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

Brain expansion in the chick embryo initiated by experimentally produced occlusion of the spinal neurocoel

The brain expands in the early chick embryo from pressure generated by accumulation of cerebrospinal fluid (CSF) in a closed neural tube. The sealing of the neural tube occurs as the result of occlusion of the spinal neurocoel rostral to and before closure of the posterior neuropore. We have previously demonstrated the dependence of normal brain expansion upon intraluminal pressure. We had yet to demonstrate, however, that brain expansion actually depends upon natural occlusion of the spinal neurocoel. To demonstrate such dependence, we experimentally occluded the spinal neurocoels of embryos 5 hr younger than stage 11 embryos (in which occlusion of the neurocoel occurs naturally). The stage 10 chick embryos were cultured ex ovo and critically staged, and their spinal neurocoels were occluded using microcautery. All embryos were photographed immediately and at 5, 12, and 24 hr after cautery. Serial sections were made of selected embryos, in which the areas of both the brain and the head were measured. Wilcoxon-Mann-Whitney rank-sum nonparametric tests, Hodges-Lehmann estimators, bootstrapping techniques, and resampling randomization tests were used to determine whether the increases in the brain and head areas for the experimental embryos were significantly different from those of the control embryos during three distinct intervals of expansion: 0-5, 5-12, and 0-12 hr. From 0 to 5 hr, the brains of the precociously occluded embryos expanded significantly more than the brains of the non-occluded controls. From 5 to 12 hr, the brains of the embryos with naturally occluded neurocoels grew significantly larger than the brains of the embryos with precociously occluded neurocoels. At 12 hr, there appeared to be no difference in brain size for these two groups. We conclude that the data support the hypothesis that brain expansion is directly dependent upon occlusion of the spinal neurocoel.

Cooperative model of epithelial shaping and bending during avian neurulation: autonomous movements of the neural plate, autonomous movements of the epidermis, and interactions in the neural plate/epidermis transition zone

Morphogenetic movements during neurulation cause a tissue to change shape within the plane of the epithelium (e.g., conversion of the oval neural plate into the narrow spinal plate and the wide brain plate), cause bending out of the plane of the epithelium (e.g., raise the neural folds and curl the neural plate into a tube), or contribute to both phenomena. In this study, pieces that contain neural plate alone, epidermis alone, or both tissues (with or without underlying tissues) are cut from chick embryos and allowed to develop for up to 24 hr. Examination of histological sections through such isolates allows analysis of the formation of neural folds. When the neural plate/epidermis transition zone is disrupted, neural folds do not form. Conversely, when the transition zone remains intact, neural folds form. Neural folds form even when most of the medial neural plate and lateral epidermis has been removed, leaving only the isolated transition zone. These data indicate that the transition zone is both necessary and sufficient for the formation of neural folds. The transition zone may play a number of roles in epithelial bending including organizing, focussing, and redirecting movements that are autonomous to the neural plate or epidermis. Time-lapse video recording, and sequential photographs allowed the documentation of such movements. Neural plate isolates exhibit autonomous rostrocaudal lengthening and mediolateral narrowing. Isolated strips of epidermis exhibit autonomous movements which, unlike wound-healing movements, are unidirectional (medial), and region-specific (beginning and reaching their greatest extent in the cranial region). Isolated pieces of neural plate or epidermis remain flat instead of bending, providing further evidence that the transition zone is necessary for the formation of neural folds.

Computer simulation of organogenesis: an approach to the analysis of shape changes in epithelial organs

Embryonic development of epithelial organ primordia often involves changes in several parameters, such as cell height, cell width, cell volume, amount of extracellular space, and cell number. Since these changes often occur simultaneously, it becomes difficult to "separate out" the role that each plays in the developmental process. A computer program has been written that allows the shape of epithelial organs to be reproduced based upon measurements of the primordium. A developmental sequence can be simulated by changing the dimensions of the primordium based upon either measurements of the developmental stages or theoretical projections of changes. The primordium is divided into blocks representing groups of cells, based upon characteristics of the different cell groups. The program allows differences in cell height and circular and spiral curvatures of the primordium to be simulated. Analysis of the optic primordium using this method has allowed recognition of several regional changes during optic cup formation. These are sequential constriction of cell apices at the margin of the optic cup, expansion of the apical surface toward the center of the retinal disc, and spreading of the future pigmented layer. Simulation of other organs permits regions of morphogenetic activity to be identified.

The early development of the nervous system in staged insectivore and primate embryos

The early development of the nervous system was studied in stage embryos of hemicentetes semispinosus, Microcebus murinus, Alouatta seniculus, Cebus appella, Cebus albifrons, macaca mulatta, and Homo sapiens. The specimens were assigned to Carnegie stages 11-13. Serial transverse sections were examined and graphic reconstructions were prepared. The early development of the neural tube is basically similar in all the species investigated but differences in detail are noticeable. The mesencephalic flexure serves in all cases as a landmark for malpighi's tripartite subdivision of the brain. The nonhuman embryos seem to show a little more variation than the human in the closure of the neuropores in relation to somitic count. With the exception of the later-appearing terminal-vomeronasal component, all major portions of the neural crest as classified by O'Rahilly ('65) are represented in both the nonhuman and the human embryos studied. No crest is present at the level of rhombomere 1, nor at rhombomere 3 except in the platyrrhines and some human embryos, nor at rhombomere 5 except in certain human specimens. An indication of the division of the trigeminal ganglion into its primary divisions is rare at stage 11 (C. apella), may be visible at stage 12 (Alouatta, macaca, Homo), and is definite (in Homo) at stage 13. Ganglionic contributions from head ectoderm (epipharyngeal placodes), as previously described in the human and some other vertebrate embryos, were sought and found in Cebus apella. In both nonhuman and human, a tendency is noted whereby the rostral limit of the occipitospinal crest, high at stage 11, seems to descend relatively at stage 12, and ascend again at stage 13 (at least in the human) to become associated with the appearance of the accessory and hypoglossal nerves. In general, the motor components of the nerves are identifiable before the sensory elements, and, in the present study, nerve fibers were first observed in the human at stage 13 in some of the cranial nerves and in the ventral roots of the spinal nerves.