Martin Catala

Embryonic expression of the human GATA-3 gene.

The spatial and temporal analysis of GATA-3 expression pattern in the human embryo revealed its expression in new anatomical sites. These include the endoderm of the primitive foregut, pharynx and allantois, the branchial arches and the mesenchymal cells surrounding the stomach and dorsal aorta. On the other hand, human (h) GATA-3 expression in the central nervous system, somites and embryonic kidney confirms the tissue specificity of this gene throughout vertebrate evolution.

Pleiotropic and diverse expression of ZFHX1B gene transcripts during mouse and human development supports the various clinical manifestations of the "Mowat-Wilson" syndrome.

ZFHX1B encodes Smad-interacting protein 1, a transcriptional corepressor involved in the transforming growth factors beta (TGFbeta) signaling pathway. ZFHX1B mutations cause a complex developmental phenotype characterized by severe mental retardation (MR) and multiple congenital defects. We compared the distribution of ZFHX1B transcripts during mouse and human embryogenesis as well as in adult mice and humans. This showed that this gene is strongly transcribed at an early stage in the developing peripheral and central nervous systems of both mice and humans, in all neuronal regions of the brains of 25-week human fetuses and adult mice, and at varying levels in numerous nonneural tissues. Northern blot analysis suggested that ZFHX1B undergoes tissue-specific alternative splicing in both species. These results strongly suggest that ZFHX1B determines the transcriptional levels of target genes in various tissues through the combinatorial interactions of its isoforms with different Smad proteins. Thus, as well as causing neural defects, ZFHX1B mutations may also cause other malformations.

Carbonic anhydrase activity during development of the choroid plexus in the human fetus

Abstract Carbonic anhydrase is one of the key enzymes responsible for the secretion of cerebrospinal fluid. This secretion increases dramatically during postnatal life in mammals. Nothing is known that can account for this regulation in the neonatal choroid plexus. However, the expression of carbonic anhydrase is developmentally regulated in several cells, such as erythrocytes and striated muscle fibers. The aim of our study was to assess the presence of carbonic anhydrase in epithelial cells of the choroid plexus during human development. We performed both histochemical and immunohistochemical detections of the enzyme on choroid plexuses between 9 and 34 weeks of gestation. We found that both carbonic anhydrase activity and the isozyme II were present as early as the 9th week of gestation. Expression of carbonic anhydrase is thus a very early event during plexus differentiation, and this enzymatic system could account for the secretion of cerebrospinal fluid during fetal life.

Human and monkey fetal brain development of the supramammillary-hippocampal projections: a system involved in the regulation of theta activity.

The supramammillary (SUM)‐hippocampal pathway plays a central role in the regulation of theta rhythm frequency. We followed its prenatal development in eight Cynomolgus monkeys (Macaca fascicularis) from embryonic day E88 to postnatal day 12 (term 165 days) and in eight human fetuses from 17.5 to 40 gestational weeks, relying on neurochemical criteria established in the adult (Nitsch and Leranth [ 1993 ] Neuroscience 55:797–812). We found that 1) SUM afferents reached the dentate juxtagranular and CA2 pyramidal cell layers at midgestation in human fetuses, earlier than in monkeys (two‐thirds of gestation [E109]). They co‐expressed calretinin, substance P, and acetylcholinesterase but not γ‐aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD); 2) the presumed parent neurons in the monkey SUM expressed calretinin or both calretinin and substance P; 3) most of them were surrounded by GAD‐containing terminals that might correspond to the septo‐SUM feedback pathway (Leranth et al. [ 1999 ] Neuroscience 88:701); and 4) in addition, a large band of calretinin‐labeled terminals that did not co‐express substance P, GAD, or acetylcholinesterase was present in the deepest one‐third of the dentate molecular layer in both the Cynomolgus monkey and human fetuses. It persisted in the adult monkey but not in adult human hippocampus; it remains questionable whether it originates in the SUM. In conclusion, the early ingrowth of the excitatory SUM‐hippocampal system in human and non‐human primates may contribute to the prenatal activity‐dependent development of the hippocampal formation. The possibility and the functional importance of an in utero generation of hippocampal theta‐like activity should also be considered. J. Comp. Neurol. 429:515–529, 2001.

The developmental potentials of the caudalmost part of the neural crest are restricted to melanocytes and glia.

The avian spinal cord is characterized by an absence of motor nerves and sensory nerves and ganglia at its caudalmost part. Since peripheral sensory neurons derive from neural crest cells, three basic mechanisms could account for this feature: (i) the caudalmost neural tube does not generate any neural crest cells; (ii) neural crest cells originating from the caudal part of the neural tube cannot give rise to dorsal root ganglia or (iii) the caudal environment is not permissive for the formation of dorsal root ganglia. To solve this problem, we have first studied the pattern of expression of ventral (HNF3beta) and dorsal (slug) marker genes in the caudal region of the neural tube; in a second approach, we have recorded the emergence of neural crest cells using the HNK1 monoclonal antibody; and finally, we have analyzed the developmental potentials of neural crest cells arising from the caudalmost part of the neural tube in avian embryo in in vitro culture and by means of heterotopic transplantations in vivo. We show here that neural crest cells arising from the neural tube located at the level of somites 47-53 can differentiate both in vitro and in vivo into melanocytes and Schwann cells but not into neurons. Furthermore, the neural tube located caudally to the last pair of somites (i.e. the 53rd pair) does not give rise to neural crest cells in any of the situations tested. The specific anatomical aspect of the avian spinal cord can thus be accounted for by limited developmental potentials of neural crest cells arising from the most caudal part of the neural tube.

Embryology of the Head and Neck

The skeleton of the head and neck represents a very complex set of bones whose development is both complicated and precise. It is by now impossible to write a precise and thorough chapter to account for the tremendous amount of new data gained by experimental embryology. So, I will focus my presentation on: (1) some elements of descriptive embryology that are mandatory to give the readers landmarks for human development; (2) the origin of the cells that are fated to form these structures and their subsequent development; (3) tissue interactions that could account for the malformative associations that can be observed in humans; and (4) genetic controls of these processes.

The role of primary cilia in corpus callosum formation is mediated by production of the GLI3 repressor

Agenesis of the corpus callosum (AgCC) is a frequent brain disorder found in over 80 human congenital syndromes including ciliopathies. Here, we report a severe AgCC in Ftm/Rpgrip1l knockout mouse, which provides a valuable model for Meckel-Grüber syndrome. Rpgrip1l encodes a protein of the ciliary transition zone, which is essential for ciliogenesis in several cell types in mouse including neuroepithelial cells in the developing forebrain. We show that AgCC in Rpgrip1l(-/-) mouse is associated with a disturbed location of guidepost cells in the dorsomedial telencephalon. This mislocalization results from early patterning defects and abnormal cortico-septal boundary (CSB) formation in the medial telencephalon. We demonstrate that all these defects primarily result from altered GLI3 processing. Indeed, AgCC, together with patterning defects and mispositioning of guidepost cells, is rescued by overexpressing in Rpgrip1l(-/-) embryos, the short repressor form of the GLI3 transcription factor (GLI3R), provided by the Gli3(Δ699) allele. Furthermore, Gli3(Δ699) also rescues AgCC in Rfx3(-/-) embryos deficient for the ciliogenic RFX3 transcription factor that regulates the expression of several ciliary genes. These data demonstrate that GLI3 processing is a major outcome of primary cilia function in dorsal telencephalon morphogenesis. Rescuing CC formation in two independent ciliary mutants by GLI3(Δ699) highlights the crucial role of primary cilia in maintaining the proper level of GLI3R required for morphogenesis of the CC.

Embryology of the Spine and Spinal Cord

The spine and spinal cord form a couple of structures whose development is highly coordinated, explaining why abnormal development of one structure is usually associated with the maldevelopment of the other. The spinal cord differentiates, as does the whole central nervous system, from the neural tube. The spine is yielded by the somites, which form the so-called paraxial mesoderm. The neural tube arises from the neural plate during neurulation, which takes place during the fourth week of gestation in humans. The neural plate results from induction of the primitive ectoderm during neural induction (a process that takes place during the third week of gestation in humans).

Development of the Cerebrospinal Fluid Pathways During Embryonic and Fetal Life in Humans

Trying to understand the mechanisms involved in human development is both a complex and fascinating problem. It is also the absolute prerequisite for an analysis of human malformations. Self-evidently, the genesis of malformation syndromes cannot be understood without a knowledge of the normal steps of development. This contradicts the classical works of human embryology, which were devoted to the analysis of malformation syndromes in the attempt to decipher the normal steps of human development — a method that may be called “reverse embryology”. Nowadays, this old method is of no value and it has been necessary to develop new paradigms to discover the normal pathways of development.

Embryology of the Brain

The construction of the brain during embryonic life is a fascinating event. Indeed, the brain is the most complex organ of the whole body, and this is particularly evident in human beings. The human brain contains a huge number of cells, and each neuron is able to connect a great number of other neurons, leading to a very complex network of circuits. The development of such a complex structure is likely to be highly regulated in order to give rise to reliable anatomical regions that can perform their normal tasks after birth. Furthermore, the capacity for growth of the human brain is fantastic during fetal life; this can be illustrated by comparing the size of the brain at the beginning and the end of gestation (Fig. 1.1).