Cytoarchitectonic study of the brain of a perciform species, the sea bass (Dicentrarchus labrax). I. The telencephalon

A cytoarchitectonic analysis of the telencephalon of the sea bass Dicentrarchus labrax, based on cresyl violet-stained serial transverse sections, is presented. Rostrally, the brain of the sea bass is occupied by sessile olfactory bulbs coupled to telencephalic hemispheres. The olfactory bulbs comprise an olfactory nerve fiber layer, a glomerular layer, an external cellular layer, a secondary olfactory fiber layer, and an internal cellular layer. Large terminal nerve ganglion cells are evident in the caudomedial olfactory bulbs. We recognized 22 distinct telencephalic nuclei which were classified in two main areas, the ventral telencephalon and the dorsal telencephalon. The ventral telencephalon displays four periventricular cell masses: the dorsal, ventral, supracommissural, and postcommissural nuclei; and four migrated populations: the lateral, central, intermediate, and entopeduncular nuclei. In addition, a periventricular cell population resembling the lateral septal organ reported in birds is observed in the ventral telencephalon of the sea bass. The dorsal telencephalon contains 13 nuclei, which can be organized into five major zones: the medial part, dorsal part, lateral part and its ventral, dorsal, and posterior divisions, the central part, and posterior part. Based on histological criteria, two cell masses are recognized in the ventral division of the lateral part of the dorsal telencephalon. The nucleus taenia is found in the caudal area of the dorsal telencephalon, close to the ventral area. This study represents a useful tool for the precise localization of the neuroendocrine territories and for the tracing of the neuronal systems participating in the regulation of reproduction and metabolism in this species.     

The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytochemically for calcium-binding proteins, gamma aminobutyric acid (GABA) or other antigens. Cyto- and myeloarchitecture revealed thalami that are fundamentally mammalian in organization, with the three principal divisions of the thalamus (epithalamus, dorsal thalamus and ventral thalamus) identifiable as in marsupials and eutherian mammals. The dorsal thalamus exhibits more nuclear parcellation than hitherto described, but lack of an internal medullary lamina, caused by splaying out of afferent fibre tracts that contribute to it in other mammals, makes identification of anterior, medial and intralaminar nuclear groups difficult. Differentiation of the ventral nuclei is evident with the ventral posterior nucleus of the platypus enormously expanded into the interior of the cerebral hemisphere, where it adopts a relationship to the striatum not seen in other mammals. Other nuclei such as the lateral dorsal become identifiable by expression of patterns of calcium-binding proteins identical to those found in other mammals. GABA cells are present in the ventral and dorsal thalamic nuclei, and in the ventral thalamus form a remarkable continuum with GABA cells of the two segments of the globus pallidus and pars reticulata of the substantia nigra.     

THE DEVELOPMENTAL BASIS OF ANISOPHYLLY IN SELAGINELLA MARTENSII. I. INITIATION AND MORPHOLOGY OF GROWTH

The dorsiventral shoot system of Selaginella martensii is characterized by opposite pairs of ventral and dorsal leaves that are dimorphic in size and form. This study was undertaken to determine if the smaller dorsal leaf can be appropriately regarded as an arrested form of the larger leaf. Although the pattern of cell divisions and cell enlargement associated with leaf initiation is similar for both leaf types, the extent of localized growth results in distinctly larger primordia on the ventral side of the shoot. Ventral leaf primordia are also distinguished by the early formation of more extensive mesophyll tissue. Regression analysis of quantitative data on leaf length vs. position and leaf width vs. length indicates that the growth pattern of ventral and dorsal leaves is significantly different. These observations indicate that the developmental pathways of the dimorphic leaves of Selaginella martensii do not diverge at a relatively late developmental stage, but rather can be distinguished from inception.     

Origin and organization of the zebrafish fate map

We have analyzed lineages of cells labeled by intracellular injection of tracer dye during early zebrafish development to learn when cells become allocated to particular fates during development, and how the fate map is organized. The earliest lineage restriction was described previously, and segregates the yolk cell from the blastoderm in the midblastula. After one or two more cell divisions, the lineages of epithelial enveloping layer (EVL) cells become restricted to generate exclusively periderm. Following an additional division in the late blastula, deep layer (DEL) cells generate clones that are restricted to single deep embryonic tissues. The appearance of both the EVL and DEL restrictions could be causally linked to blastoderm morphogenesis during epiboly. A fate map emerges as the DEL cell lineages become restricted in the late blastula. It is similar in organization to that of an amphibian embryo. DEL cells located near the animal pole of the early gastrula give rise to ectodermal fates (including the definitive epidermis). Cells located near the blastoderm margin give rise to mesodermal and endodermal fates. Dorsal cells in the gastrula form dorsal and anterior structures in the embryo, and ventral cells in the gastrula form dorsal, ventral and posterior structures. The exact locations of progenitors of single cell types and of local regions of the embryo cannot be mapped at the stages we examined, because of variable cell rearrangements during gastrulation.     

Light and electron microscopic studies on the pineal tract of rainbow trout, Salmo gairdneri.

The pineal tract of rainbow trout from the pineal end vesicle to the posterior commissure was studied by light and electron microscopy. Five types of nerve fibres (photoreceptor basal process, ganglion cell dendrite, electron-lucent fibre and synaptic vesicles, myelinated and unmyelinated axons) and two modes of synapses (photoreceptor basal process ganglion cell dendrite and axon terminal with synaptic vesicles-photoreceptor basal process synapses) are distinguishable in the proximal region of end vesicle. The two distinct synaptic associations with the photoreceptor basal process suggest two different (excitatory and inhibitory) control of pineal sensory activity. At the distal portion of stalk about two thousand nerve fibres converge into dorsal and ventral bundles. Posterior to the habenular commissure several small branches run out laterally from the ventral bundles to the basal margin of the ependyma, but not into the habenular commissure. The dorsal bundle passes through the dorsal side of the subcommissural organ and runs ventral to the posterior commissure. The pineal tract is composed of unmyelinated axons, electron-lucent nerve fibres and myelinated axons. The number of fibres increases throughout the stalk and reaches the maximum number at the opening of pineal lumen to IIIrd ventricle, however, the number of fibres then decreases through the subcommissural organ and posterior commissure. This increase and decrease of nerve fibres suggest the continuous participation of axonal fibres of pineal nerve cells and the ramification or branching of pineal tract, respectively.     

Germ line and oogenesis during paedogenetic reproduction inHeteropeza pygmaea Winnertz (Diptera: Cecidomyiidae)

The germ line during paedogenetic reproduction in the gall midgeHeteropeza pygmaea was followed cell generation by cell generation. There are altogether 8 or 9 successive divisions during one paedogenetic cycle, and all descendants of the primordial germ cell develop into occytes, while the trophocytes are of somatic origin. In the cytological race studied (from southern Finland) the germ-line cells possess 58 chromosomes, the somatic number being 10.     

Primordial germ cells and early stages of oogenesis in Ophryotrocha puerilis (Polychaeta,Dorvillediae)

Summary Each setigerous segment of the protandric polychaete Ophryotrocha puerilis contains two primordial germ cells. A ventral furrow in the gut wall together with the peritoneal lining of the gut forms a genital blood vessel. The gonocytes are located within the peritoneum of this genital blood vessel. At sexual maturity the gonocytes undergo a proliferation cycle, the first division of which gives rise to a cell which is extruded into a forming outpocketing of the coelomic lining. The stem cell remains within the peritoneum. Inside the forming gonad the detached cell goes through a series of four mitotic divisions. The resulting 16 cells are interconnected by cytoplasmic bridges. These bridges are arranged in a very regular pattern which allows the mitotic cycles to be followed. While remaining still within the gonad the 16 cells begin to synthesize yolk and to take up exogenous yolk precursors. At this stage a differentiation into oocytes and nurse cells becomes visible. The oocytes deposit yolk platelets of the definitive size whereas the polyploid nurse cells produce only small yolk bodies that are passed to the adjacent oocytes. In a later stage the cell bridges between adjacent nurse cells are cut and pairs of one oocyte and one nurse cell are released to the coelomic cavity during breakdown of the gonadal sac. Oocyte-nurse cell-complexes then freely float in the coelomic fluid. The proliferation of gonadal cells is well synchronized within one segment. In anterior segments, however, gonadal proliferation usually begins earlier than in posterior segments but smaller oocytes in posterior segments catch up within a few days. Finally a batch of oocytes is produced in which all the oocytes are of the same size (120 m). The origin of the primordial germ cells remains unknown.     

Cell lineage studies in the crayfish Cherax destructor (Crustacea,Decapoda) : germ band formation,segmentation, and early neurogenesis

Summary The cell division pattern of the germ band of Cherax destructor is described from gastrulation to segmentation, limb bud formation, and early neurogenesis. The naupliar segments are formed almost simultaneously from scattered ectoderm cells arranged in a V-shaped germ disc, anterior to the blastopore. No specific cell division pattern is recognisable. The post-naupliar segments are formed successively from front to rear. Most post-naupliar material is budded by a ring of about 39 to 46 ectoteloblasts, which are differentiated successively and in situ in front of the telson ectoderm. The ectoteloblasts give rise to 15 descendant cell rows by unequal divisions in an anterior direction, following a mediolateral mitotic wave. Scattered blastoderm cells of non-ectoteloblastic origin in front of the ectoteloblast descendants and behind the mandibular region are also arranged in rows. Despite their different origins, teloblastic and non-teloblastic rows cleave twice by mediolateral mitotic waves to form 4 regular descendant rows each. Thereafter, the resulting grid-like pattern is dissolved by stereotyped differential cleavages. Neuroblasts are formed during these differential cleavages and segmentation becomes visible. Each ectoderm row represents a parasegmental unit. Therefore, the segmental boundary lies within the area covered by the descendants of 1 row. Segmental structures (limbs, ganglia) are composed of derivatives of 2 ectoderm rows. The results are compared with the early development of other crustaceans and insects in relation to mechanisms of germ band formation, segmentation, neurogenesis, and evolution.     

The Golgi body and thiamine pyrophosphatase localization in embryonic cells in culture

Chick embryo endoblast and segmented mesoderm cells were cultured and processed using the cytochemical technique for the detection of the enzyme thiamine pyrophosphatase. Reaction product was found in cisternae of the Golgi body and associated vesicles. Random, patchy distribution of deposit was observed on the dorsal surface of the monolayer cells and between overlapping processes, but not on the ventral surface which faces the substratum. Both cell types reacted similarly. The results suggest that new membrane of Golgi origin is formed at the dorsal but not the ventral surface of cells. There was no defined locus of insertion, at the tip of the advancing lamellipodia or elsewhere.     

Electrophysiological analysis of the organization of somatosensory thalamic inputs in the frog

Potentials produced in the frog thalamus by electrical stimulation of the peripheral nerves were investigated by sink and current source-density analysis. Sinks, which are viewed as potential generation sites, were located in three regions: the cell-free zone of the ventral thalamus adjoining the ventrolateral nucleus, the ventromedial and ventrolateral nuclei, and the caudal section of the dorsal thalamus. Evoked activity was recorded in individual neurons in the area of the second and third of these sinks. The first sink failed to form after section of the dorsal tracks of the spinal cord, while the remaining two only appeared after a considerably extended latency. It is suggested that nuclei of the ventral and caudal sections of the dorsal thalamus receive somatic impulses through the systems connected with the dorsal as well as the ventrolateral columns of the spinal cord. The direct projections of the primordial nuclei of dorsal columns may be involved in afferentation the ventral thalamus.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 687–695, September–October, 1986.     

Specification of secondary mesenchyme-derived cells in relation to the dorso-ventral axis in sea urchin blastulae

To learn how the dorso-ventral (DV) axis of sea urchin embryos affects the specification processes of secondary mesenchyme cells (SMC), a fluorescent dye was injected into one of the macromeres of 16-cell stage embryos, and the number of each type of labeled SMC was examined at the prism stage. A large number of labeled pigment cells was observed in embryos in which the progeny of the labeled macromere were distributed in the dorsal part of the embryo. In contrast, labeled pigment cells were scarcely noticed when the descendants of the labeled macromere occupied the ventral part. In such embryos, free mesenchyme cells (probably blastocoelar cells) were predominantly labeled. CH3COONa treatment, which is known to increase the number of pigment cells, canceled such patterned specification of pigment cells and blastocoelar cells along the DV axis. Pigment cells were also derived from the ventral blastomere in the treated embryo. In contrast, a similar number of coelomic pouch cells was derived from the labeled macromere, irrespective of the position of its descendants along the DV axis. After examination of the arrangement of blastomeres in late cleavage stage embryos, it was determined that 17-20 veg2-derived cells encircled the cluster of micromere descendants after the 9th cleavage. From this number and the numbers of SMC-derived cells in later stage embryos, it was suggested that the most vegetally positioned veg2 descendants at approximately the 9th cleavage were preferentially specified to pigment and blastocoelar cell lineages. The obtained results also suggested the existence of undescribed types of SMC scattered in the blastocoele.     

The structures of dorsal and ventral regions of a dragonfly retina

Summary The apposition eyes of the corduliid dragonfly Hemicordulia tau are each divided by pigment colour, facet size and facet arrangement into three regions: dorsal, ventral, and a posterior larval strip. Each ommatidium has two primary pigment cells, twenty-five secondary pigment cells, and eight receptor cells, all surrounded by tracheae which probably prevent light passing between ommatidia, and reduce the weight of the eye. Electron microscopy reveals that the receptor cells are of two types: small vestigial cells making virtually no contribution to the rhabdom, and full-size typical cells. The ventral ommatidia have a distal typical cell (oriented either horizontally or vertically), four medial typical cells, two proximal typical cells and one full-length vestigial cell. The dorsal ommatidia have only four full-length typical cells, and one distal and three vestigial full-length cells. The cross-section of dorsal rhabdoms is small and circular distally, but expands to a large three-pointed star medially and proximally. The tiered receptor arrangement in the ventral ommatidia is typical of other Odonata but the dorsal structure has not been fully described in other species. Specialised dorsal eye regions are typical of insects that detect others against the sky.     

On the Significance of Larval and Juvenile Morphology for Suggesting Phylogenetic Relationships of the Vestimentifera

SYNOPSIS. Examination of larvae and juveniles ofRidgeia hasrevealed the presence of a prototroch, metatroch and neurotroch.A complete digestive tract is present in juveniles, the esophagealportion of which passes through the brain. The major longitudinal,pulsatile blood vessel is on the side opposite the nerve cordand, along with the longitudinal blood vessel adjacent to thenerve cord, is situated in the medial mesentery separating lateralcoelomic cavities. The posterior body region is multisegmented,and the lateral cavities of each segment are separated by amedial mesentery. Newly forming segments exhibit paired spacesthat expand to form coelomic cavities of that segment. All ofthese characters are held in common with the Annelida and stronglysuggest that the nerve cord is ventral and the major pulsatileblood vessel is dorsal; the brain is, in effect, a circumesophagealganglion; and coelom formation is by schizocoely and mesodermalorigin is teloblastic. It is suggested that the Vestimentiferaare allied with the gastroneuralians and may well have arisennear the base of the phylogenetic line leading to the Annelida.Avenues of future research, likely to shed more light on theposition of the Vestimentifera in invertebrate phylogeny, aresuggested     

Immunocytochemical investigation of forebrain control by somatostatin of the pituitary in the teleost Poecilia latipinna

Summary An extensive system of somatostatin-immunoreactive neurons has been localized in the forebrain and pituitary of the molly (Poecilia latipinna), using the unlabelled antibody immunocytochemical method.In the hypothalamus, reactive perikarya were scattered throughout the parvocellular divisions of the preoptic nucleus. These cells were smaller in size and more ventral in position than those which stained with antisera to the neurohypophysial hormones, vasotocin and isotocin. A few very small somatostatin-immunoreactive cells were observed in the tuberal region and in the nuclei of the lateral and posterior recesses — areas which were rich in somatostatin-immunoreactive fibres.Somatostatin cells were also found in a small area of the ventral thalamus, mainly in the dorsolateral nucleus. Some of these neurons were large and multipolar, and appeared to form tracts of fibres into the posterior hypothalamus. In the telencephalon there were a few stained cells in the ventral area, with a complex pattern of fibres occurring in parts of the dorsal area.Somatostatin-immunoreactivity was intense in the central and posterior neurohypophysis, and particularly in its finger-like projections into the proximal pars distalis, around groups of growth hormone cells. Examination of material from fishes under various experimental conditions provided evidence for the somatostatin fibres originating from the preoptic neurons being involved in the control of growth hormone secretion.     

The organization of honeybee ocelli: Regional specializations and rhabdom arrangements

We have re-investigated the organization of ocelli in honeybee workers and drones. Ocellar lenses are divided into a dorsal and a ventral part by a cusp-shaped indentation. The retina is also divided, with a ventral retina looking skywards and a dorsal retina looking at the horizon. The focal plane of lenses lies behind the retina in lateral ocelli, but within the dorsal retina in the median ocellus of both workers and drones. Ventral retinula cells are ca. 25 μm long with dense screening pigments. Dorsal retinula cells are ca. 60 μm long with sparse pigmentation mainly restricted to their proximal parts. Pairs of retinula cells form flat, non-twisting rhabdom sheets with elongated, straight, rectangular cross-sections, on average 8.7 μm long and 1 μm wide. Honeybee ocellar rhabdoms have shorter and straighter cross-sections than those recently described in the night-active bee Megalopta genalis. Across the retina, rhabdoms form a fan-shaped pattern of orientations. In each ocellus, ventral and dorsal retinula cell axons project into two separate neuropils, converging on few large neurons in the dorsal, and on many small neurons in the ventral neuropil. The divided nature of the ocelli, together with the particular construction and arrangement of rhabdoms, suggest that ocelli are not only involved in attitude control, but might also provide skylight polarization compass information.     

The evagination of the genital imaginal discs ofDrosophila melanogaster

Summary The morphology of the evaginating female genital disc ofDrosophila melanogaster was examined at different stages of metamorphosis. The observations show that the internal genital organs are derived from the anterior half of the disc and that their morphogenesis is mainly a protrusion of the different primordial areas of the disc epithelium. The external genital and anal derivatives originate from the posterior half of the disc, which undergoes complex rearrangements during metamorphosis. The disc opens along the posterior margin and the dorsal and ventral epithelia evert and thereby completely reverse their anteroposterior orientation. Dramatic elongation has been observed during the formation of the seminal receptacle. The cells of the repressed male genital primordium do not form any recognizable structures and are assumed to be eliminated during metamorphosis.     

Transmission of Surface Pattern Through a Dedifferentiated Stage in the Ciliate Paraurostyla. Evidence from the Analysis of Microtubule and Basal Body Deployment

ABSTRACT. During conjugation of the hypotrich ciliate Paraurostyla weissei , the two partners fuse to form a transient dedifferentiated stage, the zygocyst, which later redifferentiates into a vegetative cell. Immunocytochemical studies have been performed to follow the deployment of microtubules and basal bodies during the entire cycle of conjugation. They show that a superficial lattice persists during the whole zygocyst stage, after most of the infraciliature of the exconjugants has been disassembled. These superficial microtubules display different immunocytochemical properties in the mature zygocyst and during its morphogenesis, suggesting that some transient chemical modifications of the microtubules are associated with the morphogenetic activity. In the zygocyst, the superficial microtubules retain the specific orientation characteristic of the ventral and the dorsal sides of the recipient cell, respectively. In the course of subsequent morphogenesis of the zygocyst, these specific cellular territories differentiate into the ventral and dorsal sides of the new cell. Although our experiments do not resolve the question of whether superficial microtubules play an active or merely a passive role in the transmission of surface pattern, they show that no complete breakdown in cell polarity occurs, even through a profound dedifferentiated stage. Thus, the overall surface pattern appears to be retained, in a simplified form, through the conjugation cycle.     

A posterior centre establishes and maintains polarity of the Caenorhabditis elegans embryo by a Wnt-dependent relay mechanism

Cellular polarity is a general feature of animal development. However, the mechanisms that establish and maintain polarity in a field of cells or even in the whole embryo remain elusive. Here we provide evidence that in the Caenorhabditis elegans embryo, the descendants of P₁, the posterior blastomere of the 2-cell stage, constitute a polarising centre that orients the cell divisions of most of the embryo. This polarisation depends on a MOM-2/Wnt signal originating from the P₁ descendants. Furthermore, we show that the MOM-2/Wnt signal is transduced from cell to cell by a relay mechanism. Our findings suggest how polarity is first established and then maintained in a field of cells. According to this model, the relay mechanism constantly orients the polarity of all cells towards the polarising centre, thus organising the whole embryo. This model may also apply to other systems such as Drosophila and vertebrates.     

A Posterior Centre Establishes and Maintains Polarity of the Caenorhabditis elegans Embryo by a Wnt-Dependent Relay Mechanism

Cellular polarity is a general feature of animal development. However, the mechanisms that establish and maintain polarity in a field of cells or even in the whole embryo remain elusive. Here we provide evidence that in the Caenorhabditis elegans embryo, the descendants of P₁, the posterior blastomere of the 2-cell stage, constitute a polarising centre that orients the cell divisions of most of the embryo. This polarisation depends on a MOM-2/Wnt signal originating from the P₁ descendants. Furthermore, we show that the MOM-2/Wnt signal is transduced from cell to cell by a relay mechanism. Our findings suggest how polarity is first established and then maintained in a field of cells. According to this model, the relay mechanism constantly orients the polarity of all cells towards the polarising centre, thus organising the whole embryo. This model may also apply to other systems such as Drosophila and vertebrates.