Hefzibah Eyal-Giladi (1925-2017): over fifty years of embryological research in Israel.

Hefzibah Eyal-Giladi was a creative and innovative pioneering scientist in the creation of the field of early chick embryo development. She had a sharp thinking and enthusiastic attitude, which enabled her to make a deep impression that was highly valued by the general scientific community. Notably, she was a highly successful female researcher in an era which was dominated by male scientists.

Evolutionary innovations of the vertebrates.

This review concentrates on the greatest anatomical and morphological evolutionary innovations of the vertebrates. During evolution, many new species of vertebrates evolved and underwent modifications by developing new forms, structures and functions of tissues and organ systems. Evolutionary development of the chordates and vertebrates is herein examined in terms of innovations in their organ systems and organismal complexity.

The importance of the posterior midline region for axis initiation at early stages of the avian embryo.

The avian blastoderm acts during its early stages of development as an integrative system programmed to form a single embryonic axis. Here, I report the results of a variety of transplantation experiments of the midline region at stages X-XII, which were carried out to study their relevance for axis initiation. The results of the experimental series discussed herein emphasizes the importance of the posterior midline region (including the marginal zone and Koller's sickle) for axis initiation.

Functional evolution in the ancestral lineage of vertebrates or when genomic complexity was wagging its morphological tail.

Early vertebrate evolution is characterized by a significant increase of organismal complexity over a relatively short time span. We present quantitative evidence for a high rate of increase in morphological complexity during early vertebrate evolution. Possible molecular evolutionary mechanisms that underlie this increase in complexity fall into a small number of categories, one of which is gene duplication and subsequent structural or regulatory neofunctionalization.

The evolution of genome size: what can be learned from anuran development?

Differences in nuclear DNA content in vertebrates have been shown to be correlated with cell size, cell division rate, and embryonic developmental rate. We compare seven species of anuran amphibians with a three-fold range of genome sizes. Parameters examined include the number and density of cells in a number of embryonic structures, and the change in cell number in the CNS during development.

Variations in anuran embryogenesis: yolk-rich embryos of Hyperolius puncticulatus (Hyperoliidae).

Anuran development is usually described using model species, most notably Xenopus laevis and Rana pipiens. We describe the development of the East African Reed Frog, Hyperolius puncticulatus, a species displaying development that is highly divergent from the "classic" anuran developmental pattern. Although having small eggs, the eggs of H.

Variation in anuran embryogenesis: differences in sequence and timing of early developmental events.

Comparative embryology of closely related species can shed light on the evolution of developmental processes. An important mechanism in the evolution of developmental processes, which can lead to significant changes in larval or adult form, is variation in the sequence and timing of developmental events. We compared the development of 12 species of anurans, including a wide taxonomic range as well as a number of congeneric species.

Limited left-right cell migration across the midline of the gastrulating avian embryo.

During avian development the earliest phase in which the avian embryo expresses axial features of a left-right axis is at the primitive streak stage. Until the stage of definitive primitive streak (streak 4 H&H), the axis seems to possess morphological bilateral symmetry. Morphological asymmetry begins only during the next few hours of incubation, with development of overt morphological and molecular asymmetry within Hensen's node (stage 5 H&H).

The ability to initiate an axis in the avian blastula is concentrated mainly at a posterior site.

Cell interactions during early vertebrate development are crucial for embryonic mesoderm induction and axis initiation. In the avian embryo two unique layers of cells, the epiblast and the hypoblast, constitute the blastoderm before the primitive streak develops (stage XIII). It was suggested that cells of the hypoblast have the ability to induce competent cells in the epiblast to form the mesoderm and to initiate the embryonic axis.

High proliferation rate characterizes the site of axis formation in the avian blastula-stage embryo.

Localized zones of high cell proliferation have been thought to be important in determining several phases of axis formation at early stages of chick development. It was suggested that a developmental center, a center of cellular activity such as proliferation and movement, is located in the posterior half of the area pellucida in the unincubated chick blastoderm. In the work reported here, we have used the bromodeoxyuridine (BrdU) incorporation procedure followed by immunoperoxidase detection to assess the rate of cell proliferation at particular sub-regions of pregastrulating chick blastoderms (stages X-XIII).

Axis formation in half blastoderms of the chick: Stage at separation and the relative positions of fused halves influence axis development.

The blastoderm of the avian embryo acts during the early stages of development as an integrative system programmed to form a single embryonic axis. Isolated parts of the blastoderm are known to each form an axis, owing to the system's properties. In the work reported here, the regulative capability of the right and left halves of chick blastoderms to form an embryonic axis was examined systematically at different stages.

The rotated hypoblast of the chicken embryo does not initiate an ectopic axis in the epiblast.

In the amniotes, two unique layers of cells, the epiblast and the hypoblast, constitute the embryo at the blastula stage. All the tissues of the adult will derive from the epiblast, whereas hypoblast cells will form extraembryonic yolk sac endoderm. During gastrulation, the endoderm and the mesoderm of the embryo arise from the primitive streak, which is an epiblast structure through which cells enter the interior.

Genomic organization of a gene encoding the spicule matrix protein SM30 in the sea urchin Strongylocentrotus purpuratus.

We report the characterization of a genomic clone containing portions of two tandemly arranged genes that encode a spicule matrix protein, SM30, of the sea urchin Strongylocentrotus purpuratus. The isolated 18.4-kilo-base genomic clone contains the complete genomic sequence of one SM30 gene, designated SM30-alpha, and a portion of another SM30 gene, designated SM30-beta.

The potency of the first two cleavage cells in echinoderm development: the experiments of Driesch revisited.

A hundred years have passed since Driesch performed the classical experiment of separating sea urchin blastomeres from a two-cell-stage embryo, finding that each developed into a complete though smaller larva. The earlier studies of Roux using frogs showed that inactivating one of the two blastomeres by a heated needle resulted, during the early stages of development, in the formation of a half embryo. In this type of experiment, in which the two blastomeres are not separated, the live blastomere continues its development while it is still attached to an inactivated neighbour.

Interactions of different vegetal cells with mesomeres during early stages of sea urchin development.

It has been known from results obtained in the classical experiments on sea urchin embryos that cell isolation and transplantation showed extensive interactions between the early blastomeres and/or their descendants. In the experiments reported here a systematic reexamination of recombination of mesomeres and their progeny (which come from the animal hemisphere) with various vegetal cells derived from blastomeres of the 32- and 64-cell stage was carried out. Cells were marked with lineage tracers to follow which cell gave rise to what structures, and newly available molecular markers have been used to analyze different structures characteristic of regional differentiation.

The influence of cell interactions and tissue mass on differentiation of sea urchin mesomeres.

The developmental potential of different blastomeres of the sea urchin embryo was re-examined. We have employed a new method to isolate substantial numbers of different kinds of blastomeres from 16-cell-stage embryos, and we have used newly available molecular markers to analyze possible vegetal differentiation. We have found that, while isolated mesomere pairs behave according to the classical expectations and develop into ectodermal vesicles, there is a clear effect of reaggregating two or more mesomere pairs.

The chick's marginal zone and primitive streak formation. II. Quantification of the marginal zone's potencies--temporal and spatial aspects.

When a posterior fragment of the chick's marginal zone (PM) was exchanged with equal sized lateral marginal zone fragment (LM), of the same blastoderm, its capacity to initiate an ectopic primitive streak (PS) was found to be both size and stage dependent. Good correlation was demonstrated between the areas of PM fragments and the number of cells they contained. In stage X blastoderms, PM fragments containing less than 1200 cells were incapable of initiating an ectopic PS.

The chick's marginal zone and primitive streak formation. I. Coordinative effect of induction and inhibition.

A variety of transplantation experiments of posterior and lateral marginal zone fragments at stages X, XI, and XII have been carried out in order to test their relevance to the development of a primitive streak (PS). At the stages studied the marginal zone (MZ) was shown to behave as a ring-like gradient field, the maximal value of which was at the posterior end (PM). The PM was found to be capable at the same time of promoting the development of a PS and of suppressing the inductive potential of other regions of the MZ.

The embryo-forming potency of the posterior marginal zone in stages X through XII of the chick.

At stage X a small posterior marginal zone (PM) fragment, when transplanted into similar-size hole in the lateral marginal zone, can initiate the development of an ectopic axis. The laterally transplanted PM, inhibits the regeneration of an axis at the original posterior side from the lateral section of the marginal zone (LM) inserted to replace it. At stage XI both the axis-forming and inhibitory capacities of the PM fragment become weaker and an axis-forming capacity starts to build up anterior to the PM, resulting in the formation of two primitive streaks at 90 degrees to each other.

Developmental potencies of area opaca and marginal zone areas of early chick blastoderms.

The marginal zone, in pregastrulating chick blastoderms, has been defined as the intermediate ring between the epiblast proper and the most external region, the area opaca (Spratt & Haas, 1960). Azar & Eyal-Giladi (1979) have shown that the marginal zone of a stage XIII blastoderm has the capacity of regenerating an inductive layer which when in contact with a competent stage XIII epiblast can cause the formation of axial structures. The present work demonstrates that at stage X (Eyal-Giladi & Kochav, 1976) the marginal zone can both induce and respond to its own inductive stimulus by forming an embryonic axis.