Biomedical Engineering Reference
In-Depth Information
1. INTRODUCTION
A fundamental process in the development of a complex multicellular
organism is spatial and temporal control of cell proliferation, differentiation,
survival, and polarity. In the past, our understanding about regulatory mech-
anisms of cell proliferation, apoptosis, and cell-fate determination has been
greatly advanced, while relatively little attention was given to cell polarity.
Most cell types exhibit some kind of polarity, for instance, epithelial cells
show apical-basal polarities while neurons are polarized into domains spe-
cialized for either receiving (dendrites) or transmitting (axons) cellular sig-
nals (
Craig & Banker, 1994; Rodriguez-Boulan & Powell, 1992; Simons &
Fuller, 1985
). Epithelial cells lining internal and external surface of animal
bodies are characterized by apical-basal polarities with apical membrane
facing outside of the body or internal lumen and basal membrane
attaching to the underlying cells (
Simons & Fuller, 1985
). Mesenchymal
cells, such as fibroblasts, also have polarities. In order to move in certain
directions, these cells must form leading and trailing edges. Such
front-rear polarity is required for directional migration (
Ridley et al.,
2003
). These types of cell polarities have been extensively studied and
continue to be active research fields of cell biology. However, all these
polarities are studied mostly at single-cell level. The mechanisms that
establish and coordinate cell polarities with other cellular processes in a
complex multicellular organism when tissues and organs are shaped
still remain largely unknown. An emerging mechanism whereby
directional morphogenesis is regulated is planar cell polarity (PCP).
PCP, which was originally identified in
Drosophila
as the polarity of
epithelial cells within the plane orthogonal to their apical-basal
axis, controls coordinated, uniformly polarized cellular behaviors in a
field of cells (
Gubb & Garcia-Bellido, 1982
). More importantly,
regulation by PCP has been also extended to mesenchymal cells
(
Simons & Mlodzik, 2008
). With new findings made in recent years,
PCP has emerged to be critical for many basic developmental and
physiological processes as it plays important roles in morphogenesis of
most developing organs and tissues. However, fundamental questions
still remained to be answered. This review aims to summarize recent
advances in our understanding of how PCP is established, highlighting
the role of global cue in initiating and establishing PCP and potential
future research directions.
