Vertebral artery trauma

The developmental consequences of paternal exposure to acrylamide (50 mg/kg i.p. for 5 days) were assessed in preimplantation embryos. There was a significant increase in the proportion of morphologically abnormal embryos after postmeiotic treatment during spermatogenesis (88.7% vs. 14.8% in control). Abnormal embryos had an average of 1.8 +/- 3.5 cells and > 80% had at least one fragmented nucleus. In addition, morphologically normal embryos were significantly delayed (34.3 +/- 12.8 cells per embryo vs. 57.6 +/- 15.7 in control, P < 0.001). Acrylamide caused 10- and 20-fold increases in frequencies of cells with micronuclei (MN) in morphologically normal and abnormal embryos, respectively (41 and 93 MN per 1,000 cells). Both centromere-negative (MN-) and centromere-positive (MN+) were induced. Nuclei of abnormal embryos were significantly larger (900 microm2 vs. 250 microm2) than controls. In addition, MN of abnormal embryos were larger than those of normal embryos (21.2 microm2 vs. 6.5 microm2, P < 0.01). Among control embryos, MN+ were significantly larger than MN- (P < 0.05). These findings suggest that the preimplantation embryo is a sensitive indicator of paternally transmitted effects on early development. Multiple mechanisms appear to be involved, including cytogenetic damage, proliferation arrest/delay, and fertilization failure. Future studies are needed to establish how induced cytological defects in preimplantation embryos contribute to birth defects and other postimplantation abnormalities.     

Mitosis in the human embryo: the vital role of the sperm centrosome (centriole)

The pattern of sperm centrosomal (centriolar) inheritance, centrosomal replication and perpetuation during mitosis of the human embryo is reviewed with a series of electron micrographs. Embryonic cleavage involves repeated mitoses, a convenient sequence to study centriolar behaviour during cell division. After the paternal inheritance of centrioles in the human was reported (Sathananthan et al., 1991a), there has been an upsurge of centrosomal research in mammals, which largely follow the human pattern. The human egg has an inactive non-functional centrosome. The paternal centrosome contains a prominent centriole (proximal) associated with pericentriolar material which is transmitted to the embryo at fertilization and persists during sperm incorporation. Centriolar duplication occurs at the pronuclear stage (interphase) and the centrosome initially organizes a sperm aster when male and female pronuclei breakdown (prometaphase). The astral centrosome containing diplosomes (two typical centrioles) splits and relocates at opposite poles of a bipolar spindle to establish bipolarization, a prerequisite to normal cell division. Single or double centrioles occupy pivotal positions on spindle poles and paternal and maternal chromosomes organize on the equator of a metaphase spindle, at syngamy. Bipolarization occurs in all monospermic and in most dispermic ova. Dispermic embryos occasionally form two sperm asters initially and produce tripolar spindles (tripolarization). Anaphase and telophase follows producing two or three cells respectively, completing the first cell cycle. Descendants of the sperm centriole were found at every stage of perimplantation embryo development and were traced from fertilization through cleavage (first four cell cycles) to the morula and hatching blastocyst stage. Centrioles were associated with nuclei at interphase, when they were often replicating and occupied pivotal positions on spindle poles during mitosis. Sperm remnants were associated with centrioles and were found at most stages of cleavage. Centrioles were found in trophoblast, embryoblast and endoderm cells in hatching blastocysts. Pericentriolar, centrosomal material nucleated astral and spindle microtubules. Abnormal nuclear configurations observed in embryos reflect mitotic aberrations. The bovine embryo closely resembles the human embryo in centriolar behaviour during mitosis. It is concluded that the sperm centrosome is the functional active centrosome in humans and is likely the ancestor of centrioles within centrosomes in foetal and adult somatic cells. The role of the sperm centrosome in embryogenesis and male infertility is discussed, since it is of clinical importance in assisted reproduction.     

Maternal inheritance of mouse mtDNA in interspecific hybrids: segregation of the leaked paternal mtDNA followed by the prevention of subsequent paternal leakage

The transmission profiles of sperm mtDNA introduced into fertilized eggs were examined in detail in F1 hybrids of mouse interspecific crosses by addressing three aspects. The first is whether the leaked paternal mtDNA in fertilized eggs produced by interspecific crosses was distributed stably to all tissues after the eggs' development to adults. The second is whether the leaked paternal mtDNA was transmitted to the subsequent generations. The third is whether paternal mtDNA continuously leaks in subsequent backcrosses. For identification of the leaked paternal mtDNA, we prepared total DNA samples directly from tissues or embryos and used PCR techniques that can detect a few molecules of paternal mtDNA even in the presence of 10(8)-fold excess of maternal mtDNA. The results showed that the leaked paternal mtDNA was not distributed to all tissues in the F1 hybrids or transmitted to the following generations through the female germ line. Moreover, the paternal mtDNA leakage was limited to the first generation of an interspecific cross and did not occur in progeny from subsequent backcrosses. These observations suggest that species-specific exclusion of sperm mtDNA in mammalian fertilized eggs is extremely stringent, ensuring strictly maternal inheritance of mtDNA.     

Electroejaculation and assisted fertility in men with psychogenic anejaculation

OBJECTIVES: To evaluate sperm characteristics and fertility potential in ejaculates obtained after electroejaculation in men with psychogenic anejaculation. DESIGN: Retrospective clinical study. SETTING: In Vitro Fertilization Unit, Bikur Cholim Hospital, Jerusalem, Israel. PATIENTS: Twenty men with psychogenic anejaculation who underwent 55 sessions of electroejaculation and their spouses. INTERVENTIONS: Electroejaculation, assisted reproduction technologies. MAIN OUTCOME MEASURES: Semen analysis, IVF, intracytoplasmic injection (ICSI), fertilization rates, and pregnancy rates. RESULTS: In all patients, sperm density and motility rates were unsatisfactory (98 +/- 127 x 10(6) with 14.6% +/- 15% motility in the antegrade portions and 42 +/- 42 x 10(6) with 9.7% +/- 15.6% motility in the retrograde samples). Intrauterine inseminations performed in eight couples did not result in a pregnancy. Four couples underwent IVF-ET treatments. Two pregnancies were achieved with overall success rates of 22% per cycle. Five couples were treated using the ICSI procedure. Although good quality embryos were transferred, none of the treatments resulted in a pregnancy. CONCLUSIONS: Psychogenic failure to ejaculate may be treated by electroejaculation. However, the average motility of the sperm obtained is diminished. The combination of electroejaculation with IVF, including the ICSI procedure, should improve chances of fertilization and pregnancy in these cases.     

Development of normal mice from metaphase I oocytes fertilized with primary spermatocytes

Primary spermatocytes are the male germ cells before meiosis I. To examine whether these 4n diploid cells are genetically competent to fertilize oocytes and support full embryo development, we introduced the nuclei of pachytene/diplotene spermatocytes into oocytes that were arrested in prophase I (germinal vesicle stage), metaphase I, or metaphase II (Met II). Both the paternal and maternal chromosomes then were allowed to undergo meiosis synchronously until Met II. In the first and second groups, the paternal and maternal chromosomes had intermingled to form a large Met II plate, which was then transferred into a fresh enucleated Met II oocyte. In the third group, the paternal Met II chromosomes were obtained by transferring spermatocyte nuclei into Met II oocytes twice. After activation of the Met II oocytes that were produced, those microfertilized at metaphase I showed the best developmental ability in vitro, and three of these embryos developed into full-term offspring after embryo transfer. Two pups (one male and one female) were proven to be fertile. This finding provides direct evidence that the nuclei of male germ cells acquire the ability to fertilize oocytes before the first meiotic division.     

Blastocyst development from supernumerary embryos after intracytoplasmic sperm injection: a paternal influence?

The success of intracytoplasmic sperm injection (ICSI) warrants further study on the role of paternal factors in early human embryogenesis. To investigate whether poor sperm parameters can influence embryo development, we examined the development of ICSI-fertilized embryos to the blastocyst stage. We present results of blastocyst development from supernumerary ICSI embryos after co-culture on monkey kidney epithelial cells. In addition, we compare the development of supernumerary embryos to the blastocyst stage after ICSI and in-vitro fertilization (IVF). Of 168 supernumerary ICSI embryos, 45 (26.8%) developed to blastocysts. Sperm concentration and morphology did not influence blastocyst development. In contrast, blastocysts arose from spermatozoa that had a significantly higher (P = 0.015) forward progressive motility compared with spermatozoa from those patients who failed to produce blastocysts (42.7% versus 28.2%, respectively). Overall the rate of embryo development to the blastocyst stage after ICSI was lower (26.8%) than that after IVF (47.3%). When the rate of blastocyst development was calculated for patients with three or more supernumerary embryos, it remained significantly higher for the IVF patients than for the ICSI patients (45.6% versus 30.0%). There was no significant difference in the mean cell number and quality of the supernumerary embryos between the IVF and ICSI patients. This study confirms previous reports that have postulated that abnormal spermatozoa may manifest a negative paternal effect on preimplantation embryo development.     

Suppression of inflammatory neurotoxins by highly active antiretroviral therapy in human immunodeficiency virus-associated dementia

The development in culture of 1-cell hamster embryos prior to the completion of fertilization is not well understood. In this study it was observed that culture for only 6 h of these early 1-cell embryos collected before pronuclei formation (3 h post-egg activation; PEA) significantly increased intracellular free calcium levels (194.3 +/- 3.1 nM) compared to levels in similarly aged 1-cell embryos collected from the oviduct at 9 h PEA, after pronuclei formation is complete (134.2 +/- 6.8 nM). Not only was the developmental competence of cultured 3-h PEA embryos with elevated intracellular free calcium levels compromised as compared with that of embryos collected from the oviduct at 9 h PEA; these embryos also had impaired cytoplasmic mitochondrial distribution (ratio of 0.62 +/- 0. 06 for cultured embryos compared to 0.44 +/- 0.04 for in vivo-developed embryos) and decreased lactate metabolism (2.93 +/- 0. 22 pmol/embryo per 3 h for cultured embryos compared to 5.37 +/- 0. 36 for in vivo-developed embryos). This impairment in mitochondrial distribution and function and reduced development in culture by 3-h PEA embryos appears related to the ability to regulate intracellular calcium homeostasis. Intracellular free calcium levels were reduced by culture with increased medium magnesium concentrations, calcium channel inhibitors nifedipine or verapamil, or an intracellular calcium chelator. All of these treatments also stimulated development of 3-h PEA embryos to the morula/blastocyst stages and prevented impairment in mitochondrial organization and function. Conversely, culture with low medium magnesium and high calcium concentrations that increased intracellular free calcium levels resulted in low development and reduced mitochondrial function. Therefore, it appears that removal of the early embryo from the oviduct results in an inability to regulate intracellular calcium levels. As increased magnesium concentrations, nifedipine, and verapamil inhibit L-gated calcium channels, it may be a loss of regulation of these channels that alters calcium homeostasis resulting in impaired developmental competence.     

A note on assessing the superiority of a combination drug with a specific alternative

The gas atmosphere and medium composition are critical factors in the in vitro development of one- and two-cell embryos of several species. The present study evaluated the effect of different O2/CO2 concentrations (2/5, 2/10, 5/2.5, 5/5, 5/10, 10/10 and 21/5) on pig one- and two-cell embryo development. The embryos were individually cultured, for 6 days at 39 degrees C in a medium rich in bicarbonate and glutamine and containing pyruvate and lactate but lacking glucose. When the CO2 levels increased from 2.5% to 10%, the pH of the medium decreased from 8.2 to 7.5 and the development of the embryos was affected, but this depended mainly on the O2 levels. Pig embryo development was inhibited by 2 and 21% O2 levels. The optimum level for pig embryo development was 5% O2 and 5% CO2, whatever the criteria used to evaluate embryo development. At these optimal levels, the mean number of cells per embryo was 26 +/- 1.7 (ls mean +/- SE), and 50% of the one- and two-cell embryos developed to blastocysts. The substitution of 0.5% bovine serum albumin (BSA) in the medium by 0.3% polyvinyl-pyrrolidone (PVP) significantly decreased the one- and two-cell embryo development. When the calcium and chloride contents of the medium with PVP were reduced, however, the embryo development was similar to that observed in the medium containing BSA. Pig embryo development in vitro was found to be optimal under an atmosphere of 5% O2 and 5% CO2 and PVP could replace BSA as the high molecular weight supplement.     

The role of the paternal genome in the development of the mouse germ line

The mouse germ line originates at 6.5 days post coitum (dpc) in the proximal epiblast, apparently in response to signals from the primitive endoderm or the extraembryonic mesoderm [1,2]. Some studies have implied a significant role for imprinted genes in germ-line development [3,4]. These genes, whose expression is determined by their parental origin [5], serve complementary functions during mammalian development [6-9] and exert striking reciprocal phenotypic effects on androgenetic (AG: two paternal genomes) and parthenogenetic (GG/PG: two maternal genomes) cells [3,4,10]. This may include a fundamental effect on germ-cell development because PG but not AG cells can differentiate into viable gametes [3,4,11], suggesting that the maternal genome is obligatory for development of the mammalian germ line. Here we show unequivocally that AG cells can differentiate into germ cells, and that in chimeras with normal cells they produce functional sperm. These studies establish that the paternal and maternal genomes can individually provide both the signal and the response required for the specification of germ cells in mammals.     

Study of aneuploidy in normal and abnormal germ cells from semen of fertile and infertile men

This study was undertaken with the aim of investigating the cytogenetic constitution of normal as well as abnormal spermatozoa and immature germ cells found in semen of normal men and infertile patients. A specific protocol of double in-situ hybridization for chromosomes 1 and 17 based on colorimetric detection of the hybridization signals (ISH) and brightfield microscopy analysis of cellular morphology was applied. Also the influence of paternal age on sperm aneuploidy was investigated. We found that, at least in the age range analysed (28-54 years) and for semen of good quality (total normal motile counts above 10 x 10(6)) (n = 17), paternal age has no influence on baseline rates of sperm aneuploidy. However, with decreasing semen quality (total normal motile sperm counts below 5 x 10(6)) (n = 6) significantly higher rates of sperm aneuploidy for autosomes 1 and 17 were scored (0.8 versus 1.43%) (P < 0.001). Regardless of the type of semen analysed, a number of morphologically abnormal spermatozoa were found to be hyperhaploid or diploid in a high percentage of cases (20 and 10% respectively). The same was found for immature germ cells (aneuploidy rate of 18%). We conclude that in infertile men with poor quality semen a direct relationship may exist between the impairment of the spermatogenesis process (as reflected by an increased production of morphologically and cytogenetically abnormal germ cells) and rates of baseline aneuploidy occurring in normal spermatozoa. Infertile couples undergoing assisted reproduction treatment need to be counselled about the risk of using spermatozoa which may carry higher rates of non-disjunction for different chromosomes. While sperm hyper- or hypohaploidy for some chromosomes (X,Y) implies counselling couples about the risk of abnormal phenotype in their offspring, most autosomal sperm aneuploidies probably translate only into lower rates of embryo fertilization and survival.     

Purification and nucleic-acid-binding properties of a Saccharomyces cerevisiae protein involved in the control of ploidy

The mouse H19 gene is expressed exclusively from the maternal allele. The imprinted expression of the endogenous gene can be recapitulated in mice by using a 14-kb transgene encompassing 4 kb of 5'-flanking sequence, 8 kb of 3'-flanking sequence, which includes the two endoderm-specific enhancers, and an internally deleted structural gene. We have generated multiple transgenic lines with this 14-kb transgene and found that high-copy-number transgenes most closely follow the imprinted expression of the endogenous gene. To determine which sequences are important for imprinted expression, deletions were introduced into the transgene. Deletion of the 5' region, where a differentially methylated sequence proposed to be important in determining parental-specific expression is located, resulted in transgenes that were expressed and hypomethylated, regardless of parental origin. A 6-kb transgene, which contains most of the differentially methylated sequence but lacks the 8-kb 3' region, was not expressed and also not methylated. These results indicate that expression of either the H19 transgene or a 3' DNA sequence is key to establishing the differential methylation pattern observed at the endogenous locus. Finally, methylation analysis of transgenic sperm DNA from the lines that are not imprinted reveals that the transgenes are not capable of establishing and maintaining the paternal methylation pattern observed for imprinted transgenes and the endogenous paternal allele. Thus, the imprinting of the H19 gene requires a complex set of elements including the region of differential methylation and the 3'-flanking sequence.     

Influence of rat placental lactogen-I on the development of whole rat embryos in culture

Rat placental lactogen-I (rPL-I), the first prolactin-like hormone expressed in the placenta during pregnancy in the rat, is known to influence maternal functions. In the present study, we have investigated the effects of rPL-I on the growth and development of cultured whole rat embryos. Rat embryos, with or without ectoplacental cone (EPC) attached, were explanted at day 9 of gestation. After 48 h of culture, the embryos, enclosed by the yolk sacs, were assessed by the presence of visible heart contractions ('heart beats'), crown-rump length (CRL) and yolk sac diameter (YSD). When intact embryos with EPC were cultured, the concentrations of rPL-I and rPL-II (products of EPC) in the medium were 850+/-841 and 92+/-181 ng/ml respectively (means+/-s.e.m.). In embryo cultures with the EPC removed, rPL-I levels decreased to相似文献   

Fertilization and embryo development to blastocysts after intracytoplasmic sperm injection in the rhesus monkey

Notwithstanding the thousands of seemingly healthy children born after intracytoplasmic sperm injection (ICSI), it is not yet possible to conclude absolutely that the ICSI procedure might induce some altered development or that the ICSI protocol might not be improved even further. To address this in a clinically relevant system, the developmental potential of rhesus monkey embryos produced by ICSI is reported. Oocytes collected by laparoscopy from gonadotrophin-stimulated fertile females were fertilized by ICSI using spermatozoa obtained from fertile males by electro-ejaculation. Neither sperm immobilization prior to injection nor an additional chemical stimulus were necessary to achieve oocyte activation and pronuclear formation. Survival and activation of the injected oocytes were judged by the extrusion of the second polar body. Successful fertilization was confirmed by the presence of two pronuclei within 12 h post-ICSI. Some oocytes were fixed and processed for the detection of microtubules and chromatin. Fluorescent labelling revealed that by 12 h post-ICSI the male and female pronuclei were closely apposed and eccentrically positioned within a large microtubule aster. ICSI resulted in a 76.6 +/- 14.9% fertilization rate. First cleavage was completed within 24 h post-ICSI. Two-cell ICSI embryos were co-cultured in CMRL medium on a buffalo rat liver cell monolayer until the hatched blastocyst stage. Oocytes collected laparoscopically from stimulated monkeys can be fertilized by ICSI and will complete preimplantation embryo development in vitro demonstrating that the rhesus monkey is an excellent preclinical model for examining and understanding many aspects of human ICSI.     

Fatal meconium aspiration in spite of appropriate perinatal airway management: pulmonary and placental evidence of prenatal disease

The cytoskeletal components of hamster oocytes, zygotes, and spontaneously activated parthogenotes were examined after immunocytochemical labeling. Microtubules were found only in the anastral, tangentially arranged second meiotic spindle of unfertilized oocytes. Taxol treatment of unfertilized oocytes greatly augmented astral microtubules in both the metaphase II spindle and the cortex. Disruption of the meiotic spindle microtubules with nocodazole resulted in cortical chromosomal scattering. During hamster sperm incorporation and pronuclear formation, no sperm aster was detected in association with the male DNA. Instead, a large overlapping array of microtubules assembled in the cortex. By mitosis, this interphase array disassembled and an anastral metaphase spindle formed. Microtubule and chromatin configurations were also imaged in hamster oocytes injected with human sperm. Astral microtubules were absent from the sperm centrosome. The implications of these results are discussed in relation to the hamster oocyte penetration assay, a test commonly used by in vitro fertilization clinics to demonstrate the fertilizing ability of human sperm. We conclude that since hamsters and humans follow different methods of centrosome inheritance, maternal and paternal, respectively, the hamster may be an inappropriate model for exploring microtubule and centrosomal defects in humans or for assaying postinsemination forms of human male fertility defects.     

Phenotypic variation in a genetically identical population of mice

The parental alleles of an imprinted gene acquire their distinctive methylation patterns at different times in development. For the imprinted RSVIgmyc transgene, methylation of the maternal allele is established in the oocyte and invariably transmitted to the embryo. In contrast, the methylation of the paternal allele originates during embryogenesis. Here, we show that the paternal methylation pattern among mice with identical genetic backgrounds is subject to extensive variation. In addition to this nongenetic variation, the process underlying RSVIgmyc methylation in the embryo is also subject to considerable genetic regulation. The paternal transgene allele is highly methylated in an inbred C57BL/6J strain, whereas it is relatively undermethylated in an inbred FVB/N strain. Individual methylation patterns of paternal alleles, and therefore all of the variation (nongenetic and genetic) in methylation patterns within an RSVIgmyc transgenic line, are established in early embryogenesis. For each mouse, the paternal RSVIgmyc allele is unmethylated at the day-3.5 blastocyst stage, and the final, adult methylation pattern is found no later than day 8.5 of embryogenesis. Because of the strong relationship between RSVIgmyc methylation and expression, the variation in methylation is also manifest as variation in transgene expression. These results identify embryonic de novo methylation as an important source of both genetic and nongenetic contributions to phenotypic variation and, as such, further our understanding of the developmental origin of imprinted genes.     

Localization of mitochondrial large ribosomal RNA in germ plasm of Xenopus embryos

In Xenopus, factors with the ability to establish the germ line are localized in the vegetal pole cytoplasm, or germ plasm, of the early embryo [1-3]. The germ plasm of Xenopus, and of many other animal species including Drosophila, contains electron-dense germinal granules which may be essential for germ-line formation [4-5]. Several components of the germinal granules have so far been identified in Drosophila [6-10]. One of these is mitochondrial large ribosomal RNA (mtlrRNA), which is present in the germinal granules (polar granules) during the cleavage stage until the formation of the germ-line progenitors or pole cells [8-9]. MtlrRNA has been identified as a factor that induces pole cells in embryos that have been sterilized by ultraviolet radiation [11]. The reduction of mtlrRNA in germ plasm by injecting anti-mtlrRNA ribozymes into embryos leads to the inability of these embryos to form pole cells [12]. These observations clearly show that mtlrRNA is essential for pole cell formation in Drosophila. Here, we report that mtlrRNA is enriched in germ plasm of Xenopus embryos from the four-cell stage to the blastula. Furthermore, our electron microscopic studies show that this mtlrRNA is present in the germinal granules during these stages. Thus, mtlrRNA is a common component of germinal granules in Drosophila and Xenopus, suggesting that the mtlrRNA has a role in germ-line development across phylogenetic boundaries.     

Reproductive wastage in the androstenedione-immune ewe

An experiment was carried out using 320 adult Merino ewes to examine the effects of immunization against an androstenedione human serum albumin conjugate (Fecundin) on ovulation rate, fertilization rate and embryo viability at days 2, 9 and 13-14 after fertilization. The ovulation rate of immunized ewes (2.19 +/- 0.06) was greater (P < 0.001) than that of control ewes (1.43 +/- 0.04). The recovery rate of embryos or of unfertilized oocytes on day 2 was reduced in immunized ewes, but fertilization rate of recovered oocytes was unaffected by immunization. The mean number of normal embryos per ewe pregnant (prolificacy) was higher and the proportion of ewes pregnant (fertility) was lower in immunized than in the control ewes. The distribution of the number of cells per embryo showed no differences in developmental age over the period of sampling, the majority of embryos at this time being at the two- to four-cell stage of development. At day 9 of pregnancy, blastocyst recovery rates were lower in immunized than in control ewes. About 90% of blastocysts recovered were developing normally in control ewes compared with 64% in immunized ewes. The majority of blastocysts recovered on day 9 had hatched from the zona pellucida prior to recovery (mean values were 94.2% and 87.8% for control and immunized groups, respectively). In control ewes single blastocysts were larger than twin blastocysts, but for the immunized ewes this difference was not significant. Both single blastocysts (P < 0.01) and twin blastocysts (P < 0.05) from immunized ewes were smaller than those from control ewes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in ribosomal ribonucleic acid content within in vitro-produced bovine embryos

The abundance of 28S, 18S, and 5S rRNA was measured by Northern blot techniques applied to RNA samples extracted from bovine oocytes and preattachment embryos produced by in vitro procedures. Total RNA content was estimated by comparing the intensity of hybridization signals of 28S and 18S rRNA probes to embryo RNA samples and to standard curves generated from bovine ovary or bovine oviduct cell RNA. RNA content declined from the oocyte to the morula stage (2.4 +/- 0.3 ng/oocyte, 1.7 +/- 0.5 ng/1-cell embryo, 2.2 +/- 0.9 ng/2- to 4-cell embryo, 0.8 +/- 0.2 ng/6- to 8-cell embryo, and 0.7 +/- 0.2 ng/morula). A marked increase in RNA content, based on levels of hybridization to 28S and 18S rRNA, was observed in blastocysts, in which values averaged 5.3 +/- 0.6 ng/embryo. On a relative basis, 5S rRNA abundance followed a pattern similar to that of 28S and 18S rRNA across the early development period to the blastocyst stage.