Relationship between aging and periodontal disease severity in gauge-raised cynomolgus monkeys (Macaca fascicularis).

The study aimed to evaluate the periodontal disease status in different age groups and clarify the relationship between aging and the severity of periodontal disease. The test animals were cynomolgus monkeys that were born and raised at the Tsukuba Primate Research Center of the National Institutes of Biomedical Innovation, Health, and Nutrition. The participants were divided into three groups: young (5-10 years old), middle (10-19 years old), and old (≥20 years old). The plaque Index (PLI), Gingival Index (GI), probing pocket depth (PPD), and Bleeding on probing (BOP) were used for the periodontal examination. Representative teeth were also examined. Polymerase chain reaction (PCR) was used to identify Porphyromonas macacae in dental plaque. Multiple comparisons and regression analyses were used to analyze the relationship between each age group and each oral examination index. Statistically significant differences were found between the age groups and periodontal examination index. Multiple regression analysis revealed that age was strongly correlated with each oral examination index. Based on these results, oral examinations of cynomolgus monkeys kept in the same environment confirmed an association between aging and periodontal disease severity. Monkeys at this facility are expected to serve as new experimental models for elucidating the mechanisms underlying the progression of age-related periodontal disease.

Safety and Osteointegration of Titanium Screws Coated with a Fibroblast Growth Factor-2-Calcium Phosphate Composite Layer in Non-Human Primates: A Pilot Study.

Spinal instrumentation surgery for older patients with osteoporosis is increasing. Implant loosening may occur due to inappropriate fixation in osteoporotic bone. Developing implants that achieve stable surgical results, even in osteoporotic bone, can reduce re-operation, lower medical costs, and maintain the physical status of older patients. Fibroblast growth factor-2 (FGF-2) promotes bone formation; thus, coating pedicle screws with an FGF-2-calcium phosphate (FGF-CP) composite layer is hypothesized to enhance osteointegration in spinal implants. We designed a long-term implantation pilot study that estimated the safety and bone-forming efficacy of pedicle screws coated with an FGF-CP composite layer in cynomolgus monkeys. Titanium alloy screws, either uncoated (controls) or aseptically coated with an FGF-CP composite layer, were implanted in the vertebral bodies of six female adult cynomolgus monkeys (three monkeys per group) for 85 days. Physiological, histological, and radiographic investigations were performed. There were no serious adverse events, and no radiolucent areas were observed around the screws in either group. The bone apposition rate in the intraosseous region was significantly higher in the FGF-CP group than in the controls. Moreover, as analyzed by Weibull plots, the bone formation rate of the FGF-CP group exhibited a significantly higher regression line slope than the control group. These results demonstrated that there was significantly less risk of impaired osteointegration in the FGF-CP group. Our pilot study suggests that FGF-CP-coated implants could promote osteointegration, be safe, and reduce the probability of screw loosening.

A long-acting anti-IL-8 antibody improves inflammation and fibrosis in endometriosis.

Current pharmacological treatments for endometriosis are limited to hormonal agents that can relieve pain but cannot cure the disease. Therefore, the development of a disease-modifying drug for endometriosis is an unmet medical need. By studying human endometriotic samples, we found that the progression of endometriosis was associated with the development of inflammation and fibrosis. In addition, IL-8 expression was highly up-regulated in endometriotic tissues and closely correlated with disease progression. We created a long-acting recycling antibody against IL-8 (AMY109) and evaluated its clinical potency. Because rodents do not produce IL-8 and do not experience menstruation, we analyzed the lesions in cynomolgus monkeys that spontaneously developed endometriosis and in a surgically induced endometriosis monkey model. Both spontaneously developed and surgically induced endometriotic lesions demonstrated pathophysiology that was highly similar to that of human endometriosis. Once-a-month subcutaneous injection of AMY109 to monkeys with surgically induced endometriosis reduced the volume of nodular lesions, lowered the Revised American Society for Reproductive Medicine score as modified for monkeys, and ameliorated fibrosis and adhesions. In addition, experiments using cells derived from human endometriosis revealed that AMY109 inhibited the recruitment of neutrophils to endometriotic lesions and the production of monocyte chemoattractant protein-1 from neutrophils. Thus, AMY109 may represent a disease-modifying therapy for patients with endometriosis.

Safety of terminally gamma-ray-sterilized screws coated with fibroblast growth factor 2-calcium phosphate composite layers in non-human primates.

Screws coated with fibroblast growth factor 2 (FGF-2)-calcium phosphate (CP) composite layers exhibit enhanced soft tissue and bone formation and angiogenesis because of the biological activity of FGF-2. Furthermore, the mitogenic activity of the FGF-2 within the composite layers remains unchanged after gamma-ray sterilization, which may improve the storage stability prior to clinical use. However, the in vivo safeties of these screws as spinal implants remain unknown. Here, a randomized controlled trial, involving non-human primates, investigated the safety of using FGF-2-CP composite layer-coated screws after either gamma-ray sterilization or aseptic processing. Titanium alloy screws coated with FGF-2-CP composite layers and subjected to either gamma-ray sterilization at 25 kGy (GS group) or aseptic storage (AS group) were implanted into the vertebral bodies of two cynomolgus monkeys exceeding 12 weeks (day 99). Physiological, histological, and radiographic investigations were performed to evaluate the safeties of the screws. There were no serious adverse events, such as surgical site infection, significant loss of body weight, or abnormal blood test results. No radiolucent areas were observed around the screws from the GS or AS group throughout the study. In the intraosseous region, no significant differences were observed in bone and fibrous tissue apposition rates and rate of bone formation between the two groups (p = 0.49, 0.77, and 0.11, respectively). Neither tumor lesions nor accumulation of lymphocytes and neutrophils were observed in either group. Our data suggest that FGF-2-CP composite layer-coated screws subjected to terminal gamma-ray sterilization are as safe as those fabricated in aseptic processing.

The effect of betaine for mouse sperm cryopreservation.

Sperm cryopreservation is an effective method of preserving male fertility in humans, as well as domestic and experimental animals. However, various factors such as ice crystal formation, osmotic stress, and oxidative stress, negatively influence the motility and viability of post-thawed spermatozoa. Betaine, which works as an osmoprotectant is known to work as a nontoxic cryoprotectant. However, the protective effects during mouse sperm cryopreservation are still unclear. Thus, the purpose of this study was to investigate whether betaine has protective effects during the process of mouse sperm cryopreservation. In this study, betaine was found to be effective in maintaining sperm motility during the freezing procedure and 1% (85.4 mM) betaine was identified as the optimal concentration to be added to cryopreservation solutions. It was also found that betaine improves the integrity of the plasma membranes of sperm tails, suggesting that betaine has a positive effect on sperm motility.

Nuclear-to-cytoplasmic ratios of 1PN and 2PN zygotes after in vitro fertilization of mouse oocytes.

Numerous studies have reported comparisons of the nuclear-to-cytoplasmic (NC) ratio during mitosis. However, little information is known about how the pronuclear size is regulated and determined at the end of meiosis II in mammalian zygotes. The present study aims to analyze the NC ratio of female and male pronuclei, and also to compare the size of single pronuclei using photographs that were obtained during experiments to create chimeric hermaphrodites from 2-cell oocytes. The volume of both the female and the male pronucleus was found to correlate with the volume of the oocyte cytoplasm. The NC ratio of the male pronucleus was greater than that of the female pronucleus. The NC ratio of the average volume of the female and male pronuclei was greater than the NC ratio of the mononucleate oocytes. The occurrence of 1PN oocytes was significantly higher when the volume of cytoplasm was lower than the cut-off value. These results indicated that the NC ratio is retained during pronuclear formation. A higher NC ratio in male compared with the female pronucleus indicated structural and/or molecular difference between the two pronuclei. 1PN formation may occur when sperm enters close to the MII spindle.

Usefulness of cardiac hormones for evaluating valvular disease in cynomolgus monkeys (Macaca fascicularis).

Nonhuman primates are commonly used as experimental animals due to their biological resemblance to humans. In patients with cardiac disease, the levels of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) tend to increase in response to cardiac damage, and they are thus used as indicators for the diagnosis of human heart failure. However, no reference values for ANP and BNP have been reported for heart disease in nonhuman primates. In this study, we recorded the age, sex, and body weight of 202 cynomolgus monkeys, and performed evaluations to assess the ANP and BNP levels, electrocardiography and echocardiography, and accordingly divided the monkeys into two groups: healthy monkeys and those with spontaneous cardiac disease. Statistical analysis was performed to determine the relationship of ANP and BNP with the factors of age, sex, and body weight. No significant relationship was found between the levels of ANP and BNP and the factors of age, sex, and body weight. However, both the ANP and BNP levels were significantly different between the healthy monkeys and monkeys with valvular disease. Similar to humans, the ANP and BNP levels tended to increase with the progression of cardiac disease in monkeys. Based on these results, we concluded that ANP and BNP are indicators of cardiac disease in nonhuman primates, and that this nonhuman primate cardiac disease model is applicable for cardiology research in humans.

Echocardiographic evaluation of cardiac function in cynomolgus monkeys over a wide age range.

Various cardiovascular diseases can be detected and diagnosed using echocardiography. The demand for cardiovascular system research using nonhuman primates is increasing, but echocardiographic references for nonhuman primates are limited. This report describes the first comparison of echocardiographic reference values in 247 normal cynomolgus monkeys (135 females, 112 males) over a wide age range. Echocardiography, electrocardiography, blood pressure and chest X-ray images were acquired under immobilization with intramuscular ketamine hydrochloride, then cardiac structure, function, and flow velocity were assessed. Cardiac hormone levels were also tested. We found that cardiac structures positively correlated with weight, that the size of these structures stabilized after reaching maturity and that cardiac output increased according to heart size. In contrast, fractional shortening of the left ventricle, ejection fraction and flow velocity showed no significant correlations with weight or age, and age and E wave correlated negatively. These findings appear sufficiently similar to those in humans to suggest that cynomolgus monkeys can serve as a suitable model of human cardiac disease. Our data should also prove useful for surveying cardiac dysfunction in monkeys.

Establishment of a new formula for QT interval correction using a large colony of cynomolgus monkeys.

The demand for monkeys for medical research is increasing, because their ionic mechanism of repolarization is similar to that of humans. The QT interval is the distance between the Q wave and T wave, but this interval is affected by heart rate. Therefore, QT correction methods are commonly used in clinical settings. However, an accurate correction formula for the QT interval in cynomolgus monkeys has not been reported. We assessed snapshot electrocardiograms (ECGs) of 353 ketamine-immobilized monkeys, including aged animals, and contrived a new formula for the corrected QT interval (QTc) as a marker of QT interval prolongation in cynomolgus monkeys. Values for QTc were calculated using the formula [QTc] = [QT] / [RR]n, along with several other formulas commonly used to calculate QTc. We found that the optimal exponent of the QT interval corrected for heart rate, n, was 0.576. The mean value of QTc in healthy monkeys determined using the new formula was 373 ± 31 mm, and there were no significant differences between the sexes. Other ECG parameters were not significantly different between the sexes and there were no age-related effects on QTc. Prolongation of QTc to over 405 ms, as calculated by the new formula, was observed in 50 monkeys with underlying diseases. Additionally, all monkeys with QTc above 440 ms by the new formula had some underlying disease. The results resemble those in humans, suggesting that the new QTc formula could be useful for diagnosis of QT interval prolongation in cynomolgus monkeys.

Effect of blue light-emitting diode light and antioxidant potential in a somatic cell.

Light is an indispensable part of routine laboratory work in which conventional light is generally used. Light-emitting diodes (LEDs) have come to replace conventional light, and thus could be a potent target in biomedical studies. Since blue light is a major component of visible light wavelength, in this study, using a somatic cell from the African green monkey kidney, we assessed the possible consequences of the blue spectra of LED light in future animal experiments and proposed a potent mitigation against light-induced damage. COS-7 cells were exposed to blue LED light (450 nm) and the growth and deoxyribonucleic acid (DNA) damage were assessed at different exposure times. A higher suppression in cell growth and viability was observed under a longer period of blue LED light exposure. The number of apoptotic cells increased as the light exposure time was prolonged. Reactive oxygen species (ROS) generation was also elevated in accordance to the extension of light exposure time. A comparison with dark-maintained cells revealed that the upregulation of ROS by blue LED light plays a significant role in causing cellular dysfunction in DNA in a time-dependent manner. In turn, antioxidant treatment has been shown to improve cell growth and viability under blue LED light conditions. This indicates that antioxidants have potential against blue LED light-induced somatic cell damage. It is expected that this study will contribute to the understanding of the basic mechanism of somatic cell death under visible light and maximize the beneficial use of LED light in future animal experiments.

Utility of arterial blood gas, CBC, biochemistry and cardiac hormones as evaluation parameters of cardiovascular disease in nonhuman primates.

Cardiovascular disease (CVD) has a tremendous impact on the quality of life of humans. While experimental animals are valuable to medical research as models of human diseases, cardiac systems differ widely across various animal species. Thus, we examined a CVD model in cynomolgus monkeys. Laboratory primates are precious resources, making it imperative that symptoms of diseases and disorders are detected as early as possible. Thus, in this study we comprehensively examined important indicators of CVD in cynomolgus monkeys, including arterial blood gas, complete blood count (CBC), biochemistry and cardiac hormones. The control group included 20 healthy macaques showing non-abnormal findings in screening tests, whereas the CVD group included 20 macaques with valvular disease and cardiomyopathy. An increase of red blood cell distribution width was observed in the CBC, indicating chronic inflammation related to CVD. An increase of HCO3 was attributed to the correction of acidosis. Furthermore, development of the CVD model was supported by significant increases in natriuretic peptides. It is suggested that these results indicated a correlation between human CVD and the model in monkeys. Moreover, blood tests including arterial blood gas are non-invasive and can be performed more easily than other technical tests. CVD affected animals easily change their condition by anesthesia and surgical invasion. Pay attention to arterial blood gas and proper respond to their condition are important for research. This data may facilitate human research and aid in the management and veterinary care of nonhuman primates.

Relationship between menarche and fertility in long-tailed macaques (Macaca fascicularis).

We attempted to elucidate female reproduction in long-tailed macaques (Macaca fascicularis). These monkeys have a non-seasonal menstruation cycle, which makes them suitable subjects for studies in a variety fields including medical science and regenerative medicine. We analyzed individual breeding data including time of menarche, start of regular menstruation, and first pregnancy. These three events are related to the maturation of female long-tailed macaques. All research subjects were female long-tailed macaques bred at the Tsukuba Primate Research Center. The study comprised 45 females; we included time of menstruation, male-female cohabitation, and first pregnancy in their growth records. We extracted age and weight data relating to menarche, start of regular menstruation, and first pregnancy from these records. In the two years typically required from menarche to first pregnancy, the body weight increased by approximately 500 g (21% of the weight at menarche); it is clear that there is a significant physical change after menarche. Our findings suggest that female monkeys are not necessarily mature enough for pregnancy at menarche. Therefore, the use of the word "maturity" in terms of fecundity may be more accurate after the start of regular menstruation. This is what we term "adolescence" in the developmental process. Therefore, M. fascicularis monkeys are candidates for an animal model of human adolescence.

Live, full-term mouse pups from oocytes grown and matured in vitro with serum substitutes.

For in vitro growth and maturation of mouse oocytes (IVG-IVM), serum is added to media up to and including the stage of oocyte maturation; this subsequently supports oocytes through fertilization and early embryo development. However, problems may occur with sera, such as batch differences and issues of biosafety. The purpose of the present study was to determine the capacity for fertilization and pre- and post-implantation development of oocytes that underwent IVG-IVM with a serum substitute. Oocyte-granulosa cell complexes from preantral follicles were cultured in medium with either fetal bovine serum (FBS), Serum Substitute Supplement™ (SSS), or Knockout™ Serum Replacement (KSR) for 10days, and were then allowed to mature for 17 h. Subsequently, more than 90% of oocytes underwent germinal vesicle breakdown (GVBD) and more than 70% reached metaphase II, with no significant difference between the groups. A lower fertilization rate, presumably due to zona hardening, was found in the serum substitute groups. Nevertheless, more than 50% of the inseminated oocytes were fertilized and 35%-45% of them underwent first cleavage and developed to the blastocyst stage. Following embryo transfer, one and four live offspring were produced from the SSS and KSR groups, respectively. The present study demonstrated that murine IVG-IVM oocytes cultured in media with a serum substitute, achieved fertilization in vitro, pre- and post-implantation development, and the delivery of live pups, although the efficiency of the process is reduced compared to FBS supplementation.

Establishment of reference values for complete blood count and blood gases in cynomolgus monkeys (Macaca fascicularis).

Cynomolgus monkeys are closely related to humans phylogenetically, and this has resulted in their widespread use as a preclinical model. Hematological data with regard to these monkeys are thus important. Although reference values for blood components and sex hormones have been established for cynomolgus monkeys, those for arterial blood gases have not. The arterial blood gases quickly reflect respiratory and circulatory dynamics, and are thus useful for animal management and safe general anesthesia and surgical operations. Furthermore, since O2 is transported by RBC, CBC and blood gases are closely related. The present study aimed to establish reference values for arterial blood gases and CBC in cynomolgus monkeys over a wide age range. Blood gases and CBC of arterial blood, collected from 41 female and 21 male anesthetized monkeys, were measured. Age correlated with RBC, HGB and HCT in the CBC. Values differed significantly between males and females in pCO2, CO2 concentration, MCV and MCH. The pH of blood was equivalent to that of humans and pCO2 was more stable, whereas MCV and MCH were lower than those in humans. Erythrocytes were smaller and less pigmented than in other Macaca species. Several relationships between gender and age, and blood gases and CBC were identified in cynomolgus monkeys. In conclusion, these reference values will be useful as markers for veterinary applications and in the care and maintenance of these animals.

Discrimination of Stem Cell Status after Subjecting Cynomolgus Monkey Pluripotent Stem Cells to Naïve Conversion.

Experimental animal models have played an indispensable role in the development of human induced pluripotent stem cell (iPSC) research. The derivation of high-quality (so-called "true naïve state") iPSCs of non-human primates enhances their application and safety for human regenerative medicine. Although several attempts have been made to convert human and non-human primate PSCs into a truly naïve state, it is unclear which evaluation methods can discriminate them as being truly naïve. Here we attempted to derive naïve cynomolgus monkey (Cm) (Macaca fascicularis) embryonic stem cells (ESCs) and iPSCs. Several characteristics of naïve Cm ESCs including colony morphology, appearance of naïve-related mRNAs and proteins, leukaemia inhibitory factor dependency, and mitochondrial respiration were confirmed. Next, we generated Cm iPSCs and converted them to a naïve state. Transcriptomic comparison of PSCs with early Cm embryos elucidated the partial achievement (termed naïve-like) of their conversion. When these were subjected to in vitro neural differentiation, enhanced differentiating capacities were observed after naïve-like conversion, but some lines exhibited heterogeneity. The difficulty of achieving contribution to chimeric mouse embryos was also demonstrated. These results suggest that Cm PSCs could ameliorate their in vitro neural differentiation potential even though they could not display true naïve characteristics.

Effective use of the TSPY gene-specific copy number in determining fetal DNA in the maternal blood of cynomolgus monkeys.

Since the available concentration of single-copy fetal genes in maternal blood DNA is sometimes lower than detection limits by PCR methods, the development of specific and quantitative PCR detection methods for fetal DNA in maternal blood is anticipated, which may broaden the methods that can be used to monitor pregnancy. We used the TaqMan qPCR amplification for DYS14 multi-copy sequence and the SRY gene in maternal blood plasma (cell-free DNA) and fractional precipitated blood cells (cellular DNA) from individual cynomolgus monkeys at 22 weeks of pregnancy. The availability of cell-free fetal DNA was higher in maternal blood plasma than that of cellular DNA from fractional precipitated blood cells. There was a significantly higher (P < 0.001) mean copy number of fetal male DYS14 from maternal plasma (4.4 × 10(4) copies/mL) than that of detected fetal cellular DNA from fractional blood cell pellets. The sensitivity of the DYS14 PCR assay was found to be higher than that of the SRY assay for the detection of fetal DNA when its presence was at a minimum. The DYS14 assay is an improved method for quantifying male fetal DNA in circulating maternal blood in the primate model.

Detection and quantification of male-specific fetal DNA in the serum of pregnant cynomolgus monkeys (Macaca fascicularis).

Because of their developmental similarities to humans, nonhuman primates are often used as a model to study fetal development for potential clinical applications in humans. The detection of fetal DNA in maternal plasma or serum offers a source of fetal genetic material for prenatal diagnosis. However, no such data have been reported for cynomolgus monkeys (Macaca fascicularis), an important model in biomedical research. We have developed a specific, highly sensitive PCR system for detecting and quantifying male-specific fetal DNA in pregnant cynomolgus monkeys. We used multiplex quantitative real-time PCR to analyze cell-free DNA in maternal blood serum obtained from 46 pregnant monkeys at gestational weeks 5, 12, and 22. The presence of SRY gene and DYS14 Y chromosomal sequences was determined in 28 monkeys with male-bearing pregnancies. According to confirmation of fetal sex at birth, the probe and primers for detecting the Y chromosomal regions at each time point revealed 100% specificity of the PCR test and no false-positive or false-negative results. Increased levels of the SRY-specific sequences (mean, 4706 copies/mL serum DNA; range, 1731 to 12,625) and DYS14-specific sequences (mean, 54,814 copies/mL serum DNA; range, 4175-131,250 copies) were detected at week 22. The SRY- and DYS14-specific probes appear to be an effective combination of markers in a multiplex PCR system. To our knowledge, this report is the first to describe the detection of cell-free DNA in cynomolgus monkeys.

Single-step generation of rabbits carrying a targeted allele of the tyrosinase gene using CRISPR/Cas9.

Targeted genome editing of nonrodent mammalian species has provided the potential for highly accurate interventions into gene function in humans and the generation of useful animal models of human diseases. Here we show successful clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated (Cas)-mediated gene targeting via circular plasmid injection in rabbits. The rabbit tyrosinase gene (TYR) was effectively disrupted, and we confirmed germline transmission by pronuclear injection of a circular plasmid expressing humanized Cas9 (hCas9) and single-guide RNA. Direct injection into pronuclear stage zygotes was possible following an in vitro validation assay. Neither off-target mutagenesis nor hCas9 transgenesis was detected in any of the genetically targeted pups and embryos examined. Gene targeting with this rapid and simplified strategy will help accelerate the development of translational research using other nonrodent mammalian species.

Naïve-like conversion enhances the difference in innate in vitro differentiation capacity between rabbit ES cells and iPS cells.

Quality evaluation of pluripotent stem cells using appropriate animal models needs to be improved for human regenerative medicine. Previously, we demonstrated that although the in vitro neural differentiating capacity of rabbit induced pluripotent stem cells (iPSCs) can be mitigated by improving their baseline level of pluripotency, i.e., by converting them into the so-called "naïve-like" state, the effect after such conversion of rabbit embryonic stem cells (ESCs) remains to be elucidated. Here we found that naïve-like conversion enhanced the differences in innate in vitro differentiation capacity between ESCs and iPSCs. Naïve-like rabbit ESCs exhibited several features indicating pluripotency, including the capacity for teratoma formation. They differentiated into mature oligodendrocytes much more effectively (3.3-7.2 times) than naïve-like iPSCs. This suggests an inherent variation in differentiation potential in vitro among PSC lines. When naïve-like ESCs were injected into preimplantation rabbit embryos, although they contributed efficiently to forming the inner cell mass of blastocysts, no chimeric pups were obtained. Thus, in vitro neural differentiation following naïve-like conversion is a promising option for determining the quality of PSCs without the need to demonstrate chimeric contribution. These results provide an opportunity to evaluate which pluripotent stem cells or treatments are best suited for therapeutic use.

Role of retinoic acid and fibroblast growth factor 2 in neural differentiation from cynomolgus monkey (Macaca fascicularis) embryonic stem cells.

Retinoic acid is a widely used factor in both mouse and human embryonic stem cells. It suppresses differentiation to mesoderm and enhances differentiation to ectoderm. Fibroblast growth factor 2 (FGF2) is widely used to induce differentiation to neurons in mice, yet in primates, including humans, it maintains embryonic stem cells in the undifferentiated state. In this study, we established an FGF2 low-dose-dependent embryonic stem cell line from cynomolgus monkeys and then analyzed neural differentiation in cultures supplemented with retinoic acid and FGF2. When only retinoic acid was added to culture, neurons differentiated from FGF2 low-dose-dependent embryonic stem cells. When both retinoic acid and FGF2 were added, neurons and astrocytes differentiated from the same embryonic stem cell line. Thus, retinoic acid promotes the differentiation from embryonic stem cells to neuroectoderm. Although FGF2 seems to promote self-renewal in stem cells, its effects on the differentiation of stem cells are influenced by the presence or absence of supplemental retinoic acid.