In vitro fertilization and preimplantation embryo development of African green monkeys (Cercopithecus aethiops)

Assisted reproductive technologies in the common marmoset: an integral species for developing nonhuman primate models of human diseases

Generation of nonhuman primate models of human disease conditions will foster the development of novel therapeutic strategies. Callithrix jacchus, or the common marmoset, is a New World, nonhuman primate species that exhibits great reproductive fitness in captivity with an ovarian cycle that can be easily managed with pharmacological agents. This characteristic, among others, provides an opportunity to employ assisted reproductive technologies to generate embryos that can be genetically manipulated to create a variety of nonhuman primate models for human disease. Here, we review methods to synchronize the marmoset ovarian cycle and stimulate oocyte donors, and compare various protocols for in vitro production of embryos. In light of advances in genomic editing, recent approaches used to generate transgenic or genetically edited embryos in the marmoset and also future perspective are reviewed.

Assisted reproductive technology in nonhuman primates

Nonhuman primates (NHP) are the closest animal species to humans and have been widely used for studying human reproductive physiology. Assisted reproductive technology (ART) in Old World NHPs provides great opportunity for studying fertilization, embryo development, embryonic stem cell (ESC) derivation for regenerative medicine, somatic cell nuclear transfer (cloning), and transgenic NHP models of inherited genetic disorders. Here we present two ART protocols developed for rhesus monkey (Macaca mulatta) and baboon (Papio cynocephalus).

Characterization of a novel embryonic stem cell line from an ICSI-derived blastocyst in the African green monkey

Several cell types from the African green monkey (Cercopithecus aethiops), such as red blood cells, primary culture cells from kidney, and the Vero cell line, are valuable sources for biomedical research and testing. Embryonic stem (ES) cells that are established from blastocysts have pluripotency to differentiate into these and other types of cells. We examined an in vitro culture system of zygotes produced by ICSI in African green monkeys and attempted to establish ES cells. Culturing with and without a mouse embryonic fibroblast (MEF) cell monolayer resulted in the development of ICSI-derived zygotes to the blastocyst stage, while culturing with a buffalo rat liver cell monolayer yielded no development (3/14, 21.4% and 6/31, 19.4% vs 0/23, 0% respectively; P<0.05). One of the nine blastocysts, which had been one of the zygotes co-cultured with MEF cells, formed flat colonies consisting of cells with large nuclei, similar to other primate ES cell lines. The African green monkey ES (AgMES) cells expressed pluripotency markers, formed teratomas consisting of three embryonic germ layer tissues, and had a normal chromosome number. Furthermore, expression of the germ cell markers CD9 and DPPA3 (STELLA) was detected in the embryoid bodies, suggesting that AgMES cells might have the potential ability to differentiate into germ cells. The results suggested that MEF cells greatly affected the quality of the inner cell mass of the blastocysts. In addition, AgMES cells would be a precious resource for biomedical research such as other primate ES cell lines.

Kinetics of pronuclear development and the effects of vector type and timing of injection on the efficiency of gene transfer into rhesus macaque embryos

A series of experiments was performed to determine the dynamics of pronuclear development as well as the efficiency of either adenovirus-associated (AAV) or lentivirus-derived vectors to introduce a green fluorescent protein (GFP) reporter gene into rhesus macaque (Macaca mulatta) embryos. Assessment of pronuclear development at various times after fertilization revealed that the appearance of pronuclei was determined by the presence of the first and the timing of the second polar body. The dynamics of pronuclear formation was a significant determinant of whether an oocyte reached the blastocyst stage, however, when the percentage of blastocysts were based on the number of zygotes, the timing of the appearance of polar bodies did not appear to have any effect on subsequent development. Injection of different AAV-derived vectors showed that the serotype of the vector did not affect development or the proportion of transgenic embryos. Moreover, all putative transgenic embryos proved to be expression mosaics. Injection of embryos with lentiviral vectors showed that timing of injection (before or after fertilization) had no effect on subsequent transgene expression, but that the type of reporter gene determined post-injection development and rate of transgenesis. The transfer of embryos following injection of a lentiviral vector into three recipients resulted in one pregnancy which was lost during the second trimester. Analysis of fetal tissues showed ubiquitous presence of the transgene and GFP expression in all tissues examined. These results show that lentivirus-derived vectors can efficiently transform rhesus embryos and are suitable for the generation of transgenic rhesus monkeys.

Characteristics and regulation of the ovarian cycle in vervet monkeys (Chlorocebus aethiops)

This study was designed to evaluate the timecourse of ovarian and pituitary endocrine events throughout the menstrual cycle in the vervet monkey, and whether circulating luteinizing hormone (LH) or the uterus regulates the functional lifespan of the vervet corpus luteum. Daily saphenous blood samples were collected from adult females (1) during spontaneous menstrual cycles (n = 7), and (2) during cycles in which a gonadotropin-releasing hormone antagonist (acyline) was administered for 3 days at midluteal phase (n = 3), and (3) for 30 days following recovery from hysterectomy (n = 4). Estradiol (E) and progesterone (P) levels were assayed using electrochemoluminescent assays. Gonadotropin levels were measured by radioimmunoassay using reagents developed for the assay of follicle-stimulating hormone and LH in macaques. Spontaneous cycles exhibited a midcycle E rise (476+/-49 pg/ml), engendering an LH surge, 12+/-1 days after onset of menses, followed by a luteal phase with peak P levels of 4.7+/-0.9 ng/ml. Histologic evaluation of the ovaries at late follicular phase or early luteal phase revealed the presence of a single, large Graafian follicle or developing corpus luteum, respectively. Acyline treatment caused a significant (P<0.05) decline in P levels (2.9+/-0.5 vs 0.5+/-0.3 ng/ml, 0 vs 48 h post-treatment) and premature menstruation compared with untreated controls (P<0.05). Hysterectomy had no apparent effect on the monthly pattern or levels of circulating E or P. Thus, the characteristics and regulation of the ovarian cycle in vervets appear similar to those in women and macaques, with cyclicity dependent on pituitary gonadotropin hormones and independent of a uterine luteolytic factor.

Evaluation of the vervet (Clorocebus aethiops) as a model for the assisted reproductive technologies

The vervet monkey was evaluated as a primate model for use in assisted reproductive technologies (ARTs). Eight adult female vervets were hormonally monitored for their potential use as egg donors and those six females displaying regular menstrual cycles were subjected to controlled ovarian stimulation with recombinant human gonadotropins. Three animals failed to respond while laparoscopic follicular aspiration was performed on the other three females at 27-30 h post-human chorionic gonadotropin administration. A total of 62, 40, and 18 oocytes was recovered from these three animals of which 30, 20, and 4, respectively, matured to the metaphase II stage and were subsequently inseminated using intracytoplasmic sperm injection. An average of 40+/-15% (SEM) of the inseminated oocytes were fertilized based on pronucleus formation and timely cleavage. One embryo from each of the two stimulated females developed into expanded blastocysts. Two adult male vervets were assessed as sperm donors. Neither adjusted well to the restraint and collection procedure required for penile electroejaculation. Samples collected via rectal electroejaculation were very low in sperm motility and concentration; however, cauda epididymal aspirations from one male yielded an adequate concentration of motile sperm. These results emphasize the need to establish species-specific ovarian stimulation protocols and semen collection techniques if vervets are to be considered for basic and applied (ARTs) research on primate gametes or embryos.

Comparison of methods to stimulate ovarian follicular growth in cynomolgus and African green monkeys for collection of mature oocytes

The objective was to compare various gonadotropin-based methods to stimulate ovarian follicular growth in female cynomolgus (n=16) and African green monkeys (n=8) for collection of mature oocytes. On the 1st day of menstruation, the monkeys were treated with 3.75 mg leuprorelin acetate (a GnRH agonist). Starting 2-3 weeks later, ovarian follicular growth was stimulated as follows: (a) 25 IU/kg of human FSH (hFSH) in a glycerol solution given once daily for 9 d; (b) 200 IU of eCG given six times during a 9-d interval; (c) 75 IU/kg hFSH in a glycerol solution given three times (72 h intervals) during a 6-d interval. In addition, the monkeys were given 1200 or 4000 IU of hCG 36 h (Methods A and B) or 60 h (Method C) after the last gonadotropin treatment, and oocyte collection was attempted 36-38 h after hCG. Although there were no significant differences among methods in the number of oocytes collected, in cynomolgus monkeys, hFSH (Methods A and C) was better than eCG (Method B; 12 and 10 versus 7 mature oocytes, respectively), whereas in African green monkeys, eCG (Method B) was more effective than hFSH (Method A; 12 versus 7 mature oocytes). Furthermore, in cynomolgus monkeys, Method C was nearly as effective as Method A; using a glycerol solution as a solvent decreased the frequency of hFSH administration from nine to three times. In conclusion, in cynomolgus and African green monkeys, ovarian response depended on the species and on the individual, and in cynomolgus monkeys, hFSH in a glycerol solvent was effective.

Overriding follicle selection in controlled ovarian stimulation protocols: quality vs quantity

Selection of the species-specific number of follicles that will develop and ovulate during the ovarian cycle can be overridden by increasing the levels of pituitary gonadotropin hormones, FSH and LH. During controlled ovarian stimulation (COS) in nonhuman primates for assisted reproductive technology (ART) protocols, the method of choice (but not the only method) has been the administration of exogenous gonadotropins, either of nonprimate or primate origin. Due to species-specificity of the primate LH (but not FSH) receptor, COS with nonprimate (e.g., PMSG) hormones can be attributed to their FSH activity. Elevated levels of FSH alone will produce large antral follicles containing oocytes capable of fertilization in vitro (IVF). However, there is evidence that LH, probably in lesser amounts, increases the rate of follicular development, reduces heterogeneity of the antral follicle pool, and improves the viability and rate of pre-implantation development of IVF-produced embryos. Since an endogenous LH surge typically does not occur during COS cycles (especially when a GnRH antagonist is added), a large dose of an LH-like hormone (i.e., hCG) may be given to reinitiate meiosis and produce fertilizable oocytes. Alternate approaches using exogenous LH (or FSH), or GnRH agonist to induce an endogenous LH surge, have received lesser attention. Current protocols will routinely yield dozens of large follicles with fertilizable eggs. However, limitations include non/poor-responding animals, heterogeneity of follicles (and presumably oocytes) and subsequent short luteal phases (limiting embryo transfer in COS cycles). However, the most serious limitation to further improvements and expanded use of COS protocols for ART is the lack of availability of nonhuman primate gonadotropins. Human, and even more so, nonprimate gonadotropins are antigenic in monkeys, which limits the number of COS cycles to as few as 1 (PMSG) or 3 (recombinant hCG) protocols in macaques. Production and access to sufficient supplies of nonhuman primate FSH, LH and CG would overcome this major hurdle.