Mucosal microfolds augment mixing at the wall of the distal ileum of the brushtail possum

Abstract

We describe a mechanism by which peripheral mixing may be engendered by mucosal microfolds without requiring the coordinated contraction of individual villi or groups of villi.

Determination of Villous Rigidity in the Distal Ileum of the Possum (Trichosurus vulpecula)

“…Hence villi would be sufficiently rigid to allow their shafts to tilt in concert with folding of the underlying mucosa to generate crowding of the villous tips between adjacent folds [6]. The ileal villi in this species were of similar lengths to those of other common laboratory animals such as rats (250–375 µm) [30], pigs (275–310 µm) [31] and dogs (460–550 µm) [32] and were within the range of villi lengths of the human small intestine (500–1000 µm) [33], [34].…”

“…The position of the probe was such that the tissue overlying the tip lay in the focal plane of an inverted microscope (Nikon Eclipse TE2000-U) and gave a lateral view of villi with the perfusate flowing at right angles to their long axes. Shear rates in species such as the rat and the possum during either pendular or peristaltic activity has been reported to be around 0.5–1 s −1 [6], [19]. Given that the viscosity in the continuous fluid layer around the tips of ileal villi of the possum is around 1.5 mPa.s [17], then by assuming Poiseuille flow we can calculate the shear stress acting on the villi at these shear rates to be between 0.7–2.5 mPa.…”

“…This action causes the tips of the attached villi to incline towards each other in the concavities and to diverge over the apices of microfolds. Further, fluid mechanical simulations show that these actions cause fluid to be alternately expressed from and drawn into the intervillous spaces from alternate crowding and separation of villous tips, augmenting peripheral mixing and dispersion of luminal contents [6]. This mechanism is more parsimonious than one that requires coordinated endogenous contraction of groups of villi [7], [8], [9], [10] to induce such movement of fluid.…”

“…Recent experimental evidence has shown that arrays of short-lived radially disposed mucosal microfolds are formed during static pendular contractions [6]. This action causes the tips of the attached villi to incline towards each other in the concavities and to diverge over the apices of microfolds.…”

Advective mixing due to longitudinal and segmental contractions in the ileum of the rabbit

“…The study of flow and mixing within the lumen of the small intestine has been of increased interest in the last few years (Wang et al, 2010;Lentle et al, 2013;Janssen et al, 2007;Love et al, 2012;Loubens et al, 2013;Fullard et al, 2014). It is thought that the contractions of the intestinal wall may contribute to the digestion and absorption of nutrients by engendering mixing within the lumen (Wang et al, 2010;Fullard et al, 2014).…”