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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). J. Comp. Neurol. 448:337–348, 2002. © 2002 Wiley‐Liss, Inc.

Determinations of plaminogen activator (PA) activity are usually performed in Triton X-100-treated tissue homogenates or crude membrane fractions. Such preparations usually involve a single Triton X-100 treatment. In the present paper we describe the pattern of variability of PA activity measured in different fractions obtained from the developing chick CNS by a repetitive procedure of Triton X-100 treatment and ultracentrifugation. To further characterize this PA activity we have also performed zymographic analyses during the embryonic development and the early postnatal life. Our results show that: a) a single Triton X-100 treatment does not completely extract the enzyme and this lead to an underestimation of the total PA activity; b) the PA activity is associated with the particulate component of the total tissue homogenate requiring its complete solubilization more drastic Triton X-100 treatments; c) better estimations of total and specific activities are obtained by using soluble fractions derived by ultracentrifugation from Triton X-100-treated membrane fractions; d) the developing chick optic lobe expresses only one kind of PA molecule along the entire development; e) the level of PA activity vary characteristically during the ontogeny and the early postnatal life indicating the existence of a developmentally regulated mechanism of PA expression.

The cells of the mesenteric caeca in the midgut of certain insects possess a labyrinth of transepithelial cisternae. Their existence can be seen in thin sections of lanthanum-incubated tissue, where the tracer enters not only the intercellular clefts but also membranous cisternae which are inpocketings from, and, in continuity with, both the lateral clefts and basal membrane. These infoldings, which are numerous, run from the basal or lateral surfaces into the perinuclear region of the cells, where they are found, laden with lanthanum, as smooth cisternae or vesicles in the peripheral cytoplasm near the plasma membrane. These can be followed in serial sections and are quite distinct from other sub-surface cisternae of the lateral borders which are studded with ribosomes on the cytoplasmic surface. Near the luminal surface, tracer-laden structures in the form of vesicles and granules become increasingly predominant over those in the form of cisternae. Freeze-fracture replicas confirm the above observations, in that the plasma membrane of the intercellular cleft can be characterized as such unequivocally, since it exhibits smooth septate junctional E face grooves and P face ridges. Lateral infoldings, cisternae and vesicles can be seen arising directly from these junction-bearing membranes. The transepithelial cisternae and vesicles may be the morphological basis of an insect transcellular transport system, comparable to the tubulocisternal endoplasmic reticulum present in the transporting secretory and absorptive epithelia of vertebrate tissues. However, in insect midgut caecal epithelia, the cisternae appear to be, albeit presumably transiently, in direct continuity with the extracellular space, forming a plasma membrane reticular system which seems not to be the case with the tubulo-cisternal endoplasmic reticulum which terminates in subsurface cisternae.

Accessory glands of the cockroach are composed of secretory and supportive cells, the latter providing a skeleton-like framework of attentuated cytoplasmic processes into which the former are positioned. These two cell types are associated with one another laterally by adhaering, pleated septate, and gap junctions. Hemi-adhaerens junctions are also found on both luminal and basal surfaces of the gland; the former are associated with the cuticular lining of the lumen and the latter with extracellular matrix. The adhering and septate junctions are flanked by both filaments and microtubules; the former insert into the junctional membranes and are actin-like, binding both rhodamine-conjugated phalloidin and the S1 subfragment of rabbit heavy meromyosin. The role of this cytoskeletal protein with the cellular junctions has been explored by treatment with a disruptive agent, cytochalasin D. Dissociation of actin leads to changes in septate junctions and in microtubular distribution. This suggests that the latter act as anchors for the actin filaments which, in turn, appear bound to certain of the intramembranous junctional components.

The present report describes a systematic study comparing and combining methods currently used for the removal of endogenous factors known to affect the interaction of GABA with its receptor. The effects of these methods were analyzed by performing [3H]GABA binding studies, and by measuring the amount of residual GABA left in the different membrane preparations. The effectiveness of these methods were also applied to different developmental stages. The results show that: 1) an exhaustive buffer washing procedure is necessary to accurately measure the maximal binding capacity (Bmax) of the low-affinity GABA binding site, and 2) the use of more drastic methods, including freeze-thawing and Triton treatment allows a clear demonstration of receptor heterogeneity and a precise measurement of the Bmax of the high-affinity GABA binding site as well as increases the affinity of the low-affinity site. The analysis of the Bmax values obtained with these different procedure in relation to the values of GABA removal, strongly indicates that the exhaustive washing procedure removes some unknown endogenous substances required for Triton treatment to exhibit its maximal effectiveness. Finally, a detailed analysis of Kd and Bmax values obtained with these three methods in the developing nervous tissue shows the existence of significant differences with regard to their effectiveness in removing endogenous substances when applied in different developmental stages.

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.