Chimerism in cattle through microsurgical aggregation of morulae

Testes of DAZL null neonatal sheep lack prospermatogonia but maintain normal somatic cell morphology and marker expression

Multiplying the germline would increase the number of offspring that can be produced from selected animals, accelerating genetic improvement for livestock breeding. This could be achieved by producing multiple chimaeric animals, each carrying a mix of donor and host germ cells in their gonads. However, such chimaeric germlines would produce offspring from both donor and host genotypes, limiting the rate of genetic improvement. To resolve this problem, we disrupted the RNA-binding protein DAZL and generated germ cell-deficient host animals. Using Cas9-mediated homology-directed repair (HDR), we introduced a DAZL loss-of-function mutation in male ovine fetal fibroblasts. Following manual single cell isolation, 4/48 (8.3%) of donor cell strains were homozygously HDR-edited. Sequence-validated strains were used as nuclear donors for somatic cell cloning to generate three lambs, which died at birth. All DAZL null male neonatal sheep lacked germ cells on histological sections and showed greatly reduced germ cell markers. Somatic cells within their testes were morphologically intact and expressed normal levels of lineage-specific markers, suggesting that the germ cell niche remained intact. This extends the DAZL mutant phenotype beyond mice into agriculturally relevant ruminants, providing a pathway for using absolute germline transmitters in rapid livestock improvement.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

An interspecies barrier to tetraploid complementation and chimera formation

To study development of the conceptus in xenogeneic environments, we assessed interspecies chimera formation as well as tetraploid complementation between mouse and rat. Overall contribution of donor PSC-derived cells was lower in interspecies chimeras than in intraspecies chimeras, and high donor chimerism was associated with anomalies or embryonic death. Organ to organ variation in donor chimerism was greater in interspecies chimeras than in intraspecies chimeras, suggesting species-specific affinity differences among interacting molecules necessary for organogenesis. In interspecies tetraploid complementation, embryo development was near normal until the stage of placental formation, after which no embryos survived.

Contributions of Mammalian Chimeras to Pluripotent Stem Cell Research

Chimeras are widely acknowledged as the gold standard for assessing stem cell pluripotency, based on their capacity to test donor cell lineage potential in the context of an organized, normally developing tissue. Experimental chimeras provide key insights into mammalian developmental mechanisms and offer a resource for interrogating the fate potential of various pluripotent stem cell states. We highlight the applications and current limitations presented by intra- and inter-species chimeras and consider their future contribution to the stem cell field. Despite the technical and ethical demands of experimental chimeras, including human-interspecies chimeras, they are a provocative resource for achieving regenerative medicine goals.

Describing the Stem Cell Potency: The Various Methods of Functional Assessment and In silico Diagnostics

Stem cells are defined by their capabilities to self-renew and give rise to various types of differentiated cells depending on their potency. They are classified as pluripotent, multipotent, and unipotent as demonstrated through their potential to generate the variety of cell lineages. While pluripotent stem cells may give rise to all types of cells in an organism, Multipotent and Unipotent stem cells remain restricted to the particular tissue or lineages. The potency of these stem cells can be defined by using a number of functional assays along with the evaluation of various molecular markers. These molecular markers include diagnosis of transcriptional, epigenetic, and metabolic states of stem cells. Many reports are defining the particular set of different functional assays, and molecular marker used to demonstrate the developmental states and functional capacities of stem cells. The careful evaluation of all these methods could help in generating standard identifying procedures/markers for them.

Embryo splitting affects the transcriptome during elongation stage of in vitro-produced bovine blastocysts

Embryo splitting has been used for the production of identical twins and to increase the pregnancy rate per available embryo. Split blastocysts can develop to term; however, little is known about the impact on gene expression of split embryos, especially at the whole transcriptome level. This work was aimed to evaluate the effect of blastocyst splitting on global gene expression profile at the elongation stage. For that, split and time-matched nonsplit (control group) bovine blastocysts were transferred to a bovine recipient and recovered at Day 17 of development. The number of collected embryos, their size, and global gene expression was compared between both groups. From 16 transferred split embryos, six (37.5%) were collected, whereas nine elongated were recovered from 17 nonsplit (52.9%). Neither the recovery rate nor the average length of the elongated embryos was significantly different between both groups. However more than 50% of embryos from the control group had a length surpassing 100 mm, whereas only 33% of the split embryos reached that size. Global gene expression was performed in individual elongated embryos from both groups using Two-Color Microarray-Based Gene Expression Analysis. From detected genes, 383 (1.31%) were differentially expressed between both groups, among them, 185 (0.63%) were downregulated and 198 (0.67%) genes were upregulated in split embryos. Bioinformatic analysis of differentially expressed genes revealed that embryo splitting affects transcriptomes of resulting elongated embryos, mainly downregulating genes involved in matrix remodelation, control of growth, detoxification, and transport of metabolites. These in turns might have a detrimental impact on the developmental potential of produced embryos.

Stem cell potency and the ability to contribute to chimeric organisms

Mouse embryonic chimeras are a well-established tool for studying cell lineage commitment and pluripotency. Experimental chimeras were successfully produced by combining two or more preimplantation embryos or by introducing into host embryo cultured pluripotent embryonic stem cells (ESCs). Chimera production using genetically modified ESCs became the method of choice for the generation of knockout or knockin mice. Although the derivation of ESCs or ESC-like cells has been reported for other species, only mouse and rat pluripotent stem cells have been shown to contribute to germline-competent chimeras, which is the defining feature of ESCs. Herein, we describe different approaches employed for the generation of embryonic chimeras, define chimera-competent cell types, and describe cases of spontaneous chimerism in humans. We also review the current state of derivation of pluripotent stem cells in several species and discuss outcomes of various chimera studies when such cells are used.

Generation of chimeric rhesus monkeys

Totipotent cells in early embryos are progenitors of all stem cells and are capable of developing into a whole organism, including extraembryonic tissues such as placenta. Pluripotent cells in the inner cell mass (ICM) are the descendants of totipotent cells and can differentiate into any cell type of a body except extraembryonic tissues. The ability to contribute to chimeric animals upon reintroduction into host embryos is the key feature of murine totipotent and pluripotent cells. Here, we demonstrate that rhesus monkey embryonic stem cells (ESCs) and isolated ICMs fail to incorporate into host embryos and develop into chimeras. However, chimeric offspring were produced following aggregation of totipotent cells of the four-cell embryos. These results provide insights into the species-specific nature of primate embryos and suggest that a chimera assay using pluripotent cells may not be feasible.

Effect of blastomere sex and fluorescent labelling on the development of bovine chimeric embryos reconstituted at the four-cell stage

The development rate of bovine chimeric embryos reconstituted at the 4-cell stage is relatively low. If chimerism is to be used as an approach in producing transgenic livestock, it is important to investigate whether this rate is affected by the sex of the blastomeres being combined and if all blastomeres survive equally well. In Experiment 1, blastomeres from 4-cell stage embryos were inserted into surrogate zonae pellucidae either in pairs to reconstitute 4-cell chimeras, or as the original sets of four to make handled controls. The development of chimeras with one pair of blastomeres labelled with PKH26-GL was also investigated. The rate of development into blastocysts was similar in chimeras with unlabelled blastomeres (23%) and in those in which one pair of blastomeres was labelled (26%) and was lower (P < 0.001) than in the handled and IVF control groups (43 and 58%, respectively). Labelled cells were distributed approximately evenly between ICM and trophoblast. In Experiment 2, the effect of sex differences between pairs of blastomeres in chimeras was investigated; chimeras were reconstituted from pairs of blastomeres taken from 4-cell embryos in which the remaining pair was sexed by PCR. No significant differences according to the sex of constituent blastomeres were detectable (mixed sex, 27%; males, 24%; females, 21%; P > 0.05). These results suggest that, in addition to the negative effects of micromanipulation, factors other than the sex of the blastomeres are involved in the reduced rate of development of chimeric bovine embryos. They also confirm the usefulness of PKH26-GL labelling for tracking the progeny of cleaving bovine blastomeres at least to the blastocyst stage.

Fertility of sperm from a tetraparental chimeric bull

The purpose of the present study was to examine the ability of a tetraparental Chimera in producing IVF embryos. Cumulus oocytes complexes (COCs) were matured in vitro for 22 h. Frozen-thawed sperm of a Chimera (CH), as well as Japanese Black (JB), Limousin (L), Japanese Brown (JBr), Holstein (H) bulls were used for IVF. The chromosome preparations were made from peripheral lymphocytes. Based on chromosome analysis the Chimera had apparently normal chromosomes (29 acrocentric pairs, one large sub metacentric X chromosome and one small sub metacentric Y chromosome). The proportion of acrosome reacted spermatozoa after 1 h incubation was higher (P < 0.01) with the Chimera (CH) than with the Holstein and in Japanese Brown bulls, but did not differ from Japanese Black and Limousin bull sperm (79.0%, 71.2%, 72.5%, 57.8% and 57.0% for CH, JB, L, JBr and H sperm, respectively). Fertilization rates observed after 5 h of sperm-oocyte incubation with Chimera (O-CH) sperm were higher (P < 0.05) than with Japanese Brown (O-JBr) and (P < 0.01) than with Holstein (O-H) sperm, but did not differ from Japanese Black (O-JB) and Limousin (O-L) sperm (36/44, 81.8%; 28/35, 80.0%; 25/36, 69.4%; 19/43, 44.2% and 6/33, 18.2% for O-CH, O-JB, O-L, O-JBr and O-H, respectively). The cleavage rates of IVM oocytes inseminated with Chimera sperm were also higher (P < 0.001) than in Holstein, (P < 0.01) Japanese Brown and (P < 0.05) Limousin, but did not differ from Japanese Black sperm (181/239, 75.7%; 123/171, 71.9%; 108/186, 58.1%; 80/196, 40.8% and 30/186, 16.1% for O-CH, O-JB, O-L, O-JBr and O-H, respectively). The blastocyst rates of IVM oocytes inseminated with sperm were higher (P < 0.05) than in Limousin, Japanese Brown and Holstein, but did not differ from Japanese Black (69/181, 38.1%; 48/123, 39.0%; 27/108, 25.0%; 7/30, 23.3% and 16/80, 17.8% for O-CH, O-JB, O-L, O-JBr and O-H, respectively). Three findings suggested the sperm from this tetraparental Chimeric bull were able to be used for producing bovine IVF embryos.

Current status and potential of embryo transfer and reproductive technology in dairy cattle

Significant use of embryo transfer in dairy cattle commenced with the introduction of nonsurgical embryo recovery in the mid-1970s and developed with the use of nonsurgical transfers in the late 1970s. Numbers of registered Holstein calves from embryo transfer doubled yearly through 1980, after which the rate of increase slowed; the total reached nearly 19,000 calves in 1990. However, the efficacy of superovulation procedures and commercial success rates of transferred fresh embryos have not improved the past 10 to 15 yr. Fertilization rates in superovulated donors remain low. Although embryo-splitting techniques were perfected in the early 1980s, they are not used widely. A practical, commercial embryo-sexing procedure remains unavailable. Recent significant improvement is apparent in the technology of ultrasound-guided oocyte collection and in vitro oocyte maturation, fertilization, and embryo culture. In the future, this technology may be used in conjunction with sperm separated by sex with a flow cytometer. Modest numbers of embryo clones have been produced in several commercial programs via nuclear transfer techniques. However, the efficiency of gene transfer experiments involving ova of cattle and other domestic species has been low. Recently, DNA probe technology has begun to provide genotype information for cattle and will ultimately be applied to embryos.

Micromanipulation of mammalian embryos: Principles, progress and future possibilities

Numerous advances in development of techniques for manipulating mammalian embryos outside the maternal environment have been made over the past decade. Some techniques were developed primarily for use in research; others were developed in response to problems of practical livestock production but have proven useful in research as well. Embryo micromanipulation procedures are used often in conjunction with embryo transfer, and interest in these procedures was stimulated by growth of the embryo transfer industry. Included in this review are discussions of procedures for manipulation of gametes and embryos, including sperm injection into oocytes, pronuclear and nuclear transfer, embryo biopsy and splitting, experimental chimera production and isolation of embryonic stem cells.

Production of chimaeric bovine embryos and calves by aggregation of inner cell masses with morulae

Bovine inner cell masses (ICMs) were isolated by immunosurgery at day 8 or 10, or by dissection at day 14, and combined with day-5.5 morulae. Aggregation was obtained between 89%, 62%, and 0% of the day-5:day-8, day-5:day-10, day-5:day-14 composites, respectively. Chromosome analysis of composites, respectively. Chromosome analysis of composites potentially carrying the 1/29 translocation as a chromosome marker and temporarily transferred to the bovine uterus for 8 days showed that chimaeric day-14 embryos can be obtained from day-5:day-8 aggregation. The definitive transfer of eight day-5:day-8 and 11 day-5:day-10 composites resulted in the birth of six and four calves, respectively; five of the six, but none of the four, were chimaeric. The five chimaeras showed mostly the ICM phenotype. The morphological differences between ICMs at different stages of development were examined by electron microscopy and related to the success of the aggregation technique. It is concluded that bovine embryonic cells can regulate for at least 3 days difference in development but not 5 days even though aggregation is still possible.