Mechanics of epithelial tissues during gap closure

The closure of gaps is crucial to maintaining epithelium integrity during developmental and repair processes such as dorsal closure and wound healing. Depending on biochemical as well as physical properties of the microenvironment, gap closure occurs through assembly of multicellular actin-based contractile cables and/or protrusive activity of cells lining the gap. This review discusses the relative contributions of ‘purse-string’ and cell crawling mechanisms regulated by cell–substrate and cell–cell interactions, cellular mechanics and physical constraints from the environment.

Introduction

Epithelia have important roles in shaping tissues and organs during embryogenesis, as well as in protecting tissues from homeostasis loss during wound healing [1]. Many physiological and pathological processes involve the (re-)sealing of epithelial gaps. From single cell apoptosis to macroscopic wound, discontinuities of the epithelial barrier occur continuously throughout the lifetime of organisms and in various scales and geometries.

Our review hence focuses on how epithelium maintains its own integrity by examining diverse gap closure scenarios. Such discontinuities can arise either intrinsically (e.g. ventral closure and dorsal closure during development, cell extrusion during homeostasis maintenance) or extrinsically (e.g. physical and chemical injury, infection). Due to its physiological importance, a wide range of studies has strived to elucidate the mechanism of epithelial gap closure with both in vivo and in vitro techniques.

Various morphogenetic events require the collective migration of neighboring epithelium into an opening to form a continuous monolayer, including D. melanogaster dorsal closure, C. elegans ventral enclosure, eyelid closure, neural tube closure and trachea invagination [2, 4••, 5•, 6]. In all these processes, an actin cable assembles apically to form a contractile ‘purse-string’, and actin-based structures drive basal protrusion [7, 8, 9, 10]. Lessons learnt from other gap closure processes studied in vitro, thanks to their striking similarities, helped understand the analysis of tissue morphogenesis in vivo [3].

Wound healing takes place during embryogenesis but also during adult life after a stress, for instance a skin cut, asthma or acute lung injury in the airway system. Independent of the tissue, healing processes share similarities [11]. However, due to its prevalence and tissue accessibility, epidermal wound healing has been the most studied: a multi-step process including tissue growth and remodeling leading to the reconstruction of the wounded area [12]. In adult skin injuries, re-epithelization can last days, during which activated keratinocytes migrate collectively over the wound area, dragging their own basal lamina as they move forward [13]. Keratinocytes in the front remodel the underlying ECM by secreting proteolitic enzymes such as metalloproteinases and depositing new ECM proteins [14]. Cell crawling seems to be more prominent here, with leader cells extending broad lamellipodia [15, 16, 17]. Interestingly, wound healing mechanisms vary with the age of the tissue. Much attention has been devoted to the study of embryonic wound healing due to its lack of scarring, reminiscent of gap closure events during morphogenesis, typically by a purse-string mechanism including rapid recruitment and assembly of actin and myosin into a thick cable in neighboring cells around the wound [18, 19, 20].

Finally, a particular case of epithelial gap closure is apoptotic cell extrusion, in which a dying cell is excluded from an epithelial monolayer. Cell extrusion also occurs recurrently in adulthood during tissue turnover and homeostatic processes [21, 22, 23]. When one or more cells undergo apoptosis, a purse-string mechanism triggers contraction that squeezes the apoptotic cell out of the epithelium.

From the examples discussed above, it appears that two main mechanisms contribute to the restoration of the epithelial integrity: (1) acto-myosin cable contraction in a purse-string manner and (2) cell crawling driven by lamellipodial and/or filopodial protrusions. Sometimes one mechanism dominates but often the two are both present and not mutually exclusive, making it challenging to distinguish their individual contributions [24, 25•] (Table 1, Table 2). Fortunately, recent development of in vitro approaches allowed great progress in the understanding of the relative and synergistic effects of the two mechanisms as well as their regulation, by means of applying mechanical and geometrical constraints [25•, 26•, 27••, 28•, 29, 30•, 31].