Time and development

Temporal phase relation of circadian neural oscillations as the basis of testicular maturation in mice: a test of a coincidence model

To study the underlying mechanism of gonadal growth during the attainment of puberty and to test a coincidence model, 7 experimental groups of 2-week-old male mice, Mus musculus, were administered the serotonin precursor, 5-hydroxytryptophan, followed by the dopamine precursor, L-dihydroxyphenylalanine at hourly intervals of 6, 7, 8, 9, 10, 11 and 12 h (5 mg/100 g body weight per day for 13 days). At 11 days post-treatment, a suppression of gonadal activity was seen in the 7-h mice and a maximum suppression in the 8-h mice, along with a significantly increased degree of gonadal development in the 12-h mice, as compared with the controls. In addition to its known regulation of seasonal gonadal cycles, the relative position of two circadian neural oscillations may also affect the rate of gonadal development during the attainment of puberty in mice. Moreover, the present study provides an experimental paradigm to test the coincidence model of circadian oscillations.

Circadian clock genes in reproductive tissues and the developing conceptus

The circadian (near 24-h) clock is involved in the temporal organisation of physiological and biochemical activities of many organisms, including humans. The clock functions through the rhythmic transcription and translation of several genes, forming an oscillatory feedback loop. Genetic analysis has shown that the circadian clock exists in both a central circadian pacemaker (i.e. the suprachiasmatic nucleus of the hypothalamus), as well as in most peripheral tissues. In particular, the circadian clockwork genes are expressed in all female and male reproductive tissues studied so far, as well as in the conceptus itself. The current data clearly show a robust rhythm in female reproductive tissues, but whether rhythmicity also exists in male reproductive tissues remains uncertain. Although the conceptus also expresses most of the canonical circadian genes, the rhythmicity of their expression is still under investigation. Published data indicate that environmental and genetic manipulations influence reproductive function and fecundity, suggesting an important role for the circadian clock in reproduction, and possibly early development.

Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins

The pregastrula stage mammalian conceptus consists of both embryonic and non-embryonic components. The latter forms the bulk of the tissues, provides nutrition for the developing embryo and also contributes developmental signals that influence events within the embryo itself. Understanding the origins and relationships between the embryonic and extraembryonic cell lineages is thus central to understanding development in mammals. Despite the apparent gross differences in early developmental strategy and form, the conceptuses of eutherian, marsupial and monotreme mammals show some remarkable similarities in the lineage allocation to trophoblast and hypoblast and in the emergent properties of the two cell types. We suggest that the gross differences can be explained by two relatively small evolutionary timing changes affecting cell adhesion patterns and the polarisation of developmentally significant information. These changes result in the conversion of a unilaminar blastocyst to a morula form composed of blastomeres with increased regulatory capacity.

Embryo-maternal interactive factors regulating the implantation process: implications in assisted reproductive

The embryo-maternal dialogue that starts very early in the life of the embryo is crucial for its own implantation. A disturbance in this dialogue is the major reason for which 60% of all pregnancies are terminated at the end of the periimplantation period. Many studies have been performed to improve the understanding of the molecular mechanisms involved in this dialogue. Both partners, the mother and the embryo, are equally involved in this exchange of signals. Much progress has been done in understanding the role of (i) chorionic gonadotrophin, (ii) growth factors and cytokines, and (iii) steroid hormones and other mediators, produced either by the embryo, by the mother, or by both, during the peri-implantation period. Today it is clear that their production dictates changes in the endometrium, in the immunological system of the mother and in embryo metabolism, that enable the embryo to implant. Knowledge of the molecular mechanisms involved in the embryo-maternal interaction are reviewed in this article.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

Tribute to Georgeanna and Howard Jones

This tribute is divided into three sections. The first of these is a description of six weeks of research involving Georgeanna and Howard Jones and me over a few weeks in Johns Hopkins in 1965. Initially, it describes details of preliminary work and then completion of the maturation programme of the mature human oocyte and the first serious attempts at IVF. Scientific discoveries in those exciting weeks contributed to the earliest beginnings of IVF. A second section on knowledge of the human preimplantation embryo today concentrates on the problem of poor-quality embryos and how to overcome it, and on new knowledge on the regulation of human embryonic development. Lastly, the ethics of assisted conception are debated in relation to early practice and to some international features of modern ethical adjudication. A brief conclusion describes some of the friendships initiated in Johns Hopkins and still intact today.

Aspects of the molecular regulation of early mammalian development

Many regulatory systems operate in the early mammalian embryo. This brief overview surveys several systems and their integration including polarities and axes, left-right differentiation, timers in cells, tissues and in gene expression, and imprinting. Polarities are essential from the very earliest stages of oocyte formation, and maintain their significance until blastocyst stages and beyond. They determine cleavage axes and the distribution of maternal proteins in the oocyte, distinct distributions being identified at the animal pole especially. Left-right axes are no doubt expressed from the earliest embryonic stages, and perhaps even in determining slight differences in the axes of cleavage and of maternal protein distribution. Timers, equally fundamental, have been demonstrated to control many functions in oocytes and embryos. Many fundamental processes in early mammalian oocytes and embryos are closely timed. They are classified into circadian rhythms, hourglass timers, clocks regulating major aspects of development including transcription, longevity via telomere clocks and long-range systems. Imprinting and methylation, increasingly important in establishing stable phenotypes in early embryos, might develop abnormally under some circumstances including intracytoplasmic sperm injection and cloning. A general summary briefly describes some other aspects of regulation, especially chromosomal anomalies in human embryos.

Circadian clockwork genes are expressed in the reproductive tract and conceptus of the early pregnant mouse

Circadian genes are expressed in some peripheral tissues, but the expression status of the female reproductive tract and the conceptus over the preimplantation period is unknown. Oocytes, uterine, oviducal tissues and preimplantation conceptuses from days 1-4 of mouse pregnancy were analysed for transcript presence by reverse transcription polymerase chain reaction. Transcripts encoded by the seven known mammalian canonical circadian genes (Per1-3, Cry1-2, Bmal1 and Clock), plus the mammalian genetic homologue of the Drosophila canonical gene Timeless, were detected in the uteri and oviducts taken from mice on days 1-4 of pregnancy and in unfertilized oocytes. After fertilization, transcripts for Per1, Cry1, Bmal1, Clock and Tim have been detected unambiguously. Transcript levels for each of these five genes fall at the two-cell stage, but are restored rapidly for Per1, Cry1 and Bmal1, presumptively by zygotic gene expression. In contrast, transcripts for Clock and Tim recover more slowly. It is concluded that circadian genes are expressed, and may therefore have a role, during the early development of the mammal.