The central dogma of female reproductive biology has long held that oogenesis ceases prior to birth in mammals. During the first half of the last century, there was much debate about whether this was the case or whether oogenesis continued in the postnatal ovary. A report in 1951 effectively put an end to this debate and laid the foundation for the dogma. A new paper by Johnson et al. (2004)1 resurrects the debate over whether postnatal oogenesis occurs (...) in the mammalian ovary. If confirmed, this would have tremendous impact on issues related to female fertility and reproductive longevity. BioEssays 26:829–832, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The ovarian tumor (otu) gene behaves as if it encodes a product (OGP) which is required during several early steps in the transformation of oogonia into functional oocytes. The ovarian phenotypes produced by various EMS‐induced mutations can be explained as graded responses by individual mutant germ cells to the different levels of functionally active OGP they themselves synthesize. In addition, genetic evidence suggests that otu also encodes a second product that is utilized late in oogenesis. Molecular studies of the (...) otugene demonstrate that it does transcribe at least two ovaryspecific RNAs. The possible function of the otu product in the cellular divisions that give rise to the oocyte and its sister nurse cells is discussed. (shrink)

Oogenesis being performed in the ovary shows two different kinds of nuclei in the nursing cells. The above mentioned nuclei are transferred and incorporated into the nuclei of developing eggs, which become sexually differentiated and showWolanski's methyl-green reaction. The sex determination is, therefore, cytologically progamic and genotypical. The spawned eggs in the jelly strings appear first of identical shape and are all coated from the very beginning with grainy bonellian pigment, but afterwards, being reared in free water cultures in (...) the absence of females, they differentiate into distinctly different male and female larvae. The numerical ratio of the sexually differentiated larvae is discussed. The true chromosomes in gamogenesis as well as in somatic cells have not been found. L'oogénèse qui a lieu dans l'ovaire montre deux sortes de noyaux dans les cellules nourricières. Les noyaux mentionnés ci-dessus sont transportés dans les noyaux des ovules en voie de développement qui se différencient sexuellement et montrent la réaction deWolanski au vert de méthyle. Le déterminisme du sexe est, par conséquent progamique et génotypique, du point de vue cytologique. Les ovules déposés en gelée sont d'abord de forme identique et sont tous recouverts, dès le début, du pigment granulé de boneliéne, mais, élevés par la suite, dans des cultures en eau courante, en l'absence des femelles, ils se différencient distinctement en larves mâles et femelles. La proportion numérique des larves différenciées sexuellement est discutée. Dans la gamétogénèse, comme dans les cellules somatiques, les vrais chromosomes restent introuvables. Während der Oogenese, die sich im Eierstock abspielt, zeigen sich zwei differente Kerntypen in den Nährzellen. Diese Kerne werden allmählich in die Kerne des sich entwickelnden Eies übertragen und dort inkorporiert. Das Ei wird dadurch geschlechtlich differenziert und zeigt dann dieWolanski's Methyl-grün Reaktion. Die sexuelle Determination ist somit progam und genotypic. Die in schnurartigen Gallerten abgelegten Eier sind am Beginn von gleicher Form und Grösse, so wie von Anfang an mit Körnchen des Pigmentes Bonellein bekleidet. In freien Wasserkulturen,ohne Weibchen, entwickeln sie sich in deutlich differenzierte männliche und weibliche Larven. Das numerische Verhältniss der geschlechtlich differenten Larven wird diskutiert. Echte Chromosomen sind weder bei der Gamogenese noch in somatischen Zellen gefunden worden. (shrink)

RNA localization is a powerful strategy used by cells to localize proteins to subcellular domains and to control protein synthesis regionally. In germ cells, RNA targeting has profound implications for development, setting up polarities in genetic information that drive cell fate during embryogenesis. The frog oocyte offers a useful system for studying the mechanism of RNA localization. Here, we discuss critically the process of RNA localization during frog oogenesis. Three major pathways have been identified that are temporally and spatially (...) separated in oogenesis. Each pathway uses a different mechanism to effect RNA localization. In some cases, localization elements within the 3' untranslated region have been identified and have provided unique insights into the localization process. This important field is still in its infancy, however, and much remains to be learned. BioEssays 21:546–557, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Ascidians have evolved alternate modes of development in which the conventional tadpole larva is remodeled or eliminated. Adultation, the precocious development of adult features in the larval head, is caused by superimposing the larval and adult differentiation programs. Caudalization, the addition of muscle cells to the larval tail, is caused by enhancing muscle induction or increasing the number of muscle cell divisions before terminal differentiation. Adultation and caudalization are correlated with increased egg size, suggesting dependence on maternal processes. Anural development, (...) the elimination of the larval stage, is caused by maternal and zygotic events resulting in abbreviation and deletion of larval developmental programs. An example of a maternal change in anural species is the modification of the egg cytoskeleton during oogenesis, whereas a zygotic change may involve altered cell interactions during embryogenesis. Interspecific hybridization experiments suggest that some aspects of anural development may be caused by loss‐of‐function mutations. The dissociation of developmental programs is a key process in changing the mode of development in ascidians. (shrink)

Platynereis dumerilii, a marine polychaetous annelid with indirect development, can be continuously bred in the laboratory. Here, we describe its spectacular reproduction and development and address a number of open research problems. Oogenesis is easily studied because the oocytes grow while floating in the coelom. Unlike the embryos of other model spiralians, the Platynereis embryo is transparent giving insight into the dynamic structures and processes inside the cells that accompany the prevailing anisotropic cleavages. Functional studies on cell specification and (...) differential gene expression in embryos, larvae, and later stages are underway. Lifelong proliferation of uniform trunk segments qualifies Platynereis as a model for the study of gene expression and of the functional circuitry of this process. Platynereis can also become a stepping stone in the comparison of segmentation between annelids and arthropods because it comes closer to the putative ancestral morphology and style of development than other model annelids. BioEssays 26:314–325, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Extraordinary extended lampbrush chromosomes with thousands of transcription loops are favorable objects in chromosome biology. Chromosomes become lampbrushy due to unusually high rate of transcription during oogenesis. However, until recently, the information on the spectrum of transcribed sequences as well as genomic context of individual chromomeres was mainly limited to tandemly repetitive elements. Here we briefly outline novel findings and future directions in lampbrush chromosome studies in the post‐genomic era. We emphasize the fruitfulness of combining genome‐wide approaches with microscopy (...) imaging techniques using lampbrush chromosomes as a remarkable model object. We believe that new data on the spectrum of sequences transcribed on the lateral loops of lampbrush chromosomes and their structural organization push the boundaries in the discussion of their biological role. Also see the video abstract here: https://youtu.be/zexoHfzX9rM. (shrink)

The polarisation of the embryonic anteroposterior (AP) axis requires the establishment of positional cues with spatial information, and often involves complex intercellular communications, cell adhesion and cell movement. Recent work on several fronts has begun to shed light on how the initial asymmetries are established and maintained. In this review, I discuss the polarisation of the AP axis during Drosophila oogenesis, focusing on the function of the Notch signalling pathway and its relationship to the activation of the epidermal growth (...) factor receptor. I make special reference to some aspects of Notch activity regulation during oogenesis that appear to depart from the canonical pathway. Finally, I hypothesise on possible similarities between these activities of Notch signalling during Drosophila oogenesis and vertebrate somitogenesis. BioEssays 25:781–791, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The mechanisms governing anterior‐posterior and dorsal‐ventral polarity in Drosophila melanogaster had previously been considered as independent processes. However, two papers(1,2) now reveal that both axes are initiated during oogenesis by the same pathway, and also clearly demonstrate that one is dependent on the other.

The creeping vole Microtus oregoni exhibits remarkably transformed sex chromosome biology, with complete chromosome drive/drag, X‐Y fusions, sex reversed X complements, biased X inactivation, and X chromosome degradation. Beginning with a selfish X chromosome, I propose a series of adaptations leading to this system, each compensating for deleterious consequences of the preceding adaptation: (1) YY embryonic inviability favored evolution of a selfish feminizing X chromosome; (2) the consequent Y chromosome transmission disadvantage favored X‐Y fusion (“XP”); (3) Xist‐based silencing of Y‐derived (...) XP genes favored a second X‐Y fusion (“XM”); (4) X chromosome dosage‐related costs in XPXM males favored the evolution of XM loss during spermatogenesis; (5) X chromosomal dosage‐related costs in XM0 females favored the evolution of XM drive during oogenesis; and (6) degradation of the non‐recombining XP favored the evolution of biased X chromosome inactivation. I discuss recurrent rodent sex chromosome transformation, and selfish genes as a constructive force in evolution. (shrink)

Although female and male gametes are presumably equivalent in their genetic contribution to embryos, they carry specific information, perhaps reversibly imprinted into the genomes during oogenesis and spermatogenesis, as to their maternal or paternal origin. This information is crucial for embryogenesis and, in the absence of at least one haploid set of chromosomes from each parent, embryos do not develop to term. The paternal genome is probably required for proliferation of extraembryonic tissues and the maternal genome for some stages (...) of embryogeneis. Furthermore, the effects of certain mutations are determined by the paternal or maternal origin of the inheritance. (shrink)

RNA localization is a powerful strategy used by cells to localize proteins to subcellular domains and to control protein synthesis regionally. In germ cells, RNA targeting has profound implications for development, setting up polarities in genetic information that drive cell fate during embryogenesis. The frog oocyte offers a useful system for studying the mechanism of RNA localization. Here, we discuss critically the process of RNA localization during frog oogenesis. Three major pathways have been identified that are temporally and spatially (...) separated in oogenesis. Each pathway uses a different mechanism to effect RNA localization. In some cases, localization elements within the 3' untranslated region have been identified and have provided unique insights into the localization process. This important field is still in its infancy, however, and much remains to be learned. BioEssays 21:546–557, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Mammalian sex chromosomes exhibit marked sexual dimorphism in behavior during gametogenesis. During oogenesis, the X chromosomes pair and participate in unrestricted recombination; both are transcriptionally active. However, during spermatogenesis the X and Y chromosomes experience spatial restriction of pairing and recombination, are transcriptionally inactive, and form a chromatin domain that is markedly different from that of the autosomes. Thus the male germ cell has to contend with the potential loss of X‐encoded gene products, and it appears that coping strategies (...) have evolved. Genetic control of sex‐chromosome inactivation during spermatogenesis does not involve pairing or the presence of the Y chromosome or an intact X chromosome, and may therefore be under exogenous control by the gonad. Sex‐chromosome reactivation during oogenesis and inactivation during spermatogenesis probably reflect specific meiotic events such as recombination. Understanding these phenomena may help explain other sex‐related differences in genetic recombination. (shrink)

Centrosomes are the main microtubule organizing centers in animal cells. In particular during embryogenesis, they ensure faithful spindle formation and proper cell divisions. As metazoan centrosomes are eliminated during oogenesis, they have to be reassembled upon fertilization. Most metazoans use the sperm centrioles as templates for new centrosome biogenesis while the egg's cytoplasm re-prepares all components for on-going centrosome duplication in rapidly dividing embryonic cells. We discuss our knowledge and the experimental challenges to analyze zygotic centrosome reformation, which requires (...) genetic experiments to enable scrutinizing respective male and female contributions. Male and female knockout animals and mRNA injection to mimic maternal expression of centrosomal proteins could point a way to the systematic molecular dissection of the process. The most recent data suggest that timely expression of centrosome components in oocytes is the key to zygotic centrosome reformation that uses male sperm as coordinators for de novo centrosome production. Centrosome reformation at fertilization is essential for metazoan development. It requires a complicated interplay of sperm centrioles and the cytoplasm of the oocyte, which has eliminated its centrioles during oogenesis. The male centrioles launch centrosome duplication using female centrosome proteins for the first metazoan cell divisions. (shrink)

The ovarian reserve defines female reproductive lifespan, which in humans spans decades. The ovarian reserve consists of oocytes residing in primordial follicles arrested in meiotic prophase I and is maintained independent of DNA replication and cell proliferation, thereby lacking stem cell‐based maintenance. Largely unknown is how cellular states of the ovarian reserve are established and maintained for decades. Our recent study revealed that a distinct chromatin state is established during ovarian reserve formation in mice, uncovering a novel window of epigenetic (...) programming in female germline development. We showed that an epigenetic regulator, Polycomb Repressive Complex 1 (PRC1), establishes a repressive chromatin state in perinatal mouse oocytes that is essential for prophase I‐arrested oocytes to form the ovarian reserve. Here we discuss the biological roles and mechanisms underlying epigenetic programming in ovarian reserve formation, highlighting current knowledge gaps and emerging research areas in female reproductive biology. (shrink)