Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1192 Chapter 21: Development of Multicellular Organisms
FOLDING BUDDING CORD HOLLOWING CELL HOLLOWING CELL ASSEMBLY
An Epithelium Can Bend During Development to Form a Tube or
Vesicle
The creation of systems of tubes such as blood MBoC6 vessels n22.225-22.53 and airways is a complex
process, and it can involve various additional forms of cell behavior, as sketched
in Figure 21–55.
As explained in Chapter 19, the process that converts an epithelial sheet into a
tube depends on contraction of specific bundles of actin filaments. With the help
of myosin motor proteins, actin filament bundles can shorten, causing the epithelial
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.
Figure 21–55 The forms of cell behavior
involved in tube formation. Folding
generates the neural tube, budding
underlies the formation of lungs and
trachea, cord hollowing occurs during
the formation of mammalian salivary
glands, cell hollowing is involved in the
formation of tracheal terminal cell tubes,
and cell assembly generates the heart tube
that forms at the earliest stage of heart
development.
sheet of epithelial cells
adhesion belt with
associated actin filaments
INVAGINATION OF EPITHELIAL SHEET CAUSED
BY AN ORGANIZED TIGHTENING ALONG
ADHESION BELTS IN SELECTED REGIONS OF
CELL SHEET
EPITHELIAL TUBE PINCHES OFF
FROM OVERLYING SHEET OF CELLS
epithelial tube
or vesicle
(A)
(B)
forming neural tube
50 µm
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.)