Biology Reference
In-Depth Information
cases, though, adults are not present at a high enough density for this to be feasible, and
gametes have to migrate by crawling or swimming. In both plants and animals, migration
has favoured the loss of isogamy and the specialization of gametes either to be nutrient-rich
and relatively immobile (ova), or numerous, small and specialized for migration (sperm).
Sperm use a variety of methods for migration (
Figure 7.1
). Those of nematodes are
approximately amoeboid in shape and move by amoeboid motion (
Figure 7.1
b). Those of
decapod crustaceans (crabs, lobsters, and so on) have a large number of actin-containing
processes
3
(
Figure 7.1
c). Mammalian sperm, and those of many other animals and algae,
move by the whiplash action of flagella (
Figure 7.1
d). In many species, sperm is attracted
towards eggs of the correct species by chemotaxis (see Chapter 9). Male gametes of primitive
plants, such as liverworts (Hepaticae), also swim using flagella but in flowering plants the
meeting of sperm and egg nuclei is achieved by the extension of a tube from the pollen,
which invades the female parts of the flower and brings the male gamete nucleus to the
ovum (
Figure 7.1
e).
Important as it is to development, fertilization is not morphogenesis. During post-
fertilization development, cell migration and locomotion contribute directly to morphogen-
esis in four main ways:
1.
Bringing dispersed cells together to aggregate in one place
2.
Moving a group of cells from one place in the body to another
3.
Dispersing cells created in one place to a variety of sites
4.
Connecting cells and cell processes together in a specific network, such as the nervous
system.
There are also many cell rearrangements that operate on too short a range to be considered
true migrations and are therefore discussed in Chapter 16 rather than here.
MORP
HOGENESIS BY COALESCENCE OF DISPERSED
CELLS
One of the simplest examples of cell migration effecting true morphogenetic change is
seen in the social amoeba Dictyostelium discoideum. Where food sources are plentiful, D. dis-
coideum exists as a community of scattered myxamoebae, which are amoeba-like cells that
roam decaying wood on forest floors and prey on bacteria. When food becomes scarce,
the myxamoebae begin to emit a chemotactic signal and, through sensing and information
processing systems that will be discussed in detail in Chapter 9, they move toward sources
of this signal and therefore toward each other. Their migration results in the formation of
streams of moving cells that converge to produce a tight, conical aggregate (
Figure 7.2
).
This aggregate then behaves as a single organism, and later processes of differentiation
within it turn it into a migratory grex, sometimes called a 'slug' because of its superficial
resemblance to these animals. The grex moves away and, as it does so, some cells differen-
tiate to form spores. It then stops migrating, raises these spores into the air atop a long stalk,
and releases them so that they disperse. Where they land, they form further myxamoebae
and the cycle repeats.
Morphogenesis by coalescence of migrating cells is seen many times in the development of
higher animals too. The great blood vessels of vertebrates, for example, form in this way. In
