Explaining and learning to control large-scale pattern is a central unsolved problem, with implications for mitigation of birth defects, and the advancement of regenerative medicine and synthetic bioengineering. The dynamics of signals orchestrating large-scale order in vivo are a key area of research, as understanding these signals is an essential first step in developing interventions that alter anatomical outcomes. The dynamics of chemical signals and their gradients are becoming increasingly well-understood (Reingruber and Holcman, 2014; Slack, 2014; Werner et al., 2015). However, endogenous bioelectric signals represent a parallel regulatory system that exerts instructive control over large-scale growth and form. Recent work has demonstrated that ionic and bioelectrical signaling of various cell types underpins a powerful system of biological pattern control [reviewed in Nuccitelli (2003a), McCaig et al. (2005), Levin (2012, 2014), Levin and Stephenson (2012), and Tseng and Levin (2013)]. Importantly, endogenous bioelectric gradients across tissues can be a very early pre-pattern for subsequent transcriptional and morphogenetic events. For example, during craniofacial development of frogs, specific transmembrane voltage (Vmem) patterns determine the downstream shape changes and gene expression domains of the developing face (Vandenberg et al., 2011; Adams et al., 2016) and brain (Pai et al., 2015). Furthermore, experimental modulation of cell Vmem states can radically alter large-scale anatomy, for example, inducing eye formation in ectopic body areas, such as the gut, where the master eye regulator Pax6 cannot induce eyes (Pai et al., 2012), reprograming the regeneration blastemas of planaria to produce heads instead of tails (Beane et al., 2011), or rescuing normal brain patterning despite the presence of mutated neurogenesis genes, such as Notch (Pai et al., 2015).

On the scale of single cells, the Vmem spanning every living cell’s plasma membrane is a demonstrated regulator of key processes, such as cell proliferation (Blackiston et al., 2009), programed cell death (Boutillier et al., 1999; Wang et al., 1999), and differentiation (Ng et al., 2010), and is known to be a factor in the activation of immune cells (Bronstein-Sitton, 2004). For example, despite the action of growth factors, stem cells have been inhibited from differentiation by preventing the cells from developing a hyperpolarized Vmem (Sundelacruz et al., 2008). The bioelectric properties of single cells are fairly well-understood (Lodish et al., 2000; Wright, 2004). However, bioelectric states often regulate large-scale anatomical properties, such as axial polarity (Marsh and Beams, 1952; Beane et al., 2011), organ size (Perathoner et al., 2014) and shape (Beane et al., 2013), and induction of formation of whole appendages (Adams et al., 2007; Tseng et al., 2010). Moreover, pattern control involves long-range coordination of bioelectric states. In metastatic conversion (Morokuma et al., 2008; Blackiston et al., 2011; Lobikin et al., 2012), tumor suppression (Chernet and Levin, 2014; Chernet et al., 2015), brain size regulation (Pai et al., 2015), and head–tail polarity in planarian regeneration (Beane et al., 2011), the patterning outcome in one region of the animal is a function of the bioelectric states of both local and remote cells. Thus, it is imperative to understand not only how ion channel and pump activity controls single-cell electrical properties but also how electrical gradients self-organize, propagate, and evolve in multicellular networks. Moreover, understanding the origin of developmental order also requires that we understand how tissue-level gradients of bioelectric properties arise.

In a multicellular collective, endogenous patterns of Vmem and electric fields provide positional information and achieve long-range coordination of cell activity. As in the central nervous system, this occurs because cells in a tissue are not isolated, but are electrochemically connected (and, therefore, communicating) in several ways, including intracellular channels known as gap junctions [GJ (Goodenough and Paul, 2009)], and by ephaptic coupling created by local field potentials, which enable one cell’s Vmem activity to influence that of its neighbor’s (Zhou et al., 2012). These connections between cells create bioelectrical circuits involving long-range signal patterns through whole structures, which have been determined crucial for developing embryos (Jaffe, 1981; Hotary and Robinson, 1990; Hotary and Robertson, 1994; Shi and Borgens, 1995), normal limb development of animals (Altizer et al., 2001), healing of wounds (Nuccitelli, 1992, 2003a; McCaig et al., 2005; Zhao, 2009), and even in continuous tumor suppression in adult animals (Chernet and Levin, 2013, 2014). The ability for cells to couple and communicate makes local changes to cell Vmem relevant in terms of long-range signals capable of affecting the whole. Likewise, the inability for cells to form communication networks, for instance, due to improper expression or function of GJ connections, is observed in disease processes, such as cancer (Leithe et al., 2006; Trosko, 2007). Even briefly altering the bioelectric connectivity of a cellular network enables rewriting of an organism’s target morphology. For example, genomically normal fragments of planarian flatworms can be induced to regenerate heads with shapes and internal anatomy belonging to other extant species (Emmons-Bell et al., 2015), or changed to a two-headed form that regenerates with two heads in perpetuity, illustrating the ability to stably re-wire bioelectric circuits with permanent changes to the overall anatomy (Oviedo et al., 2010).

Another important bioelectrical signal relevant to multicellular clusters is a voltage gradient known as the trans-epithelial potential (TEP), which forms at the outer boundary of an organ or organism. The TEP is also implicated in normal developmental processes (Shi and Borgens, 1995), wound healing (Zhao, 2009), and disease processes, such as cystic fibrosis (Hay and Geddes, 1985), fungal infection (Gow and Morris, 1995), inflammation, and cancer (Soler et al., 1999). The TEP is created when multicellular structures develop impermeable tight junctions (TJ) between cells at the exterior boundary (Hay and Geddes, 1985); disruptions to this process induce electric fields that serve as guidance cues for many migratory cell types during injury response (McCaig, 1990; Zhao, 2009; Yamashita, 2013) and limb development (Borgens, 1984; Borgens et al., 1987). Understanding plasma membrane voltage gradients and transepithelial potentials, and their spatio-temporal transitions in vivo, is a key enabling step for the field of developmental bioelectricity and its applications.