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Biomedical subjects

A G Jacobson

Publications and source records attributed to A G Jacobson.

At least 19 recordsLinked to original sources
PubMed1990

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.

Animals↗
PubMed1990

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.

Ambystoma↗
PubMed1990

The role of the dorsal lip in the induction of heart mesoderm in Xenopus laevis.

We have examined the tissue interactions responsible for the expression of heart-forming potency during gastrulation. By comparing the specification of different regions of the marginal zone, we show that heart-forming potency is expressed only in explants containing both the dorsal lip of the blastopore and deep mesoderm between 30 degrees and 45 degrees lateral to the dorsal midline. Embryos from which both of these 30 degrees-45 degrees dorsolateral regions have been removed undergo heart formation in two thirds of cases, as long as the dorsal lip is left intact. If the dorsal lip is removed along with the 30 degrees-45 degrees regions, heart formation does not occur. These results indicate that the dorsolateral deep mesoderm must interact with the dorsal lip in order to express heart-forming potency. Transplantation of the dorsal lip into the ventral marginal zone of host embryos results in the formation of a secondary axis; in over half of cases, this secondary axis includes a heart derived from the host mesoderm. These findings suggest that the establishment of heart mesoderm is initiated by a dorsalizing signal from the dorsal lip of the blastopore.

Animals↗
PubMed1989

Neural fold formation at newly created boundaries between neural plate and epidermis in the axolotl.

According to a recent model, the cortical tractor model, neural fold and neural crest formation occurs at the boundary between neural plate and epidermis because random cell movements become organized at this site. If this is correct, then a fold should form at any boundary between epidermis and neural plate. To test that proposition, we created new boundaries in axolotl embryos by juxtaposing pieces of neural plate and epidermis that would not normally participate in fold formation. These boundaries were examined superficially and histologically for the presence of folds, permitting the following observations. Folds form at each newly created boundary, and as many folds form as there are boundaries. When two folds meet they fuse into a hollow "tube" of neural tissue covered by epidermis. Sections reveal that these ectopic folds and "tubes" are morphologically similar to their natural counterparts. Transplanting neural plate into epidermis produces nodules of neural tissue with central lumens and peripheral nerve fibers, and transplanting epidermis into neural plate causes the neural tube and the dorsal fin to bifurcate in the region of the graft. Tissue transplanted homotypically as a control integrates into the host tissue without forming folds. When tissue from a pigmented embryo is transplanted into an albino host, the presence of pigment allows the donor cells to be distinguished from those of the host. Mesenchymal cells and melanocytes originating from neural plate transplants indicate that neural crest cells form at these new boundaries. Thus, any boundary between neural plate and epidermis denotes the site of a neural fold, and the behavior of cells at this boundary appears to help fold the epithelium. Since folds can form in ectopic locations on an embryo, local interactions rather than classical neural induction appear to be responsible for the formation of neural folds and neural crest.

Ambystoma mexicanum↗
PubMed1989

The specification of heart mesoderm occurs during gastrulation in Xenopus laevis.

The establishment of heart mesoderm during Xenopus development has been examined using an assay for heart differentiation in explants and explant combinations in culture. Previous studies using urodele embryos have shown that the heart mesoderm is induced by the prospective pharyngeal endoderm during neurula and postneurula stages. In this study, we find that the specification of heart mesoderm must begin well before the end of gastrulation in Xenopus embryos. Explants of prospective heart mesoderm isolated from mid- or late neurula stages were capable of heart formation in nearly 100% of cases, indicating that the specification of heart mesoderm is complete by midneurula stages. Moreover, inclusion of pharyngeal endoderm had no statistically significant effect upon either the frequency of heart formation or the timing of the initiation of heartbeat in explants of prospective heart mesoderm isolated after the end of gastrulation. When the superficial pharyngeal endoderm was removed at the beginning of gastrulation, experimental embryos formed hearts, as did explants of prospective heart mesoderm from such embryos. These results indicate that the inductive interactions responsible for the establishment of heart mesoderm occur prior to the end of gastrulation and do not require the participation of the superficial pharyngeal endoderm.

Animals↗
PubMed1988

A survey by scanning electron microscopy of the extracellular matrix and endothelial components of the primordial chick heart.

Scanning electron microscopy was used to study the morphogenesis of the primitive embryonic chick heart (stage 5 late primitive streak through stage 9+). Components of the developing heart (myocardium, endocardial endothelium, and extracellular matrix) were viewed from the ventral surface after removal of the endoderm. The myocardial component of the heart can first be seen by light microscopy at stage 5 as two darker oval-shaped areas located on either side of the embryonic axis in the cranial region of the embryo. Scanning electron microscopy demonstrates that as early as stage 6 an area of extracellular matrix, enriched in comparison to more lateral and medial splanchnic mesoderm, can be identified ventral to the myocardial primordium. As heart formation progressed we observed primordial endothelial elements in the splanchnic mesoderm lateral to the myocardial primordia. By late stage 7 these lateral primordial elements had anastomosed into small, loose plexuses. This process of anastomosis progressed rapidly, and by stage 8 the entire cranial surface of the myocardial primordium was covered with vascular plexuses. By late stage 8 the progressive fusion of these plexuses resulted in the formation of large multiple tubular elements near the midline. More medially the fusion of tubular elements resulted in a continuous endothelial sheet at the midline.

Animals↗
PubMed1988

Features of embryonic induction.

The patterned distribution of different organs in the amphibian embryo begins with the establishment of two domains, the animal and vegetal regions, that differ in developmental potency. Differences amplify as inductive interactions occur across boundaries between areas of different potency. Embryonic induction establishes a temporally and spatially dynamic area of developmental potency - a morphogenetic field. The final arrangement and differentiation of cell types within the field emerge from subsequent interactions occurring primarily within the field. These principles are illustrated in a review of the induction of the lens and the heart. Recent studies show that the induction of the lens of the eye and the induction of the heart begin early in development. Most of lens inductions occurs before the formation of the optic vesicle, and the heart appears to be part of a complex of dorsal structures whose formation is dependent upon the establishment of the dorsoventral axis. Suppressive as well as inductive tissue interactions occur during the determination of both of these organs, affecting their position and time of appearance. The complex processes of induction defined by the past nine decades of experimental work present many challenging questions that can now be addressed, especially in terms of the molecular events, cellular behaviour and regulatory physiology of the responding tissue.

Animals↗
PubMed1988

Somitomeres: mesodermal segments of vertebrate embryos.

Well before the somites form, the paraxial mesoderm of vertebrate embryos is segmented into somitomeres. When newly formed, somitomeres are patterned arrays of mesenchymal cells, arranged into squat, bilaminar discs. The dorsal and ventral faces of these discs are composed of concentric rings of cells. Somitomeres are formed along the length of the embryo during gastrulation, and in the segmental plate and tail bud at later stages. They form in strict cranial to caudal order. They appear in bilateral pairs, just lateral to Hensen's node in the chick embryo. When the nervous system begins to form, the brain parts and neuromeres are in a consistent relationship to the somitomeres. Somitomeres first appear in the head, and the cranial somitomeres do not become somites, but disperse to contribute to the head the same cell types contributed by somites in the trunk region. In the trunk and tail, somitomeres gradually condense and epithelialize to become somites. Models of vertebrate segmentation must now take into account the early presence of these new morphological units, the somitomeres. Somitomeres were discovered in the head of the chick embryo (Meier, 1979), with the use of stereo scanning electron microscopy. The old question of whether the heads of the craniates are segmented is now settled, at least for the paraxial mesoderm. Somitomeres have now been identified in the embryos of a chick, quail, mouse, snapping turtle, newt, anuran (Xenopus) and a teleost (the medaka). In all forms studied, the first pair of somitomeres abut the prosencephalon, but caudal to that, for each tandem pair of somitomeres in the amniote and teleost, there is but one somitomere in the amphibia. The mesodermal segments of the shark embryo are arranged like those of the amphibia.

Animals↗
PubMed1987

Mesodermal metamerism in the teleost, Oryzias latipes (the medaka).

Previous studies of the metameric pattern in mesodermal tissues of chick, mouse, turtle, and amphibian embryos have indicated that segmental characteristics exist along the entire length of the embryo. This paper describes this phenomenon in a fish embryo, for some differences in the cranial segmental plan exist between the anamniote and the amniote embryos hitherto studied. Embryos of the cyprinodont, Oryzias latipes, were fixed at various times, the examined by means of stereo scanning electron microscopy. As in other vertebrate embryos, the first indication of mesodermal metamerism in this fish embryo is the occurrence of somitomeres, which are orderly, tandemly arranged units of uncondensed mesenchymal cells in the paraxial mesoderm. As many as ten somitomeres can be observed caudal to the last formed somite to the elongating tail region. In addition, 7 somitomeres are present rostral to the first definitive somite, which is segment number eight. As in other vertebrate embryos examined, somitomeres in Oryzias embryos are circular, bilaminar arrays of paraxial mesoderm that form before any indications of segmentation can be seen with the light microscope. In the trunk region these mesodermal units condense to give rise to definitive somites, but in the head they eventually disperse. Despite a fundamentally different mode of gastrulation and a relatively small number of cells in the newly formed somitomeres, cranial segmentation in Oryzias embryos was found to be more similar in number to the metameric pattern of the embryos of the bird, reptile, and mammal than to the situation found in the two amphibians studied thus far.

Animals↗
PubMed1986

Neurulation and the cortical tractor model for epithelial folding.

We present here a new model for epithelial morphogenesis, which we call the 'cortical tractor model'. This model assumes that the motile activities of epithelial cells are similar to those of mesenchymal cells, with the added constraint that the cells in an epithelial sheet remain attached at their apical circumference. In particular, we assert that there is a time-averaged motion of cortical cytoplasm which flows from the basal and lateral surfaces to the apical region. This cortical flow carries with it membrane and adhesive structures that are inserted basally and resorbed apically. Thus the apical seal that characterizes epithelial sheets is a dynamic structure: it is continuously created by the cortical flow which piles up components near where they are recycled in the apical region. By use of mechanical analyses and computer simulations we demonstrate that the cortical tractor motion can reproduce a variety of epithelial motions, including columnarization (placode formation), invagination and rolling. It also provides a mechanism for driving active cell rearrangements within an epithelial sheet, while maintaining the integrity of the apical seal. Active repacking of epithelial cells appears to drive a number of morphogenetic processes. Neurulation in amphibians provides an example of a process in which all four of the above morphogenetic movements appear to play a role. Here we reexamine the process of neurulation in amphibians in light of the cortical tractor model, and find that it provides an integrated view of this important morphogenetic process.

Amphibians↗
PubMed1976

Changes in the shape of the developing vertebrate nervous system analyzed experimentally, mathematically and by computer simulation.

Two forces are necessary and sufficient to produce the transformation of the newt neural plate from a hemispheric sheet of cells one cell thick to a keyhole shape. These forces are: (1) a regionally programmed shrinkage of the surface of the neural plate (accomplished by contraction of the apical surfaces of the neural plate cells and elongation of these cells perpendicular to the plate); and (2) displacement of the whole sheet caused by elongation of either the notochord or the overlying neural plate cells in the antero-posterior direction. A computer simulation and mathematical analysis ("morphodynamics"), together with experiments and observations on embryos, were used to deduce the morphogenesis of the neural plate from these forces.

Animals↗
PubMed1976

Nature and origin of patterns of changes in cell shape in embryos.

Spatial patterns of the future elongation of cells exist in the early embryo. In the newt, such a pattern of changes of cell shape contributes to the formation of the neural plate. Regardless of where neural plate cells are transplanted, they change shape as prescribed by the pattern. Embryonic induction has a role in establishing this pattern.

Animals↗