Organs mainly attain their size by cell growth and proliferation, but sometimes also grow through recruitment of undifferentiated cells. Here we investigate the participation of cell recruitment in establishing the pattern of Vestigial (Vg), the product of the wing selector gene in Drosophila. We find that the Vg pattern overscales along the dorsal-ventral (DV) axis of the wing imaginal disc, i.e., it expands faster than the DV length of the pouch. The overscaling of the Vg pattern cannot be explained by differential proliferation, apoptosis, or oriented-cell divisions, but can be recapitulated by a mathematical model that explicitly considers cell recruitment. By impairing cell recruitment genetically, we find that the Vg pattern almost perfectly scales and adult wings are approximately 20% smaller. Furthermore, using fluorescent reporter tools, we provide direct evidence that cell recruitment takes place in a specific time between early and mid third-instar larval development. Altogether, our work quantitatively shows when, how, and by how much cell recruitment shapes the Vg pattern and drives growth of the Drosophila wing.
-Spatio-temporal quantification of Vg pattern in wing discsThe Vg pattern in each disc was obtained by quantifying the Vg intensity of a maximum projection image of the z-stack confocal images. We then used the line selection tool in ImageJ (ImageJ RRID: SCR_003070) from the ventral to dorsal fold (yellow rectangle in Fig. S1A). The width of the line (in pixels) was taken to be 0.6 of the distance between the folds (converted to pixels) so that we can get an average pattern along the DV axis (yellow rectangle in Fig. S1A). Figure S1. Outline of Vg quantification in wing discs. Raw data from confocal images stained for Vg and DAPI were processed as follows: (A-B) Three-dimensional data were converted into a Vg profile first by obtaining a maximum projection and then collecting the average intensity along vertical lines of pixels within an area of interest (yellow rectangle in A). This gives a Vg profile in arbitrary units (AU) as a function of DV position from the ventral to the dorsal fold (in m, B). (C) Background levels of Vg from a non-pouch area (*) were substracted to all positions in the Vg pattern. (D) Since the pouch area of the disc is not flat (** shows a cross section of the disc), we used DAPI (which we assumed is homogeneous, ***) as a normalizing recombination. Diagram representing an imaginal wing disc with red clones within the pouch, major (L) and minor (l) axis for each clone and the ratio was determinate. The long axis of each dividing clone is oriented relative to the P-D axis and plotted against its relative distance from the center to the first fold (edge) of the pouch (C).(B) Third instar imaginal wing discs containing clones expressing lacZ. (C) Box plots show median and first and third quartiles. Only cells with elongation ratios (long/short axis) >1.2 are plotted.Figure S15. Vg is established beyond the region of Wg signaling. Immunofluorescence images of two representative vg QE Gal4 UAS-TransTimer wing discs with DV length in the 60 -80 μm range. Images show the expression of GFP (A, B), RFP (A', B'), or both (A'', B''). At the right of each panel, an amplification of the region inside the white rectangle is shown and cells from the DV border are counted as in Chaudhary et al. 2019. The cyan arrow points to newly-recruited cells (GFP positive, RFP negative) located 14 (A) or 16 (B) cells away.
Cell differentiation, proliferation, and morphogenesis are generally driven by instructive signals that are sent and interpreted by adjacent tissues, a process known as induction. Cell recruitment is a particular case of induction in which differentiated cells produce a signal that drives adjacent cells to differentiate into the same type as the inducers. Once recruited, these new cells may become inducers to continue the recruitment process, closing a feed-forward loop that propagates the growth of a specific cell-type population. So far, little attention has been given to cell recruitment as a developmental mechanism. Here, we review the components of cell recruitment and discuss its contribution to development in three different examples: the Drosophila wing, the vertebrate inner ear, and the mammalian thyroid gland. Finally, we posit some open questions about the role of cell recruitment in organ patterning and growth.
The COVID-19 pandemic affected in-person learning worldwide due to fears that schools could contribute to the propagation of the virus within their communities. Using computational modeling, we compare the reopening of schools with mitigation measures with a strategy in which schoolchildren are segregated into small isolated groups or bubbles, where children physically interact without restrictions while receiving remote instruction from their teachers. This strategy is robust to common perturbations and is more flexible and stable than reopening of schools. Our modeling results and a real-world implementation of a bubbles program in an elementary school in Mexico City support that this strategy is an effective transition alternative, especially in communities with low vaccination rates or where operational costs associated to safely reopening schools cannot be afforded.
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