Histology of Epithelial
Tissue
Last Updated: August 16, 2022
access_time24 min
Learning Objectives (5)
After completing this brick, you will be able to:
● List the two types of epithelium (covering/lining and glandular) and
describe their functions.
● 1
● Describe the structure and histologic features of epithelial tissue.
● 2
● Explain how covering/lining epithelium is classified.
● 3
● Describe the histologic features of glandular epithelium.
● 4
● Describe the five types of epithelial intercellular junctions.
● 5
CASE CONNECTION
VS is a 47-year-old man with a recurrent epidermoid cyst (sebaceous
cyst) on his upper back. Two months ago, he had the cyst incised and
drained in another clinic after he had tried to squeeze out the contents
a few months before that. “It just keeps coming back,” he says. “Now
it’s so big, you can see it through my shirt.” On his back, you note a
firm, mildly tender, smooth ovoid nodule within the skin. It is not
fluctuant. “Can you remove it so this doesn’t keep happening?”
,What will need to be done to achieve complete removal of the lesion?
Consider your answer as you read, and we’ll revisit at the end of the
brick.
What Is Epithelial Tissue and How Is
It Organized?
Epithelium is one of the four basic tissue types (the other three are
muscle tissue, nerve tissue, and connective tissue). It is found
throughout the body—covering it; lining organs, vessels, and cavities;
and forming glands. It absorbs nutrients, transports electrolytes,
secretes hormones, and regulates body temperature by producing
sweat.
We begin with some general principles of how epithelial tissue is
organized, and then we describe its various components.
General Organization
Epithelial tissue can be organized into layers that line or cover
surfaces (like the skin, blood vessels, or gastrointestinal [GI] tract) or
into glands that specialize in secreting different products either into
the body or the external environment. Sometimes the specific role of
,an epithelial tissue gives it a unique name, like urothelium (that lines
the genitourinary tract) or endothelium (that lines the blood vessels).
Epithelial tissue is composed of aggregates of epithelial cells that are
tightly connected to each other by different types of intercellular
junctions. Aggregates of epithelial cells rest on a supportive structure
called the basement membrane. This separates them from an
underlying layer of loose connective tissue (of variable thickness) that
contains blood vessels. It is important to note that epithelia are
avascular, so the blood vessels in the underlying connective tissue are
their source of oxygen, nutrients, signaling molecules, and other
proteins. These molecules passively diffuse through the connective
tissue and across the basement membrane to supply the epithelial
cells. The underlying connective layer in most of our internal organ
systems is called the lamina propria; in the skin, it is called the
dermis.
Epithelial Cells
Epithelial cells are notable for their polarity (or “sidedness”). This
means that, unlike many cells, different sides of the epithelial cell
have different membrane receptors or channels, allowing differential
function between one side of the cell and the opposite side. The
opposite sides are called the apical membrane (facing away from the
, basement membrane) and the basolateral membrane, sitting on the
basement membrane. The two sides of the cell connecting the apical
and basolateral membranes are called the lateral membranes.
We see this polarity, for example, in a renal tubular epithelial cell
(Figure 1), which has a special function responsible for transporting
ions and molecules from one side of the cell to the other. In the cell
depicted in the figure, we see that the cell moves molecules from the
lumen of the renal tubule (left) to the blood vessels, contained in the
renal interstitium (right). On these cells the specialized apical
membrane and basolateral membrane each have different transporters
that help move the molecules through the cell to the other side. Once
transported, the molecules can freely pass through the basement
membrane into the interstitium, where they are absorbed by blood
vessels.
Tissue
Last Updated: August 16, 2022
access_time24 min
Learning Objectives (5)
After completing this brick, you will be able to:
● List the two types of epithelium (covering/lining and glandular) and
describe their functions.
● 1
● Describe the structure and histologic features of epithelial tissue.
● 2
● Explain how covering/lining epithelium is classified.
● 3
● Describe the histologic features of glandular epithelium.
● 4
● Describe the five types of epithelial intercellular junctions.
● 5
CASE CONNECTION
VS is a 47-year-old man with a recurrent epidermoid cyst (sebaceous
cyst) on his upper back. Two months ago, he had the cyst incised and
drained in another clinic after he had tried to squeeze out the contents
a few months before that. “It just keeps coming back,” he says. “Now
it’s so big, you can see it through my shirt.” On his back, you note a
firm, mildly tender, smooth ovoid nodule within the skin. It is not
fluctuant. “Can you remove it so this doesn’t keep happening?”
,What will need to be done to achieve complete removal of the lesion?
Consider your answer as you read, and we’ll revisit at the end of the
brick.
What Is Epithelial Tissue and How Is
It Organized?
Epithelium is one of the four basic tissue types (the other three are
muscle tissue, nerve tissue, and connective tissue). It is found
throughout the body—covering it; lining organs, vessels, and cavities;
and forming glands. It absorbs nutrients, transports electrolytes,
secretes hormones, and regulates body temperature by producing
sweat.
We begin with some general principles of how epithelial tissue is
organized, and then we describe its various components.
General Organization
Epithelial tissue can be organized into layers that line or cover
surfaces (like the skin, blood vessels, or gastrointestinal [GI] tract) or
into glands that specialize in secreting different products either into
the body or the external environment. Sometimes the specific role of
,an epithelial tissue gives it a unique name, like urothelium (that lines
the genitourinary tract) or endothelium (that lines the blood vessels).
Epithelial tissue is composed of aggregates of epithelial cells that are
tightly connected to each other by different types of intercellular
junctions. Aggregates of epithelial cells rest on a supportive structure
called the basement membrane. This separates them from an
underlying layer of loose connective tissue (of variable thickness) that
contains blood vessels. It is important to note that epithelia are
avascular, so the blood vessels in the underlying connective tissue are
their source of oxygen, nutrients, signaling molecules, and other
proteins. These molecules passively diffuse through the connective
tissue and across the basement membrane to supply the epithelial
cells. The underlying connective layer in most of our internal organ
systems is called the lamina propria; in the skin, it is called the
dermis.
Epithelial Cells
Epithelial cells are notable for their polarity (or “sidedness”). This
means that, unlike many cells, different sides of the epithelial cell
have different membrane receptors or channels, allowing differential
function between one side of the cell and the opposite side. The
opposite sides are called the apical membrane (facing away from the
, basement membrane) and the basolateral membrane, sitting on the
basement membrane. The two sides of the cell connecting the apical
and basolateral membranes are called the lateral membranes.
We see this polarity, for example, in a renal tubular epithelial cell
(Figure 1), which has a special function responsible for transporting
ions and molecules from one side of the cell to the other. In the cell
depicted in the figure, we see that the cell moves molecules from the
lumen of the renal tubule (left) to the blood vessels, contained in the
renal interstitium (right). On these cells the specialized apical
membrane and basolateral membrane each have different transporters
that help move the molecules through the cell to the other side. Once
transported, the molecules can freely pass through the basement
membrane into the interstitium, where they are absorbed by blood
vessels.
