On the long-term control of nuclear activity during cell differentiation

Signals of anticipation of reward and of mean reward rates in the human brain

Theoretical models of dopamine function stemming from reinforcement learning theory have emphasized the importance of prediction errors, which signal changes in the expectation of impending rewards. Much less is known about the effects of mean reward rates, which may be of motivational significance due to their role in computing the optimal effort put into exploiting reward opportunities. Here, we used a reinforcement learning model to design three functional neuroimaging studies and disentangle the effects of changes in reward expectations and mean reward rates, showing recruitment of specific regions in the brainstem regardless of prediction errors. While changes in reward expectations activated ventral striatal areas as in previous studies, mean reward rates preferentially modulated the substantia nigra/ventral tegmental area, deep layers of the superior colliculi, and a posterior pontomesencephalic region. These brainstem structures may work together to set motivation and attentional efforts levels according to perceived reward opportunities.

Attmepts to analyse the biochemical basis of regional differences in animal eggs

The microinjection of cell material or macromolecules into living cells is potentially useful for identifying cell components responsible for positional information. Experiments with purified globin mRNA injected into frog oocytes and eggs show that macromolecules can retain their activity when injected into living cells. Experiments with histones show that injected macromolecules can take up their normal intracellular location in living cells. Attempts have been made to identify the molecules involved in regulating DNA synthesis, ribosomal RNA synthesis, and early developmental events by fractionating egg cytoplasm, and by rescuing maternal effect mutants. The present state, and theoretical basis, of such experiments is discussed.

Chromosomal transplantation. The nuclear transplantation of colchicine-treated cells

Dissociated cells of middle-to-late blastulae were exposed to 0.1 mg colchicine/ml and achieved 92% metaphase arrest. These cells contained a haploid set of Bombina maxima (Anura:Discoglossidae) chromosomes. When transplanted into the enucleated eggs of B. orientalis, some donor cells stimulated development to the late blastula and middle gastrula stages. - Most (17/20) of the embryos resulting from chromosomal transplantation were nonmosaic aneuploids. A high percentage of recipient egg enucleation (93%), the ratio of long-to-short chromosomes, and the presence of species-specific marker chromosomes proved that chromosomes were transplanted from the donor cells. Therefore, metaphase chromosomes lacking intact spindle apparatuses were injected into and incorporated by amphibian eggs. These chromosomes were replicated in all cells of the resulting embryos. The aneuploidy of these embryos is explained by an inability of the recipient egg to locate and replicate many transplanted chromosomes (44%) before first cleavage.

Pattern of RNA synthesis during the regeneration by Owenia fusiformis (polychaete annelid)

The binding of [3H] Uridine labelled RNAs and [32P] RNAs to poly U filters using the Sheldon et al. technique (1972) shows an increase in the poly A+ RNA populations from total RNAs and cytoplasmic RNAs in the prereplicative phase of the regenerating process in Owenia fusiformis. This increase confirms that the first stage of this process consists in an activation of the transcription of the genetic material concerning mainly mRNAs synthesis activation.

The croonian lecture, 1976. Egg cytoplasm and gene control in development

This article is concerned with how a fertilized egg develops into a complete individual. In nearly all animal species (the main exceptions being mammals), ferti­lized eggs develop entirely independently of their mother. Commonly, eggs are surrounded by layers of materials such as membranes, jelly layers, or a shell, which isolate the egg and developing embryo from their environment. Embryos remain inside these coverings until they hatch as a mobile, free-living larva capable of feeding itself. Such a stage is usually reached only a few days or even hours after fertilization. During this time, development appears to take place without any chemical or physical instructions from the embryo’s environment. Development involves the formation of hundreds or thousands of cells from a single fertilized egg cell, as well as the conversion of yolk, a food-reserve, into the numerous different kinds of proteins which make up the cells of a complete larva. The question of how a superficially structureless egg converts itself, in a relatively short time, into a complex and highly organized structure has interested scientists since the time of Aristotle, 2000 years ago. However, specific concepts or explanations of early development were not well formulated until the eighteenth century. In 1779, for example, Bonnet made the explicit proposal that in each egg is a miniature embryo which itself contains an ovary with eggs, each of which themselves contain miniatures with ovaries, and eggs, and so on - the so-called doctrine of ‘emboitement’. Even Bonnet did not believe this doctrine in its strict sense, which would demand, as Bonnet’s own calculations showed, that Eve would have had 27 million embryos in her ovary. Throughout the nineteenth century there was extensive discussion of the relative merits of epigenesis and preformation. † In the later part of the nineteenth century, there arose the concept of neopreformation, according to which the preformed components of a fertilized egg were thought of as molecules and not morphological structures. For example, Lankester (1877) stated that: ‘Though the substance of a cell may appear homo­geneous under the most powerful microscope, excepting for the fine granular matter suspended in it, it is quite possible, indeed certain, that it may contain,already formed and individualised, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already estab­lished, and not visible.’ This concept of the existence of determinant molecules in eggs may be taken as the point of departure for the present article. A concise account of early theories of development is included in Davidson’s (1968) book, and a history of embryology has been published by Needham (1934) and Oppenheimer (1955).

Transcription of DNAs of known sequence after injection into eggs and oocytes of Xenopus laevis

1. When the synthetic polynucleotide, poly[d(A-T) - d(A-T)] is injected into the eggs and oocytes of Xenopus laevis, a stimulation of RNA synthesis results. Analysis of extracted RNA by high-voltage electrophoresis, shows that this stimulation of RNA synthesis is due to the production of poly[r(A-U)] transcripts. The rate of poly[r(A-U)] synthesis is calculated to be at least ten-fold greater in eggs than in oocytes. The amount of poly[r(A-U)] detectable in injected eggs has reached a maximum by 1.5 h after injection; in oocytes, however, poly[r(A-U)] continues to accumulate between the third and 16th hour after injection. The transcripts range in length from less than 80 nucleotides up to over 2000 nucleotides long. The co-injection of poly[d(A-T)] - d(A-T)] and alpha-amanitin into oocytes, has shown that the synthesis of poly[r(A-U)] is approximately 90% inhibited at a concentration of alpha-amanitin which has no effect on the capacity of the oocyte to synthesize ribosomal and 4-S RNA; thus the nucleoplasmic RNA polymerases IIa and/or IIb, are implicated as playing a major role in poly[r(A-U)] synthesis in oocytes. 2. When poly(dG) - poly(d-C), poly(dA), poly(dA) - poly(dT) and poly[d(I-C) - d(I-C)] are individually injected into eggs only poly[d(I-C) - d(I-C)] is transcribed as efficiently as poly[d(A-T) - d(A-T)]. 3. When calf thymus native or denatured DNA, polyoma, T2, T4 and phiX DNAs are individually injected into eggs only the injection of calf thymus native DNA causes a detectable stimulation of RNA synthesis. 4. The activities of crude preparations of egg and oocyte RNA polymerases are tested in vitro with different DNA templates. In contrast to the situation in vivo, it is found that poly[d(A-T1 - d(A-T)] is as efficiently transcribed in vitro by oocyte polymerase as by egg polymerase. Additionally poly[d(A-T) - d(A-T)] is transcribed ten-fold more efficiently in vitro than calf thymus native DNA. When poly(dA) - poly(dT), poly(dA), phiX, T2, and calf thymus denatured DNA are tested in vitro, only calf thymus denatured DNA is transcribed to a significant extent. The above results are discussed in relation to the known synthetic activities of Xenopus eggs and oocytes.