Chapter twelve

Neurulation

‘Ce qui caractérise le développement de l’embryon des Téléostéens, c’est que j’appellerai le développement massif.’

P. Henneguy (1891)

During gastrulation the neural chord (neurocord, neurochord) is formed, induced by the underlying notochord, as mentioned in the previous chapter. The process is called neurulation and the ensueing embryonic stage the neurula.

The thickening fi rst observed in the embryonic shield is the neural plate1. In amphibians, birds and mammals this plate invaginates to form a neural groove, which subsequently closes to form the neural tube. This is often referred to as the traditional, or normal, or primary neurulation. Textbooks of vertebrate morphol-ogy, when referring to teleostean development, usually stress that neurulation in bony fi sh differs markedly from that in the other vertebrates though the end result is the same. In contrast to other vertebrates the neural plate of teleosts thickens but does not fold. It separates from the rest of the ectoderm and begins to sink below the surface and the overlying putative epidermis fuses. The solid neural rod2, often referred to as neural keel, ridge or carina, which projects ventrally towards the yolk, later becomes a hollow neural tube by cavitation. This process is referred to as ‘secondary neurulation’. In higher vertebrates secondary neurula-tion is observed in the tailbud region. In addition to the neural cord the teleostean eyecup and ear arise as solid (massif) thickenings and only subsequently acquire a lumen. All authors who mention cavitation in the teleostean CNS agree that the lumen is fi rst observed in the primordium of the eye. The separation of the solid neural rod from the epiblast (which additionally gives rise to the epidermic strata) takes place relatively very late. Before it has been completed the fi rst traces of the olfactory, auditory and optic primordia are already visible. Clusters of neural crest cells, wedged between the cord and the overlying epidermis, are observed.

The majority of the material for the nervous system (NS) of the teleostean embryo lies within the early embryonic shield. The presumptive NS undergoes considerable antero-posterior stretching and very little convergence. However, the cells lying immediately laterally to it in the shield (Fig. 11/8E) do converge

Y. W. Kunz, Developmental Biology of Teleost Fishes© Springer Science+Business Media Dordrecht 2004

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toward the midline, contributing to the formation of the mid-brain, hind-brain, neural chord, and the NS in the tailbud-blastema. The embryonic axis coincides most frequently with the second cleavage plane. The ectoderm forming the dor-sal section of the embryonic shield is thicker than the extra-embryonic ectoderm and increases in thickness as the ectoderm cells move toward the midline of the embryonic shield, creating a deep keel that is especially well developed at the anterior end of the shield — the future brain. The early brain appears in living embryos as a pronounced ventral swelling.

In contrast to the evidence of the so-called secondary neurulation in teleostei, recent investigations with dye labelling suggest that the neural cord in Brachydaniorerio is formed as a result of infolding similar to primary neurulation in the other vertebrates ‘(Papan and Campos-Ortega, 1994)’. However, close analysis of the early literature reveals that more than 100 years earlier, and intermittently dur-ing the meantime, it was shown already that the neural cord, usually referred to as keel, in certain teleosts represents a ‘closed fold’ ‘(Goette, 1873, 1878)’ (Fig. 12/1ABC-12/3ABC). But these results were overlooked for a long time and have not been included in textbooks on comparative embryology.

12.1 EARLY INVESTIGATORS

12.1.1 Neural fold

The following early investigators took it for granted that the cerebro-spinal axis in teleosts is formed as a tube as found in amphibia and amniotes: ‘Rathke (1833)’ for Blennius, ‘von Baer (1835)’ for Cyprinus blicca, ‘Vogt (1842)’ for Coregonuspalea, ‘Lereboullet (1861, 1862)’ for the pike, perch and trout3, ‘Stricker (1865)’ for the trout, Salmo fario, ‘Truman (1869)’ for the pike, ‘Hoffmann (1881)’ for teleosts generally. They noted that a thin skin (epidermic stratum) covers the open furrow. Already ‘Vogt (1842)’ had noted that the ‘cellules épidermoida-les’ (epidermic stratum) do not take part in neurulation. According to ‘Calberla (1877)’ the keel (ventral ridge) in Syngnathus and Salmo is formed as a result of the folding together of the two sides of the primitively uniform epiblastic layer. The epidermic stratum is carried down into the keel as a double layer just as if it, too, had been ‘folded in’. He suggested that the ‘epidermic stratum’ will form the ependyma while the ‘nervous layer’ will give rise to the true nervous tissue. He was unaware of the results by ‘Romiti (1873)’ (see 12.1.2). ‘His (1878)’described the appearance of a shamrock-shaped anlage of the brain with cross-shaped lumi-na leading to the neural cord with its canal. He, too, concluded that brain and neural cord are formed as a result of folding. ‘Kowalewski (1886b)’ mentioned briefl y the presence in teleosts of a connection between the alimentary canal and the ‘neural groove’, in other word a to a ‘neurenteric canal’. Equally, ‘Kingsley and Conn (1883)’ and ‘Raffaele (1888)’ observed medullary folds but were unable to observe their fusion.

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12.1.2 Solid CNS

‘Kupffer (1868, 1884)’, analyzing the embryos of Gasterosteus and Gobius, was the fi rst to specify that the teleostean nervous system is laid down as a solid con-vex keel and suggested that the central canal (canalis centralis) results from a dehiscence of the cells in the centre of the keel. He also stressed that the anlage of the brain is solid. ‘Romiti (1873)’ accepts the solid formation of the neural cord but observed that the ‘epidermic stratum’ evaginates dorsally into it forming the future ependyma. According to him, the central canal arises as a result of the sep-aration of the ependymal cells. Solid formation of the neural system was upheld also by ‘Schapringer (1871)’, ‘Weil (1872)’, ‘van Bambeke (1876)’, ‘Ryder (1884, 1885a)’, ‘Henneguy (1885)’and ‘Reinhard (1888)’. The neural keel forms in an anterior to posterior progression along the axis as does the neural fold in higher vertebrates. According to the fi rst two authors, the central canal is formed as a result of the simple separation (dehiscence) of the medially situated and radially arranged cells whereas ‘Oellacher (1873)’ in his treatise on the development of the trout stated that the ependymal canal in the solid neural cord is formed by the destruction and following resorption of the median cells. An internal lumen is first observed ventrally and then proceeds dorsally. ‘Balfour (1881)’ stressed that the separation of the solid nervous system from the epiblast takes place relatively very late; and before it has been completed, the fi rst traces of the auditory, optic and olfactory anlage are evident. ‘Hoffmann (1882a)’, on the basis of different staining methods, suggested that the central canal in the nervous system of the trout is partly formed by a dissolution of median cells. ‘Cunningham (1885a)’ in a treatise on Kupffer’s vesicle mentioned that ‘the neurocord is present as a thick cord of cells derived from the epiblast, and containing no canal’. ‘Henneguy (1888)’ described that in the solid CNS the lumina both in the optic lobes and the neural cord are formed by the separation of cells before the ventricles in the brain proper appear. The neural keel forms in an anterior to posterior progression along the axis as does the neural fold in higher vertebrates.

12.1.3 Closed neural fold

‘Goette (1873, 1878)’ was the first to stress that in the trout the open neural (medullary) furrow observed in amphibia is present as a ‘closed fold’. The pres-ence of a fold becomes especially clear when the lateral sheets of the neural cord separate after the epidermis has been cut off (Fig. 12/1A,B-12/3A,B).

‘Goette’ suggested that the formation of this ‘closed fold’ takes place as fol-lows: The broad shieldlike ectodermal medullary plate1 is medially thinned by the inpushing ventral entoderm and more posteriorly by the notochord. This results in an incomplete division of the CNS into two halves and the clear presence of a closed fold. ‘His (1878)’, ‘Hoffmann (1881-1883)’, ‘Ziegler (1888)’, Henneguy (1888)’, ‘Wilson (1891)’and ‘Kopsch (1898)’ agreed essentially with Goette’s interpretations. ‘Wilson (1891)’ gave detailed illustrations of the development of

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the neural chord in the sea bass Serranus atrarius. ‘Ziegler (1902)’ in his textbook on the comparative development of lower vertebrates confi rmed that the ecto-derm forms medially the solid medullary plate which only later acquires a lumen as previously shown in Lepidosteus. The median part of the plate sinks ventrally thereby forming a closed fold. The epidermic stratum does not participate in this fold. ‘Jablonowski (1899)’ described a closed fold in the region of the fore- and middle-brain of the pike.

All teleosts tested showed a dorsal ‘dip’ situated above the keel. This dorsal groove, by some authors called neural furrow4, subsequently straightens and closes. However, this furrow is nothing but a transient longitudinal indentation and is not homologous with the neural groove of the higher vertebrates, which later folds to form the neural tube [‘Kupffer (1868, 1878, 1884)’; ‘Goette (1869)’;

Fig. 12/1 (Goette, 1878) Transverse sections through the trunk regions of trout embryo. (A) Histological section. (B) Diagrams near the head. Notochord (ch), neural keel (axI) and side plates (axII) can be distinguished. Entoderm (db) is continuous under the notochord (ch). The EVL (d) covers the embryo.

Fig. 12/2 (Goette, 1878) Transverse sections through the trunk regions of trout embryo older than Fig. 12/1 (A). (B) Diagrams. The side plates of the neural keel have ‘shrunk’ and the neural keel presents a pseudostratified structure. A ‘dorsal dip’ (f) is visible and putative epidermal cells (g) have emerged from the axial plate.

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‘Hoffmann (1881)’; ‘M’Intosh and Prince (1890)’; ‘Eycleshymer (1895)’;]. According to ‘Goette (1878)’ this transitory ‘dip’ is due to the abovementioned cellular movements resulting in a neural chord (see ‘f’ in Fig. 12/2). M‘Intosh and Prince (1890)’ mention that the dorsal groove in teleosts ‘wholly passes away’.

‘Goette (1878)’ stressed that Carlbera’s statements (12.1.2) were not to be trusted and goes on to say ‘if they could be accepted, the difference in the forma-tion of the medullary canal in teleostei and in other vertebrates would become altogether unimportant and consist simply in the fact that the ordinary open medullary groove is in teleostei obliterated in its inner part by the two sides of the groove coming together. Both layers of epiblast would thus have a share in the formation of the CNS: the epidermic layer giving rise to the lining epithe-lial cells of the central canal, and the nervous layer to the true nervous tissues.’ He stressed, too, that if Calberla’s interpretation could be accepted, the difference between the neurulation in teleosts and other vertebrates would become alto-gether unimportant. ‘Balfour (1881)’ put forward that ‘the explanations of

Fig. 12/3 (Goette, 1878) Transverse sections through the trunk regions of trout embryo older than 12/2. (A) Historical section. (B) Diagram. The previously solid primordium of the neurochord exhibits a crevasse (G. Spalt) which eventually changes the neural chord into a neural tube with a lumen. The neurochord has become separated from the epidermis(ob). The entoderm (db) has thickened, which forecasts the formation of the intestine.Abbreviations fi gs 12/1-3, for A: axI - median keel of neural plate (axial plate); axII - lateral parts of neural plate; ch - notochord; d - EVL/periderm; db - entoderm; f - dorsal ‘fold’ (dip); g - future epidermis; ob - epidermis; r - neurochord; rr - lumen of neurochord; sp - the continuation of the neural plate (sensory plate) into the trunk region; st - somite. ‘Goette’ refers to the early neural plate as ‘axial plate’ because it still contains the cells of the future epidermis. Once deprived of these cells the neural plate is made up of a ‘neural keel’, which represents the brain anlage, and the ‘wings’, which represent the ‘sensory plates’ which give rise to the main sensory organs.Abbreviations fi gs 12/1-3, for B: CH - notochord; ENT - entoderm; EP - epidermis; EVL - enveloping layer (periderm): F - ‘dip’ in dorsal neurochord; NC - neurochord; NP - neural plate; SO - somite.

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Goette and Calberla appear to me to contain between them the truth in this mat-ter. The groove above in part represents the medullary groove; but the closure of the groove is represented by the folding together of the lateral parts of the epiblast plate to form the medullary keel.’

‘Hertwig (1906)’ stressed that there is no special difference between the pri-mary and secondary formation of the neural tube. The former displays an exter-nally protruding fold with an open and broad furrow versus the latter with an internally oriented fold with the folding sheets tightly pressed together. ‘Assheton (1907)’ refers to the neurulation in teleosts as a secondary modifi cation of the usual method of folding and mentions that a trace of such a folding is seen in the head region of the pike and in the middle trunk region of Gymnarchus.

12.2 MORE RECENT INVESTIGATIONS

‘Holmdahl (1932)’ stressed that although the way of neural tube formation is different in primary and secondary neurulation, the difference is not very pronounced. He referres to the observations by ‘Goronowitsch (1885)’ and ‘Jablonowski (1899)’ on the pike, which seemed to represent both types of neu-ral tube formation: the cranial part was said to be formed as a result of primary neurulation while the rest of the body followed the secondary type. According to ‘Holmdahl (1932)’ secondary neurulation is restricted to the caudal region of the trunk in many vertebrates that otherwise exhibit primary neurulation, from Petromyzon, through Gallus domesticus to Homo sapiens. ‘Oppenheimer (1936b)’ showed with vital staining of Fundulus embryos that most of the material for the nervous system lies within the early embryonic shield, while a smaller part is found in the extraembryonic region anterior to the shield (see Fig. 13/3). The cells in the shield form the solid keel and the hind- and midbrain while regions of the anterior shield and the neighbouring extraembryonic region will form the solid eye and forebrain primordia. ‘Price (1934a,b)’ described the primordia of the CNS in the whitefi sh Coregonus clupeaformis as a solid neural keel and three solid primary brain lobes and the optic outgrowths. The indifferently placed cells in the neural keel become later arranged into two parallel rows. Their subsequent sepa-ration will form the cavity of the neural canal. The rearrangement of the brain cells starts in the prosencephalon and continues posteriorly.

In a biological study on vertebrate development ‘Peter (1947)’ reported that ectodermal organs of teleosts develop from a ‘compact mass’. In the neural keel only later a lumen is produced by separation of cells. Alternatively, a ‘fold without a lumen’ is found. Its walls adhere to each other and later separate. In the textbook of ‘Comparative Embryology of Vertebrates’ by ‘Nelsen (1953)’ the teleostean neural keel has been described as an axial cord, the cells of which are organized according to an irregular pattern.‘Gihr (1957)’ in her thesis on the development of the pike, Esox lucius, mentions that at the 4-somite stage the ‘neural material’ is shifted from a transverse into a dorso-ventrally directed arrangement.

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The embryology of the English sole, Parophrys vetulus, was investigated by ‘Orsi (1968)’. The fusiform cells of the solid early neural keel run ventro-laterally from an apex in the dorsal midline, while in the brain region the deeper cells ‘run more or less straight across it’. This author, too, noticed a temporary dorsal furrow prior to the end of epiboly. He stressed two basic changes in the neural system: 1) the main areas of the brain are defi ned by constrictions into fore-, mid- and hindbrain, 2) the cells simultaneously rearrange themselves to permit the open-ing of the brain ventricles and the neural canal. The formation of the latter begins in the medulla and then moves down the neural cord. This is followed by the for-mation of the third ventricle and the dorsal part of the fourth ventricle. Apart from changing position, the neural cells become smaller, more numerous and densely packed. However, all other authors who mention cavitation agree that the lumen is fi rst observed in the primordium of the eye. It appears as a slit and later becomes a ventricle (see Chapter 17).

Organogenesis of the viviparous embiotocid Hyperprosopon argenteum wasdescribed by ‘Engen (1968)’. He points out that the CNS development is of the typical teleostean type. ‘The neural plate tissue proliferates inward to form a solid cord of cells along the midline called the neural keel’. The lumen develops second-arily and is first noted in association with the formation of the optic vesicle.

‘T. Yamamoto (1975)’ in his extensive treatise of the medaka Oryzias latipesassumed a solid primordium for the whole CNS with the neurocoele appearing at a later stage. This was followed by ‘Miyayama and Fujimoto (1977)’ who reported light microscopical studies of the neural tube formation in the medaka. These authors were the fi rst to perform additional analyses by EM. They observed, too, a transient neural groove (dip), which was very shallow. The outermost cells of the solid neural chord were continuous with the deep cells of the epidermis. The central cells of the neural chord were randomly arranged and desmosomes were observed between the apical part of neighbouring cells. During development the cells moved apart into groups at each side of the midline. In both groups, starting from the ventral and proceeding to the dorsal side, cells arranged them-selves into a pseudostratifi ed epithelium. As development proceeded, a distinct zig-zag midline was observed separating the left from the right pseudostratifi ed epithelium. [No separating membrane was observed, as was suggested on the basis of light microscopical investigations by ‘Goette (1873, 1878)’ and ‘Carlbera (1877)’.] The primitive neurocoele started to open in the ventral part of the chord, then widened and extended to the dorsal part. Transitory cytoplasmic processes, originating near the desmosomal junctions, as well as short cilia, began to pro-trude into the lumen. At the neck of these cells microfi laments and microtubules were observed; the latter were also present in the cell body. When the neural chord changed from its wedge-shape into an elliptical form, the appearance of cluster of neural crest cells, wedged between the chord and the overlying epider-mis, became evident. These cells later separated and moved ventrad. ‘Raible et al.(1992)’ described the segregation and early dispersal of trunk neural crest cells in Brachydanio (Danio) rerio, which is similar to the development of these cells in

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other species. ‘Reichenbach et al. (1990)’, using SEM, observed a neural groove and a narrow neurocoele in the neural keel of Cichlasoma nigrofasciatum.

The zebrafi sh, Brachydanio (Danio) rerio, has been the subject of the most recent investigations. A paper by ‘Hisaoka and Battle (1958)’contained the fi rst complete description of the normal stages of the zebrafi sh. They mention that a solid nerve chord becomes visible when the blastopore is closed and two somites have developed and the neurocoele makes its appearance at the fi ve somite stage. ’Schmitz et al. (1993)’ studied the process of neurulation by iontophoretically labelling cells either with a dye (DiI) or fl uorescine dextran amine. They observed that the neural plate developed between 90% epiboly and the 10-somite stage. They mention that a solid nerve chord becomes visible when the blastopore is closed and two somites have developed. At this stage the neural tube anlage con-sisted of a medial thickening above the notochord with a smaller one on each side. This was followed by a fusion of the two bilateral ectodermal thickenings with the median thickening which gave rise to the neural keel. Labelling of cells and observations at the late gastrula stage revealed that the medial region contains exclusively neural progenitor cells (from the 90-100% epiboly stage onwards), while the lateral regions contain a mixture of neural precursor, prospective epi-dermal and neural crest cells. On the basis of these results the authors concluded that the lateral thickenings are homologous to the neural folds in higher verte-brates. The prospective neural and epidermis cells separated within 1-2 h, during which time some cells of the lateral thickenings would have had to move a dis-tance of up to 400 microns to reach their destination. EM analyses revealed the presence of intercellular junctions along the dorsal-ventral extent of the neural cord. From the 14-somite stage onwards clefts were observed along the midline, first ventrally then dorsally, until they eventually fused to form the neural cav-ity. In the meantime epidermal cells had overgrown the neural tube. Within the neural tube of higher vertebrates most cell divisions take place at the ventricular side of the epithelium and, as mentioned above, in teleosts, due to the absence of a neurocoele in early stages of their development, cell divisions are observed along the midline of the neural keel and the nervechord. ‘Papan and Campos-Ortega (1994)’ further studied the morphogenetic movements that transform the neural plate into the neural tube by injecting a dye marker into single neural plate cells. They established that the latero-medial position of injected cells in the neural plate was correlated with the dorso-ventral position of their progeny in the neural tube. Based on these results, which confi rm those of ‘Schmitz et al. (1993)’, the authors suggested that the neural plate ‘folds in’ at the dorsal midline to form the neural keel and, therefore, that neurulation in zebrafi sh does not differ essentially from that of higher vertebrates. Moreover, they observed mitoses of labelled cells along the medial divide of the neural chord preceding neurocoele formation.

‘Kimmel et al. (1994)’ reported cell lineage analyses by video time-lapse record-ing in vivo in the CNS of zebrafi sh embryos. When cells of a single clone become arranged as a bilateral pair of discontinuous lines, these are referred to as ‘strings’. These strings are oriented along the neural axis as described already by ‘Kimmel

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and Warga (1986)’ and ‘Kimmel (1993)’. Cell intercalations during interphases, alternating with mitotic divisions, generate these strings. In the more recent pub-lication the relationship between cell cycles and cellular morphogenetic behav-iour of single strings was investigated. Cells in single clones were shown to divide together and to maintain this synchrony up to the end of gastrulation (cycle 16). As a result of convergence, cells drift away from the EVL and are moved, as antero-posteriad oriented strings, towards the dorsal midline. After cycle 16 an extension of the strings, oriented preferentially in the mediolateral direction, follows due to cell intercalations. On reaching the dorsal midline, the cells often divide and dur-ing the following interphase the sister cells move, one to the left, the other to the right, producing a bilaterally symmetrical pattern. By division 18 the solid neural keel hollows into a neural tube and the neurocoele probably blocks cell passage across the midline. The authors suggested that this divergent lateral movement of sister cells takes place underneath, not into, the stream of cells still converging medially, and that such inward movement is important for the formation of the neural tube. It has been proposed that the progressive thickening of the neural anlage, as well as its dorso-ventral organization, is the result of the following cell movements: cells near the dorsal midline will go to form the bottom of the stack, i.e. the ventral part of the neural tube, and later arriving cells will divide and ‘dive’ later, and consequently will form the more dorsal parts of the neural tube.

In vertebrates with primary neurulation, i.e. folding of the neuroectoderm, the dorsoventral position of a cell in the neural tube can be predicted from its mediolateral position on the neural plate. Medial cells of the plate will form ven-tral structures and lateral cells will form dorsal structures of the neural tube. The same holds true for the zebrafi sh neural keel despite its peculiar mechanism of formation. Dye-labeling of single cells in the zebrafi sh neural plate showed that the mediolateral positon of a cell in it is projected faithfully into the dorso-ventral axis of the neural keel. However, in contrast to other vertebrates, the left-right separation of cells in the neural plate is not preserved in the zebrafi sh neural keel. Cells can cross the midline to colonize the contralateral side of the neural keel.

It has been proposed again and again that the mechanism of teleost neurula-tion is similar to the secondary neurulation seen in the tailbud of higher verte-brates. In many respects, however, zebrafi sh neural tube formation resembles that of other vertebrates and may thus be just a variant of primary neurulation. This idea is supported by the observation that evolutionarily more primitive fi sh, like the sturgeon, generate the neural tube by folding of the neural plate. It is tempting to speculate that the teleost type of neurulation is an adaptation to the altered egg architecture characteristic of teleosts.

As mentioned above, due to this infolding, cells from both halves of the neural plate are juxtaposed at the midline. As in the case of other epithelia, cells of the neural keel and nerve rod divide at their apical side; thus, sister cells can be dis-tributed on either side of the nerve rod and this leads to a bilateral distribution of cell clones. This is a peculiarity that clearly distinguishes zebrafi sh neurulation from that in other vertebrates.

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The neural keel like the neural plate is organized as a two-layered pseudostrati-fied columnar epithelium which would suggest that the neural keel forms by infolding of the neural plate, probably caused by the continued convergence of ectodermal cells towards the prospective dorso-medial regions of the embryo. These results endorse the conclusion of ‘Reichenbach et al. (1990)’ regarding the ‘epithelial genesis’ of the neural keel of teleosts.

Most cell divisions within the neural tube of vertebrates take place at the ven-tricular side of the epithelium, i.e. at the side of the neurocoele. Although there is no neurocoele in early stages of neurulation of teleosts, cell divisons can be observed in the middle of the neural keel and neural rod, where the neurocoele will form ‘(Reichenbacha et al., 1990; Schmitz et al., 1993)’.

‘Strähle and Blader (1994)’ and ‘Takeda and Miyagawa (1994)’ have produced a review of the neurogenesis and cell fate in the zebrafi sh embryo.

12.3 OTHER MIDLINE STRUCTURES

The hypochord was thought to be an endodermal derivative induced by the notochord though doubts were recently expressed (see Chapter 13). Its role dur-ing embryogenesis is still unknown. The ectodermal fl oor plate, another median structure, is formed due to a secondary embryonic induction by the notochord (Fig. 12/4). It is present along the ventral neural tube from the forebrain/mid-brain junction to the tail. As shown in the zebrafi sh, the fl oor plate forms ante-riorly a two cell-wide strip, which narrows to a single row in the spinal cord. Along most of the length of the spinal cord the fl oor plate is a single linear cell row but changes at about the fourth spinal segment to a staggered row, which continues into the hindbrain. The fl oor plate can readily be distinguished from the neural tube since its small cells have a very distinctive cuboidal epithelial shape. Moreover, antibodies are available that label selectively the fl oor plate ‘(Hatta and Kimmel (1993)’. ‘Schmitz et al. (1993)’ observed ciliary structures and basal bod-ies in the ectodermal fl oor plate. ‘Kimmel (1993)’ reported that ‘ectoderm, like the mesodermal prechordal plate, also seems to make a singular midline derivative anterior to the fl oor plate: Along the fl oor of the diencephalon is a midline tissue that, argued from genetic and transplantation studies, is somewhat equivalent to the fl oor plate, both developmentally and in its signaling function’. The fl oor plate serves as a guidepost for the growth of commissural axons ‘(Hatta et al.,1991)’. The medial longitudinal fascicles (MLF), which fl ank the fl oor plate, con-sist of axons the nuclei of which are located in the midbrain [‘Hatta et al., (1991)’; ‘Kimmel (1993)’; ‘Strähle and Blader (1994)’]. According to ‘Hatta et al. (1994)’the fl oor plate may participate in a signaling cascade that establishes dorsoventral patterning of the neural primordium. As mentioned above, the formation of the floor plate is the result of a so-called secondary embryonic induction because its genesis depends on inductive signals from the underlying notochord. It has been suggested that in the early neurogenesis of the zebrafi sh Reissner’s substance,

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expressed in a temporal-spatial pattern, may play a role in axonal decussa-tion ‘(Lichtenfeld et al., 1999)’. Reissner’s fi bres were fi rst described by ‘Ollsson (1964)’. ‘Portmann, 1976)’ mentions the presence of so-called Rohon-Beard cells which are expressed in the embryos of selachians, ‘yolksac stages’ of teleosts and in the larvae of amphibia. These cells are formed at the same time as those of the neural crest and occur along the dorsal neural tube. They send long processes to the periphery and in teleosts reach as far as the ventral part of the yolksac. Their function is unknown.

Fig. 12/4 Schematic representation of midline structures: spinal cord (neural tube), floor plate, notochord, hypochord (subnotochordal rod), dorsal aorta.

Summary

Although the mechanisms of neurulation in teleosts differ from those observed in amphibia and higher vertebrates, the endresult is the same in all embryos examined. Primary neurulation, as observed in amphibia, birds and mammals, proceeds by invagination of the neural plate to form a neural groove, which sub-sequently closes to form a neural tube. In secondary neurulation, as displayed by the teleosts, cells initially form a dense nerve rod, in which a lumen originates secondarily by cavitation. However, the observation of a ‘closed neural fold’ by some early investigators in the trout and other fi sh, and recently observed in the medaka and zebrafi sh, reveal that teleostean neurulation resembles primary neurulation of other vertebrates. It differs only from primary neurulation in that neurocoele formation occurs secondarily (12.1, 12.2).

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At the ventral aspect of the neural tube, and in close contact with the noto-chord, the ectodermal fl oor plate extends anteriorly through the midbrain. The floor plate has been shown to be implicated in inductive interactions that pattern the neural tube and the somites (12.3).

Endnotes

1 G. Axenplatte of Goette, Medullarplatte of ‘Ziegler’.2 G. Axenstrang of ‘His’

F. cordon axial of ‘Henneguy’3 Salar Ausonii. Val., Salmo fario L.Bt.4 G. Rückenfurche of von Baer, Primitivrinne, Primitivstreif, Achsenstreif

F. Sillon dorsal of ‘Vogt’ and ‘Lereboullet’; gouttière primitive, sillon primitif, sillon médullaire de ‘Henneguy’, ligne primitive