (Smooth) Muscle
Visceral muscle, is found in various parts of the body such as the arteries, the bladder, the digestive tract, as well as in many other organs. Visceral muscle is also called smooth muscle because it doesn’t have cross striations. Visceral muscle contracts slower than skeletal muscle, but the contraction can be sustained over a longer period of time.
Nervous Tissue: Neurons
Nerve tissue , nervous tissue the specialized tissue making up the central and peripheral nervous systems, consisting of neurons with their processes, other specialized or supporting cells, and extracellular material.
Nervous tissue is responsible for sensing stimuli and transmitting signals to and from different parts of an organism. Neurons are the basic unit of nervous tissue.
As we have previously discussed, structure and function are very much intertwined within biology and the structure of a neuron is uniquely suited to its function within nervous tissue.
A neuron consists of two major parts:
The central cell body contains the neuron’s nucleus, associated cytoplasm, and other organelles. Nerve Processes
Nerve processes are “finger-like” projections from the cell body that are able to conduct and transmit signals. There are two types:
Axons – typically carry signals away from the cell body.
Dendrites – typically carry signals toward the cell body.
Neurons usually have one axon (can be branched however). Axons usually terminate at a synapse through which the signal is sent to the next cell, most often through a dendrite.
Unlike axons, dendrites are usually more numerous, shorter and more branched. As with other structures in organisms, there are exceptions.
Bundles of axons and dendrites are called nerves. They are sensory if they consist of dendrites only, motor if they consist of axons only and mixed if they consist of both.