Polarized CellsWhat are
Polarized cells?
Polarized cells are cells that display structurally and functionally
different plasma domains namely the apical and the
basolateral domains. Tight
junctions separate the apical and the basolateral domains. They are essential
for a variety of cellular processed such as growth control, differentiation,
and motility and tissue formation.
Key features of polarized cells
·
A distribution of plasma
membrane proteins and lipids into three distinct surface domains: apical,
lateral and basolateral. The apical membrane faces the luminal compartment and
contains proteins that determine the cells primary function as secretion of
absorption. The lateral domain consists of junctional complexes. The
basolateral domain faces the extra cellular-fluid compartment.
·
Tight junctions (formed by
claudin and occludin proteins, joining the cytoskeletons of the adjacant cells)
that separate apical and lateral surface domains and form barriers to
intercellular diffusion of ions and macromolecules.
·
Cohesive cell-cell interactions
are formed in the lateral domain by cell adhesion molecules and a highly
developed junctional complex consisting of tight junctions, desmosomes and gap
junctions.
·
A polarized distribution of
cytoplasmic organelles and cytoskeleton within the cell.
Functions of polarized cells
1. Sheets of polarized epithelial cells
form barriers of very selective permeability between compartments.
2.They regulate
the composition of the two compartments by carrying out specialized
transport between the compartments, such as absorption, transcytosis and secretion.
Glucose
transport by polarized epithelial cells that line the lumen of the small
intestine.
The function of the small intestine is to
absorb glucose i.e, to transport it from the lumen into blood. Glucose cannot
pass through the tight junctions, so it must be transported through the
epithelial cells. The apical membrane of the cell contains a sodium-dependent
glucose transporter that allows the cell to absorb glucose. In the basolateral
membrane are sodium-independent glucose transporters that allow the cell to
export glucose into extracellular fluid and hence into blood. Thus, polarized
phenotype of the cell allows a one-way transport of glucose across the cell, as
well as an efficient mechanism for the transport of sodium.
Functions of tight junctions
1.Tight junctions prevent most molecules from
crossing the epithelium between cells.
Water
is able to diffuse through tight junctions relatively unimpeded, but most ions
and even small macromolecules are completely blocked. Simple sugars and amino
acids, for example, are unable to penetrate tight junctions.
2.Tight junctions contribute to maintenance of
membrane asymmetry in epithelial cells, which is necessary for much of the
transcellular transport.
T-cell
polarization and the formation of an immunological synapse.
A crucial step of the immune response is
the recognition by T lymphocytes of antigens on the surface of antigen
presenting cells. Following antigen recognition, T cells polarize towards the
antigen-presenting cell. This polarization is characterized by morphological
changes of the T lymphocyte, and by the concentration at the cell-cell contact
zone of a number of molecules, such as the T cell receptor, adhesion molecules,
signaling effectors and cytoskeleton components. This organized cell-cell
junction has been called the immunological synapse, since ensures the complex
communication between the T cell and the antigen-presenting cell.
Development of cell-cell and
cell-substratum contacts induces a reorganization of membrane proteins.
The attachment of a single Madin-Darby
canine kidney (MDCK) cell to
an artifical substrate is sufficient to induce polarization of apical membrane
marker proteins to the non-attached portion of the plasma membrane in less than
one day. In contrast to MDCK cells, mouse L cells are not normally polarized
and do not express E-cadhedrin (a transmembrane glycoprotein that mediates
epithelial cell-to-cell adhesion).
If however, the E-cadhedrin gene is introduced and expressed in L cells, they
rapidly develop extensive cell-cell contacts and assume a polarized phenotype,
including redistribution of the Na/K ATPase molecules into discrete domains.
Polarization in embryonic cells
This is seen in development of the early
embryo - in cells of cleavage-stage mouse embryos, the Na/K ATPase is uniformly
distributed in the plasma membrane, but as cell junctions form at the morula
stage (a process called compaction, which establishes a type of polarized
epithelium called trophectoderm), the ATPase gradually becomes localized to the
basolateral membrane.