Branching morphogenesis

Abstract

Tubular structures are a fundamental anatomical theme recurring in a wide range of animal species. In mammals, tubulogenesis underscores the development of several systems and organs, including the vascular system, the lungs, and the kidneys. All tubular systems are hierarchical, branching into segments of gradually diminishing diameter. There are only two cell types that form the lumen of tubular systems – either endothelial cells in the vascular system, or epithelial cells in all other organs. The most important feature in determining the morphology of the tubular systems is the frequency and geometry of branching. Hence, deciphering the molecular mechanisms underlying the sprouting of new branches from pre-existing ones is the key to understanding the formation of tubular systems. The morphological similarity between the various tubular systems is underscored by similarities between the signaling pathways which control their branching. A prominent feature common to these pathways is their duality – an agonist counterbalanced by an inhibitor. The formation of the tracheal system in Drosophila melanogaster is driven by fibroblast growth factor (FGF) and inhibited by Sprouty/Notch. In vertebrates, the analogous pathways are FGF and transforming growth factor β in epithelial tubular systems, or vascular endothelial growth factor and Notch in the vascular system.

Figure 1. Signaling pathways regulating the branching of the Drosophila tracheal system

a. The anterior-posterior and dorsal-ventral patterning genes induce bnl expression in mesenchymal cell clusters, which secrete and form a gradient of Bnl (green). Bnl induces btl expression (red) in the epithelial cells of the tracheal placodes that closest to the Bnl source, and acts as a chemoattractant. The migrating cells coalesce into a primary branch. b. Bnl induces secondary branch tip-cell genes in epithelial cells sensing the highest Bnl signal, which then suppress tip-cell gene expression in stalk cells via Notch. Pointed is a pivotal tip-cell gene which upregulates MAPK signaling and Sprouty. Sprouty forms a negative feedback loop by inhibiting Btl signaling. c. Hypoxia induces local expression of bnl via Fatiga, leading to Btl signaling via Pointed and Blistered, and resulting in terminal branch sprouting.

Figure 2

Signaling pathways regulating branching in vertebrate lung development. a. FGF10 (green), possibly induced by homeotic transcription factors, is secreted by visceral mesenchyme and induces FGFR2b (red) expression in the cells at the tip of the primordial buds or in the buds of subsequent generations. b. FGFR2b signaling induces Spry2 which forms a negative feedback loop by inhibiting FGFR2b signaling. Wnt5a (blue) expressed in the surrounding mesenchyme induces BMP4, which antagonizes FGF10 in an autocrine manner, but has a paracrine agonist effect on budding. Netrin1,4 (pink) restricts FGFR2b signaling to the tip of the growing bud.

Figure 3

Signaling pathways regulating branching in the ureteric system. a. Morphology of the developing ureteric system. New tubes are formed mostly by budding from ampullae as bifid or trifid branches, but some tubes branch out laterally from stalks. b. GDNF (blue) stimulates proliferation of the ureteric epithelial cells via the Ret receptor. GDNF induces expression of Spry1 and BMP4, which form a negative feedback loop by inhibiting Ret. FGF7,10 (green) signal via FGFR2b and probably act as chemoattractants. GDNF induces expression of Wnt11 (pink), which forms a positive feedback loop by upregulating GDNF.

Figure 4

Major components in the signaling pathways regulating vascular branching. Hypoxia induces expression of VEGF-A (green), which promotes vessel growth. VEGF-A signals via VEGFR (VEGFR2 in mouse retinal vessels and VGFR3/Flk4 in zebrafish intersomitic vessels) and neuropilin (Nrp) as both a chemoattractant and a cytokine. Induction and expression of Dll4 by VEGFR in individual cells confers a cell-tip phenotype (red) and activates Notch in adjacent cell. Notch activation suppresses VEGFR expression and prevents these cells from conversion into tip-cells.