Pattern formation and developmental mechanisms

Characterizing embryonic gene expression patterns in the mouse using nonredundant sequence-based selection

This article investigates the expression patterns of 160 genes that are expressed during early mouse development. The cDNAs were isolated from 7.5 d postcoitum (dpc) endoderm, a region that comprises visceral endoderm (VE), definitive endoderm, and the node-tissues that are required for the initial steps of axial specification and tissue patterning in the mouse. To avoid examining the same gene more than once, and to exclude potentially ubiquitously expressed housekeeping genes, cDNA sequence was derived from 1978 clones of the Endoderm library. These yielded 1440 distinct cDNAs, of which 123 proved to be novel in the mouse. In situ hybridization analysis was carried out on 160 of the cDNAs, and of these, 29 (18%) proved to have restricted expression patterns.

Testing the model of optic chiasm formation in human beings

Recent data from animal studies allow us to understand how the optic commissure formed. However, their validity in humans has not yet been demonstrated. Clues from human teratological cases provide useful information to test the validity of animal models and suggest other morphogenetic mechanisms. Janiceps are non-viable, rare cases of co-joined embryos fused along the frontal plane, with two composite faces, half formed by each of the embryos. Their development after fusion is of interest to study the optic chiasm. The analysis of the optic nerves and of the skull by magnetic resonance imaging or direct inspection of three different cases of Janiceps showed a shared hypophyseal fossa that laterally could contain a compound optic commissure between the eyes of the same composite face (heterologues), but not between the eyes from the same individual (homologue). The current model of the optic commissure formation correctly predicts these findings in humans because the optic commissure are driven toward the midline by local factors and not by target regions. However, our data suggest that the overall geometry of the region is important for a successful decussation of the axons, and additional mechanisms are involved in midline differentiation.

Aberrations of cell cycle and cell death in normal development of the chick embryo growth plate

The epiphyses of femurs from 7.5-15 day chicken embryos were studied by electron microscopy. Several forms of aberrant cell cycles were present: (1) in the perichondrium, polyploid metaphases, segmentating large (giant) cells, and mitotic catastrophe (midway between mitosis and apoptosis) were observed; (2) in the resting zone, premature chromosome condensation was found; (3) in the proliferative zone, approximately 5% of divisions were aberrant, representing most often mitosis restitution from metaphase and more seldom from the anaphase; (4) in all layers, 'dark chondrocytes' representing a premortal form of hypersecretory cells undergoing often a-mitotic nuclear segmentation were present. Many of the aberrations of cell cycle were combined with cell death. These deviations omitting or adapting the cell cycle check-points represent evidently the normal epigenetic mechanisms of development and repair. At the same time, by origin and appearances they seem very close to the loss of the growth control displayed by malignant tumours. This connection is briefly analysed in view of some current concepts of carcinogenesis.