Several ontogenetic studies have been devoted to the structural organization of the developing tectum opticum. They disagree in many respects because they are based on histological preparations performed with differently oriented planes of section. According to our results the differences found in the literature mainly result from the fact that the developmental gradient axis undergoes remarkable positional changes with respect to both optic lobe and neural tube longitudinal anatomical axes during the early stages of development. The present work is a dynamic description of the tectum opticum lamination based on sections coinciding with the developmental gradient. Since this latter displays a curved disposition, several slightly modified planes of section had to be used to obtain a complete picture along the developmental gradient. The development of the tectal architecture proceeds from a relatively simple organization through increasingly complex multilaminated patterns. A dynamic interpretation of successive images of a particular region observed at increasing developmental stages or of images observed at a particular stage along the entire length of the developmental gradient axis, allows us to propose that embryonic laminae are only transient spatial arrangements of cells actively migrating from the sites where they were generated to those where they will definitively reside. These considerations led us to define a nomenclature that establishes clear correlations between the early transient organizations and the definitive one of the fully developed optic tectum. This type of nomenclature could be usefully applied to describe dynamically the development of structures displaying multilaminated patterns such as other cortical zones of the central nervous system.

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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, β-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.

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 dosedependent manner. After stimulation, uPAR interacts with a 5 -and b 1 -integrin subunits, which may constitute an abheterodimer 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. Developmental Dynamics 243:676-689, 2014. V C 2014 Wiley Periodicals, Inc.

Background: Cell proliferation plays an important morphogenetic role. This work analyzes the temporalspatial 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 (b) 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. Developmental Dynamics 241:1043-1061, 2012. V C 2012 Wiley Periodicals, Inc.Key words: developing CNS; mitotic cell organization; nonlinear analyses; morphogenesis Key findings:The signals representing the mitotic cell spatial organization display 1/f type spectrum. The signals subsume deterministic components that reveal the existence of long-range influences. The signals subsume stochastic fluctuations revealing short-range anti-correlations between adjacent cells. The differential displacements of zones of high mitotic density regulate planar expansion. The asymmetric proliferation prints space-dependent histogenetic changes to the tectal corticogenesis.

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.

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BackgroundRetinotopic 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 FindingsBy 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.ConclusionsWe 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.