The developing optic tectum: An asymmetrically organized system and the need for a redefinition of the notion of sensitive period.

The present article summarizes the main events involved in the isthmic organizer and optic tectum determination and analyses how optic tectum patterning is translated, by the organized operation of several specific cell behaviors, into the terminally differentiated optic tectum. The paper proposes that this assembling of temporally/spatially organized cell behaviors could be incorporated into a wider notion of patterning and that, given the asymmetric organization of the developing optic tectum, the notion of "sensitive period" does not capture the whole complexity of midbrain development and the pathogenesis of congenital disorders. The cell behaviors involved in the optic tectum development are organized in time and space by the isthmic organizer. A comprehensive description of the normal optic tectum development, and also its alterations, should consider both domains. Significantly, the identity of each neuronal cohort depends critically on its "time and place of birth". Both parameters must be considered at once to explain how the structural and functional organization of the optic tectum is elaborated. The notion of "patterning" applies only to the early events of the optic tectum development. Besides, the notion of "sensitive period" considers only a temporal domain and disregards the asymmetric organization of the developing optic tectum. The present paper proposes that these notions might be re-defined: (a) a wider meaning of the term patterning and (b) a replacement of the term "sensitive period" by a more precise concept of "sensitive temporal/spatial window".

NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system.

A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase -4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.

Corrigendum: Morphogenetic and Histogenetic Roles of the Temporal-Spatial Organization of Cell Proliferation in the Vertebrate Corticogenesis as Revealed by Inter-specific Analyses of the Optic Tectum Cortex Development.

[This corrects the article on p. 67 in vol. 10, PMID: 27013978.].

Morphogenetic and Histogenetic Roles of the Temporal-Spatial Organization of Cell Proliferation in the Vertebrate Corticogenesis as Revealed by Inter-specific Analyses of the Optic Tectum Cortex Development.

The central nervous system areas displaying the highest structural and functional complexity correspond to the so called cortices, i.e., concentric alternating neuronal and fibrous layers. Corticogenesis, i.e., the development of the cortical organization, depends on the temporal-spatial organization of several developmental events: (a) the duration of the proliferative phase of the neuroepithelium, (b) the relative duration of symmetric (expansive) versus asymmetric (neuronogenic) sub phases, (c) the spatial organization of each kind of cell division, (e) the time of determination and cell cycle exit and (f) the time of onset of the post-mitotic neuronal migration and (g) the time of onset of the neuronal structural and functional differentiation. The first five events depend on molecular mechanisms that perform a fine tuning of the proliferative activity. Changes in any of them significantly influence the cortical size or volume (tangential expansion and radial thickness), morphology, architecture and also impact on neuritogenesis and synaptogenesis affecting the cortical wiring. This paper integrates information, obtained in several species, on the developmental roles of cell proliferation in the development of the optic tectum (OT) cortex, a multilayered associative area of the dorsal (alar) midbrain. The present review (1) compiles relevant information on the temporal and spatial organization of cell proliferation in different species (fish, amphibians, birds, and mammals), (2) revises the main molecular events involved in the isthmic organizer (IsO) determination and localization, (3) describes how the patterning installed by IsO is translated into spatially organized neural stem cell proliferation (i.e., by means of growth factors, receptors, transcription factors, signaling pathways, etc.) and (4) describes the morpho- and histogenetic effect of a spatially organized cell proliferation in the above mentioned species. A brief section on the OT evolution is also included. This section considers how the differential operation of cell proliferation could explain differences among species.

Temporal-spatial correlation between angiogenesis and corticogenesis in the developing chick optic tectum.

The developing chick optic tectum is a widely used model of corticogenesis and angiogenesis. Cell behaviors involved in corticogenesis and angiogenesis share several regulatory mechanisms. In this way the 3D organizations of both systems adapt to each other. The consensus about the temporally and spatially organized progression of the optic tectum corticogenesis contrasts with the discrepancies about the spatial organization of its vascular bed as a function of the time. In order to find out spatial and temporal correlations between corticogenesis and angiogenesis, several methodological approaches were applied to analyze the dynamic of angiogenesis in the developing chick optic tectum. The present paper shows that a typical sequence of developmental events characterizes the optic tectum angiogenesis. The first phase, formation of the primitive vascular bed, takes place during the early stages of the tectal corticogenesis along which the large efferent neurons appear and begin their early differentiation. The second phase, remodeling and elaboration of the definitive vascular bed, occurs during the increase in complexity associated to the elaboration of the local circuit networks. The present results show that, apart from the well-known influence of the dorsal-ventral and radial axes as reference systems for the spatial organization of optic tectum angiogenesis, the cephalic-caudal axis also exerts a significant asymmetric influence. The term cortico-angiogenesis to describe the entire process is justified by the fact that tight correlations are found between specific corticogenic and angiogenic events and they take place simultaneously at the same position along the cephalic-caudal and radial axes.

uPA-uPAR molecular complex is involved in cell signaling during neuronal migration and neuritogenesis.

BACKGROUND: In the development of the central nervous system (CNS), neuronal migration and neuritogenesis are crucial processes for establishing functional neural circuits. This relies on the regulation exerted by several signaling molecules, which play important roles in axonal growth and guidance. The urokinase-type plasminogen activator (uPA)-in association with its receptor-triggers extracellular matrix proteolysis and other cellular processes through the activation of intracellular signaling pathways. Even though the uPA-uPAR complex is well characterized in nonneuronal systems, little is known about its signaling role during CNS development. RESULTS: In response to uPA, neuronal migration and neuritogenesis are promoted in a dose-dependent manner. After stimulation, uPAR interacts with α5- and ß1-integrin subunits, which may constitute an αß-heterodimer that acts as a uPA-uPAR coreceptor favoring the activation of multiple kinases. This interaction may be responsible for the uPA-promoted phosphorylation of focal adhesion kinase (FAK) and its relocation toward growth cones, triggering cytoskeletal reorganization which, in turn, induces morphological changes related to neuronal migration and neuritogenesis. CONCLUSIONS: uPA has a key role during CNS development. In association with its receptor, it orchestrates both proteolytic and nonproteolytic events that govern the proper formation of neural networks.

Sonic hedgehog (Shh)/Gli modulates the spatial organization of neuroepithelial cell proliferation in the developing chick optic tectum.

BACKGROUND: Sonic hedgehog (Shh)/Gli pathway plays an important regulatory role on the neuroepithelial cells (NEc) proliferation in the dorsal regions of the developing vertebrate Central Nervous System. The aim of this paper was to analyze the effect of the Shh/Gli signaling pathway activation on the proliferation dynamics and/or the spatial organization of the NEc proliferation activity during early stages of the developing chick optic tectum (OT). In ovo pharmacological gain and loss of hedgehog function approaches were complemented with in vivo electroporation experiments in order to create ectopic sources of either Shh or Gli activator (GliA) proteins in the OT. NEc proliferating activity was analyzed at ED 4/4.5 by recording the spatial co-ordinates of the entire population of mitotic NEc (mNEc) located along OT dorsal-ventral sections. Several space signals (numerical sequences) were derived from the mNEc spatial co-ordinate records and analyzed by different standardized non-linear methods of signal analysis. RESULTS: In ovo pharmacologic treatment with cyclopamine resulted in dramatic failure in the OT expansion while the agonist purmorphamine produced the opposite result, a huge expansion of the OT vesicle. Besides, GliA and Shh misexpressions interfere with the formation of the intertectal fissure located along the dorsal midline. This morphogenetic alteration is accompanied by an increase in the mNEc density. There is a gradient in the response of NEcs to Shh and GliA: the increase in mNEc density is maximal near the dorsal regions and decrease towards the OT-tegmental boundary. Biomathematical analyses of the signals derived from the mNEc records show that both Shh and GliA electroporations change the proliferation dynamics and the spatial organization of the mNEc as revealed by the changes in the scaling index estimated by these methods. CONCLUSIONS: The present results show that the Shh/Gli signaling pathway plays a critical role in the OT expansion and modelling. This effect is probably mediated by a differential mitogenic effect that increases the NEc proliferation and modulates the spatial organization of the NEc proliferation activity.

EphA3 expressed in the chicken tectum stimulates nasal retinal ganglion cell axon growth and is required for retinotectal topographic map formation.

BACKGROUND: Retinotopic projection onto the tectum/colliculus constitutes the most studied model of topographic mapping and Eph receptors and their ligands, the ephrins, are the best characterized molecular system involved in this process. Ephrin-As, expressed in an increasing rostro-caudal gradient in the tectum/colliculus, repel temporal retinal ganglion cell (RGC) axons from the caudal tectum and inhibit their branching posterior to their termination zones. However, there are conflicting data regarding the nature of the second force that guides nasal axons to invade and branch only in the caudal tectum/colliculus. The predominant model postulates that this second force is produced by a decreasing rostro-caudal gradient of EphA7 which repels nasal optic fibers and prevents their branching in the rostral tectum/colliculus. However, as optic fibers invade the tectum/colliculus growing throughout this gradient, this model cannot explain how the axons grow throughout this repellent molecule. METHODOLOGY/PRINCIPAL FINDINGS: By using chicken retinal cultures we showed that EphA3 ectodomain stimulates nasal RGC axon growth in a concentration dependent way. Moreover, we showed that nasal axons choose growing on EphA3-expressing cells and that EphA3 diminishes the density of interstitial filopodia in nasal RGC axons. Accordingly, in vivo EphA3 ectodomain misexpression directs nasal optic fibers toward the caudal tectum preventing their branching in the rostral tectum. CONCLUSIONS: We demonstrated in vitro and in vivo that EphA3 ectodomain (which is expressed in a decreasing rostro-caudal gradient in the tectum) is necessary for topographic mapping by stimulating the nasal axon growth toward the caudal tectum and inhibiting their branching in the rostral tectum. Furthermore, the ability of EphA3 of stimulating axon growth allows understanding how optic fibers invade the tectum growing throughout this molecular gradient. Therefore, opposing tectal gradients of repellent ephrin-As and of axon growth stimulating EphA3 complement each other to map optic fibers along the rostro-caudal tectal axis.

Optic tectum morphogenesis: a step-by-step model based on the temporal-spatial organization of the cell proliferation. Significance of deterministic and stochastic components subsumed in the spatial organization.

BACKGROUND: Cell proliferation plays an important morphogenetic role. This work analyzes the temporal-spatial organization of cell proliferation as an attempt to understand its contribution to the chick optic tectum (OT) morphogenesis. RESULTS: A morphogenetic model based on space-dependent differences in cell proliferation is presented. Step1: a medial zone of high mitotic density (mZHMD) appears at the caudal zone. Step2: the mZHMD expands cephalically forming the dorsal curvature and then duplicates into two bilateral ZHMDs (bZHMD). Step3: the bZHMDs move toward the central region of each hemitectum. Step4: the planar expansion of both bZHMD and a relative decrement in the dorsal midline growth produces a dorsal medial groove separating the tectal hemispheres. Step5: a relative caudal displacement of the bZHMDs produces the OT caudal curvature. Numerical sequences derived from records of mitotic cells spatial coordinates, analyzed as stochastic point processes, show that they correspond to 1/f((ß)) processes. The spatial organization subsumes deterministic and stochastic components. CONCLUSIONS: The deterministic component describes the presence of a long-range influence that installs an asymmetric distribution of cell proliferation, i.e., an asymmetrically located ZHMD that print space-dependent differences onto the tectal corticogenesis. The stochastic component reveals short-range anti-correlations reflecting spatial clusterization and synchronization between neighboring cells.

The chick optic tectum developmental stages. A dynamic table based on temporal- and spatial-dependent histogenetic changes: A structural, morphometric and immunocytochemical analysis.

Development is often described as temporal sequences of developmental stages (DSs). When tables of DS are defined exclusively in the time domain they cannot discriminate histogenetic differences between different positions along a spatial reference axis. We introduce a table of DSs for the developing chick optic tectum (OT) based on time- and space-dependent changes in quantitative morphometric parameters, qualitative histogenetic features and immunocytochemical pattern of several developmentally active molecules (Notch1, Hes5, NeuroD1, ß-III-Tubulin, synaptotagmin-I and neurofilament-M). Seven DSs and four transitional stages were defined from ED2 to ED12, when the basic OT cortical organization is established, along a spatial developmental gradient axis extending between a zone of maximal and a zone of minimal development. The table of DSs reveals that DSs do not only progress as a function of time but also display a spatially organized propagation along the developmental gradient axis. The complex and dynamic character of the OT development is documented by the fact that several DSs are simultaneously present at any ED or any embryonic stage. The table of DSs allows interpreting how developmental cell behaviors are temporally and spatially organized and explains how different DSs appear as a function of both time and space. The table of DSs provides a reference system to characterize the OT corticogenesis and to reliably compare observations made in different specimens.

Acute hypoxia differentially affects the NMDA receptor NR1, NR2A and NR2B subunit mRNA levels in the developing chick optic tectum: stage-dependent plasticity in the 2B-2A ratio.

It is known that the NMDA-R NR1 subunit is needed for the receptor activity and that under hypoxia the evolution toward apoptosis or neuronal survival depends on the balance NR2A/NR2B subunits. This paper analyzes the effect of acute hypoxia on the above mentioned subunits mRNAs during development. The mean percentage of NR1+ neurons displayed the higher plasticity during development while the NR2A+ neurons the higher stability. Acute hypoxia increased the mean percentage of NR1+ and NR2B+ neurons at ED12 but only that of NR1+ neurons at ED18. Acute hypoxia increased the levels of expression of NR1 and NR2B mRNAs at ED12 without changes in the NR2A mRNA. During early stages there is a higher sensitivity to change the subunits mRNA levels under a hypoxic treatment. At ED12 acute hypoxia increased the probability of co-expression of the NR1-NR2A and NR1-NR2B subunits combinations, the level of NR1 and NR2B and the ratio NR2B/NR2A. These conditions facilitate the evolution towards apoptosis.

Brachytherapy versus radical hysterectomy after external beam chemoradiation: a non-randomized matched comparison in IB2-IIB cervical cancer patients.

BACKGROUND: A current paradigm in the treatment of cervical cancer with radiation therapy is that intracavitary brachytherapy is an essential component of radical treatment. This is a matched retrospective comparison of the results of treatment in patients treated with external beam chemoradiation (EBRT-CT) and radical hysterectomy versus those treated with identical chemoradiation followed by brachytherapy. METHODS: In this non-randomized comparison EBRT-CT protocol was the same in both groups of 40 patients. In the standard treated patients, EBRT-CT was followed by one or two intracavitary Cesium (low-dose rate) applications within 2 weeks of finishing external radiation to reach a point A dose of at least 85 Gy. In the surgically treated patients, radical hysterectomy with bilateral pelvic lymph node dissection and para-aortic lymph node sampling were performed within 7 weeks after EBRT-CT. Response, toxicity and survival were evaluated. RESULTS: A total of 80 patients were analyzed. The patients receiving EBRT-CT and surgery were matched with the standard treated cases. There were no differences in the clinicopathological characteristics between groups or in the delivery of EBRT-CT. The pattern of acute and late toxicity differed. Standard treated patients had more chronic proctitis while the surgically treated had acute complications of surgery and hydronephrosis. At a maximum follow-up of 60 months, median follow-up 26 (2-31) and 22 (3-27) months for the surgery and standard therapy respectively, eight patients per group have recurred and died. The progression free and overall survival are the same in both groups. CONCLUSION: The results of this study suggest that radical hysterectomy can be used after EBRT-CT without compromising survival in FIGO stage IB2-IIB cervical cancer patients in settings were brachytherapy is not available. A randomized study is needed to uncover the value of surgery after EBRT-CT.

Acute hypoxia differentially affects the gamma-aminobutyric acid type A receptor alpha(1), alpha(2), beta(2), and gamma(2) subunit mRNA levels in the developing chick optic tectum: stage-dependent sensitivity.

This investigation analyzes the effect of an acute hypoxic treatment on the level of four (alpha(1), alpha(2), beta(2), and gamma(2)) subunit mRNAs of the GABA(A) receptor in layer "i" of the developing chick optic tectum. Our results show that 1 hr of normobaric acute hypoxia significantly changes the subunit mRNA levels. Different subunit mRNAs display different sensitivity to hypoxia: alpha(1), beta(2), and gamma(2) mRNAs are highly sensitive, whereas alpha(2) mRNA is almost not affected. The sensitivity of the mRNA levels to hypoxia is stage dependent. The mean percentages of variation produced by the hypoxia in the level of expression of the four subunits were 20% at ED12, 5% at ED16, and only 2% at ED18. These changes in the mean percentages of expression modify the probability of coexpression. In the case of double mRNA combinations, the hypoxia produced a mean variation in the probability of coexpression of 37% at ED12, 8% at ED16, and only 4% at ED18. With regard to the triple subunit mRNAs combinations, the variations were 206% at ED12, 11% at ED16, and only 7% at ED18. The quadruple combination values were 1,500% at ED12, 21% at ED16, and only 11% at ED18. This study demonstrates that the subunit mRNA levels are highly sensitive during the early stages, suggesting that GABA(A) receptor composition might undergo environment-dependent plastic changes providing a high degree of plasticity to the GABA neurotransmitter system development.

Grupos de simetría de codones sinónimos revelados en un nuevo espacio de representación / Symmetric groups of codon usage correlations in a new space of representation

El presente trabajo presenta un nuevo espacio de configuración de codones fundado en el carácter dual que las bases poseen considerando dos propiedades fisicoquímicas esenciales: (a) especie química (Y o R) y (b) número de puentes de H (3 ó 2) involucrados en la interacción codón-anticodón. La concepción de esta nueva representación de codones se basa en la construcción de un espacio" bidimensional definido por dos ejes ortogonales cuyos "valores" quedan definidos por las ocho combinatorias posibles de las propiedades Y y R para uno de los ejes y por las ocho combinatorias posibles de las propiedades 2 y 3 para el otro eje...En este trabajo se presentan, discuten contribuciones a la teoría y utilidades prácticas

Grupos de simetría de codones sinónimos revelados en un nuevo espacio de representación / Symmetric groups of codon usage correlations in a new space of representation

El presente trabajo presenta un nuevo espacio de configuración de codones fundado en el carácter dual que las bases poseen considerando dos propiedades fisicoquímicas esenciales: (a) especie química (Y o R) y (b) número de puentes de H (3 ó 2) involucrados en la interacción codón-anticodón. La concepción de esta nueva representación de codones se basa en la construcción de un espacio" bidimensional definido por dos ejes ortogonales cuyos "valores" quedan definidos por las ocho combinatorias posibles de las propiedades Y y R para uno de los ejes y por las ocho combinatorias posibles de las propiedades 2 y 3 para el otro eje...En este trabajo se presentan, discuten contribuciones a la teoría y utilidades prácticas(AU)

Tissue-type plasminogen activator activity in morphologically normal tissues adjacent to gastrointestinal carcinomas is associated with the degree of tumor progression.

PURPOSE: To investigate whether the level of plasminogen activator (PA) activity assayed in gastrointestinal carcinomas and the "morphologically normal tissues" adjacent to them is associated with the degree of tumor progression. METHODS: Tumor and "normal tissues" were obtained from gastrointestinal surgical samples to assess urokinase-type (u-PA) and tissue-type plasminogen activator (t-PA) activities by radial caseinolytic assay and the expression of PA inhibitor-1 (PAI-1) by ELISA. We compared the PA system between the tumor and "normal tissues" and we investigated the existence of correlations between: (a) PA production in the tumor and "normal tissues", (b) different components of the PA system, and (c) PA system and the degree of tumor progression. RESULTS: (1) Total PA activity, u-PA activity and PAI-1 expression are significantly higher in tumor than in "normal tissues", whereas t-PA activity does not differ between them. (2) Total PA activity mainly correlates with u-PA activity in tumor tissues and similarly with u-PA and t-PA activities in "normal tissues". (3) There is a significant association between t-PA activity in tumor and "normal tissues" and the degree of tumor progression. CONCLUSIONS: "Morphologically normal tissues" adjacent to carcinomas present abnormal t-PA activity that is associated with the degree of tumor progression. Assaying of this activity could be useful as a predictive parameter.

Developmental pattern of NADPH-diaphorase positive neurons in chick optic tectum is sensitive to changes in visual stimulation.

The chick retinotectal system is a suitable model to investigate the mechanisms involved in the establishment of synaptic connections in whose refinement nitric oxide was implicated. The purpose of this work was to describe the developmental pattern of the nitric oxide synthase (NOS)-positive neurons as well as to determine if it is sensitive to changes in visual stimulation. The NADPH-diaphorase histochemical method was used to describe and quantify NOS neurons in normally stimulated and subnormally stimulated chickens. Nine types of NOS neurons were identified; seven of them express NOS until adulthood, while two of them show only a transient expression. The developmental pattern of NOS neurons follows the process of laminar segregation. It can be divided into three phases. The first includes the onset of NOS expression in periventricular neurons and the formation of a deep network of NOS fibers during early development. These neurons do not show any significant change in subnormally stimulated animals. The second phase includes the appearance of two transient NOS populations of bipolar neurons that occupy the intermediate layers during the optic fibers ingrowth. One of them significantly changes in subnormally stimulated chicks. The third phase occurs when the transitory expression of bipolar neurons decreases. It includes NOS expression in six neuronal populations that innervate the superficial retinorecipient layers. Most of these cells suffer plastic changes in subnormally stimulated chicks. The diversity of neuronal types with regard to their morphology, location, and sensitivity to visual stimulation strongly suggests that they serve different functions.

Development and localisation of GABA(A) receptor alpha1, alpha2, beta2 and gamma2 subunit mRNA in the chick optic tectum.

An in situ hybridisation technique was used to analyse the spatial and temporal pattern of expression of the mRNA encoding the four gamma-aminobutyric acid A (GABA(A)) receptor subunits (alpha1, alpha2, beta2, and gamma2) in the developing chick optic tectum. As a rule, layer i, layer h, and transient cell compartment 3 (TCC3) show the highest levels of expression, especially of alpha1, alpha2 and beta2, which undergo striking changes as a function of time. Apart from these common features, the global pattern is highly complex and dynamic. Such complexity derives from the fact that each subunit exhibits a characteristically distinct pattern of expression and the temporal evolution of each differs in the different layers of the tectum. The influence of several developmental cell behaviours such as proliferation, neuronal migration, programmed cell death, and differentiation must be taken into account to understand pattern complexity and dynamics. Our results suggest that differences in the rate of subunit expression, particularly of alpha1, alpha2, and beta2, could have significant consequences on GABA(A) receptor complex subunit composition along development and on the functional properties of the GABA neurotransmitter system.

Developmental changes in the spatial pattern of mesencephalic trigeminal nucleus (Mes5) neuron populations in the developing chick optic tectum.

The developing mesencephalic trigeminal nucleus (nucleus of the fifth cranial nerve; Mes5) is composed of four neuron populations: 1) the medial group, located at the tectal commissure; 2) the lateral group distributed along the optic tectum hemispheres; 3) a group outside the neural tube; and 4) a population located at the posterior commissure. The present work aims to elucidate the site of appearance, temporal evolution, and spatial distribution of the four Mes5 populations during development. According to detailed qualitative observations Mes5 neurons appear as a primitive unique population along a thin dorsal medial band of the mesencephalon. According to quantitative analyses (changes in cell density along defined reference axes performed as a function of time and space), the definitive spatial pattern of Mes5 neurons results from a process of differential cell movements along the tangential plane of the tectal hemispheres. Radial migration does not have a relevant developmental role. Segregation of medial and lateral group populations depends on the intensity of the lateral displacements. The mesenchymal population appears as an outsider subset of neurons that migrate from the cephalic third of the neural tube dorsal midregion to the mesenchymal compartment. This process, together with the intensive lateral displacements that the insider subset undergoes, contributes to the disappearance of this transient population. We cannot find evidence indicating that neural crest-derived precursors enter the neural tube and differentiate into Mes5 neurons. Our results can be better interpreted in terms of the notion that a dorsal neural tube progenitor cell population behaves as precursor of both migrating peripheral descendants (neural crest) and intrinsic neurons (Mes5).

The formation of intercellular junctions in insect stem cell progeny (cockroach intestinal epithelium).

The stages in the development of intercellular junctions have been followed in the mesenteric caecal cells of the cockroach midgut, where two types of mature cell, the columnar and the secretory, exist. 'Nests' of undifferentiated replacement cells occur at intervals along the basal lamina, consisting of central, dividing cells and peripheral semi-lunar cells; the former act as proliferative stem cells to give rise to either pre-columnar or pre-secretory cells. The semi-lunar cells are pre-columnar and produce an attenuated process which gradually projects up to the luminal surface, producing microvilli and a dense extracellular substance en route. Intercellular gap junctions appear between these maturing columnar cell borders first, while septate junctions differentiate later; these are assembled from two different sets of intramembranous particle which become organized into either plaques or rows in parallel alignment, possibly mediated by actin filaments and microtubules. The pre-secretory cells, which are much fewer in number, remain associated only with the basal lamina and never reach the lumen; they develop into one of three distinct mature secretory cell types which release their secretory product in different ways.