Cells
Cells are the smallest structural unit of living organisms, capable of
maintaining life and reproducing. An adult body consists of about 100 trillion
cells.
There are many different types of cells in the body. However, they all
contain three
parts: the cell membrane, the nucleus, and the cytoplasm.
The cell membrane separates the material outside the cell from the material
inside the cell. It maintains the integrity of a cell and controls passage of
materials into and out of the cell. The cell membrane is a double layer of
phospholipid molecules, embedded with proteins which may form channels for passage of
materials or act as receptor sites to transmit signals from outside into the
cell.
The nucleus is formed by a nuclear membrane around a fluid nucleoplasm. It
is the control center of the cell. DNA is located in the nucleus. In response
to signals transmitted to DNA, specific proteins can be produced based on the
genetic information encoded in the DNA.
The cytoplasm is the gel-like fluid between the cell membrane and the
nucleus. It contains hundreds or
even thousands of miniscule but distinct structures called organelles. Examples of cytoplasmic organelles are mitochondrion, ribosomes, endoplasmic reticulum, golgi apparatus, and lysosomes.
Tissues
Tissue is a group of cells that have similar structure and that function together as a unit. A nonliving material, called the intercellular matrix, fills the spaces between the cells. This may be abundant in some tissues and minimal in others. The intercellular matrix may contain special substances such as salts and fibers that are unique to a specific tissue and gives that tissue distinctive characteristics. There are four main tissue types in the body: epithelial, connective, muscle, and nervous.
Epithelial tissues
Epithelial tissues are widespread throughout the body. They form the
covering of all body surfaces, line body cavities and hollow organs, and are
the major tissue in glands. They perform a variety of functions that include
protection, secretion, absorption, excretion, filtration, diffusion, and
sensory reception.
The cells in epithelial tissue are tightly packed together with very little
intercellular matrix. Because the tissues form coverings and linings, the
cells have one free surface that is not in contact with other cells. Opposite
the free surface, the cells are attached to underlying connective tissue by a
non-cellular basement membrane. This membrane is a mixture of carbohydrates
and proteins secreted by the epithelial and connective tissue cells.
Epithelial cells may be squamous, cuboidal, or columnar in shape and may be
arranged in single or multiple layers.
Connective tissues
Connective tissues bind structures together, form a framework and support
for organs and the body as a whole, store fat, transport substances, protect
against disease, and help repair tissue damage. They occur throughout the
body. Connective tissues are characterized by an abundance of intercellular
matrix with relatively few cells. Connective tissue cells are able to
reproduce but not as rapidly as epithelial cells. Most connective tissues have
a good blood supply but some do not. Examples of connective tissues include
loose connective tissue, adipose tissue, dense fibrous connective tissue,
elastic connective tissue, cartilage, osseous tissue (bone), and blood.
Muscle Tissues
Muscle tissue is composed of cells that have the special ability to shorten
or contract in order to produce movement of the body parts. The tissue is
highly cellular and is well supplied with blood vessels. The cells are long
and slender so they are sometimes called muscle fibers, and these are usually
arranged in bundles or layers that are surrounded by connective tissue. Actin
and myosin are contractile proteins in muscle tissue.
Muscle tissue can be categorized into skeletal muscle tissue, smooth muscle
tissue, and cardiac muscle tissue.
Skeletal muscle fibers are cylindrical, multinucleated, striated, and under
voluntary control. Smooth muscle cells are spindle shaped, have a single,
centrally located nucleus, and lack striations. They are called involuntary
muscles. Cardiac muscle has branching fibers, one nucleus per cell,
striations, and intercalated disks. Its contraction is not under voluntary
control.
Nervous tissues
Nervous tissue is found in the brain, spinal cord, and nerves. It is
responsible for coordinating and controlling many body activities.
It stimulates muscle contraction, creates an awareness of the
environment, and plays a major role in emotions, memory, and
reasoning. To do all these things, cells in nervous tissue need to
be able to communicate with each other by way of electrical nerve
impulses.
The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon. The main part of the cell, the part that carries on the general functions, is the cell body. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. An extension or process called an axon carries impulses away from the cell body. Nervous tissue also includes cells that do not transmit impulses, but instead support the activities of the neurons. These are the glial cells (neuroglial cells), together termed the neuroglia. Supporting, or glia, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, while others provide nutrients by binding blood vessels to the neurons.
Membranes
Body membranes are thin sheets
of tissue that cover the body, line body cavities, and cover organs
within the cavities in hollow organs. They can be categorized into
epithelial and connective tissue membrane.
Epithelial Membranes
Epithelial membranes consist of epithelial tissue and the connective tissue
to which it is attached. The two main types of epithelial membranes are the
mucous membranes and serous membranes
Mucous Membranes
Mucous membranes are epithelial membranes that consist of epithelial tissue
that is attached to an underlying loose connective tissue. These membranes,
sometimes called mucosae,
line the body cavities that open to the outside. The entire digestive tract is
lined with mucous membranes. Other examples include the respiratory,
excretory, and reproductive tracts.
Serous Membranes
Serous membranes line body cavities that do not open directly to the outside,
and they cover the organs located in those cavities. Serous membranes are
covered by a thin layer of serous fluid that is secreted by the epithelium.
Serous fluid lubricates the membrane and reduces friction and abrasion when
organs in the thoracic or abdominopelvic cavity move against each other or the
cavity wall. Serous membranes have special names given according to their
location. For example, the serous membrane that lines the thoracic cavity and
covers the lungs is called pleura.
Connective Tissue Membranes
Connective tissue membranes contain only connective tissue. Synovial
membranes and meninges belong to this category.
Synovial Membranes
Synovial membranes are connective tissue membranes that line the cavities of
the freely movable joints such as the shoulder, elbow, and knee. Like serous
membranes, they line cavities that do not open to the outside. Unlike serous
membranes, they do not have a layer of epithelium. Synovial membranes secrete
synovial fluid into the joint cavity, and this lubricates the cartilage on the
ends of the bones so that they can move freely and without friction.
Meninges
The connective tissue covering on the brain and spinal cord, within the dorsal
cavity, are called meninges. They provide protection for these vital
structures.
Source:
SEER's Training Website.
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