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We have investigated the expression patterns of five LIM-homeodomain (LIM-hd) genes, x-Lhx1, x-Lhx2, x-Lhx5, x-Lhx7, and x-Lhx9 in the forebrain of the frog Xenopus laevis during larval development and in the adult. The results were analyzed in terms of neuromeric organization of the amphibian brain and of combinatorial LIM-hd code and showed that LIM-hd developmental transcription factors are particularly powerful to highlight the coherence of several groups or nuclei, to delineate subdivisions, and/or to clarify structures that are still a matter of debate. Among other findings, we bring substantial evidence for the following: (1) a dual origin of olfactory bulb neurons, based on x-Lhx5 expression; (2) the existence of a ventralpallium in frog, based on x-Lhx9 expression; (3) a multiple (pallial and subpallial) origin for the nuclei of the amygdaloid complex, based on distinct combinations of the five studied genes; (4) a clear homology between the Xenopus medialpallium and the mammalian hippocampus, based on x-Lhx2 pattern; and (5) a confirmed prosomeric organization of the diencephalon, based on alternating x-Lhx1/5 and x-Lhx2/9 expressions. In addition, the important expression levels for LIM-hd factors found throughout development and in the adult brain suggest a role for these genes in development and maintenance of neuronal specification and phenotype, as for example in the case of x-Lhx7 and cholinergic neurons. Moreover, following LIM-hd patterns throughout development points out to some of the migrations and morphogenetic movements, which give rise to the adult structures. Finally, the detailed description of the LIM-hd code in the developing and adult Xenopus forebrain provides interesting cues for the possible mechanisms of evolution of the vertebrate forebrain.
Fig. 2. A–Q: Schematic drawings of transverse sections through the brain of adult Xenopus laevis
illustrating the distribution of x-Lhx1-, x-Lhx5-, and x-Lhx1/5-expressing cells in the forebrain. The
appropriate levels of the sections are indicated in Figure 1. For abbreviations, see list. [Color figure can
be viewed in the online issue, which is available at www.interscience.wiley.com.]
Fig. 3. Photomicrographs of transverse sections showing x-Lhx1-
and x-Lhx5-expressing cells at different levels of the adult Xenopus
forebrain. a: The mitral cell layer of the AOB. b: BST and the ventral
septal expressing cells in the mediocaudal hemisphere. c: Scattered
cells in the lateral and medial part of the amygdaloid complex.
d: Caudal tip of the BST and the POa. e: Nucleus of the eminentia
thalami. f,g: Thalamus, with abundant cells in the ventralthalamus
and scattered cells in the dorsal thalamus. h: Retromamillary complex,
showing two subdivisions, and the mamillary complex. i: Caudal
diencephalic levels with robust staining in the pretectal juxtacommissural
nucleus, nucleus of the medial longitudinal fascicle, and the
caudal retromamillary nucleus. For abbreviations, see list. Scale
bars 100 m in a–i.
Fig. 4. Photomicrographs of dissected brains treated in toto (a–c)
or transverse sections (d–i) showing the expression of x-Lhx1 and
x-Lhx5 in the developing Xenopus forebrain at different stages (St).
a: Lateral view of the brain of a late embryo illustrating the regional
pattern of x-Lhx5 expression at the end of the embryonic period.
b: Dorsal view of the brain of an early premetamorphic larva showing
the x-Lhx1 expression pattern. c: Detail of a dorsal view of the telencephalon
of a mid-premetamorphic larva stained for x-Lhx5 expression
showing the developing olfactory bulb and ventralpallium. Asterisks
in b and c indicate expression in the ventral portion of the
medial telencephalic wall. d: The pallium at embryonic stages. e: The
olfactory bulb at prometamorphic stages. f: Amygdaloid complex
showing a dense staining in the lateral portion but only scattered cells
in the medial portion (arrow). g–i: Transverse sections through the
developing thalamus from rostral (g,h) to caudal (i). The approximate
orientation of the sections shown in d and g is indicated by lines in a.
For abbreviations, see list. Scale bars 100 m in a–i.
Fig. 5. A–Q: Schematic drawings of transverse sections through the brain of adult Xenopus laevis
illustrating the distribution of x-Lhx2-, x-Lhx9-, and x-Lhx2/9-expressing cells in the forebrain. The
appropriate levels of the sections are indicated in Figure 1. For abbreviations, see list. [Color figure can
be viewed in the online issue, which is available at www.interscience.wiley.com.]
Fig. 6. Photomicrographs of transverse sections showing x-Lhx2-
and x-Lhx9-expressing cells at different levels of the adult Xenopus
forebrain. a,b: Labeled cells in the medialpallium, at low (a) and high
(b) magnification. c: x-Lhx9 expression in the ventralpallium, with
abundant cells in the lateralamygdala and scattered cells dorsally in
the ventralpallium. d: Suprachiasmatic nucleus. e: Habenular complex.
f–i: The dorsal thalamus from rostral to caudal levels showing a
conspicuous staining in the medial nuclei (anterior and central) and
scattered cells in the lateral nuclei and pretectal region. For abbreviations,
see list. Scale bars 100 m a–i.
Fig. 7. Photomicrographs of dissected brains treated in toto (a–c)
or transverse sections (d–i) showing the expression of x-Lhx2 and
x-Lhx9 in the developing Xenopus forebrain at different stages (St).
a: Lateral view of the brain of an early premetamorphic larva illustrating
the regional pattern of x-Lhx2 expression. b: Dorsal view of
the brain of a late embryo illustrating the regional pattern of x-Lhx9
expression. The asterisk indicates expression in the ventral portion of
the medial telencephalic wall. c: Detail of a dorsal view of the diencephalon
and tectum of the brain of a mid-premetamorphic larva
stained for x-Lhx2 expression, showing the developing habenula and
dorsal thalamus. d: Section through the pallium at embryonic stages.
e: Habenular complex showing a strong expression in the ventral
portion. f: Ventralpallium with a strong staining in the lateral portion
of the amygdala and scattered cells in the lateralseptum.
g: Lateralamygdala and BST. h,i: Developing thalamus from rostral
(h) to caudal levels (i) with the characteristic strong expression in the
dorsal thalamus. The orientation of the sections showed in h and i is
indicated by lines in a. For abbreviations, see list. Scale bars 100
m in a–i.
Fig. 8. A–Q: Schematic drawings of transverse sections through the brain of adult Xenopus laevis
illustrating the distribution of x-Lhx7-expressing cells in the forebrain. The appropriate levels of the
sections are indicated in Figure 1. For abbreviations, see list. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
Fig. 9. Photomicrographs showing x-Lhx7-expressing cells at different
levels of the adult Xenopus forebrain (a–c) and through the
development (d–g; St, stage). a: Septal complex and BST. b: Suprachiasmatic
nucleus. c: Caudal telencephalic levels showing strong
expressing cells in the pallidum, anterior preoptic area, BST and
scattered cells in the medialamygdala. d: In totolateral view of the
brain of an early premetamorphic larva showing in toto the x-Lhx7
expression in the medial ganglionic eminence and the hypothalamus;
the arrow points to scattered cells located in the mamillary complex.
e: Transverse section through the medial ganglionic eminence.
f: Lateralseptum at prometamorphic stages. g: Hypothalamus at
premetamorphosis. The orientation of the sections showed in e and g
is indicated by lines in d. For abbreviations, see list. Scale bars 100
m in a–g.
Fig. 10. A–Q: Schematic drawings of transverse sections through the brain of adult Xenopus laevis
illustrating the distribution of x-Lhx1-, x-Lhx5-, x-Lhx2-, x-Lhx9-, x-Lhx7-, Lhx1/5-, and x-Lhx2/9-
expressing cells in the forebrain. The appropriate levels of the sections are indicated in Figure 1. For
abbreviations, see list.
Fig. 11. a,b: Photomicrographs of sagittal sections through the
adult Xenopus diencephalon showing x-Lhx9 (a) and x-Lhx1 (b) expression
in complementary banded patterns. For abbreviations, see
list. Scale bar 100 m in b (applies to a,b).
