Tissues are structures with discrete structural and functional properties; in combination
they form organs, such as the heart or liver, and in turn organs can be grouped into 11
organ systems.
Cells working together form tissues -collections of specialized cells and cell products that
carry out a limited number of functions.
There are four basic types of tissue:
- Epithelial tissue covers surfaces, lines internal passageways and chambers and form
glands.
- Connective tissue fills internal spaces, provides structural support for other tissues,
transports material within the body, and stores energy.
- Muscle tissue is specialized for contraction and includes the skeletal muscles of the
body, the muscle of the hearts, and the muscular walls of hollow organs.
- Nervous tissue carries information from one part of the body to another in the form
of electrical impulses.
Epithelia are layers of cells that cover external or line internal surfaces.
Glands are structures that produce fluid secretions: they are either attached or derived
from epithelia.
Functions of epithelial tissue:
- Provide physical protection: epithelia protect exposed and internal surfaces from
abrasion, dehydration, and destruction by chemical or biological agents.
- Control permeability: any substance that enters or leaves your body must cross an
epithelium.
Many epithelia contain the molecular ‘machinery’ needed for absorbing or
secreting specific substances.
Some epithelia are easily crossed, whereas others are relatively impermeable.
The epithelial barrier can also be regulated and modified in response to stimuli
(hormones, physical stress).
- Provides sensation: most epithelia are extremely sensitive to stimulation, because
they have a large sensory nerve supply.
These sensory nerves continually provide information about the external and
internal environments.
These sensory nerves continually provide information about the external and
internal environments.
- Produce specialized secretions: epithelial cells that produce secretions are called
gland cells.
Individual gland cells are usually scattered among other cell types in an
epithelium.
In a glandular epithelium, most or all of the epithelial cells produce secretions.
These cells either discharge their secretions onto the surface of the
epithelium (to provide physical protection or temperature regulation) or release
them into the surrounding interstitial fluid and blood (to act as chemical
messengers).
, Characteristics of epithelial tissue:
- Polarity: an epithelium has an exposed surface, either facing the external
environment or an internal space (apical surface), and a base (basal surface), which
is attached to underlying tissues.
The two surfaces differ in plasma membrane structure and function: often, the
apical surface has microvilli; sometimes it has cilia.
The term polarity refers to the presence of structural and functional differences
between the exposed and attached surfaces.
The functional polarity is also evident in the uneven distribution of organelles
between the exposed surface and the basement membrane.
- Cellularity: epithelia are made almost entirely of cells bound closely together by
interconnections known as cell junctions.
- Attachments: the base of an epithelium is bound to a thin noncellular basement
membrane, which is formed from the fusion of several successive layers (basal
lamina and reticular lamina), a collagen matrix, and proteoglycans (intercellular
cement).
The basement membrane adheres to the basal surface and to the underlying
tissues to establish the cell’s border and resist stretching.
- Avascularity: epithelia are avascular, which means that they lack blood vessels.
Epithelial cells get nutrients by diffusion or absorption across either the exposed
or the attached epithelial surface.
- Regeneration: epithelial cells that are damaged or lost at the exposed surface are
continuously replaced through stem cell divisions in the epithelium.
Epithelial cells have several structural specializations: for the epithelium as a whole to
perform the functions just listed, individual epithelial cells may be specialized for:
1) The movement of fluids over the epithelial surface, providing protection and
lubrication.
2) The movement of fluids through the epithelium, to control permeability.
3) The production of secretion that provide physical protection or acts as
chemical messengers.
The specialized epithelial cell is often divided into two functional regions,
which means the cell has a strong polarity.
One is the apical surface, where the cell is exposed to an internal or
external environment.
The other consists of the baso-lateral surfaces, which include both the
base (basal surface), where the cell attaches to underlying epithelial cells or
deeper tissues, and the sides (lateral surfaces), where the cell contacts its
neighbors.
Many epithelial cells that line internal passageways have microvilli on their exposed
surfaces: especially abundant on epithelial surfaces where absorption and secretion take
place, such as along portions of the digestive system and kidneys.
Motile cilia are characteristics of surfaces covered by a ciliated epithelium.
