Embryonic Patterning of Xenopus Mesoderm by Bmp-4
167
mesoderm, induces and patterns the neural plate, and sets in motion a
cascade of secondary inductions (Hamburger 1988; Harland and Gerhart 1997).
In the early amphibian embryo, mesoderm originates from the socalled marginal zone, a ring of cells present in the equatorial region of
the early gastrula. The marginal zone arises as the consequence of an
inductive process, mesoderm induction (Nieuwkoop 1969; Nieuwkoop
1973; Harland and Gerhart 1997). Based on self-differentiation capacity, four different areas of the marginal zone can be distinguished: the
dorsal domain, which is equivalent to the Spemann organizer, differentiates into notochord; the dorsolateral domain into muscle; the lateral
and ventrolateral domain into pronephros and mesenchyme; and the
ventral domain into blood and mesenchyme (Fig. I A) (Dale and Slack
1987). Thus, the dorsoventral (d/v) pattern of these tissues is already
specified in the gastrula marginal zone. Hence, the elucidation of the
molecular mechanisms underlying pattern formation in the marginal
zone is of prime importance for understanding mesodermal development.
Bone morphogenetic proteins (BMPs) play an important role in
mesodermal patterning. They are members of the TGF-~ growth factor
superfamily. BMPs can form homo- and heterodimers which exhibit
distinct activities (Aono et al. 1995; Suzuki et al. 1997b; Nishimatsu and
Thomsen 1998), and they are involved in a variety of developmental
processes (Hogan 1996). Here we will discuss progress made in understanding the role of BMP signalling during mesodermal patterning of
the amphibian gastrula. The role of BMP signalling in early zebrafish is
discussed by M. Hild et al. in this volume (see Chap. 6).
10.2 An Evolutionarily Conserved System
for Dorsoventral Patterning
Grafting experiments have shown that during gastrulation, signals emanating from the Spemann organizer dorsalize the ventral marginal zone
to form intermediate types of mesoderm (for review see Kimelman et al.
1992; Slack 1993). Ventral mesoderm, therefore, used to be considered
ground state mesodermal tissue which serves as the passive substrate
upon which the organizer acts.
167
mesoderm, induces and patterns the neural plate, and sets in motion a
cascade of secondary inductions (Hamburger 1988; Harland and Gerhart 1997).
In the early amphibian embryo, mesoderm originates from the socalled marginal zone, a ring of cells present in the equatorial region of
the early gastrula. The marginal zone arises as the consequence of an
inductive process, mesoderm induction (Nieuwkoop 1969; Nieuwkoop
1973; Harland and Gerhart 1997). Based on self-differentiation capacity, four different areas of the marginal zone can be distinguished: the
dorsal domain, which is equivalent to the Spemann organizer, differentiates into notochord; the dorsolateral domain into muscle; the lateral
and ventrolateral domain into pronephros and mesenchyme; and the
ventral domain into blood and mesenchyme (Fig. I A) (Dale and Slack
1987). Thus, the dorsoventral (d/v) pattern of these tissues is already
specified in the gastrula marginal zone. Hence, the elucidation of the
molecular mechanisms underlying pattern formation in the marginal
zone is of prime importance for understanding mesodermal development.
Bone morphogenetic proteins (BMPs) play an important role in
mesodermal patterning. They are members of the TGF-~ growth factor
superfamily. BMPs can form homo- and heterodimers which exhibit
distinct activities (Aono et al. 1995; Suzuki et al. 1997b; Nishimatsu and
Thomsen 1998), and they are involved in a variety of developmental
processes (Hogan 1996). Here we will discuss progress made in understanding the role of BMP signalling during mesodermal patterning of
the amphibian gastrula. The role of BMP signalling in early zebrafish is
discussed by M. Hild et al. in this volume (see Chap. 6).
10.2 An Evolutionarily Conserved System
for Dorsoventral Patterning
Grafting experiments have shown that during gastrulation, signals emanating from the Spemann organizer dorsalize the ventral marginal zone
to form intermediate types of mesoderm (for review see Kimelman et al.
1992; Slack 1993). Ventral mesoderm, therefore, used to be considered
ground state mesodermal tissue which serves as the passive substrate
upon which the organizer acts.