Pursuit of Markers to Assess and Select Competence of <em>invitro</em>-Produced Embryos

Review Article

Austin J In Vitro Fertili. 2015;2(2): 1018.

Pursuit of Markers to Assess and Select Competence of invitro-Produced Embryos

Rodriguez-Alvarez L¹* and Velasquez AE

Department of Animal Science, Universidad de Concepcion, Chile

*Corresponding author: Rodriguez-Alvarez Ll, Department of Animal Science, Faculty of Veterinary Sciences, Universidad de Concepcion, Avenida Vicente Mendez 595, Chillan, Chile

Received: April 28, 2015; Accepted: June 16, 2015; Published: June 18, 2015


Morphological classification has been used as a conventional and noninvasive method to select pre-implantation embryos for transferring to recipients. However, low implantation rate and embryonic mortality after embryo transfer, specially using invitro produced or manipulated embryos, indicate than this method is not reliable enough to reflect the developmental competence of an embryo and it needs to be improved for biological and applied needs. In fact, embryo viability and competence correlate better with gene expression pattern than with embryo morphology. Nevertheless, the analysis of gene expression in pre-implantation embryos is an invasive procedure that most frequently implies the lysis of the embryo or a part of it. In this context, the identification of secreted markers linked to embryo quality and development competence may be a useful tool to classify pre-implantation embryos. Candidates may be studied in order to define suitable markers for embryo selection, widely expressed across species and correlated to developmental capacity and survival up to term. This paper presents a review of the literature on the different methods that may be used for embryo scoring as well of those used to predict embryo competence and quality. The most practical methods are those that consider embryo morphology. However, it seems that the molecular signature of each individual embryo is more predictive of its competence and capability to produce a healthy offspring. Many studies and experiments are required to propose a consistent method for embryo selection applicable in the assisted reproductive technologies in both humans and animals.

Keywords: Embryo competence; Embryo selection; Embryo development; ART


Establishment and maintenance of a pregnancy able to produce a healthy offspring is the main goal of any reproductive system. In a natural conception, pregnancy lost before the 20th week of gestation reaches 75% in humans and it is attributed to implantation failure [1]. In farm animals, fertility defined as pregnancy rate per cycle might vary from 50 to 80 % and most of pregnancy loss occurs during the first three weeks due to defective embryo development [2-4]. It has been determined that pregnancy success depends on both embryo quality and uterine environment. In fact, embryonic loss is most likely attributed to maternal conditions such as animal age, nutritional factors, stress and uterine infections [3-9] in naturally occurring conceptions. However, invitro-produced embryos have a reduced developmental potential, resulting in a low implantation rate and an increase in the frequency of early pregnancy loss. Most of the in-vitro produced embryos are not able to function within a normal development schedule so that embryos can stop developing at any stage. Moreover, some of the embryos that reach the implantation stage do not induce a proper signal for pregnancy recognition and embryo-maternal crosstalk.

The development of Assisted Reproductive Technologies (ARTs) represents a great advance in both commercial and basic studies in animals, as well as in the treatment of infertility in humans [10-16]. However, despite continuous efforts to improve embryo development and competence, competent embryos produced invitro nowadays do not exceed 50 % of the total number of fertilized oocytes [17,18], while less than 40 % of transferred embryos produce a healthy offspring [18-21]. In humans, 8 out of 10 transferred embryos will not result in a pregnancy [22]. The low developmental potential of in-vitro produced embryos is mainly due to suboptimal conditions provided during oocyte collection and maturation, fertilization and embryo culture [23-25].

Scoring and selection of invitro-produced embryos

Morphology as a primary criterion for embryo selection: After invitro embryo production, selecting healthy embryos with the best potential to implant and produce an offspring is one of the major tasks for an embryologist. In humans, low embryonic competence is often handled by transferring several embryos to ensure a birth [26]. This practice is frequently the cause of preterm deliveries and other health complications for both the baby and the mother [27]. In farm animals, the main goal of ARTs is to multiply high value animals, which in the long-term contributes to the development of animal agriculture [17]. However, transfers of low competent embryos have a negative economic impact due to the direct costs associated with the maintenance of empty receptors, the price of supplies for embryo transfer and costs of the embryo or fetus lost.

In order to avoid the above-mentioned problems, it is mandatory to perform an accurate embryo selection before transferring. In general, morphological parameters are widely used criteria for embryo selection in all species. Morphology and timing of embryo development (the time of first cell cleavage and when embryos reach the morula or the blastocyst stages) are simple and non-invasive for the embryo. In those species in which embryos are transferred at early stages, the morphological selection is based on the Pronucleus (PN) size and location within either the zygote or the number and size of the blastomeres, as well as the fragmentation percentage in later stages (2-8 cells) [28-30]. In humans, some studies have demonstrated that embryo scoring using the PN characteristics might improve embryo selection. However, the use of this parameter is restricted to those species with visible PN at the zygote stage. Embryos from ruminants or pigs have a dark cytoplasm so that visualizing the PN is almost impossible.

When embryos at more advanced stages are transferred, for instance, at the blastocyst stage, criteria such as blastocyst expansion, quality of the Inner Cell Mass (ICM) and Trophoectoderm (TE) and grade of fragmentation are used for embryo classification [31,32]. Gardner and Schoolcraft [31] determined that the pregnancy rate in humans can be grater than 60 % by transferring a blastocyst with an ICM containing many tightly packed cells and a TE with many cells forming a cohesive layer. However, in some cases, blastocyst scoring might be challenging and depend on the subjective criterion of the embryologist. Moreover, the development schedule of invitroproduced embryos is very heterogeneous in concordance with their competence. In fact, even grade I embryos are often unable to maintain a normal pregnancy [18,21,33-36]. This statement has been demonstrated in several species; transferring grade I embryos produced by invitro fertilization or somatic cell nucleus transfer generate low rate of implantation and development to term [18,21,33- 37]. As a concrete example, we found that transferring bovine cloned blastocyst with a very similar morphology (Figure 1) produced only 33 % of pregnancy (day 35) and 11 % of calving [18]. Furthermore, selection of human embryos based on morphology cannot predict chromosome aneuploidies [38]. It is true that blastocyst morphology correlates with the incidence of aneuploidy. However, a high proportion of good and fair human blastocyst are aneuploidy (32 and 41 %, respectively) [37].

Citation: Rodriguez-Alvarez Ll and Velasquez AE. Pursuit of Markers to Assess and Select Competence of invitro-Produced Embryos. Austin J In Vitro Fertili. 2015;2(2): 1018. ISSN:2471-0628