HUMAN BIOLOGY

Development anD aging 335

lengthen and rings of microfilaments in cells tighten like

purse strings.

Section 17.1 mentioned that morphogenesis also requires

cells to move from one place to another. Migrating cells

find their way in part by following so-called adhesive cues.

For instance, as the nervous system is developing, migrat-

ing Schwann cells stick to adhesion proteins on the surface

of axons but not on blood vessels. Adhesive cues also tell

the cells when to stop. Cells migrate to places where the

signals are strongest, then stay there once they arrive.

Successful embryonic development requires that body

parts form according to normal patterns, in a certain

sequence. Genetically programmed cell death, the process

called apoptosis (a-poh-toe-sis) introduced in Chapter 9,

helps sculpt body parts. Inside cells that are destined to

die, enzymes begin digesting cell parts. For instance, mor-

phogenesis at the ends of limb buds first produced paddle-

shaped hands at the ends of your arms (Figure 17.9A). Then

epithelial cells between the lobes in the paddles died on

cue, leaving separate fingers (Figure 17.9B). Figure 17.9C

shows what can happen when apoptosis doesn’t occur

normally while a hand is forming.

hoW does morphogenesis change an embryo?

• After the embryo’s basic body plan is established, morphogenesis
  produces the shape and proportions of body parts.


• During morphogenesis, cells divide and migrate to their proper
  locations, tissues grow and fold, and certain cells die by
  apoptosis (programmed cell death).

taKe-Home message

next, organs develop and take on
the proper shape and proportions

After gastrulation, organs and organ systems begin to
form. An example is neurulation, the first stage in the devel-
opment of the nervous system. Figure 17.8 shows how cells
of ectoderm at the embryo’s midline elongate and form a
neural plate. This is the first sign that a region of ectoderm
is starting to develop into nervous tissue. Next, cells near
the middle become wedge-shaped. The changes in cell
shape cause the neural plate to fold over and meet at the
embryo’s midline to form the neural tube.
The folding of sheets of cells is extremely important in
morphogenesis. The folding takes place as microtubules

F i g u r e 17.9 Programmed cell death separates the digits

when fingers form. A At first, webs of tissue connect the

digits. B Then, cells in the webs die by apoptosis and the

digits are separated. C These fingers remained attached

when embryonic cells did not die on cue.

A

B C

A and B: © Kathleen K. Sulick, Bowles Center for Alcohol Studies, The University of North Carolina at Chapel Hill;

C: JPD/Custom Medical Stock Photo

Figure 17.8 Animated! Formation of the neural tube is an
example of morphogenesis. A neural tube is the forerunner of
the brain and spinal cord. It forms as certain ectodermal cells
change shape. In some cells, microtubules lengthen, and the
elongating cells form a neural plate. In other cells, microfilament
rings at one end constrict and the cells become wedge-shaped.
Their part of the ectodermal sheet folds over the neural plate to
form the tube. (© Cengage Learning)

neural tube

neural groove

notochord

(mesoderm)

neural plate (ectoderm)

1 Substances produced by the notochord

induce the ectoderm above it to thicken and

form a neural plate.

2 As cells in the neural plate change shape,

the plate’s edges fold toward the plate center to

form a neural groove.

3 As inward folding continues, the edges of

the neural plate meet and fuse to form the

neural tube.

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