The structure of the mammalian egg

Melatonin protects oogenesis from hypobaric hypoxia-induced fertility damage in mice

Environmental hypoxia adversely affects reproductive health in humans and animals at high altitudes. Therefore, how to alleviate the follicle development disorder caused by hypoxia exposure and to improve the competence of fertility in plateau non-habituated female animals are important problems to be solved urgently. In this study, a hypobaric hypoxic chamber was used for 4 weeks to simulate hypoxic conditions in female mice, and the effects of hypoxia on follicle development, proliferation and apoptosis of granulosa cells, reactive oxygen species (ROS) levels in MII oocyte and 2-cell rate were evaluated. At the same time, the alleviating effect of melatonin on hypoxic exposure-induced oogenesis damage was evaluated by feeding appropriate amounts of melatonin daily under hypoxia for 4 weeks. The results showed that hypoxia exposure significantly increased the proportion of antral follicles in the ovary, the number of proliferation and apoptosis granulosa cells in the follicle, and the level of ROS in MII oocytes, eventually led to the decline of oocyte quality. However, these defects were alleviated when melatonin was fed under hypoxia conditions. Together, these findings suggest that hypoxia exposure impaired follicular development and reduced oocyte quality, and that melatonin supplementation alleviated the fertility reduction induced by hypoxia exposure.

Intercellular communication in the cumulus-oocyte complex during folliculogenesis: A review

During folliculogenesis, the oocyte and surrounding cumulus cells form an ensemble called the cumulus-oocyte complex (COC). Due to their interdependence, research on the COC has been a hot issue in the past few decades. A growing body of literature has revealed that intercellular communication is critical in determining oocyte quality and ovulation. This review provides an update on the current knowledge of COC intercellular communication, morphology, and functions. Transzonal projections (TZPs) and gap junctions are the most described structures of the COC. They provide basic metabolic and nutrient support, and abundant molecules for signaling pathways and regulations. Oocyte-secreted factors (OSFs) such as growth differentiation factor 9 and bone morphogenetic protein 15 have been linked with follicular homeostasis, suggesting that the communications are bidirectional. Using advanced techniques, new evidence has highlighted the existence of other structures that participate in intercellular communication. Extracellular vesicles can carry transcripts and signaling molecules. Microvilli on the oocyte can induce the formation of TZPs and secrete OSFs. Cell membrane fusion between the oocyte and cumulus cells can lead to sharing of cytoplasm, in a way making the COC a true whole. These findings give us new insights into related reproductive diseases like polycystic ovary syndrome and primary ovarian insufficiency and how to improve the outcomes of assisted reproduction.

Transzonal projections: Essential structures mediating intercellular communication in the mammalian ovarian follicle

The development of germ cells relies on contact and communication with neighboring somatic cells that provide metabolic support and regulatory signals. In females, contact is achieved through thin cytoplasmic processes that project from follicle cells surrounding the oocyte, extend through an extracellular matrix (ECM) that lies between them, and reach its surface. In mammals, the ECM is termed the zona pellucida and the follicular cell processes are termed transzonal projections (TZPs). TZPs become detectable when the zona pellucida is laid down during early folliculogenesis and subsequently increase in number as oocyte growth progresses. They then rapidly disappear at the time of ovulation, permanently breaking germ-soma contact. Here we review the life cycle and functions of the TZPs. We begin with an overview of the morphology and cytoskeletal structure of TZPs, in the context of actin- and tubulin-based cytoplasmic processes in other cell types. Next, we review the roles played by TZPs in mediating progression through successive stages of oocyte development. We then discuss two mechanisms that may generate TZPs-stretching at pre-existing points of granulosa cell-oocyte contact and elaboration of new processes that push through the zona pellucida-as well as gene products implicated in their formation or function. Finally, we describe the signaling pathways that cause TZPs to be retracted in response to signals that also trigger meiotic maturation and ovulation of the oocyte. The principles and mechanisms that govern TZP behavior may be relevant to understanding communication between physically separated cells in other physiological contexts.

Control of Mammalian Oocyte Development by Interactions with the Maternal Follicular Environment

Development of animal germ cells depends critically on continuous contact and communication with the somatic compartment of the gonad. In females, each oocyte is enclosed within a follicle, whose somatic cells supply nutrients that sustain basal metabolic activity of the oocyte and send signals that regulate its differentiation. This maternal microenvironment thus plays an indispensable role in ensuring the production of fully differentiated oocytes that can give rise to healthy embryos. The granulosa cells send signals, likely membrane-associated Kit ligand, which trigger oocytes within resting-stage primordial follicles to initiate growth. During growth, the granulosa cells feed amino acids, nucleotides, and glycolytic substrates to the oocyte. These factors are necessary for the oocyte to complete its growth and are delivered via gap junctions that couple the granulosa cells to the oocyte. In a complementary manner, growing oocytes also release growth factors, notably growth-differentiation factor 9 and bone morphogenetic protein 15, which are necessary for proper differentiation of the granulosa cells and for these cells to support oocyte growth. During the late stages of oocyte growth, cyclic GMP that is synthesized by the granulosa cells and diffuses into the oocyte is required to prevent its precocious entry into meiotic maturation. Finally, at the early stages of maturation, granulosa cell signals promote the synthesis of a subset of proteins within the oocyte that enhance their ability to develop as embryos. Thus, the maternal legacy of the follicular microenvironment is witnessed by the fertilization of the ovulated oocyte and subsequent birth of healthy offspring.

History, origin, and function of transzonal projections: the bridges of communication between the oocyte and its environment

Development and differentiation of a functional oocyte that following fertilization is able to give rise to a new individual requires continuous physical contact with the supporting somatic cells of the ovarian follicle. As the oocyte is surrounded by a thick extracellular coat, termed the zona pellucida, this essential contact is mediated through thin cytoplasmic filaments known as transzonal projections (TZPs) that project from the somatic granulosa cells adjacent to the oocyte and penetrate through the zona pellucida to reach the oocyte. Gap junctions assembled where the tips of the TZPs contact the oocyte plasma membrane, and other contact-dependent signaling may also occur at these sites. Here, I describe early studies of TZPs, which were first identified in the late 19th century, discuss their similarities with classical filopodia, review their structure and function, and compare two models that could account for their origin. Possible priorities and directions for future studies close this contribution.

Mammalian Oocytes Locally Remodel Follicular Architecture to Provide the Foundation for Germline-Soma Communication

Germ cells develop in a microenvironment created by the somatic cells of the gonad [1-3]. Although in males, the germ and somatic support cells lie in direct contact, in females, a thick extracellular coat surrounds the oocyte, physically separating it from the somatic follicle cells [4]. To bypass this barrier to communication, narrow cytoplasmic extensions of the follicle cells traverse the extracellular coat to reach the oocyte plasma membrane [5-9]. These delicate structures provide the sole platform for the contact-mediated communication between the oocyte and its follicular environment that is indispensable for production of a fertilizable egg [8, 10-15]. Identifying the mechanisms underlying their formation should uncover conserved regulators of fertility. We show here in mice that these structures, termed transzonal projections (TZPs), are specialized filopodia whose number amplifies enormously as oocytes grow, enabling increased germ-soma communication. By creating chimeric complexes of genetically tagged oocytes and follicle cells, we demonstrate that follicle cells elaborate new TZPs that push through the extracellular coat to reach the oocyte surface. We further show that growth-differentiation factor 9, produced by the oocyte, drives the formation of new TZPs, uncovering a key yet unanticipated role for the germ cell in building these essential bridges of communication. Moreover, TZP number and germline-soma communication are strikingly reduced in reproductively aged females. Thus, the growing oocyte locally remodels follicular architecture to ensure that its developmental needs are met, and an inability of somatic follicle cells to respond appropriately to oocyte-derived cues may contribute to human infertility.

Regulation of germ cell development by intercellular signaling in the mammalian ovarian follicle

Prior to ovulation, the mammalian oocyte undergoes a process of differentiation within the ovarian follicle that confers on it the ability to give rise to an embryo. Differentiation comprises two phases-growth, during which the oocyte increases more than 100-fold in volume as it accumulates macromolecules and organelles that will sustain early embryogenesis; and meiotic maturation, during which the oocyte executes the first meiotic division and prepares for the second division. Entry of an oocyte into the growth phase appears to be triggered when the adjacent granulosa cells produce specific growth factors. As the oocyte grows, it elaborates a thick extracellular coat termed the zona pellucida. Nonetheless, cytoplasmic extensions of the adjacent granulosa cells, termed transzonal projections (TZPs), enable them to maintain contact-dependent communication with the oocyte. Through gap junctions located where the TZP tips meet the oocyte membrane, they provide the oocyte with products that sustain its metabolic activity and signals that regulate its differentiation. Conversely, the oocyte secretes diffusible growth factors that regulate proliferation and differentiation of the granulosa cells. Gap junction-permeable products of the granulosa cells prevent precocious initiation of meiotic maturation, and the gap junctions also enable oocyte maturation to begin in response to hormonal signals received by the granulosa cells. Development of the oocyte or the somatic compartment may also be regulated by extracellular vesicles newly identified in follicular fluid and at TZP tips, which could mediate intercellular transfer of macromolecules. Oocyte differentiation thus depends on continuous signaling interactions with the somatic cells of the follicle. WIREs Dev Biol 2018, 7:e294. doi: 10.1002/wdev.294 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Signaling Pathways > Cell Fate Signaling Early Embryonic Development > Gametogenesis.

Feed restriction as a biostimulant of the production of oocyte, their quality and GDF-9 gene expression in rabbit oocytes

The use of short-term feed restriction (R) without or with subsequent refeeding (F) as biostimulant of rabbit fertility was examined in this study. A total of 40 mature, non-pregnant, non-lactating New Zealand white female rabbits were allocated to five treatments. The rabbits were individually caged and fed a complete pelleted diet (16.7% CP; 13.1 CF; 2490kcalDE/kg). Rabbits on the control (C) treatment received 150g/d of the diet. Two groups of 8 rabbits received 70% of the control daily feed intake (105g/d; moderate restriction; M) and the other two groups received 50% of the control feed intake (75g/d; severe restriction; S) for 21d. At the end of this period, one group each of M and S fed rabbits were slaughtered for oocyte recovery. Rabbits in the remaining three groups (C, MF and SF) were retained for a further 8d before slaughter and fed the control level of the diet during this period. The effects on body weight, oocyte number and quality, GDF-9 gene expression in oocytes, and changes in serum levels of leptin and IGF-1 were recorded. Initial mean body weights were not significantly different ranging from 2.50±0.33kg (S) to 2.58±0.24kg (C). After 3wk on treatment the C rabbits were significantly heavier (2.65±0.32kg; P<0.05) than rabbits on the M (2.26±0.33kg) or S (2.10±0.33kg) treatments. Following 8d of refeeding, the remaining group of S treated rabbits (SF) were still significantly lighter (2.40±0.21kg; P<0.05) than C (2.71±0.31kg) with MF rabbits having an intermediate weight (2.50±0.20kg). The number of mature grade A oocytes recovered per ovary was significantly lower for control (3.3±0.35) than the refed treatments (MF 4.0±0.30; SF 4.5±0.39; P<0.05). Semi-quantitative PCR analysis of GDF-9 expression showed that control mature grade A oocytes had significantly lower levels of expression (1.27±0.20; P<0.05) than those of refed rabbits (MF 1.60±0.10; SF 1.39±0.01). Leptin and IGF-1 values for refed rabbits were significantly higher (P<0.05) than at the end of feed restriction and the start point. It was concluded that this biostimulant method has the potential to improve the fertility of rabbits.

ZP2 and ZP3 traffic independently within oocytes prior to assembly into the extracellular zona pellucida

The extracellular zona pellucida surrounds mammalian eggs and mediates taxon-specific sperm-egg recognition at fertilization. In mice, the zona pellucida is composed of three glycoproteins, but the presence of ZP2 and ZP3 is sufficient to form a biologically functional structure. Each zona pellucida glycoprotein is synthesized in growing oocytes and traffics through the endomembrane system to the cell surface, where it is released from a transmembrane domain and assembled into the insoluble zona pellucida matrix. ZP2 and ZP3 colocalize in the endoplasmic reticulum and in 1- to 5-microm post-Golgi structures comprising multivesicular aggregates (MVA), but a coimmunoprecipitation assay does not detect physical interactions. In addition, ZP2 traffics normally in growing oocytes in the absence of ZP3 or if ZP3 has been mutated to prevent incorporation into the zona pellucida matrix, complementing earlier studies indicating the independence of ZP3 secretion in Zp2 null mice. N glycosylation has been implicated in correct protein folding and intracellular trafficking of secreted proteins. Although ZP3 contain five N-glycans, enhanced green fluorescent protein-tagged ZP3 lacking N glycosylation sites is present in MVA and is incorporated into the zona pellucida matrix of transgenic mice. Thus, ZP2 secretion is seemingly unaffected by ZP3 lacking N-glycans. Taken together, these observations indicate that ZP2 and ZP3 traffic independently through the oocyte prior to assembly into the zona pellucida.

Mutation of a conserved hydrophobic patch prevents incorporation of ZP3 into the zona pellucida surrounding mouse eggs

Three glycoproteins (ZP1, ZP2, and ZP3) are synthesized in growing mouse oocytes and secreted to form an extracellular zona pellucida that mediates sperm binding and fertilization. Each has a signal peptide to direct it into a secretory pathway, a "zona" domain implicated in matrix polymerization and a transmembrane domain from which the ectodomain must be released. Using confocal microscopy and enhanced green fluorescent protein (EGFP), the intracellular trafficking of ZP3 was observed in growing mouse oocytes. Replacement of the zona domain with EGFP did not prevent secretion of ZP3, suggesting the presence of trafficking signals and a cleavage site in the carboxyl terminus. Analysis of linker-scanning mutations of a ZP3-EGFP fusion protein in transient assays and in transgenic mice identified an eight-amino-acid hydrophobic region required for secretion and incorporation into the zona pellucida. The hydrophobic patch is conserved among mouse zona proteins and lies between a potential proprotein convertase (furin) cleavage site and the transmembrane domain. The cleavage site that releases the ectodomain from the transmembrane domain was defined by mass spectrometry of native zonae pellucidae and lies N-terminal to a proprotein convertase site that is distinct from the hydrophobic patch.

Conserved furin cleavage site not essential for secretion and integration of ZP3 into the extracellular egg coat of transgenic mice

The extracellular zona pellucida surrounding mammalian eggs is formed by interactions of the ZP1, ZP2, and ZP3 glycoproteins. Female mice lacking ZP2 or ZP3 do not form a stable zona matrix and are sterile. The three zona proteins are synthesized in growing oocytes and secreted prior to incorporation into the zona pellucida. A well-conserved furin site upstream of a transmembrane domain near the carboxyl terminus of each has been implicated in the release of the zona ectodomains from oocytes. However, mutation of the furin site (RNRR --> ANAA) does not affect the intracellular trafficking or secretion of an enhanced green fluorescent protein (EGFP)-ZP3 fusion protein in heterologous somatic cells. After transient expression in growing oocytes, normal EGFP-ZP3 and mutant EGFP-ZP3 associate with the inner aspect of the zona pellucida, which is distinct from the plasma membrane. These in vitro results are confirmed in transgenic mice expressing EGFP-ZP3 with or without the mutant furin site. In each case, EGFP-ZP3 is incorporated throughout the width of the zona pellucida and the transgenic mice are fertile. These results indicate that the zona matrix accrues from the inside out and, unexpectedly, suggest that cleavage at the furin site is not required for formation of the extracellular zona pellucida surrounding mouse eggs.

Differential expression of ZPC in the bovine ovary, oocyte, and embryo

Using nonradioactive in situ hybridization (ISH), the mRNA encoding the zona glycoprotein bZPC was localized in bovine ovaries, oocytes, and embryos. In the ovary, the distribution of the mRNA was correlated with the developmental stage of the follicle. Whereas in primordial and primary follicles the mRNA was predominantly seen in the oocyte, it was found in both the oocyte and the follicle cells of secondary and tertiary follicles. In 2-day-old embryos produced by in vitro fertilization (IVF), no mRNA encoding ZPC could be demonstrated. Immunoblotting using monospecific polyclonal antibodies against porcine ZPC revealed a distinct band at a molecular weight of 47 kD in the ovarian cortex of cows, calves, and fetuses as well as in bovine follicle cells. Immunohistochemistry using the ZPC antibody displayed a strong signal in the zona pellucida of bovine oocytes and 2- to 6-day-old embryos as well as in the follicle cells. Our results show that during follicular development bovine ZPC is synthesized by the oocyte of the primary follicle and by both the oocyte and the follicle cells of the secondary and tertiary follicle. After fertilization, the synthesis of the zona protein is finished.

Localization of the mRNA encoding the zona protein ZP3 alpha in the porcine ovary, oocyte and embryo by non-radioactive in situ hybridization

The mRNA of the zona pellucida glycoprotein ZP3 alpha was localized in frozen sections of pig ovaries, isolated oocytes and early embryos by in situ hybridization using biotinylated oligonucleotide probes. In follicles, the distribution of mRNA for ZP3 alpha was correlated with the developmental stage: in primordial and primary follicles, the mRNA was shown to be predominantly localized in the oocyte. In secondary follicles, mRNA was found in both the oocyte and follicle cells. In tertiary and preovulatory follicles, the follicle cells showed distinct staining, whereas the oocyte was labelled weakly. In the early embryo, i.e. 2 days after fertilization, mRNA for ZP3 alpha could not be demonstrated. Our results suggest that, in the pig, the zona pellucida protein ZP3 alpha is synthesized by the oocyte and the follicle cells in sequence. After fertilization, synthesis of ZP3 alpha is terminated.

Coordinate expression of the three zona pellucida genes during mouse oogenesis

The mammalian zona pellucida is an extracellular matrix that surrounds growing oocytes, ovulated eggs and early embryos. The mouse zona is composed of three sulfated glycoproteins: ZP1, ZP2 and ZP3. Each is critically involved in fertilization, the postfertilization block to polyspermy and protection of the preimplantation embryo. We have previously isolated cDNAs encoding mouse ZP2 and ZP3 and now report the isolation of a full-length cDNA encoding ZP1. Mouse ZP1 is composed of a 623 amino acid polypeptide chain with a signal peptide and a carboxyl terminal transmembrane domain, typical of all zona proteins. Sequence comparison demonstrate that mouse ZP1 is an orthologue of a rabbit zona protein, R55. The expression of R55 has been reported previously in both oocytes and granulosa cells. However, by northern analysis and in situ hybridization with 33P-labelled antisense probes to each of the three mouse zona mRNAs, we have determined that the expression of each mouse zona gene is restricted to the oocyte. ZP2 transcripts, but not ZP1 or ZP3, are detected in resting (15 microns diameter) oocytes, and all three zona transcripts coordinately accumulate as oocytes begin to grow. Together they represent approximately 1.5% of the total poly(A)+ RNA in 50–60 microns oocytes. In the latter stages of oogenesis, their abundance declines and each zona transcript is present in ovulated eggs at less than 5% of its maximal level. No zona transcripts were detected above background signal in granulosa cells. We conclude that, in mice, the three zona pellucida genes are expressed in a coordinate, oocyte-specific manner during the growth phase of oogenesis. Our data support the hypothesis that the transcription of the zona genes is controlled, in part, by shared regulatory element(s).

Differences in membrane properties between unfertilised and fertilised single rabbit oocytes demonstrated by electro-rotation. Comparison with cells from early embryos

The apparent membrane capacity of tubular rabbit oocytes increases from 1.7-2.0 microF/cm2 before fertilisation to 3.7-4.0 microF/cm2 after fertilisation. The membrane conductivity measured on single cells was also increased by fertilisation from less than 1 mS/cm2 to 14 mS/cm2. Cells obtained from 2-, 4- or 8-cell embryos exhibited intermediate values of membrane capacity (2.3-2.8 microF/cm2) and conductivity (5-22 mS/cm2). The values quoted are those effective between 1 and 10 kHz, the frequency of the rotating field used. The large apparent capacities are probably due to the presence of structures such as microvilli which cause the actual membrane area to exceed the smooth sphere area. It must be assumed that these structures change in form or number on fertilisation, and that they persist in embryos, at least up to the 8-cell stage. No difference was apparent between cells fertilised in vitro or in vivo. Comparison of the above zona-free data with measurements on zona-complete oocytes indicate how fertilised and unfertilised rabbit eggs may be distinguished from one another, even in the presence of the zona pellucida.

Human antral follicles: oocyte nucleus and the karyosphere formation (electron microscopic and autoradiographic data)

The functional organization of the nucleus in the oocytes from human antral follicles was examined by morphological and autoradiographic analysis methods at the light and electron microscopic level. According to the position of the nucleus, the level of its transcriptional activity, and the pattern of distribution of structures in it, oocytes fall into two groups. In the first one, the oocytes with the nucleus in the central position are characterized by the distribution of numerous structures all over the nucleus or by a different extent of aggregation of chromatin around the nucleolus. The nuclei of these oocytes are characterized by [3H]uridine incorporation, the label being localized over purely fibrillar, agranular nucleoli and over dispersed fibrillar chromatin adjacent to either the regions of densely packed chromatin or fibrillar-granular material of the nucleolus-like bodies. The latter, the same as condensed chromatin, do not incorporate [3H]uridine. In the second group, the nuclei are displaced towards the oocyte's periphery, and chromosomes surround the nucleolus as a continuous mass closely adjacent to its surface, thus forming a karyosphere. The karyosphere formation takes place on the background of cessation of nuclear transcriptional activity. A fully formed karyosphere represents a complex of closely associated inactivated structures: Nucleolus, chromosomes, and nucleolus-like bodies. The karyosphere nucleolus bears no granules and consists of densely packed finely fibrillar material (fibrils 3 nm thick). Two zones (central and peripheral) can be distinguished in a nucleolus. Nucleolus-like bodies, consisting of granules 20 nm in diameter embedded in finely fibrillar material, are often associated with chromosomes. In this study, data obtained by observations on the loss of association between the oocyte (with karyosphere) and corona radiata cells are evaluated. The relation of the karyosphere formation to the atresia process and the duration of karyosphere existence in human antral follicles are also discussed.

Oocyte-specific expression and developmental regulation of ZP3, the sperm receptor of the mouse zona pellucida

The mouse zona pellucida is composed of three sulfated glycoproteins, encoded by the oocyte genome, that have important biological functions in preimplantation development. One of the zona gene products, ZP3, functions as the sperm receptor at fertilization. Our present data demonstrate that the ZP3 gene is transcribed in oocytes where its expression is developmentally regulated. Resting primordial oocytes do not express ZP3 mRNA, but these transcripts rapidly accumulate in growing oocytes so that they represent 0.1-0.2% of the total poly(A+) RNA. As oocytes complete their growth and undergo meiotic maturation, the abundance of ZP3 transcripts falls off dramatically; ovulated eggs contain less than 15% of peak levels. The oocyte-specific accumulation of ZP3 transcripts serves as an attractive system for further studies of factors that modulate developmentally regulated genes during mammalian oogenesis.

Formation of the rabbit zona pellucida and its relationship to ovarian follicular development

The zona pellucida is the unique extracellular glycoprotein matrix which is assembled during growth of the mammalian oocyte. The present studies were carried out to examine the formation of this structure in relation to the differentiation of ovarian cell types during follicular development. Specific antibodies were developed against total rabbit ZP proteins as well as against ZP proteins electrophoretically purified by high-resolution two-dimensional polyacrylamide electrophoresis gels (2D-PAGE). Antibodies were characterized by (a) immunoelectrophoresis, (b) a Staphylococcus aureus protein A binding assay, and (c) immunoblotting following 2D-PAGE separation of ZP proteins. Immunoperoxidase localization with these antibodies was used to determine the stage of ovarian follicular development at which ZP antigens first appear as well as to evaluate the cellular and extracellular distribution of these proteins throughout folliculogenesis. The ZP proteins were first observed in the cytoplasm and at the periphery of the oocytes surrounded by a thin squamous follicular cell layer. No staining was observed in the cytoplasm of follicle cells during early folliculogenesis. As the ZP matrix was assembled extracellularly, the intensity of staining of the outer and inner regions could be distinguished. This differentiation of the matrix coincided with the differentiation of the follicular cells into a multilayer cell complex. At this stage, specific ZP proteins are localized within the cytoplasm of the inner layers of these follicular cells. The staining is then diminished in cells of preantral follicles. These studies demonstrate that the formation of the ZP is an excellent model system to study the early stages of follicular development and cell differentiation.

Surface characteristics of oocytes from juvenile mice as observed in the scanning electron microscope

The in vivo and in vitro development of the zona pellucida as well as the vitelline membrane surface of oocytes isolated from juvenile mice aged between 8 and 30 days p.p. were investigated by scanning electron microscopy. In vivo a consistent development of the zona pellucida surface can be observed, namely the formation of a fibrous network like structure interspersed with numerous pores. After enzymatic removal of the zona pellucida the surface of the vitelline membrane was studied. In "normal" oocytes with intact germinal vesicles, microvilli were distributed over the entire oocyte surface in all age groups investigated. After resumption of meiosis in vitro a characteristic differentiation on the vitelline membrane occurs. A glabrous polar region appears in primary oocytes and a glabrous polar body in secondary oocytes. The same differentiation in surface organization could be observed in the in vivo precociously matured oocytes.

Patterns of RNA synthesis in early meiotic prophase oocytes from fetal mouse ovaries

In a study of the early meiotic prophase stages of mouse oogenesis from d12 of gestation to 10d post-partum the patterns of RNA synthesis during these stages of oogenesis using H3-uridine incorporation as visualized by light microscope autoradiography are reported. We find that chromosomal RNA synthesis occurs in all stages except early to mid-pachytene, the time of maximum chromosome condensation. Diplotene and dictyate nuclei are the most heavily labelled stages. Nucleolar labelling ceases before leptotene and reappears in late pachytene or early diplotene, even though nucleoli can be identified in all stages except early to mid-pachytene.

Oocyte development in the mouse: an ultrastructural comparison of oocytes isolated at various stages of growth and meiotic competence

An ultrastructural comparison of mouse oocytes isolated at various stages of growth and meiotic competence has been carried out. Progressive changes in the nucleoli, ribosomes, mitochondria, endoplasmic reticulum, Golgi complex, and other organelles and inclusions of the oocyte have been examined as a function of oocyte size by transmission electron microscopy. The observations presented support the idea that growth of the mammalian oocyte involves not just tremendous enlargement of the cell, but extensive alterations in its overall metabolism as reflected in the ultrastructure of the oocyte at various stages of growth.

Variations in mitochondrial size and ultrastructure during germ cell development

Size variations and ultrastructural changes in mitochondria of developing germ cells of the female hamster were analyzed. Mitochondria in oogonia of foetus and newborn were elongate with transverse cristae. During pre-dictyate meiotic prophase they became small, rounded, and electron-dense with pleomorphic cristae. These changes were largely reversed when dictyate was reached. Maximum mitochondrial size and complexity of cristae were reached just at the beginning of the phase of rapid oocyte growth, and thereafter declined. As mitochondrial size and number of cristae decreased in the rapidly enlarging oocyte, the ratio of length to width increased, as did electron density of the matrix, until the formation of an antrum within the follicle. After antrum formation, the mitochondria again became more rounded and cristae were seldom seen. An attempt is made to correlate changes of mitochondrial morphology with other events occurring during oogenesis.

The formation and cytochemical characterization of cortical granules in ovarian oocytes of the golden hamster (Mesocricetus auratus)

The formation and cytochemical characterization of cortical granules in the ovarian oocytes of the golden hamster have been investigated by use of light and electron microscopical techniques. Particular emphasis is given to the changing population of organelles associated with cortical granule formation. Our observations indicate that cortical granules are produced by the participation of both the Golgi complex and the rough endoplasmic reticulum. Ultrastructural cytochemistry reveals that the cortical granules are composed of glycoprotein. The cortical granules are released at fertilization by a merocrine-type of secretory process.

Ultrastructure of the oocytes of the Egyptian sping mouse (Acomys cahirinus)

The oocytes of types 2, 3, 4 and 5 follicles from the normal spiny mouse were examined withe the electron microsome. Multiple juxtanuclear Golgi bodies, mitochondria associated with flattened granular endoplasmic reticulum, and large nucleus are the main features of the type 2 follicle oocyte. The numbers of mitochondria and Golgi apparati increase significantly at later stages. Small mitochondrial aggregates lacking "intermitochondrial cement" are seen in the ooplasm of types 3, 4 and 5 follicles. "Lamellar complexes" comprising two to six elongate flattened rough ER cisternae and intercisternal filaments begin to appear in the occyte of the type 3 follicle. The intercisternal filaments may be observed as punctate-, dashed-, and solid-lines in cross sections. In tangential sections the filaments display a paracrystalline structure. In the type 4 follicle oocytes, the "lamellar complex" becomes more extensive; polysomes and ribosomal fibrils are juxtaposed to the "lamellar complexes." Bundles of ribosomal fibrils are abundant in the ooplasm of the type 5 follicle. The origin of ribosomal filaments and the functional significance of "lamellar complexes" are discussed.