Segmentation in the vertebrate nervous system

Although there is good evidence that growing axons can be guided by specific cues during the development of the vertebrate peripheral nervous system, little is known about the cellular mechanisms involved. We describe here an example where axons make a clear choice between two neighbouring groups of cells. Zinc iodide-osmium tetroxide staining of chick embryos reveals that motor and sensory axons grow from the neural tube region through the anterior (rostral) half of each successive somite. 180 degrees antero-posterior rotation of a portion of the neural tube relative to the somites does not alter this relationship, showing that neural segmentation is not intrinsic to the neural tube. Furthermore, if the somitic mesoderm is rotated 180 degrees about an antero-posterior axis, before somite segmentation, axons grow through the posterior (original anterior) half of each somite. Some difference therefore exists between anterior and posterior cells of the somite, undisturbed by rotation, which determines the position of axon outgrowth. It is widespread among the various vertebrate classes.

The roles of node regression and elongation of the area pellucida in the formation of somites in avian embryos

Experiments have been carried out on explanted chick embryos to test certain widely accepted concepts about the role of Hensen's node in somite formation. The relationship between elongation of the area pellucida and regression of Hensen's node has also been investigated.

We conclude from these experiments that:

(a) The timing of somite formation is not controlled by the regression of Hensen's node, nor by the shearing of the mesoderm into right and left halves.

(b) Somite size and shape are probably controlled by local conditions in the chick embryo.

(c) Elongation and regression are two different events.

(d) The position of the somites probably depends on mechanical tensions in the area pellucida.

(e) The notochord is not required for the stability of somites in vivo.

The roles of node regression and elongation of the area pellucida in the formation of somites in avian embryos

Experiments have been carried out on explanted chick embryos to test certain widely accepted concepts about the role of Hensen's node in somite formation. The relationship between elongation of the area pellucida and regression of Hensen's node has also been investigated. We conclude from these experiments that: (a) The timing of somite formation is not controlled by the regression of Hensen's node, nor by the shearing of the mesoderm into right and left halves. (b) Somite size and shape are probably controlled by local conditions in the chick embryo. (c) Elongation and regression are two different events. (d) The position of the probably depends on mechanical tensions in the area pellucida. (e) The notochord is not required for the stability of somites in vivo.

A simple model for early morphogenesis

Despite the large amount of knowledge which continues to accumulate about early developmental events, very little is known about the processes which control them. Part of the problem may lie in that workers applying different approaches and techniques have different points of view and appear to be reluctant to read each others' literature. My aim in this paper is not to give a generative, formal model for early development, but rather to suggest several connecting strands between the physiological, biochemical, cell biological and experimental embryological approaches which may stimulate new research in fields between those already exploited.

Mitotic activity during somite segmentation in the early chick embryo

The mitotic activity of the somites, segmental plate and posterior mesoderm were investigated in colchicine-treated and untreated chick embryos at st. 7-14. The mitotic figures in the somites are restricted to the proximity of the lumen and have their spindles orientated predominantly tangentially to the cavity. In the segmental plate there is no pattern in terms of the position or orientation of the mitotic spindles, but there is a single region, often found close to the cranial end of the segmental plate, with an elevated mitotic index. This may indicate a certain degree of synchrony among groups of segmental plate cells. These results are discussed in relation to the process of somite segmentation.

Sodium transport and the control of epiblast polarity in the early chick embryo

The sodium transport properties of chick epiblast during gastrulation were studied by various techniques. It was found that the epiblast is capable of unidirectional apical to basal sodium transport, in towards the underlying intraembryonic space. The Na-K-ATPase was localized by [3H]ouabain binding and autoradiography near the basal surfaces of the cells, and the number of pump sites was quantified. The transport rate of sodium was determined with 22Na. Electrophysiological studies on embryos at primitive streak stages showed a transepithelial potential of about +16 +/- 5 mV (basal side positive) which was sensitive to strophanthidin. Applying similar voltages but of reverse polarity to isolated sheets of epiblast caused a rapid reversal of some of their morphological polarity markers as well as some of their physiological functions. The relevance of these results to development is discussed.

Sodium transport and the control of epiblast polarity in the early chick embryo

The sodium transport properties of chick epiblast during gastrulation were studied by various techniques. It was found that the epiblast is capable of unidirectional apical tobasal sodium transport, in towards the underlying intraembryonic space. The Na-K-ATPasewas localized by [3H]ouabain binding and autoradiography near the basal surfaces of the cells, and the number of pump sites was quantified. The transport rate of sodium was determined with 22Na. Electrophysiological studies on embryos at primitive streak stages showed a transepithelial potential of about + 16 ± 5mV (basal side positive) which was sensitive to strophanthidin. Applying similar voltages but of reverse polarity to isolated sheets of epiblast caused a rapid reversal of some oftheir morphological polarity markers as well as some of their physiological functions. The relevance of these results to development is discussed.

Cell-substrate contacts in cultured chick embryonic cells: an interference reflection study

Cell-substrate contacts in explants of different regions of early chick tissues were investigated using the technique of interference reflection microscopy. All the explants spread as epithelial sheets. During initial spreading a peripheral zone of 2–3 cells formed broad contacts with the substrate. In spread explants some cells in the centre made broad substrate contacts. A mat of extracellular material containing fibronectin was found under the explants. Focal contacts and focal adhesions increased in number during culture, and stress fibres were associated with them. These changes in cell contacts appeared more quickly in some tissues than in others. After 24 h, explants of hypoblast and definitive endoblast could easily be distinguished but by 7 days they were very similar. In the absence of serum, specialized cell contacts developed more quickly; in higher concentrations of serum, more slowly. Confrontations between explants were also examined. The most conspicuous feature was that cells in invading explants normally underlapped invaded cells. Invasion from above by an unspread explant could occur even if the invaded explant had formed many focal adhesions.

An integrated experimental study of endoderm formation in avian embryos

The formation of the endoderm during primitive streak stages in avian embryos was studied by combining several of the following techniques for each embryo. These included microsurgery, time-lapse filming, use of chick-quail chimaeras, tritiated thymidine autoradiography and a novel technique for identifying the morphology of the cells after small pieces of tissue from known areas had been maintained in culture for 24 h. Using these techniques we have confirmed that the ventral layer of the early chick embryo receives contributions from both the marginal and the central regions of the area pellucida. The former seems to consist of yolky cells derived from the germ wall, whilst the latter consists of smaller, less yolky cells derived from the more dorsal layers of the embryo. The movement of the lower layer anteriorly during these stages appears to be dependent upon mechanical constraints imposed upon it by the expanding tissue in more caudal regions. The extent of each of the two contributions to the lower layer was determined as a function of stage and presence or absence of a lower layer, and the findings are discussed in the light of the existing literature.

The behaviour of embryonic chick and quail tissues in culture

Pieces of tissue were dissected from early chick and quail embryos (Stages XIII and XIV of Eyal-Giladi & Kochav, 1976; and stages 3-5 of Hamburger & Hamilton, 1951). These tissues were taken from three different regions of the early embryos, and from eight different regions of the older ones, and were derived mainly from the lower layer. Epiblast tissues were also used. The experiments were designed to test the ability of one tissue to penetrate another. A single tissue was grown in culture in a Falcon dish for 18-24 h until it had formed a coherent sheet of cells (Explant I). A second tissue was then combined with it in one of two ways: (a) A small piece of tissue (Explant II) was explanted on top of Explant I. In most cases Explant II penetrated through Explant I and spread on the Falcon dish. (b) Another small piece of tissue (Explant III) was explanted beside (in confrontation with) Explant I. Usually, Explant III penetrated into Explant I rather than vice versa. The results were analysed to see if there were any variations in behaviour of the different tissues. The main result was that important differences were found to exist between certain types of chick and quail cells when grown in culture; the implications of this finding for the widely used technique of xenoplastic grafting are mentioned. Another result was that Explant I was more likely to be penetrated when the second tissue was placed on top of it (Explant II) than when it was confronted with it (Explant III). The significance of these results is discussed.

Strong electrical currents leave the primitive streak of chick embryos

The electrical fields above chick embryos were explored with a vibrating probe. These fields indicate that steady currents with exit densities of the order of 100 microamperes per square centimeter leave the whole streak and return elsewhere through the epiblast. The epicenter of these strong exit currents lies near Hensen's node. They are probably pumped into the intraembryonic space by the epiblast and then leak out of the streak because it is a zone of junctional disruption.

A re-examination of mitotic activity in the early chick embryo

As a result of extensive mitotic index analysis in colchicine-arrested chick embryos during gastrulation, it was ascertained that the primitive streak is a region of elevated mitotic index as compared to the surrounding tissue. Along the cephalo-caudal axis, the embryo displays two large peaks of mitotic index, one at the posterior end of the primitive streak and the other just anterior to Hensen's node. The length of the various phases of the mitotic period was determined in vitro by time-lapse filming, and the colchicine-arrested mitotic indices in vivo and in vitro were determined and compared for various regions. Some observations regarding the orientation of mitotic spindles and abnormal mitosis in vitro are also included, and the relevance of the above observations to early embryonic development is discussed.

Waves and periodic events during primitive streak formation in the chick

Morphogenetic movements occurring during formation of the primitive streak in the chick embryo are of a periodic nature, with a mean frequency of one pulse every 2·6 min. The period of the oscillatory movement is shown to be temperature-dependent. The onset of these pulses of movement can be seen as a slow wave starting at the posterior end of the embryo and making its way towards the anterior end. An interpretation of this behaviour is discussed.

Waves and periodic events during primitive streak formation in the chick

Morphogenetic movements occurring during formation of the primitive streak in the chick embryo are of a periodic nature, with a mean frequency of one pulse every 2.6 min. The period of the oscillatory movement is shown to be temperature-dependent. The onset of these pulses of movement can be seen as a slow wave starting at the posterior end of the embryo and making its way towards the anterior end. An interpretation of this behaviour is discussed.