Tissue

The Living Fabric

Chapter 4
 Tissues
Tissues are groups of cells similar in structure
and function.
Histology is the study of tissues
• Most organs contain several tissues and the
arrangement of the tissues determines each
organ’s structure and what it is able to do.
Four Basic Tissue Types:
• Epithelial – lining, covering and glandular tissue
• Connective – supportive and nutritive tissue
• Muscular – contracts to produce movement
• Nervous – integration and control
 Characteristics of EPITHELIAL
TISSUE
• Closely packed cells of a • The lowermost layer (basal
mostly uniform type form layer) of cells is attached
continuous sheets to a basement membrane
• May be in the form of 1 or more consists of a basal lamina
layers made of glycoprotein
• The uppermost layer (apical Functions
layer) is exposed to body’s • Protection
exterior or to the cavity of an • Absorption
internal organ. The exposed • Filtration
surface may be smooth or may • Secretion
have modifications such as
microvilli or cilia
• Epithelial cells have no
blood supply of their own
and depend on diffusion
from the capillaries for
food and oxygen
• Epithelial tissue is
supplied by nerve fibers
• Epithelium has a high
regeneration capacity.
However only those cells
that are well nourished
can reproduce.
• Cells are joined by a
junctional complex
•Tight junctions
• Desmosomes
• Gap junctions
Epithelium is named according to shape,
structure, and arrangement of cells.
Shapes of Epithelium
1. Squamous - thin
and flat cell
2. Cuboidal - cube
shaped cells
3. Columnar - column
shaped cells
 Arrangement of Epithelial Cells
•Simple - single layer of cells
•Stratified - multilayered cells
•Pseudostratified - false stratified
•Epithelial cells can be:
Transitional – stretchable
•Ciliated - cells possess cilia
 Naming an epithelial tissue
• Each epithelial tissue is given two names.
a.The first name indicates the number of layers
present, either simple (one) or stratified (more
than one).
b.The second name describes the shape of the
cells.
 Simple Squamous Epithelium
•This is the thinnest tissue in the body, used where a
semipermeable membrane allows transport.
•Example: Alveolar membrane in the lungs –transports respiratory
gases, Capillary walls –transports gases, nutrients, wastes, &
other molecules, Kidneys –filtration. The basement membrane
forms the outer connective covering of the tubes/sacks formed
from it & is an important filter in what gets in or out.
 Endothelium and Mesothelium
• 2 special squamous epithelia in the body have
special names based on their location –
Endothelium is the epithelium that forms the
inner covering. Example: lining of the lymphatic
vessels and the cardiovascular system
• Mesothelium is the epithelium that forms the
middle covering. Example: Serous membranes
lining the ventral body cavity and its organs.
 Simple Cuboidal Epithelium
• Simple cuboidal epithelium is a single layer of
cube-shaped cells forming the smallest ducts of
glands, walls of kidney tubules, & ovary surface
 Simple Columnar Epithelium
•Columnar epithelium comes as short & tall columnar cells &
lengths in between. Nuclei occur closer to the basal layer of
the cells.
•Apical surface may be non-ciliated, or have microvilli, or cilia
•Non-ciliated is found in the GI tract, e.g. stomach &
intestinal lining.
Simple Columnar Epithelium with microvilli
• Microvilli are finger-like projections on the apical
surface of the cells which increase the surface
area of the small intestine for secretion &
absorption.
• Goblet cells are secretory cells found in
columnar epithelium which lines the GI tract.
They secrete mucus which serves to protect the
lining cells & functions as a lubricant.
 Ciliated Simple Columnar Epithelium
• Ciliated simple columnar is found in large bronchioles
of the respiratory tract & in the urinogenital tract.
• The cilia beat in waves to move mucus up and out of
the respiratory tract. This removes particulate matter
which is subsequently eliminated by swallowing.
• In the female reproductive tract ciliary movement is
used to transport the egg down the fallopian tube
 Stratified Epithelia
• Stratified Epithelia contain 2 or more cell layers.
They regenerate from below. The basal cells
divide and push upwards to replace the older
surface cells.
• Function of the stratified epithelium: Protection
• Stratified epithelium is identified based on the
shape of the upper layer of cells.
 Stratified Squamous
• Stratified squamous epithelium is the most widespread stratified
epithelium. Surface cells are squamous. Deeper cells are
cuboidal or columnar.
• Since epithelial cells do not get a direct blood supply, the basal
cells which are close to other tissues are well nourished &
mitotically active but the apical layers have flattened cells in a
degenerative state which are often lost due to abrasion.
• It forms the outer layer/epidermis of the skin & extends into
body openings continuous with the skin-mouth, anus, & vagina
 Stratified cuboidal epithelium
• Typically has 2 layers of cuboidal cells
• It is rare
• Found in the ducts of large glands like the mammary
glands, and sweat glands
 Stratified columnar epithelium
• Stratified columnar epithelium also has a limited
distribution in the body.
• Small amounts are found in the pharynx, the
male urethra, & lining some glandular ducts.
• This epithelium also occurs at transition areas
or junctions between two other types of
epithelia.
• Only its apical layer of cells is columnar.
 Pseudostratified Ciliated Columnar Epithelium
•Has columnar cells arranged in 1 layer. However the cells are of
uneven height & their nuclei appear at different heights making them
appear columnar.
•A non-ciliated pseudostratified epithelium is found in large glands &
parts of male urethra. Ciliated kind are found lining the respiratory tract
 Transitional Epithelium
•It is a highly modified stratified squamous epithelium.
•Found in the bladder, ureters, & urethra which bring urine from
the kidneys, stretchability helps prevent excessive pressure on the
kidneys when urination occurs.
•The basal cells are cuboidal or columnar. The cells at the free
end vary in shape. The cells are cuboidal or columnar when the
organ is not filled with urine. When urine fills the organ, the cells
flatten & become squamous.
 Glandular Epithelium
• A gland consists of 1 or more cells. Their
product is called a secretion which is
usually a protein molecule in an aqueous
solution. Some glands secrete a lipid rich
secretion (Example: oil glands in your
skin)
• There are 2 types of glands:
1.Exocrine
2.Endocrine
 Endocrine glands
• They are ductless
glands
• Their secretions
called hormones,
are secreted directly
into the blood or
lymph.
• Example: Thyroid
and Pituitary
 Exocrine Glands
•They have ducts.
•They may be unicellular (goblet
cells) or multicellular (sweat & oil
glands).
•Secretions empty through the
ducts onto the epithelial surface
which could be external to the
body or within the body.
•Goblet cells are scattered
between the columnar epithelium
of the intestinal and respiratory
tracts. They secrete mucin, a
glycoprotein which dissolves in
water to form mucus. Mucus
protects & lubricates the surface.
 Multicellular Exocrine Glands
• Have a complex structure
with 2 basic parts:
1. An epithelium derived duct
2. A secretory unit consisting of
secretory cells
• A supportive connective
tissue surrounds the
secretory unit & supplies it
with blood vessels & nerve
fibers, & forms a fibrous
capsule that extends into the
gland & divides it into lobes.
Types of multicellular exocrine glands
 Simple and compound multicellular
exocrine glands
• Depending upon the type of duct, they are
called simple glands (unbranched ducts) or
compound glands (branched ducts)
• The glands are further classified by their
secretory units.
• Tubular – Secretory cells form tubes
• Alveolar – Secretory cells form flask like sacs
• Tubuloalveolar – Both types of structures.
Mode of Secretion – Classification based on Function
1. Merocrine glands
secrete their products
by exocytosis.
Secretory cells are not
altered in any way.
• Example: Pancreas,
sweat glands, &
salivary glands
Mode of Secretion – Classification based on Function
2. Holocrine glands
accumulate their
products within them until
they rupture. (They are
replaced by the division
of underlying cells.)
Holocrine gland
secretions include the
synthesized product plus
dead cell fragments
Example: Sebaceous
(oil) glands of the skin