Download The Epithelia

The Tissues
A tissue consists of a group of cells which are similar in structure and
carry out the same function(s). There are four primary types of tissues
found in the body:
1. Epithelia – An epithelium is a layer(s) of contiguous cells that covers
the external and internal free surfaces of the body, e.g., surface of skin
or inner surface of a small blood The epithelia carry out the functions
of protection, absorption, filtration, excretion, secretion and sensation.
2. Connective Tissues – These tissues are the most widely distributed
tissues in the body. They function to bind structure together, support
the body, protect other organs, insulate the body from impact and heat
loss and (as blood) transport materials throughout the body.
3. Muscle Tissues – Permit the movements of the body and its parts
(skeletal muscle), move blood throughout the body (cardiac muscle)
and regulate the activities of internal organs (smooth muscle).
4. Nerve Tissue – Regulate and coordinate the activities of the entire
body.
Remove skin
cell from adult
Collect
unfertilized
oocyte
Fuse skin cell
with oocyte
Collect DNA
from oocyte
Formation of Stem Cells
to Produce Various
Tissues
Embryo forms stem
cells (arrow)
Pancreatic beta cells
blood cells
cardiac muscle cells
liver cells
neurons
The Epithelial Tissues
Classification of Epithelia by Cell Shape:
There are four categories of epithelia based on cell shape:
Squamous – Cells are flat or shield-like.
Cuboidal – The cells have the same height as width. They appear box-like.
The nucleus is spherical and found in the center of the cell.
Columnar – The cells are taller than wide. The nucleus is oval and tends
to be found near the bottom of the cell. The apical surface of columnar
epithelial cells often shows the presence of microvilli and cilia.
Transitional epithelium – The appearance of transitional epithelial cells
will vary with distension of the urinary organ which they line.
Classification of Epithelia by
Number of Layers or Strata of Cells
1. Simple – An epithelium that is one
cell thick.
1.
2. Stratified epithelium - the identity
of the epithelium is based on the
appearance of the uppermost cells.
3. Pseudostratified - a columnar
epithelium that appears stratified but
is not. The misleading appearance
of the epithelium is due to the
crowding of adjacent cells.
2.
3.
Characteristics of Epithelia
1. Epithelial cells exhibit polarity, i.e., they have a top (apical end) and a
bottom (basal end).
2. All epithelia have a basement membrane. This structure is not a cell
membrane or membranous layer. It consists of two components:
a. Basal lamina - a filamentous sheet attached to the basal surface of
the epithelial cell. It is a product of the epithelial cell, itself.
b. Reticular lamina - located under the basal lamina of most basement
membranes, consisting of a condensed ground substance mixed
with collagenous fibers.
The basement membrane supports the epithelium and functions as
a semi permeable filter separating the epithelium from the
underlying tissues.
3. Epithelia are avascular, i.e., epithelia lack a direct blood supply.
The cells of an epithelium must receive nutrients and oxygen from
and eliminate wastes to capillaries across the basement
membrane. Transport of these materials is achieved by diffusion.
4. Epithelia are capable of considerable regeneration.
Surface cells are constantly lost due to abrasion, microbial
activity, toxic substances or extremes of temperature.
5. The free or apical surface of the epithelial cell often shows
specializations in structure:
a. Nonmotile processes such as microvilli, serve to increase
the surface area of the cell and, therefore, facilitate transport
of materials into and out of the cells. Sensory hairs are
receptor processes found in certain sensory epithelia
concerned with taste (taste buds), smell (olfactory
epithelium).
b. Motile Processes like the cilia are short extentions of the
cell membrane. Cilia can be seen on the free surfaces of
epithelia lining the respiratory tract and oviducts tubes in
mammals. A flagellum has the same internal structure as a
cilium but is much longer and is found on a sperm cell.
6. A variety of intercellular junctions can be seen between the cells of an
epithelium.
1. Desmosomes are dense regions of attachment between epithelial
cells.
2. "tight junction" seems to prevent materials from the intestinal
lumen from leaking into the intercellular spaces of the epithelium.
3. A gap junction represents a very small (20A) continuity between the
cytoplasms of adjacent cells. They appear to represent sites of cell to
cell communication.
• Simple squamous epithelium
consists of a single layer of
flat, shield-like cells. In a
cross sectional view, the
cells bulge due to the
presence of the nucleus This
tissue may have a protective
function or perform
secretory functions. The
image shows this tissue
lining the alveoli of the
lungs. It also lines the blood
vessels (endothelium) and
the chambers of the heart
(endocardium). It also lines
the body cavities as the
mesothelium of the
peritoneum.
Simple Squamous
Epithelium
Simple Columnar
Epithelium
Simple columnar epithelium
consists of cells which
are taller than wide (*).
This tissue lines the
alimentary canal from the
stomach to the rectum.A
number of mucous
producing cells can be
seen in this view.
*
Simple Cuboidal
Epithelium
• Simple cuboidal epithelium
consists of short cube, prism
or trapezoid-shaped cells (*).
The nuclei are large, spherical
and centrally located in the
cells. This tissue often has a
secretory function. Found in
many glands both exocrine
and endocrine, for example,
lining thyroid follicles and
ducts of sweat glands.
• They are especially common
in kidney tissue.
*
Stratified Squamous Epithelium
(non-keratinized)
• The stratified squamous
epithelium that lines interior
spaces of the body does not
form keratin or a dead surface
layer. In a vertical cross
section, the deepest layer
rests on a basement
membrane and is cuboidal in
shape This epithelium is
located lining the mouth,
esophagus and portions of
the pharynx and larynx
• Stratified squamous
consists of several layers
of generally flat cells (*)
which rest on a
supporting layer of
connective tissue. In skin
the cells of the stratified
squamous produce a
tough fibrous protein
called keratin which
waterproofs the skin. On
the top of the epithelium
the cells die (arrow)
forming the stratum
corneum.
Stratified Squamous
(keratinized)
*
Pseudostratified Ciliated Columnar
lp
As its name implies, pseudostratified columnar appears
stratified but is not. Each cell in this tissue touches the
basement membrane. This tissue is often ciliated (arrow)
and is richly endowed with mucous producing cells.
Pseudostratified epithelium rest on a supporting
connective tissue called the lamina propria (lp). The
lamina propria consists mainly of areolar connective
tissue.
Transitional
Epithelium
Transitional epithelium is located exclusively in the urinary system.
It is found lining the pelvis of the kidney, the ureter, urinary bladder
and a portion of the urethra. This epithelium rests on a lamina
propria of areolar tissue. It is capable of great distension. The cells
of transitional epithelium appear balloon-like when the bladder is
empty of urine.