Belief vs. scientific observation: The curious story of the precardiac mesoderm

Abstract

The retinas of three species of shrews (Sorex araneus, S. coronatus, and S. minutus) were analyzed. Two kinds of photoreceptors were identified according to (among other characteristics) the traits of the mitochondria of their inner segments. The rod inner segments contained several round or oval mitochondria distributed longitudinally inside the ellipsoid. The cone inner segment showed a few mitochondria, which we classified as megamitochondria (maximum length = 4.22 μm in S. araneus, 5.68 μm in S. coronatus, and 2.42 μm in S. minutus). An analysis of serial thin sections in S. coronatus showed that these large organelles occurred in the apical and central portions of the ellipsoid. In the peripheral and basal regions of the ellipsoid, megamitochondria were frequently accompanied by smaller mitochondria. The giant mitochondria were irregular in form and densely packed, and a reduced cytosol was observed between each mitochondria. In general, they exhibited an electron‐dense matrix and a complex system of cristae, which varied in length and array. In mammalian retina, megamitochondria have only been described in the ellipsoid of the tree shrews Tupaia glis and T. belangeri, two diurnal Scandentia with a rich‐cone retina. In general terms, Sorex megamitochondria are morphologically very similar to those reported for Tupaia, especially in their arrangement in the cone ellipsoid. However, they differ in the orientation of the cristae. We propose that the ellipsoid of Sorex may serve two functions: as a source of energy for receptor cells, and as a device for improving the cone outer segment optics. Anat Rec Part A 272A:484–490, 2003. © 2003 Wiley‐Liss, Inc.

The Prechordal Plate, the Rostral End of the Notochord and Nearby Median Features in Staged Human Embryos

“…Although this rostral marginal crescent is thought to be 'a feature common to many mammals' , further studies, particularly in the human, need to be undertaken. Further possible characteristics of rostral crescents are suggested by the Nkx2.5-positive domain in the chick embryo at HH5 [Eisenberg, 2002, fig. 1A].…”

Stem cells and the formation of the myocardium in the vertebrate embryo

“…When Nkx2.5 expression in the chick embryo was first described, it was suggested that this molecule is first exhibited by precardiac cells as early as HH stage 5. However, it now appears that the Nkx2.5-positive cells within the HH stage 5 embryo are not cells that will contribute to the heart, as the recent cardiac fate map generated by Redkar et al (2001) clearly indicates that the Nkx2.5 domain does not coincide with the heart-forming fields until HH stage 6 (Eisenberg, 2002). Instead, it is likely that the Nkx2.5-expressing cells at HH stage 5 are either endoderm or ectoderm, which would be consistent with the demonstration that Nkx2.5 is expressed in all three germ layers within an HH stage 6 embryo (Schultheiss et al, 1995).…”

“…Since markers for these stem cells have not been described, the only way to confront this objection would be to plate individual cells at clonal density and look at their differentiated progeny. To study the properties of myocardial progenitors in the embryo, the clonal stem cell line QCE6 was derived from precardiac mesoderm of HH stage 4 quail embryos (Eisenberg andBader, 1995, 1996). As may be expected considering their precardiac mesoderm origin, QCE6 cells are able to give rise to cardiomyocytes (Fig.…”

A molecular signature for the “master” heart cell

“…Moreover, in chicken, isl-1 expression in the primary heart field precedes the expression of Nkx2.5. (23,24) Interestingly, the D4.25 bry þ /Flk-1 þ cell population is also positive for MesP1, which is expressed in the nascent heart precursor cells in the embryo and has also been suggested to be involved in the specification of endothelial lineages. (21,22) Probably due to technical limitations, the issue of when the two heart field lineages segregate is difficult to resolve in the mouse embryo.…”