The primary brain vesicles revisited: Are the three primary vesicles (forebrain/midbrain/hindbrain) universal in vertebrates?

It is widely held that three primary brain vesicles (forebrain, midbrain, and hindbrain vesicles) develop into five secondary brain vesicles in all vertebrates (von Baer\u27s scheme). We reviewed previous studies in various vertebrates to see if this currently accepted scheme of brain morphogenesis is a rule applicable to vertebrates in general. Classical morphological studies on lamprey, shark, zebrafish,frog, chick, Chinese hamster, and human embryos provide only partial evidence to support the existence of von Baer\u27s primary vesicles at early stages. Rather, they suggest that early brain morphogenesis is diverse among vertebrates. Gene expression and fate map studies on medaka, chick, and mouse embryos show that the fates of initial brain vesicles do not accord with von Baer\u27s scheme, at least in medaka and chick brains. The currently accepted von Baer\u27s scheme of brain morphogenesis, therefore, is not a universal rule throughout vertebrates. We propose here a developmental hourglass model as an alternative general rule: Brain morphogenesis is highly conserved at the five-brain vesicle stage but diverges more extensively at earlier and later stages. This hypothesis does not preclude the existence of deep similarities in molecular prepatterns at early stage

Studies on the morphology of the inner ear and semicircular canal endorgan projections of ha, a medaka behavior mutant.

The morphology of the inner ear was investigated in the mutant strain ha of medaka (Oryzias latipes). The ha is a recessive mutant and ha homozygotes are viable but show abnormal circular swimming behavior. In adult ha/ha medaka, more than one semicircular canals are absent. In the most abnormal cases, no canals are present at all and the membranous labyrinth of the inner ear exhibits a simple rugby-ball-like structure. In spite of the apparent absence of the canals, however, receptor endorgans of the canals (crista ampullaris) and the nerves innervating the cristae (ampullar nerves) are present. Otoliths and associated receptor epithelia (maculae) as well as octaval nerve branches innervating maculae are also present, except utricular otoliths that are absent or extremely small if present. Projections of the ampullar nerves were also investigated, because central connections of the nerves may be also abnormal. Tract-tracing studies, however, revealed similar central projection patterns of primary afferents in the mutant and wild-type brains. These results suggest that membranes of prospective semicircular canals fail to form tubular structures and fuse with the membrances of otolith organs in ha/ha medaka. These results also suggest that abnormal morphology of the semicircular canals as well as the utricular otolith underlies the abnormal swimming behavior of the ha/ha medaka, in spite of apparently normal central projections of the ampullar nerves

Morphogenesis of the cerebellum in teleost fish

The J.B. Johnston Clu

Developmental Origin of Diencephalic Sensory Relay Nuclei in Teleosts

We propose here a novel interpretation of the embryonic origin of cells of deiencephalic sensory relay nuclei in teleosts based on our recent studies of gene expression patterns in the medaka (Oryzias latipes) embryonic brain and comparative hodological studies. It has been proposed that the diencephalic sensory relay sysytem in teleosts is unique among vertebrates. Teleost relay nuclei, the preglomerular complex (PG), have been assumed to originate from the basal plate (the posterior tuberculum) of the diencephalon, whereas relay nuclei in mammals are derived from the alar plate (dorsal thalamus) of the diencephalon.Our results using in situ hybridizaion show, however, that many pax6-or dlx2-positive cells migrate laterally and ventrocaudally from the diencephalic alar plate to the basal plate during development. Massive clusters of the migrated alar cells become localized in the mantle layer lateral to the posterior tubercular neuroepithelium, from which main nuclei of the PG appear to differentiate. We therefore consider most if not al neurons in the PG to be of alar, not basal, origin. Thus, the releost PG, at least in pact, can be regarded as migrated alar nuclei. Developmental and hodological data strongly suggest that the teleost PG is homologous to part of the mammalian dorsal thalamus. The organization and origi of the diencephalic sensory relay system might have been conserved across vertebrates

Axonogenesis in the medaka embryonic brain

In order to know the general pattern of axonogenesis in vertebrates, we examined axonogenesis in the embyonic brain of a teleost fish, medaka (Oryzias latipes), and the results were compared with previous studies in zebrafish and mouse.The axons and somata were stained immunocytochemically using antibodies to a cell surface marker(HNK-1) and acetylated tubulin and visualized by retrograde and anterograde labeling with a lipophilic dye. The fiber systems developed correlating with the organization of the longitudinal and transverse subdivisions of the embryonic brain. The first axons extended from the synencephalic tegmentum, forming the first fiber tract (fasciculuus longitudinalis medialis) in the ventral longitudinal zone of the neural rod, 38 hours after fertilization. In the neural tube, throughout the entire brain two pairs of longitudinal fiber systems, one ventral series and one dorsal or intermediate series, and four pairs of transverse fiber tracts in the rostral brain were formed sequentially during the first 16 hours of axon production. In one of the dorsal longitudinal tracts, its branch retracted and disappeared at later stages. One of the transverse tracts was found to course in the telencephalon and hypothalamus. The overall pattern of the longitudinal fiber systems in medaka brain is similar to that in mouse, but apparently different from that in zebrafish. We propose that a ventral tract reported in zebrafish partially belongs to the dorsal fiber system, and that the longitudinal fiber systems in all vertebrate brains pass through a common layout defined by conserved genetic and developmental programs