Blueprint for an intestinal villus: Species-specific assembly required - PubMed (original) (raw)
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Blueprint for an intestinal villus: Species-specific assembly required
Katherine D Walton et al. Wiley Interdiscip Rev Dev Biol. 2018 Jul.
Abstract
Efficient absorption of nutrients by the intestine is essential for life. In mammals and birds, convolution of the intestinal surface into finger-like projections called villi is an important adaptation that ensures the massive surface area for nutrient contact that is required to meet metabolic demands. Each villus projection serves as a functional absorptive unit: it is covered by a simple columnar epithelium that is derived from endoderm and contains a mesodermally derived core with supporting vasculature, lacteals, enteric nerves, smooth muscle, fibroblasts, myofibroblasts, and immune cells. In cross section, the consistency of structure in the billions of individual villi of the adult intestine is strikingly beautiful. Villi are generated in fetal life, and work over several decades has revealed that villus morphogenesis requires substantial "crosstalk" between the endodermal and mesodermal tissue components, with soluble signals, cell-cell contacts, and mechanical forces providing specific dialects for sequential conversations that orchestrate villus assembly. A key part of this process is the formation of subepithelial mesenchymal cell clusters that act as signaling hubs, directing overlying epithelial cells to cease proliferation, thereby driving villus emergence and simultaneously determining the location of future stem cell compartments. Interestingly, distinct species-specific differences govern how and when tissue-shaping signals and forces generate mesenchymal clusters and control villus emergence. As the details of villus development become increasingly clear, the emerging picture highlights a sophisticated local self-assembled cascade that underlies the reproducible elaboration of a regularly patterned field of absorptive villus units. This article is categorized under: Vertebrate Organogenesis > From a Tubular Primordium: Non-Branched Comparative Development and Evolution > Organ System Comparisons Between Species Early Embryonic Development > Development to the Basic Body Plan.
Keywords: epithelial-mesenchymal cross-talk; fetal intestine; morphogenesis; villus development.
© 2018 Wiley Periodicals, Inc.
Figures
Figure 1. Progressive bending of the chick epithelium into ridges and then zigzags is driven by sequential differentiation of smooth muscle layers
Prior to muscle formation (E6) the epithelium is flat. Ridges are formed following the differentiation of the inner circular muscle marked by anti-alpha-smooth muscle actin (green; E8–12). Zigzags evolve with the formation of the outer longitudinal muscle (E13–15). Villus emergence is concomitant with the formation of the muscular mucosa at E16. Scalebars are 100 μm. Arrowheads denote the new muscle layer formed at that stage. All images are reproduced with permission from.
Figure 2. Surface views of the intestinal mucosa during development
A–C) Chick at E11, E16 and E18, showing progressive formation of ridges, zigzags and villi. Images are reproduced with permission from. DR and IR in panel A denote two different rounds of ridge formation. The arrows in B mark areas along the zigzags that are beginning to bulge where villi will emerge. I and II in panel C denote villi emerging atop alternating zigzags. D–F) Mouse intestinal surface at E14, E14.5 and E15.5, demonstrating the flat epithelial surface prior to villus formation. ^ in D indicate mitotically rounded cells along the flat apical surface. Apical invaginations begin to demarcate villi (arrows in E), which emerge as domes from the flat epithelium. Images D and E are reproduced with permission from. Domes of newly emerged villi in the rat (G) and human (H), reproduced with permission from and . Scalebars are 10 μm in D and E and 100 μm in F.
Figure 3. Cell shape during villus morphogenesis in the mouse
A–B) Epithelium (ECADHERIN, green) thickens from 15 μm at E12.5 to 50 μm at E14.5. (C) Epithelial cells on the growing villi shorten and widen to become columnar. D) The pre-villus epithelium is pseudostratified, with cells touching both the apical and basal surfaces. Cells were sparsely labeled with myristylated EGFP to outline cell shapes. E) Scanning electron micrograph of an E13.5 intestine shows epithelial cells touching the apical and basal surfaces. Images A–E were reproduced with permission from. F,G) Schematic representations of stratified and pseudostratified layers. Green illustrates expected cell shapes in each type of epithelium. Note that mitotic cells are basal in the stratified epithelium and apical in the pseudostratified epithelium. H) Nuclei of cells undergoing inter kinetic nuclear migration move between the apical and basal surface in accord with the cell cycle. F–H are reproduced with permission from. I) Active apical invagination acts to demarcate villi. Apical and basal surfaces are marked by anti- aPKC and anti-COLLAGEN IV, respectively (red) and dividing cells are marked with anti-pHH3 (green). J–K) Constriction of the T invagination around an apically rounded cell (green in K, marked by anti-pHH3) is shown with phalloidin staining (white and red). * in J and K indicate the F-actin rich tether extending from the cell body to the basal surface. Images in J and K are reproduced with permission from. Scalebars in A–D, J and K are 20 μm.
Figure 4. “Invasion” of the pseudostratified epithelium by mesenchyme (arrows) is an early feature of villus morphogenesis in mammals
Arrowheads indicate invading mesenchymal clusters in developing intestines from A) mouse at E15 (Walton, K.D., unpublished), B) rat at E18, C) human at stage 21, D) pig at 35 dpf, E) sheep at 39 dpf and F) cow at 30 dpf. Images in B–F are reproduced with permission from the noted references.
Figure 5. Changes in epithelial proliferation accompany villus morphogenesis in chick and mouse
A,B) Top panels depict ridge (E12) and late zigzag/early villus (E15) stages in chick. EdU staining (pink) marks proliferative cells, which are seen throughout the epithelium at the ridge stage (A) and confined to the base of emerging villi at later stages(B) (reprinted with permission from). C,D) Bottom panels illustrate robust proliferation, as marked by BrdU staining (green), throughout the pre-villus mouse pseudostratified epithelium (C, E13.5) and restriction of proliferative activity from the tips of emerging villi at E16 (D). Anti-ECADHERIN staining (red) outlines epithelial cells in C,D. DAPI staining marks the nuclei (blue).
Figure 6. Mesenchymal clusters participate in epithelial/mesenchymal crosstalk
A–D) In situ hybridization analysis of the Pdgf signaling pathway, as demonstrated by Karlsson, et al.Pdgfa ligand transcripts are seen throughout the pre-villus pseudostratified epithelium and cells in the muscle layer (A) and are later concentrated in intervillus epithelium (C, arrows). The receptor, Pdgfra is highly expressed in the subepithelial mesenchyme in the pre-villus intestine (B) and later robustly expressed in mesenchymal clusters in the tips of emerged villi as well as in nascent forming clusters (D, arrows). A–D are reprinted with permission from. Abbreviations are: pep, pseudostratified epithelium; sm, submucosal mesenchyme; ml, muscle layer; ep, epithelium. (E–L) Mesenchymal clusters are signaling centers that express transcripts corresponding to multiple soluble signaling proteins, including Bmp2 (E), Bmp4 (F), Twsg1 (G), Bmp1 (H), Wnt5a (I), Fgf9 (J), Hgf (K) and Nog (L). E, F,G,H, and L are reproduced with permission from.
Figure 7. Epithelial/mesenchymal crosstalk via the Hedgehog (Hh) pathway in chick and mouse
(A–C) In situ hybridization of transcripts for Shh (A), Ihh (B) and Ptch1 (C) in the mouse pre-villus epithelium. Hh transcripts are confined to the epithelium while the receptor is seen in surrounding mesenchyme and concentrated in the sub-epithelial region (reprinted with permission from). (D–I) Eosin and X-Gal staining of intestinal sections from Ptc1LacZ/+ (D–F) or Gli1LacZ/+ (G–I) animals at E13.5 (D, G), E14.5 (E, H) and E15.5 (F, I). Red arrows indicate clustered mesenchymal cells that are receiving Hh signals (reprinted with permission from). (J-R) In situ hybridization for Shh, Ptc, Bmp4 and Pdgfra in the developing chick intestine. The three columns represent early fold stage (J,M,P,S), zigzag stage (K,N,Q,T) and early villus (L,O,R,U). J-U are reproduced with permission from .
Figure 8. Schematic representation of the steps involved in villus emergence in the chick
Muscular development plays a major role in patterning villi in that species by deforming epithelium so as to cause pockets of high Hh concentration and cluster gene induction. Blue = Epithelium; Pink = underlying mesenchyme; Dark red = muscle groups.
Figure 9. Schematic representation of the steps involved in villus emergence in the mouse
Cluster patterning is via a Turing-like field, driven by Bmp signals. Clusters change overlying epithelial cell shape, resulting in regions of high intraepithelial pressure; cell division in these regions aid in apical invagination, demarcating villus boundaries. Blue = epithelium; Pink = mesenchymal clusters. In the lower panel, blue cells are proliferative; white cells are withdrawing from the cell cycle.
Figure 10. Evolutionary relationships among species and the appearance of the intestinal mucosa in those species
The appearance of villi may have arisen independently three times during evolutionary time, but the survey needs broadening to clarify this issue.
References
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