Antone G. Jacobson

The origins of neural crest cells in the axolotl

We address the question of whether neural crest cells originate from the neural plate, from the epidermis, or from both of these tissues. Our past studies revealed that a neural fold and neural crest cells could arise at any boundary created between epidermis and neural plate. To examine further the formation of neural crest cells at newly created boundaries in embryos of a urodele (Ambystoma mexicanum), we replace a portion of the neural folds of an albino host with either epidermis or neural plate from a normally pigmented donor. We then look for cells that contain pigment granules in the neural crest and its derivatives in intact and sectioned host embryos. By tracing cells in this manner, we find that cells from neural plate transplants give rise to melanocytes and (in one case) become part of a spinal ganglion, and we find that epidermal transplants contribute cells to the spinal and cranial ganglia. Thus neural crest cells arise from both the neural plate and the epidermis. These results also indicate that neural crest induction is (at least partially) governed by local reciprocal interactions between epidermis and neural plate at their common boundary.

Embryonic brain enlargement requires cerebrospinal fluid pressure.

Abstract The brain of the chick embryo begins to enlarge abruptly on the second day of incubation. Shortly thereafter, major flexures and torsions of the brain occur, and many bulges and furrows appear. The onset of enlargement coincides with closure of the spinal canal which makes the neural tube a closed compartment filled with cerebrospinal fluid. We propose that cerebrospinal fluid pressure is a necessary driving force for normal brain enlargement. We have experimentally tested this hypothesis by intubating brains of chick embryos and comparing brain cavity and tissue volumes in normal and intubated embryos. The increase in cavity volume is greatly reduced, whereas brain tissue continues to grow at a reduced rate and folds into the ventricles.

Nuclear elongation and cytokinesis in Drosophila montana

Abstract We have used time-lapse cinematography and electron microscopy to study Drosophila montana embryos during formation of the cellular blastoderm, which occurs shortly after the twelfth mitotic division of the nuclei. At the end of this division, the 3500 spherical nuclei in the peripheral cytoplasm elongate to 2.6 × their diameter, increasing their volume 2.4-fold. Bundles of microtubules, oriented parallel to the direction of elongation, surround the elongating nuclei. We propose that the tubules serve to constrain the enlarging nuclei so that they elongate rather than becoming larger spheres. Cytokinesis begins as the nuclei elongate, resulting in the simultaneous division of the embryo into 3500 peripheral cells surrounding the internal yolk mass. We observe that some cleavage furrows form through spindles, but most form between asters belonging to adjacent nuclei. These observations support the ideas of Rappaport that furrow initiation results from the interaction of pairs of asters with the egg surface. Some evidence is given for the existence of a system of contractile fibers which may be responsible for furrowing. We demonstrate that there are both slow and fast phases of furrowing, and we present the possibility that there are at least two sources of membrane for furrow growth.

Normal stages of development of the axolotl, Ambystoma mexicanum

Illustrations and descriptions of normal stages of the axolotl Ambystoma mexicanum are given. The time to reach each morphological stage is compared with similar stages in two other species, Ambystoma maculatum and Taricha torosa.

The influence of axial structures on chick somite formation

Abstract The influence of the axial structures on somite formation was investigated by culturing, on a nutritive agar substrate, segmental plates from chick embryos having 8 to 20 pairs of somites. In the first set of experiments, segmental plate was explanted together with adjacent notochord and approximately the lateral halves of the neural tube and node region. These explants formed 18 to 20 somites within 30 hr. In a second series of experiments, the notochord and neural tube were included as before, but further regression movements in the explants were prevented by removing the node region. These explants formed only 11.9 ± 1.1 somites. Finally, explants of segmental plate that included no neural tube, notochord, or node region were made. These explants had formed 10.7 ± 1.1 somites 14 to 17 hr later. When such explants were cultured for periods longer than 17 hr, there was a marked tendency for the more posterior somites to disperse and for all of the somites to develop a peculiar “hollow” morphology. It was concluded from these results that during the period of development when chick embryos possess 8 to 20 pairs of somites, the segmental plate mesoderm (1) represents about 12 prospective somites, (2) may segment into its full complement of somites without further contact with the axial structures, but (3) requires continued intimate contact with the axial structures for normal somite morphologic differentiation and stability.

Analysis of morphogenetic movements in the neural plate of the newt Taricha torosa

Abstract These time-lapse cinematographic studies provide measurements and descriptions of the pathways and speeds of displacement of cell groups in the forming neural plate of a newt. At speeds ranging from 4 to 95 μ per hour, cell groups are displaced almost as much as 1 mm (896 μ) on a 2.5 mm embryo during plate development prior to neural tube formation. Changes in area of regions of the surface of the neural plate are presented and are found to correlate inversely with changes in height of the neural plate epithelium. Observations with high magnification time-lapse films establish that cells retain their contact relationships with neighboring cells throughout the period of study. The newt neural plate is a sheet of cells just one cell thick. Observed displacements of groups of neural plate cells are the consequence of deformations of the sheet. Observations and correlations indicate that these deformations are the result of regional differences in the amounts of change in cell shape of the constituent cells. Thus one consequence of primary embryonic induction is a patterned change in the height of the cells in the forming neural plate with concomitant displacements of cell groups and regional changes in area that give the neural plate its characteristic shape.

The restriction of the heart morphogenetic field in Xenopus laevis

We have examined the spatial restriction of heart-forming potency in Xenopus laevis embryos, using an assay system in which explants or explant recombinates are cultured in hanging drops and scored for the formation of a beating heart. At the end of neurulation at stage 20, the heart morphogenetic field, i.e., the area that is capable of heart formation when cultured in isolation, includes anterior ventral and ventrolateral mesoderm. This area of developmental potency does not extend into more posterior regions. Between postneurula stage 23 and the onset of heart morphogenesis at stage 28, the heart morphogenetic field becomes spatially restricted to the anterior ventral region. The restriction of the heart morphogenetic field during postneurula stages results from a loss of developmental potency in the lateral mesoderm, rather than from ventrally directed morphogenetic movements of the lateral mesoderm. This loss of potency is not due to the inhibition of heart formation by migrating neural crest cells. During postneurula stages, tissue interactions between the lateral mesoderm and the underlying anterior endoderm support the heart-forming potency in the lateral mesoderm. The lateral mesoderm loses the ability to respond to this tissue interaction by stages 27-28. We speculate that either formation of the third pharyngeal pouch during stages 23-27 or lateral inhibition by ventral mesoderm may contribute to the spatial restriction of the heart morphogenetic field.

Influences of ectoderm and endoderm on heart differentiation in the newt

Presumptive heart mesoderm from early neurulae of the newt Taricha torosa was isolated alone in salt solution or was combined in explants with ectoderm, or ectoderm and endoderm, from various regions of the embryo. The presumptive heart mesoderm formed a beating heart when isolated alone in only 14% of the cases. When explanted in combination with anterior presumptive epidermis, beating hearts developed in about one-third of the cases. Beating hearts formed in all explants of presumptive heart mesoderm combined with anterior presumptive epidermis and endoderm from the future heart region, and in two-thirds of the cases if the endoderm came from more dorsal or posterior regions. In the series where the endoderm was tested from the dorsal or posterior regions, all explants that failed to produce beating hearts contained neural tissue that had differentiated from the presumptive epidermis whereas each explant that formed a heart lacked neural tissue. In no case did a beating heart develop from presumptive heart mesoderm in an explant that included part of the anterior neural plate and fold as well as anterior presumptive epidermis. Hearts failed to develop in these explants even when endoderm from the future heart region was added. Endodermless animals failed to produce hearts, but endodermless animals also deprived of their neural plate and fold did produce hearts. It is concluded that both ectoderm and endoderm can influence the differentiation of the presumptive heart mesoderm. Heart formation is suppressed by anterior neural plate and fold, but is stimulated by endoderm.

Analysis of normal somite development

Abstract We describe how the first 6 somite pairs form, using the third somites as examples. This history is based upon time-lapse movies of carbon-marked embryos and histological studies by light and electron microscopy of embryos fixed in situ with glutaraldehyde and osmium tetroxide. At head-process stage a continuous sheet of mesoblast occupies the regions of the future third somites. Mesoblast cells attach either to hypoblast or to overlying neural plate which is already a simple pseudostratified columnar epithelium. Prospective somite cells are those attached to the neuroepithelium, and they extend laterally exactly as far as the neural plate does. By head-fold stage, regression of the node down the midline is shearing the sheet of mesoblast into right and left halves. Somite cells hang from the bottom of the neural plate. As the neural plate condenses toward the midline, attached somite cells are compacted. When the somite segments, somite cells are tightly apposed to one another, and, in addition to junctions binding their basal ends, new junctions appear between their apical ends. This leads to reorganization into the typical somite rosette configuration. Spaces filled with extracellular materials form around the whole somite.

Paired gill slits in a fossil with a calcite skeleton.

The chordates, hemichordates (such as acorn worms) and echinoderms (such as starfish) comprise the group Deuterostomia, well established as monophyletic. Among extant deuterostomes, a skeleton in which each plate has the crystallographic structure of a single crystal of calcite is characteristic of echinoderms and is always associated with radial symmetry and never with gill slits. Among fossils, however, such a skeleton sometimes occurs without radial symmetry. This is true of Jaekelocarpus oklahomensis, from the Upper Carboniferous of Oklahoma, USA, which, being externally almost bilaterally symmetrical, is traditionally placed in the group Mitrata (Ordovician to Carboniferous periods, 530–280 million years ago), by contrast with the bizarrely asymmetrical Cornuta (Cambrian to Ordovician periods, 540 to 440 million years ago). Using computer X-ray microtomography, we describe the anatomy of Jaekelocarpus in greater detail than formerly possible, reveal evidence of paired gill slits internally and interpret its functional anatomy. On this basis we suggest its phylogenetic position within the deuterostomes.