Histology - extensive summary

Histology notes

LC: Epithelia

Epithelial functions
 Lining or covering (of secondary body cavities, blood vessels, excretory ducts).
 Mechanical protection
 Absorption and resorption of nutrients in the intestines
 Secretion of products, e.g. in glands
 Transport across surfaces, e.g. mucus and air particles in the airways
 Movement by means of cilia in larval and adult invertebrates.

Intracellular adhesions and junctions
Epithelial tissue contains very little extracellular matrix. The epithelial cells form a continuous layer
due to the intracellular adhesions and junctions:
 Desmosomes: cadherins that adhere cells to each other.
o Calcium-dependent mechanism
o Hemidesmosomes: cadherins that anchor the epithelial cells to the basement
membrane. Also involved in the filtration of macromolecules
 Gap junctions: intercellular communication where liquid and ion transport between cells
occur.
o Transmit chemical signals that serve to synchronize the activity of neighbouring cells.
 Tight junctions: sealing intercellular spaces.
o Little to no water or ions can pass through the tight junctions.
o Allows strict regulation of transport through epithelia.
 Adhaerens junction (Zonula adhaerens): band-shaped adhesive connection.
 Junctional complex: combination of several intercellular connections mentioned above.




Basement membrane
Epithelial cells are connected to the underlying connective tissue by the basement membrane. The
basement membrane is sometimes referred to as the basal lamina, but actually the basal lamina is
just a part of the basement membrane. The basement membrane contains components of epithelial
and connective tissue.

, The basement membrane consists of 2 layers: the basal lamina and reticular lamina. The basal lamina
(lamina basalis) is made by epithelial cells and can be subdivided into the lamina lucida, containing
laminin, and the lamina densa. The reticular lamina (lamina reticularis) is made by fibroblasts from
the connective tissue. The basal and reticular lamina are connected by collagen fibres.
Functions of the basement membrane:
 Provides support and attachment between epithelia and connective tissue, and between
other tissues.
 Molecular filter for the blood, e.g. in the kidney
 Regulation cell division and differentiation of epithelial cells
 Involved in regeneration of epithelial tissue

Categorisation of epithelia
The categorisation of epithelia is based on the shape and the number of layers of the cells.
Simple epithelia consist of one layer of cells. The nuclei are all on the same level. The cells are lined
by the basement membrane. Simple epithelia cells can be squamous, cuboidal, columnar or
pseudostratified.

 Simple: one layer of cells, nuclei are all on the same level, all cells make contact with the
basement membrane.
o Squamous: flattened cells
o Cuboidal: cube shaped (isodiametric) cells, can be ciliated or non ciliated
o Columnar: prism-shaped cells (tall and not that wide), can be ciliated or non ciliated
o Pseudostratified: looks stratified, because the nuclei are located at different height,
but it’s simple. Can be ciliated or non ciliated

 Stratified: more than one layer of cells
o Squamous: can be keratinized or non keratinized (others are always non keratinized)
 Squamous keratinized epithelia consist of a layer of basal cells that flatten
towards the surface and form a dead, horny layer.
 Squamous nonkeratinized epithelia consist of a similar layer of basal cells
that flatten outwards, but doesn’t form a dead, horny layer.
o Cuboidal: specific for sweat glands and transitional epithelia
o Columnar: multiple layers of polygonal cells with a top layer of columnar cells.
o Transitional: only found in one place



Simple epithelia
 Simple squamous epithelium
o Flat nucleus and flat cytoplasm
o Found in alveoli, endothelium of blood
vessels and cover some body cavities.
o Facilitates uptake and excretion of
substances.
o In the photo the simple squamous
epithelium is found around the Bowman’s
capsule.