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are excluded from migrating through the caudal half of each somite.
Two studies have now shown that this pattern of crest cell migration is
due to an inhibition of crest cell movement through the caudal regions of
the somite, a process that is mediated by Eph signalling. In the chick,
EphB3 is expressed on crest cells and cells of the rostral half of the
somite, whereas ephrin-B 1, is expressed in the caudal half of the somite
(Krull et al. 1997; Wang and Anderson 1997). In the rat, the receptor
involved in this process is not clear, although the ephrin expressed in the
caudal half of the somite is ephrin-B2, and is therefore a different class
B ephrin to that performing the task in the chick.
A similar process of spatial exclusion underlies directed neural crest
cell movement in the hindbrain where cells migrate from specific rhombomeres (see below) to specific branchial arches (Smith et al. 1997).
This study concentrated on the migration of streams of neural crest from
rhombomeres (r) 4, 5 and 6, which distribute crest cells to branchial
arches 2, 3 and 4 respectively (Fig. 2). Expression of ephrin-B2 by crest
cells from r4 prevents them from mixing with crest cells migrating from
r5, which express EphA4 and EphBI, receptors that are both activated
following binding to ephrin-B2.
Inhibition of migration through repulsive interactions is a theme
underlying the roles of Eph receptors and ephrins in axon guidance and
in neural crest cell migration. Time-lapse studies of neural crest cells
have shown that contact between a crest cell and a cell carrying a
repulsive signal results in the collapse of the crest cell filopodia in a
similar manner to the axonal growth cone collapse seen in the retinotectal system (Jesuthasan 1996).
8.4 Function of Eph/Ephrin Signalling in the Formation
of Boundaries in the Somites and Hindbrain
Segmentation is a basic process in embryogenesis of many invertebrate
and all vertebrate embryos. In vertebrates the two regions of the body
axis that are clearly segmented are the paraxial mesoderm, which gives
rise to the somites - the precursors of the segmented vertebral column
(Gossler and Hrabe de Angelis 1998) - and the hindbrain region of the
neural plate. The hindbrain is divided up into regular units called rhombomeres, which are the basis for patterning of the neural epithelium and
N. Holder et al.
are excluded from migrating through the caudal half of each somite.
Two studies have now shown that this pattern of crest cell migration is
due to an inhibition of crest cell movement through the caudal regions of
the somite, a process that is mediated by Eph signalling. In the chick,
EphB3 is expressed on crest cells and cells of the rostral half of the
somite, whereas ephrin-B 1, is expressed in the caudal half of the somite
(Krull et al. 1997; Wang and Anderson 1997). In the rat, the receptor
involved in this process is not clear, although the ephrin expressed in the
caudal half of the somite is ephrin-B2, and is therefore a different class
B ephrin to that performing the task in the chick.
A similar process of spatial exclusion underlies directed neural crest
cell movement in the hindbrain where cells migrate from specific rhombomeres (see below) to specific branchial arches (Smith et al. 1997).
This study concentrated on the migration of streams of neural crest from
rhombomeres (r) 4, 5 and 6, which distribute crest cells to branchial
arches 2, 3 and 4 respectively (Fig. 2). Expression of ephrin-B2 by crest
cells from r4 prevents them from mixing with crest cells migrating from
r5, which express EphA4 and EphBI, receptors that are both activated
following binding to ephrin-B2.
Inhibition of migration through repulsive interactions is a theme
underlying the roles of Eph receptors and ephrins in axon guidance and
in neural crest cell migration. Time-lapse studies of neural crest cells
have shown that contact between a crest cell and a cell carrying a
repulsive signal results in the collapse of the crest cell filopodia in a
similar manner to the axonal growth cone collapse seen in the retinotectal system (Jesuthasan 1996).
8.4 Function of Eph/Ephrin Signalling in the Formation
of Boundaries in the Somites and Hindbrain
Segmentation is a basic process in embryogenesis of many invertebrate
and all vertebrate embryos. In vertebrates the two regions of the body
axis that are clearly segmented are the paraxial mesoderm, which gives
rise to the somites - the precursors of the segmented vertebral column
(Gossler and Hrabe de Angelis 1998) - and the hindbrain region of the
neural plate. The hindbrain is divided up into regular units called rhombomeres, which are the basis for patterning of the neural epithelium and