Figure 21–54 Branching morphogenesis of airways in a fly. (a) Drosophila embryonic tracheal system. (B) FGF (produced in Drosophila by the Branchless gene) signals from surrounding cells to the tracheal epithelium and activates its FGF receptors, leading to filopodia formation and tube elongation. [a, from G. Manning and M.a. Krasnow, in the Development of Drosophila (a. Martinez-arias and M. Bate, eds), Vol. 1, pp. 609–685. New York: cold Spring harbor Laboratory press, 1993.]
Figure 21–53 Branching morphogenesis of the lung. how FGF10 and Sonic hedgehog are thought to induce the growth and branching of the buds of the bronchial tree. Many other signal molecules, such as BMp4, are also expressed in this system, and the suggested branching mechanism is only one of several possibilities.
as indicated, FGF10 protein is expressed in clusters of mesenchyme cells near the tips of the growing epithelial tubes, and its receptor is expressed in the epithelial cells themselves. the Sonic hedgehog signal is sent in the opposite direction, from the epithelial cells at the tips of the buds back to the mesenchyme. the patterns of gene expression and their timing suggest that the Sonic hedgehog signal may serve to shut off FGF10 expression in the mesenchyme cells closest to the growing tip of a bud, splitting the FGF10-secreting cluster into two separate clusters, which in turn cause the bud to branch into two.
Figure 21–55 The forms of cell behavior an epithelium can Bend During Development to Form a tube or involved in tube formation. FoldingVesicle generates the neural tube, budding underlies the formation of lungs and
The creation of systems of tubes such as blood vessels and airways is a complex trachea, cord hollowing occurs during process, and it can involve various additional forms of cell behavior, as sketched the formation of mammalian salivary in Figure 21–55. glands, cell hollowing is involved in the
As explained in Chapter 19, the process that converts an epithelial sheet into a formation of tracheal terminal cell tubes, and cell assembly generates the heart tube tube depends on contraction of specific bundles of actin filaments. With the help that forms at the earliest stage of heart of myosin motor proteins, actin filament bundles can shorten, causing the epithe development.
lial cells to narrow at their apex. These actin bundles are connected from cell to cell by adherens junctions, and if their contraction is coordinated along a specific axis, the result will be that the sheet bends and rolls up into a tube (Figure 21–56). The vertebrate neural tube, which we discuss in the last section of this chapter, originates in this way.
sheet of epithelial cells
INVAGINATION OF EPITHELIAL SHEET CAUSED BY AN ORGANIZED TIGHTENING ALONGadhesion belt with ADHESION BELTS IN SELECTED REGIONS OFassociated actin flaments CELL SHEET
Figure 21–56 Bending of an epithelial sheet to form a tube. contraction of apical bundles of actin filaments linked from cell to cell via adherens junctions causes the epithelial cells to narrow at their apex. Depending on whether the contraction is oriented along one axis of the sheet or is equal in all directions, the epithelium will either roll up into a tube or invaginate to form a vesicle. (a) Diagram showing how an apical contraction along one axis of an epithelial sheet can cause the sheet to form a tube. (B) Scanning electron micrograph of a cross section through the trunk of a two-day chick embryo, showing the formation of the neural tube by the process diagrammed in (a). (B, courtesy of Jean-paul revel.)