SummaryThe basement membrane (BM) is a thin layer of extracellular matrix (ECM) beneath nearly all epithelial cell types that is critical for cellular and tissue function. It is composed of numerous components conserved among all bilaterians [1]; however, it is unknown how all of these components are generated and subsequently constructed to form a fully mature BM in the living animal. Although BM formation is thought to simply involve a process of self-assembly [2], this concept suffers from a number of logistical issues when considering its construction in vivo. First, incorporation of BM components appears to be hierarchical [3, 4, 5], yet it is unclear whether their production during embryogenesis must also be regulated in a temporal fashion. Second, many BM proteins are produced not only by the cells residing on the BM but also by surrounding cell types [6, 7, 8, 9], and it is unclear how large, possibly insoluble protein complexes [10] are delivered into the matrix. Here we exploit our ability to live image and genetically dissect de novo BM formation during Drosophila development. This reveals that there is a temporal hierarchy of BM protein production that is essential for proper component incorporation. Furthermore, we show that BM components require secretion by migrating macrophages (hemocytes) during their developmental dispersal, which is critical for embryogenesis. Indeed, hemocyte migration is essential to deliver a subset of ECM components evenly throughout the embryo. This reveals that de novo BM construction requires a combination of both production and distribution logistics allowing for the timely delivery of core components.

Highlights d Labeled ECM in fly embryos can be examined from initiation to homeostasis d Quantifying ECM levels to homeostasis allows for modeling of basal turnover rate d Embryonic ECM has a half-life of 10 h, which was confirmed by pulse-chase analysis d Inhibiting MMPs or ECM interactions alters the basal turnover rate

SummaryThe most prominent developmental function attributed to the extracellular matrix (ECM) is cell migration. While cells in culture can produce ECM to migrate, the role of ECM in regulating developmental cell migration is classically viewed as an exogenous matrix presented to the moving cells. In contrast to this view, we show here that Drosophila embryonic hemocytes deposit their own laminins in streak-like structures to migrate efficiently throughout the embryo. With the help of transplantation experiments, live microscopy, and image quantification, we demonstrate that autocrine-produced laminin regulates hemocyte migration by controlling lamellipodia dynamics, stability, and persistence. Proper laminin deposition is regulated by the RabGTPase Rab8, which is highly expressed and required in hemocytes for lamellipodia dynamics and migration. Our results thus support a model in which, during embryogenesis, the Rab8-regulated autocrine deposition of laminin reinforces directional and effective migration by stabilizing cellular protrusions and strengthening otherwise transient adhesion states.

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SummaryThe forces controlling tissue morphogenesis are attributed to cellular-driven activities and any role for extracellular matrix (ECM) is assumed to be passive. However, all polymer networks, including ECM, can theoretically develop autonomous stresses during their assembly. Here we examine the morphogenetic function of an ECM prior to reaching homeostatic equilibrium by analyzing de novo ECM assembly during Drosophila ventral nerve cord (VNC) condensation. Asymmetric VNC shortening and a rapid decrease in surface area correlate with exponential assembly of Collagen-IV (Col4) surrounding the tissue. Concomitantly, a transient developmentally-induced Col4 gradient leads to coherent long-range flow of ECM, which equilibrates the Col4 network. Finite element analysis and perturbation of Col4 network formation through the generation of dominant Col4-truncations that affect assembly, reveals that VNC morphodynamics is driven by a sudden increase in ECM-driven surface tension. These data highlight that ECM assembly stress and associated network instabilities can actively participate in tissue morphogenesis.