Abstract
Animal tissues are composed of multiple cell types arranged in complex and elaborate patterns. In sensory epithelia, including the auditory epithelium and olfactory epithelium, different types of cells are arranged in unique mosaic patterns. These mosaic patterns are evolutionarily conserved, and are thought to be important for hearing and olfaction. Recent progress has provided accumulating evidence that the cellular pattern formation in epithelia involves cell rearrangements, movements, and shape changes. These morphogenetic processes are largely mediated by intercellular adhesion systems. Differential adhesion and cortical tension have been proposed to promote cell rearrangements. Many different types of cells in tissues express various types of cell adhesion molecules. Although cooperative mechanisms between multiple adhesive systems are likely to contribute to the production of complex cell patterns, our current understanding of the cooperative roles between multiple adhesion systems is insufficient to entirely explain the complex mechanisms underlying cellular patterning. Recent studies have revealed that nectins, in cooperation with cadherins, are crucial for the mosaic cellular patterning in sensory organs. The nectin and cadherin systems are interacted with one another, and these interactions provide cells with differential adhesive affinities for complex cellular pattern formations in sensory epithelia, which cannot be achieved by a single mechanism.
Introduction
Vertebrates possess highly developed sense organs such as the eyes, ears, nose, and tongue. These sense organs detect information about different environments and convert extracellular stimuli into electrical signals. These signals are mediated by specialized sensory epithelia. For example, in vertebrates, the perception of sound is mediated by the auditory epithelium located in the cochlea of the inner ear, and the perception of smell is mediated by the olfactory epithelium inside the nasal cavity. These sensory epithelia are typically composed of sensory cells and non-sensory supporting cells. Interestingly, the same types of sensory cells in the sensory epithelia are separated from one another to form alternating mosaic patterns. These mosaic patterns observed in the sensory organs are evolutionarily conserved among a wide range of species, and are thought to be important for the sensory functions. During the development of the auditory epithelium, sensory hair cells and supporting cells are thought to be segregated through the process of lateral inhibition mediated by Notch–Delta signaling, and such processes themselves might contribute to the spatial separation of these cells (Zhang et al., 2000; Zine et al., 2001). Expression of Notch 1 and its ligand Jagged 2 is restricted to supporting cells and hair cells, respectively, in the auditory epithelium. In a previous study, genetic inactivation of Notch signaling did not impair the cellular pattern, but did result in an increased number of hair cells (Lanford et al., ). These observations suggest that lateral inhibition alone is insufficient to create the checkerboard-like cellular pattern. Recent progress has suggested that cellular rearrangements could also play a role in the mosaic pattern formation in the auditory epithelium. The question then arises as to how cell adhesion molecules regulate the complex and elaborate cellular patterns in the sensory organs. In this review, recent advances in cellular patterning within the sensory epithelia are introduced and discussed.
Cell–cell adhesion and cellular patterning
Cell–cell adhesion systems are involved in many aspects of morphogenesis during development, including not only adhesion, but also movement, proliferation, survival, differentiation, and polarization. Various morphogenetic changes in tissues in developing embryos can be brought about by local cell rearrangements. Differential adhesion and cortical tension have been proposed to promote cell rearrangements and cell sorting in cell aggregates (Foty and Steinberg, ; Steinberg, ). Steinberg's theory of differential cell adhesion explains cell sorting as follows: when cell adhesion between cells of the same type is stronger than that between cells of different types, the two types of cells should become segregated; conversely, when cell adhesion is stronger between cells of different types, the two types of cells should become intermingled with one another. Although this differential cell adhesion theory suggests roles for self-organized cell movements in various epithelia, it remains unclear how this strategy for cell sorting is used during morphogenesis in vivo. Differential mechanical tension also plays an important role in cellular rearrangements (Lecuit, ; Heisenberg and Bellaiche, ). For example, during Drosophila germband extension, junctions are remodeled through the polarized recruitment of myosin II within the epithelium (Bertet et al., ). The contractile activity of myosin II creates local tension that orients the disassembly of E-cadherin junctions. In the case of neural-tube closure, polarized constriction of neuroepithelial adherens junctions (AJs) induces the convergence of their apical domains toward the midline of the neural plate (Nishimura et al., ). These observations suggest that anisotropic extensions and contractions of cell–cell junctions are used for cellular rearrangements in various epithelia.
Roles of cadherins and nectins in cellular patterning
The major cell adhesion molecules at AJs are cadherins and nectins (Figure 1A; Takai et al., 2008; Meng and Takeichi, ). Cadherins are essential for maintaining multicellular structures, and play a role in vital processes such as embryogenesis, pattern formation, and maintenance of specific tissue architectures. Cadherins are Ca2+-dependent cell–cell adhesion molecules that constitute a superfamily, and are grouped into subfamilies designated classic cadherins and proto-cadherins. Here, for convenience, classic cadherins are simply referred to as cadherins. Cadherin molecules associate with p120 catenin and β-catenin via their cytoplasmic domain, and β-catenin in turn binds to α-catenin. α-Catenin can bind to F-actin, an interaction thought to be crucial for cadherins to create firm cell adhesions (Meng and Takeichi, ). The major role of cadherins is to connect cells expressing the same cadherins through homophilic interactions. Through these properties of cadherins, cells in mixed cultures of cell lines expressing E- or N-cadherin were observed to form separate aggregates (Nose et al., ; Katsamba et al., ), while differential levels of cadherin expression in two transfected cell populations caused one cell population to segregate internally or externally from the other cell population (Friedlander et al., ; Steinberg and Takeichi, ). The multicellular hexagonal lattice formation of the Drosophila retina is thought to arise through a cell-sorting process (Tepass and Harris, 2007). All cells in the retina express DE-cadherin, whereas only the cone cells express DN-cadherin (Figure 1B). The cone cell shape is formed by differential cadherin-mediated adhesion (Hayashi and Carthew, ). Differential expression of DN-cadherin within cone cells causes these cells to form an overall shape that minimizes their surface contact with surrounding cells. These observations indicated that simple patterned expression of cadherin results in a complex spatial pattern of cells in the visual system of Drosophila.
Figure 1
Nectins comprise a family of immunoglobulin-like molecules with four members, nectin-1, -2, -3, and -4. The most important property of this family is that nectins interact with other nectins in homophilic or heterophilic manners. Furthermore, nectins prefer heterotypic partners to homotypic partners, and their heterophilic interactions produce stronger cell–cell adhesions than their homophilic interactions (Figure 1C; Fabre et al.,
The cadherin and nectin systems are associated during cell–cell junction formation through their intercellular interactions (Figure 1A). It has been suggested that nectins and afadin initially form cell–cell adhesions and then recruit cadherins to the nectin-based cell–cell adhesion sites to accelerate the formation of cadherin-dependent junctions (Tachibana et al., 2000; Honda et al.,
Checkerboard-like pattern formation in the auditory epithelium
The auditory epithelium of the mammalian inner ear consists of inner hair cells, outer hair cells, and at least four different types of supporting cells. The hair cells and supporting cells are organized in a checkerboard-like pattern, such that each hair cell is separated from another hair cell by a supporting cell, forming an alternating mosaic. Cellular rearrangements are a possible mechanism for the patterning in the inner ear. In the developing auditory epithelium, the hair cells and supporting cells continue to change their position and alignment beyond the period of terminal mitoses for cells (Figure 1D; Chen et al.,
Mosaic pattern formation in the olfactory epithelium
The olfactory epithelium, which is located inside the nasal cavity in mammals, is a specialized sensory epithelium involved in odor perception. The olfactory epithelium is a pseudostratified columnar epithelium composed of olfactory cells, supporting cells, and basal cells. These cells are stereotypically layered from the apical to basal side in the olfactory epithelium. When the luminal surface of the olfactory epithelium is observed from the apical side, ciliated olfactory cells and several types of supporting cells are arranged in a specialized mosaic pattern (Figure 2A; Cuschieri and Bannister,
Figure 2

Mosaic cellular pattern in the mouse olfactory epithelium. (A) Schematic illustrations of the apical surface of the mouse olfactory epithelium from embryonic day (E) 14 to postnatal day (P) 28 (Katsunuma et al.,
Concluding remarks
Herein, the roles of the cell–cell adhesion molecules nectins and cadherins in the unique cellular pattern formation of sensory organs have been introduced and discussed. Many different types of cells in tissues express various types of cell adhesion molecules. As multiple adhesion systems appear to cooperate at the organogenesis level, it will be of great interest to determine whether they act in parallel or hierarchical manners. As mentioned above, the nectin-dependent differential distribution of cadherins leads to extension or shrinkage of cell–cell junctions, thereby contributing to the cell intercalations and mosaic patterning. However, cooperative mechanisms between nectins, cadherins, and cortical tension are likely to contribute to the production of the complex cell patterns, and our current understanding of the cooperative roles between these mechanisms is insufficient to explain the complex mechanisms underlying cellular patterning. Elucidating the combined mechanisms between differential adhesion and actin–myosin contractility will provide a more detailed picture of cellular patterning. How does the cooperative action of nectins and cadherins regulate actin–myosin contraction and generate differential adhesion? Cell cortical tension is caused by actin–myosin contraction and cell–cell adhesion, and actin cytoskeletal anchoring of cell adhesion molecules is achieved by various actin-binding proteins. Previous studies have shown that mechanical stress on cadherin adhesion complexes modifies their cytoskeletal anchoring strength (Yonemura et al., 2010), and that cadherin oligomerization stiffens these molecules anchored to the actin cytoskeleton (Strale et al., 2015). To understand the mechanism for the production of the complex cell patterns and structures in epithelia, deciphering the intracellular and intercellular signaling mechanisms will provide insights into how the cooperation of cell–cell adhesion and cortical tension contributes to self-organization of cells into tissues.
Funding
The author's laboratory was supported by KAKENHI Grant Numbers 25111716, 25127710, 25440107, and 26400205, and by a grant from the Takeda Science Foundation.
Conflict of interest statement
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Statements
Author contributions
The author confirms being the sole contributor of this work and approved it for publication.
Acknowledgments
The author apologizes to all authors whose work could not be cited because of space constraints. The author thanks S. Katsunuma for critical comments on the manuscript.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
cell adhesion molecules, cadherins, nectins, sensory organs, cell sorting, mosaic cellular pattern, self-organization
Citation
Togashi H (2016) Differential and Cooperative Cell Adhesion Regulates Cellular Pattern in Sensory Epithelia. Front. Cell Dev. Biol. 4:104. doi: 10.3389/fcell.2016.00104
Received
01 August 2016
Accepted
31 August 2016
Published
15 September 2016
Volume
4 - 2016
Edited by
Takaaki Matsui, Nara Institute of Science and Technology, Japan
Reviewed by
René-Marc Mège, Centre National de la Recherche Scientifique, France; Mitsugu Fujita, Kindai University, Japan
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© 2016 Togashi.
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*Correspondence: Hideru Togashi htogashi@med.kobe-u.ac.jp
This article was submitted to Cell Adhesion and Migration, a section of the journal Frontiers in Cell and Developmental Biology
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