September 13, 2011
Lecture 1 – Epithelia Part 1
Different population of cells will express different subset of genes that are present which
is selective gene expression
It makes two different cells different. Ex. A muscle cell contracts and generates tension,
being able to move things around in the body. Different cells lead to different functions
Tissues are a homogeneous group of cells, a bunch of hepatocyte cells will be a tissue, a
bunch of muscle cells will be a muscle tissue
Tissues aren’t just a bunch of cells; they produce products that are excreted in the
surrounding cells. Tissues are the cells itself and its products.
There are 4 different tissue types. Epithelial, connective, muscle and neural. These are the
building blocks of our body.
Epithelium covers an exposed body surface. Free surfaces are opened to an open face.
Non- free surfaces are attached to something and do not have an exposed surface. Ex.
Carpets and tiles are physically attached onto its underside (a free surface and a non-free
surface). It doesn’t necessarily have to be exposed to air, it can be fluid, it isn’t fixed, it
can still move along.
There are internal body surfaces that are covered by epithelia (all free surfaces). One
class is the internal passageways that open to the exterior of the body, ex. the digestive
and respiratory tract (a big tube, there is a free surface inside the digestive and respiratory
tract, exposed to fluid and air respectively). Blood vessels are lined with epithelium that
is specialized to deal with blood, to prevent clotting. It is called an endothelium (within):
a type of epithelium. Body cavities that are not opened to the outside but are still lined
with epithelium, known as mesothelium. These body cavities are lined with epithelium
that is histologically identical. It gets different names depending on which cavity it lines.
Pleura, pericardium and peritoneum are the same but are named differently depending on
The most basic epithelium is simple epithelium which is a single layer of cells. Multiple
cell layers are stratified epithelium, ex. skin is a stratified epithelium, many layers for
External body surfaces like skin are lined with epithelium
Covering epithelia cover a surface. Epithelia tissue are very cellular, there’s not a whole
lot of cell products, which is very different from connective tissue; which are mostly cell
product. Epithelial cells have polarity, because structurally one side is different from
another all the way around. The apex of the cell is the top, exposed to the free surface.
The lateral sides are adjacent from each other. The basal (base) surface is the non-
exposed surface, the bottom. Surfaces can each have specializations the others don’t
have, ex. microvilli on the apical surface but not on any other. What is between the tiles is important as well, there is specialized contact between the cells. Sometimes, you want
substances to cross freely; sometimes you don’t, depending on location.
Some epithelia are permissive, some are not. They have to be structurally different to be
Cells are attached to the basement membrane. Always, under the basement membrane is
the connective tissue.
Epithelia have no blood supply but there is a nerve supply. Blood vessels run through the
connective tissue but they don’t extend beyond the basement membrane. Connective
tissue diffuses gases and nutrients across the basement membrane to the epithelial cells
for metabolic support.
Epithelia can regenerate