Muscle actin genes are the earliest yet described to show cell type-specific activation in amphibian embryos. Gene-specific probes show that alpha-skeletal and alpha-cardiac actin genes start to be transcribed simultaneously at the end of gastrulation, but only in those regions of the mesoderm that subsequently form embryonic muscle. Their expression provides a molecular marker for early cell determination.

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Fertilized Xenopus eggs have been ligated with a hair loop into separate fragments before the first cleavage. The plane of the ligation was varied in relation to the animal-vegetal and dorso-ventral axes. The fragments that contained a nucleus were cultured for 24 hr until controls reached the neurula stage; they were then analyzed by S1 nuclease protection for their content of muscle-specific actin mRNA, using a gene-specific probe. We find that all egg components required for the eventual activation of these actin genes are localized, already at the 1-cell stage, in a region below the equator, and mostly on the dorsal (grey crescent) side. This material subsequently occupies the equivalent position in 8-cell and 32-cell embryos. We interpret our results, in combination with the previous work of others, to mean that mesoderm (including muscle) formation in Amphibia depends both on cytoplasmic substances already localized in the egg as well as on inductive cell interactions during cleavage.An important question in embryology is whether a fertilized but uncleaved egg contains substances already localized in a way that is related to future cell differentiation, or whether such substances first appear and become localized at a later multicellular stage. The question is significant because the localization of materials in a single cell, the egg, is most likely to depend on large molecules or complexes, whereas cell interactions may well involve small molecules capable of passing rapidly from cell to cell.There are several examples in invertebrate animals in which one region of egg cytoplasm seems to be related to a particular type of differentiation. The most celebrated cases are insect pole plasm (1) (related to egg and sperm formation) and the yellow cytoplasm in ascidian eggs (related to tail muscle formation) (reviewed in refs. 2 and 3). A more common situation is that in which a particular region of egg cytoplasm becomes localized within a certain cell lineage by asymmetric early cleavage divisions, as in the photocyteand cilia-forming cells of Ctenophores (4). In the vertebrates, the only definite example of either kind is the germ plasm of Amphibia (5), which appears to be functionally equivalent to the pole plasm of insects. Here we describe egg ligation experiments in Xenopus and the use of a gene-specific probe to investigate the localization of materials required for the activation of muscle-specific a-actin genes. Rather surprisingly, we find that these materials are already localized in a particular region of the uncleaved egg. MATERIALS AND METHODSEggs and Embryos. Eggs laid after hormone injection were artificially fertilized as described (6). Developmental stages are those of Nieuwkoop and Faber (7).Hair-Loop Ligations. Eggs were dejellied in saline containing 2% cysteine (pH 8.0), washed, and the vitelline membrane was removed manually with forceps. Eggs were ligated with a thin brunette hair-loop in the desired orientation in modified Barth saline (MBS) (8), between 30 and 60 min after fert...

Hydrocephalus was induced in 12-day-old rats by the cisternal infusion of concentrated kaolin suspension. At 19 days of age, a lesion, 100 or 150 microns in diameter, was made in the ependymal lining of the lateral ventricles. Animals were killed at intervals from 1 h to 20 days after the lesion was made. The damaged area was examined by scanning electron microscopy, light and transmission electron microscopy. Between 1 h and 48 h the hole was still open. Small round cells, identified as free subependymal cells, were associated with the edges of the hole from 1 h after the lesion was made. From 48 h, the lesion was completely covered with cell bodies and their processes and no hole was present. Signs of differentiation were seen in the free subependymal cells from 4 days, the cells becoming more electron lucent. By 15 days, three types of cell arrangement were seen within the damaged area: 1 clusters of small cells, with few processes, resembling subependymal cells; 2 small numbers of cells with flat cytoplasmic processes which formed the lining of the ventricular wall; 3 clusters of cells with long thin processes attached to the surface of the ventricular wall but not forming the ependymal lining. The results of this study suggest that, in the hydrocephalic brain, ependymal damage and the repair of a defect within the ventricular wall is initiated by subependymal cells.