A&P

1. Introduction to Tissues:
Epithelium
BIO 351
Dr. Barbara T. Wizer

2. Introduction
• Tissues are groups of cells that are
similar in structure and function.
• There are four primary tissue types:
epithelial (covering), connective
(support), nervous (control), and
muscular (movement).

3. Features of Epithelium
• An epithelium is a sheet of cells that
covers a body surface or lines a cavity.
• Epithelium occurs in the body as a
covering or lining epithelium, and as
glandular epithelium.

4. Epithelial Characteristics
• Composed of closely packed cells with little
extracellular material between.
• Adjacent epithelial cells are bound together by
specialized contacts such as desmosomes and
tight junctions.
• Exhibits polarity by having an apical surface
(free) and a basal surface (attached).
• Supported by the underlying connective tissue.
• Innervated but avascular.
• Has a high regeneration capacity.

5. Classification of Epithelia
• Each epithelial tissue is given two
names.
–The first name indicates the number
of layers present, either simple (one)
or stratified (more than one).
–The second name describes the
shape of the cells.

6. Simple Epithelia
• Simple Epithelia are mostly concerned
with absorption, secretion, and filtration.
• Simple squamous epithelium is a single
layer of fish scale-shaped cells.
• Simple squamous epithelium – found in
places where substances have to cross
the epithelium quickly, such as the air
sacs (alveoli) of lungs and lining of
blood vessels

7. Simple Squamous Epithelium
• Common locations:
– Lining of blood vessels, termed endothelium
– Lining of the heart, termed endocardium
– Serous membranes (pericardium, pleura, and peritoneum)
that line closed body cavities, termed mesothelium
– Alveoli of lungs

8. Endothelium of Blood vessel
Note flattened
nuclei and
indistinct
cytoplasm
characteristic of
simple
squamous
epithelium

10. Alveolar Epithelium (Lung)

11. Clinical Correlation
• Why is a simple squamous epithelium an
ideal design for both capillaries as well as
alveolar epithelium?

12. Simple Epithelia cont.
• Simple cuboidal epithelium is a single layer of
cube-shaped cells forming secretory portion
and the ducts of glands, certain respiratory
passages, and many kidney tubules.
• Simple columnar epithelium is a single layer
of column-shaped cells that line most of the
digestive tract, certain respiratory passages,
and the genitourinary tract. Goblet cells may
be present.
• The apical surfaces of simple columnar
epithelial cells often have cilia or microvilli

13. Simple Cuboidal Epithelium
• Common locations -
– Glandular epithelium (e.g. thyroid) and ducts
– Renal tubules
– Surface of ovary (“germinal epithelium”)

14. Simple Cuboidal Epithelium
Thyroid Gland
Notice two things:
1) The centrally
located, spherical
cellular nuclei help to
identify the type of
epithelium
2) The glandular
lumen is filled with a
protein (colloid)
which is the
precursor to the
thyroid hormones (T3
and T4)

15. Simple Cuboidal Epithelium
Renal Tubules
Note the round,
spherical nuclei
characteristic of
simple cuboidal
epithelium

16. Laboratory Tip
When viewing a
hollow, tubular
structure under
the microscope,
remember that the
shape of the
structure will
depend upon the
plane of section.

17. Simple Columnar Epithelium

18. Simple Columnar Epithelium
Small Intestine
Notice that
the nuclei
tend to be
oval or
oblong and
located
closer to the
basal
surface of
the cells.

19. Stratified Epithelia
• Stratified epithelia’s main function is
protection.
• Stratified squamous epithelium is
composed of several layers with the
cells on the free surface being
squamous-shaped and the underlying
cells being cuboidal or columnar in
shape.

20. Stratified Squamous Epithelium
• Basal layer of cuboidal or columnar cells which
become flatter and more typically squamous as the
move progressively toward the surface
• Found in locations subject to friction or attrition:
– Skin
– Mouth
– Esophagus
– Anal Canal
– Vagina

21. Stratified Squamous
Epithelium
To differentiate
stratified squamous
epithelium form
transitional
epithelium (it’s
“look-alike”),
examine the cells at
the surface.
Stratified squamous
epithelium will
exhibit flattened
squamous cells
whereas the surface
cells of transitional
epithelium are more
rounded and dome-
shaped.

22. Stratified Epithelia
• Stratified cuboidal epithelium is rare,
found mostly in the ducts of some of the
larger glands.
• Stratified columnar epithelium is found
in limited distribution with small
amounts in the pharynx, male urethra,
and lining some glandular ducts.

23. Stratified Cuboidal Epithelium
• Locations:
– Sweat glands, developing ovarian follicles
– Lining of larger ducts of mammary glands, sweat
glands, salivary glands, and pancreas

24. Pseudostratified Columnar
Epithelia
• Pseudostratified columnar epithelium
contains cells of varying heights giving
the false impression of the presence of
many layers.

25. Pseudostratified Columnar
Epithelium
• Although the nuclei appear to lie in various
layers, all cells are attached to the basal lamina
(though some do not reach the surface)
• Found along the linings of the respiratory tract
• Epithelium is ciliated and includes goblet cells
• Responsible for the “mucociliary elevator”
which protects the terminal airways and alveoli

26. Ciliated Pseudostratified
Columnar Epithelium

27. Ciliated Pseudostratified Columnar
Epithelium

28. Transitional Epithelium
• Transitional epithelium forms the lining of the
hollow organs of the urinary system that stretch as
they fill (renal pelvis, ureter, bladder, upper urethra)
• Stratified epithelium with dome-shaped surface cells
• When the bladder is empty, the epithelium is at least
5-6 cells thick with rounded superficial cells that
bulge into the lumen. When the epithelium is
stretched as when the bladder is full of urine, the
epithelium is only 3-4 cells thick and the superficial
cells become more squamous in appearance

29. Transitional Epithelium
Urethra

30. Transitional
Epithelium
Bladder
Nondistended
(Empty)
Distended
(Full)

31. Basal Lamina & Basement
Membrane
• The basal surface of epithelial cells rests upon a basal
lamina which anchors the epithelium to the underlying
connective tissue (lamina propria)
• The basal lamina is an organized sheet of organic
molecules (including collagen), most of which are
synthesized by the epithelial cells, visible by EM
• Some texts use the term basal lamina
interchangeably with basement membrane. Others
reserve the term basement membrane for a thicker
structure composed of the basal lamina and reticular
lamina (a layer of anchoring reticular fibers attaching
the basal lamina to the underlying connective tissue)
which is visible using specific staining techniques by
LM

32. Basement Membrane

33. Apical Surface Modifications:
Microvilli
• Microvilli – fingerlike projections of plasma
membrane extending from the apical surface of
epithelial cells into a lumen to increase surface
area for absorption.
• Under light microscopy, the microvilli form a
striated apical border referred to as a “brush
border”
• Common locations: simp. cuboidal cells of
some renal tubules and simp. columnar cells of
small intestine

34. Small Intestine
Simple Columnar Epithelium w/ Brush Border

35. Apical Surface Modifications:
Cilia
• Cilia – cylindrical structures on the apical
surface of some epithelial cells that exhibit
coordinated wavelike motion to propel a
current of fluid or mucus in one direction over
the epithelial surface
• Under EM, exhibit 9 + 2 arrangement of
microtubules and are attached to basal bodies
just beneath the cell membrane
• Common locations: respiratory epithelium and
fallopian tube

36. Cilia on the surface of the
pseudostratified columnar
epithelium of the trachea

37. Cilia
Electron Microscopy

38. Goblet Cells
• Goblet Cells – epithelial cells that produce
and secrete large amounts of mucus
• Commonly seen in the simple columnar
epithelium that lines the intestines and the
pseudostratified epithelium that lines the
respiratory tract

39. Epithelial Membranes
• Thin structures composed of epithelium and underlying
connective tissue (C.T.) covering body surfaces and
lining body cavities
• Three major types:
1) Serous membranes – line body cavities that do not
open to the outside and also cover the organs within
these cavities. The epithelium is called a
“mesothelium”.
2) Mucous membranes – line the cavities and organs
that open to the outside of the body. Goblet cells within
mucous membranes secrete mucus.
3) Cutaneous membrane – “skin”; covers the surface of
the body.

40. Glandular Epithelium
• Cells derived from surface epithelium which are
specialized to produce secretions
• Unicellular vs multicellular glands
• Endocrine glands – ductless glands, connection to
surface epithelium is lost during fetal development
and secretions (“hormones”) are excreted directly
into bloodstream
• Exocrine glands – connection with surface
epithelium is transformed into ducts which carry
secretions onto surface and/or into a cavity (lumen)

42. Epithelial Slides Studied in Lab
• Simple squamous epithelium
– polar view--mesothelium
– lung alveoli
– Bowman’s capsule of the kidney
• Simple cuboidal epithelium
– thyroid follicles
– kidney tubules
• Simple columnar epithelium
– duodenum

48. Epithelial Slides Studied in Lab
• Pseudostratified ciliated columnar
epithelium
– trachea
• Stratified squamous epithelium
– esophagus
• Transitional epithelium
– Urinary bladder

49. trachea