Biology Reference
In-Depth Information
the lateral mesoderm also gives rise to other parts of the vasculature, and the process by
which the precursor pool is divided is not yet understood. Morphogenesis of blood vessels
by coalescence of migratory cells, usually called 'vasculogenesis', is not the only way that
blood vessels form; smaller vessels often form by branching from existing vessels in a process
that is called 'angiogenesis' and that will be described in Chapter 20.
Coalescence of cells is not confined to foetal development; the coming together of
migrating cells is an important mechanism in the morphogenesis of lymphatic tissue, which
continues to develop and remodel through adult life. In the spleen, arterioles that derive from
the splenic artery ramify through the organ and become sheathed by lymphoid cells, mainly
in the form of lyphoid follicles. The follicles form when stromal cells at the sites of presump-
tive follicles secrete a chemokine, CXCL13.
6
B lymphocytes bear a receptor for CXCL13,
CXCR5,
6
and it is assumed that CXCL13, perhaps with other chemokines, is responsible
for attracting migrating B cells to the presumptive follicle. Certainly, B cells will migrate
towards sources of CXCL13 in culture
7
and mice lacking CXCR5 fail to form follicles.
8
Rela-
tively short-range movements of mesenchymal cells are also involved in the formation of cell
'condensations', a process so important to development that it will be discussed separately in
Chapter 14.
TRANSLOCATION OF GROUPS OF CELLS FROM ONE PLACE TO
ANOTHER
As well as driving morphogenesis through cell aggregation, cell migration can be used as
a means of cell dispersal or for the translocation of cells from the place of their birth to the
place of their final use. An illustration is provided by the primordial germ cells of vertebrate
embryos, which give rise to the gametes discussed at the beginning of this chapter. Primor-
dial germ cells arise, early in development, far outside the gonads that will eventually be
their home (
Figure 7.4
). For instance, in frogs, they develop in the posterior region of the
gut and then migrate anteriorly along its dorsal surface, out along the dorsal mesentery (a
sheet of tissue that connects the gut to the rest of the abdomen), up the abdominal walls
into the genital ridges and along the ridges to the developing gonads.
9
e
11
In mammals,
primordial germ cells arise near the junction between the embryo-proper and the allantois,
an extraembryonic structure, and invade the endoderm of the embryo, which will form
the gut.
12
Once there, they migrate anteriorly along the gut and enter the genital ridges
13
(
Figure 7.4
). It used to be believed that mammalian primordial germ cells migrated via the
allantois, but it now seems that those cells taking this route are 'lost' and do not contribute
to the germ line; cells migrating into the endoderm are the true source of mammalian
gametes.
12,14
Sometimes, groups of migrating cells really do move as a coherent, cooperative group,
integrating the guidance cues they receive and passing 'traffic control' signals to one another
to avoid cellular traffic jams. Although the existence of collective cell migration has been sus-
pected for decades, there has been a sudden surge of interest in the phenomenon, stimulated
partly by studies of the principles of efficient traffic flow in the human-scale world. For
this reason, a new chapter (13), devoted to the topic, has been added to this second edition
of this topic.
