The first digestive movements in the embryo are mediated by mechanosensitive smooth muscle calcium waves

Abstract

Peristalsis enables transport of the food bolus in the gut. Here, I show by dynamic ex-vivo intra-cellular calcium imaging on living embryonic gut transverse sections that the most primitive form of peristalsis that occurs in the embryo is the result of inter-cellular, gapjunction dependent calcium waves that propagate in the circular smooth muscle layer. I show that the embryonic gut is an intrinsically mechanosensitive organ, as the slightest externally applied mechanical stimulus triggers contractile waves. This dynamic response is an embryonic precursor of the "law of the intestine" (peristaltic reflex). I show how characteristic features of early peristalsis such as counterpropagating wave annihilation, mechanosensitivity and nucleation after wounding all result from known properties of calcium waves. I finally demonstrate that inter-cellular mechanical tension does not play a role in the propagation mechanism of gut contractile waves, unlike what has been recently shown for the embryonic heartbeat. Calcium waves are an ubiquitous dynamic signaling mechanism in biology: here I show that they are the foundation of digestive movements in the developing embryo.

Embryogenesis of the peristaltic reflex

“…We previously demonstrated that early stage guts (E7–E9) are intrinsically mechanosensitive: when mechanical pressure is applied to the gut locally by pinching, it triggers two outgoing CC waves that propagate away from the point of compression (Chevalier, ). We investigate here the development of this reflex response on duodenal segments at the TTX‐insensitive and TTX‐sensitive stages E12 and E16, respectively.…”

“…). We have previously demonstrated by direct calcium imaging that spontaneously generated, intercellular, gap‐junction‐dependent circular smooth muscle calcium waves underpin this early motility pattern (Chevalier, ). Contractions at stage E5–E6 propagate rapidly (100–1000 μm/s), leading to apparent, immediate whole‐gut contractions (Chevalier et al .…”

“…). We have recently demonstrated that early digestive movements are underpinned by calcium waves that travel through a circular smooth muscle syncytium (Chevalier, ). All attributes of early motility are virtually identical to those of calcium waves: constant speed propagation without attenuation, counter‐propagating wave annihilation, and sensitivity of wave generation to mechanical stimulation or wounding.…”

Genetic and Mechanical Regulation of Intestinal Smooth Muscle Development

“…To test this, we cultured whole intestinal explants from E9, prior to outer smooth muscle differentiation and alignment, for 72 h while decreasing or increasing circumferential muscle contraction frequency. We took advantage of the fact that fetal smooth muscle contraction depends on the activity of L-type calcium channels and myosin light chain kinase (MLCK), with calcium transients propagated between the cells via gap junctions to generate the contractile waves (Chevalier, 2018;Schultz et al, 2003). Consistent with our hypothesis, preventing these contractions during formation of the outer layer with nifedipine (a calcium channel inhibitor), ML-7 (an MLCK inhibitor), or carbenoxolone (a gap junction inhibitor) inhibited alignment of the outer muscle layer, whereas increasing the contractile frequency using the L-type calcium channel activator (S)-(-)-BayK8644 enhanced alignment (Figures 5A, S6B, and S6E; Video S3).…”

Mechanics of development

“…cortical folding constrained by skull, lung branching), as covered by Jaslove & Nelson [26] and Garcia et al [15]. Then there are the forces ( pre-stresses) that are inherently within tissues and structures, which may change over development and may direct aspects of it, as covered by Chevalier [23] in this issue, and others [20,42]. At the cell-, rather than tissue-, level, mechanical forces can affect aspects such as cell shape, organization and orientation, as described by Tetley & Mao [28] and Bagnat et al [12].…”