Chapter 4. Tissue Level of Organization
4.1
Human body consists of trillion cells but only 200 types of cells. Specialized cells work together
to form tissues and carry out certain functions. The study of such tissues is called histology.
There are four types of tissues:
● Epithelial – exposed surfaces, internal passageways and chambers and forms glands.
● Connective – internal spaces, structural support for other tissues, transports materials
within the body and stores energy.
● Muscle – contraction and includes the skeletal muscles of the body, the muscle of the
heart, and the muscular walls of hollow organs.
● Nervous – carries information from one part of the body to another in the form of
electrical impulses.
4.2
Epithelia
Epithelial tissue includes epithelia and glands. Epithelia are the layers of cells that cover external
or line internal surfaces. Glands are the structures that produce fluid secretions.
They are either attached to or derived from epithelia.
Epithelia form the surface of the skin and line the digestive, respiratory, reproductive, and
urinary tracts. The more delicate epithelium lines internal cavities and passageways, for example,
chest cavity, fluid filled spaces of the brain, the inner surfaces of blood vessels,and the chambers
of the heart.
Functions
● Provide physical protection – epithelia protect exposed and internal surfaces from
abrasion, dehydration, and destruction by chemical and biological agents.
● Control permeability – some epithelia are relatively impermeable, others let large
compounds pass through. The epithelial barrier can also be regulated and modified in
response to stimuli, containing the molecular machinery for absorbing/secreting specific
substances. For example, hormones can affect the transport of ions and nutrients through
, epithelial cells. Physical stress like calluses form on our hands when manual labor is
done.
● Provide sensation – most epithelia have a large sensory nerve supply which provide
information about in and out. Neuroepithelium is an epithelium that is specialized to
perform a particular sensory function, it contains sensory cells that provide the sensations
of smell, taste, sight, equilibrium, and hearing.
● Produce specialized secretions – epithelial cells that produce secretions are called gland
cells. In a glandular epithelium, most if not all the epithelial cells produce secretions, they
either discharge the secretions onto the surface of epithelium in order to provide physical
protection or temperature regulation or release them into the surrounding interstitial fluid
and blood to act as chemical messengers.
, Characteristics
● Polarity – it is a presence of structural and functional differences between the exposed
and attached surfaces. If an epithelium consists of a single layer of cells then it has an
exposed apical surface and an attached basal surface above the connective tissue. Apical:
has microvilli or cilia. It has an uneven distribution of organelles between in and out. For
example, in kidney tubules, mitochondria are concentrated near the base of the cell,
where energy is in high demand for the cell’s transport activities.
● Cellularity – epithelia are made of cell junctions that are cells closely bound together by
interconnections.
● Attachment – the base of an epithelium is attached to a thin noncellular basement
membrane which is formed from the fusion of basal and reticular lamina, collagen matrix
and proteoglycans (intercellular cement). This basement adheres to the basal surface and
to the underlying tissues to establish the cell’s border and resist stretching.
● Avascularity – this means that they lack blood vessels, and get nutrients by diffusion or
absorption across either in and out.
● Regeneration – damaged epithelial cells are replaced through stem cell divisions.
Regeneration rate in specifically epithelial cells is very high compared to others.
