Natural Causes for Low Human Fertility

Review Article

Austin J In Vitro Fertili. 2014;1(2): 5.

Natural Causes for Low Human Fertility

Delimitreva S*, Zhivkova R and Markova M

Department of Biology, Medical University of Sofia, Bulgaria

*Corresponding author: Delimitreva S, Department of Biology, Medical University of Sofia, 2 Zdrave St, 1431 Sofia, Bulgaria

Received: October 08, 2014; Accepted: October 27, 2014; Published: October 29, 2014

Abstract

Despite the efforts to improve the conditions of gamete and embryo culture, the success rate of assisted reproductive techniques in humans is limited. This suggests that the reduced embryo potential could not be a result only of suboptimal in vitro conditions, but real features of oogenesis and early embryogenesis are involved. Low developmental potential of embryos is related to high rate of chromosomal abnormalities. The chromosomal segregation errors are based on disturbance of interactions between chromatin, cytoskeleton and other factors involved in the last stages of mammalian oocyte meiosis.

Keywords: Mammalian oocyte; Meiosis; Preimplantation embryo; In vitro fertilization; Aneuploidy

Abbreviations

IVF: In Vitro Fertilization; IVM: In Vitro Maturation; ART: Assisted Reproductive Techniques; aCGH: Array Comparative Genomic Hybridization

Introduction

Studies on early mammalian embryogenesis started with the work of Carl Hartman. In 1931, he published his observations on bovine preimplantation embryos obtained by oviductal flushing [1]. Later, a similar approach was applied to obtain oocytes and preimplantation embryos from other mammalian species with easily predictable or controllable oestrus with multiple ovulations - rabbits, rodents and ungulates [2-5]. The techniques of human IVF (in vitro fertilization) were based on these investigations. Studies of mammalian oocytes and preimplantation embryos became the base for development of assisted human reproduction, as well as for fundamental investigations of human oogenesis and early embryogenesis.

Limited Success Rate of IVF

In vitro fertilization was developed as an approach for treatment of human infertility. However, its introduction was soon followed by disappointment of the relatively limited success rate of the method. It was initially assumed to be a result of suboptimal conditions of in vitro culture systems, hormonal stimulation and techniques for oocyte aspiration. Hence, efforts for improving success rates were directed towards optimization of these factors. A major step in the development of ART (assisted reproductive techniques) was provided by the introduction of gamete manipulations. As a result of their application, the success rate of ART was brought close to that of natural conception-according to current official data of ESHRE, the success rate of ART in European clinics is approximately 30%, and this proportion has remained stable [6,7]. Moreover, even this rate is due to the fact that in each treatment cycle, typically 2 or 3 embryos were transferred into the uterus. Now, it is accepted that the chance for each embryo to be implanted is approximately 15% [8,9]. These facts indicate that the potential to improve the success rate of ART, though not yet exhausted, is limited. Hence, the decisive factor for the treatment success must be some natural characteristics of the embryo.

Morphological Defects of Preimplantation Embryos

The most obvious abnormality of human IVF embryos is the high rate of morphological disorders - cell fragments, differences in blastomere size, enucleated and multinucleated cells, apoptotic-like cells and inadequate blastomere number are observed in more than half of the embryos [9-12]. These data can be summarized in Table 1. For the last decade, elaboration of culture and manipulation techniques has not brought significant improvement of these data.

Table 1: Morphological defects of preimplantation embryos.




  

    
Morphological abnormality

    
Embryos able to

      
form blastocyst

  

  

    
Cell fragments occupy up to 15% 

      
of the volume of cleavage    embryo

    
33 %

      
 

  

  

    
Cell fragments occupy more than 15% 

      
of the volume of cleavage embryo 

    
17 %

      
 

  

  

    
The number of blastomeres -    more

      
than expected 

    
28 %

  

  

    
The number of blastomeres -    less

      
than expected 

    
14 %

      
 

  









Table 1:  Morphological defects of preimplantation embryos.

Cytogenetic Abnormalities of Human Oocytes

The first published cytogenetic study of human oocytes was performed by Edwards, 1968, and of human embryos by Angell, 1983 [13,14]. The metaphase state of ovulated mammalian oocytes facilitates karyotyping of unfertilized eggs because pretreatment with cytostatic is not required. Data accumulated so far indicate that a substantial part of oocytes derived both by natural and stimulated ovulation, carry numerical chromosomal anomalies. In different periods, different authors have published widely varying aneuploidy rates for human oocytes unfertilized after IVF. If extreme values are excluded, the reported percentages of aneuploid oocytes are between 25% and 35% [15,16]. Although data about human oocytes obtained after natural ovulation are scarce, they are very similar to those mentioned above [17]. Data for non-primate mammals show substantial differences - their rate of meiotic chromosomal abnormalities is ten times lower than those observed in human [18], while oocytes of rhesus monkeys and marmosets produce results similar to those of humans [19,20]. Therefore, it is assumed that chromosomal anomalies of human oocytes are not a methodological artifact but are associated with natural imperfection of primate female meiosis. It was initially hypothesized that abnormal oocyte karyotype would prevent gamete fusion, but this was not confirmed [21]. Rather, chromosomal errors in the embryo, which are in most cases oocyte-derived, interfere with its potential for post-fertilization development. It can become evident as failure of implantation or at an even later stage, as spontaneous abortion.

Cell Cycle Control

Cell cycle control machinery in eukaryotes prevents the completion of cell division in cells which do not fulfill a set of criteria. These control mechanisms are universal and evolutionary conserved. However, in mammalian meiosis the division control shows some uncommon features - first, its efficiency is not full, and second, the efficiency is different for males and females.

Numerical chromosome aberrations are observed in 10-25% of human fetuses. Nearly 80% of them are due to errors during metaphase-anaphase transition of the first meiotic division in oocytes [18]. They are result of prematurely segregated bivalents or sister chromatics, unnoticed by the cell cycle control.

The meiotic checkpoint responsible for the exactness of metaphase I- anaphase I transition (spindle checkpoint) corresponds to the mitotic checkpoint which permits anaphase onset after estimation of spindle integrity and chromosome-tubulin interactions. Meiotic spindle checkpoint is supposed to arrest chromosomally abnormal oocytes at metaphase I, preventing their further maturation, fertilization and cleavage. But in oocytes the efficiency of the control during this passage is diminished (compared with male meiosis and mitosis). This fact is pointed out as one of the major reasons for human oocyte aneuploidy [22,18]. Chromosomal errors are possible also in spermatogenesis, but they are eliminated by the cell cycle control far more effectively. For example, several point mutations in mice, which cause malsegregation of bivalents, in males lead to apoptosis of spermatocytes with abnormal karyotype. In female mice these mutations do not diminish the number of ovulated oocytes, but increase the proportion of aneuploid oocytes [22]. A type of male infertility in human is associated with presence of monovalents at metaphase I and lack of progress to metaphase II, which is a result of effective spindle control. It is proved that the efficiency of the control in females is not full both for hormonally stimulated ovulation and natural cycles. Experimental data for mouse show that the efficiency of the spindle checkpoint depends on genetic background (mated mouse strains) and on the oocyte maturation conditions [18,23].

Cell cycle control is relaxed also during the first zygotic mitoses. This is illustrated by the following facts: the proportion of chromosomally abnormal embryos exceeds that of aneuploid oocytes; blastomeres with chromosomal anomalies divide normally and can be seen also in embryos without morphological defects; the high level of mosaicism shows that segregation errors arise with high probability in each of the mitoses until 16-cell stage and the morula compaction. It is supposed that normal cell cycle control is established during the morula-blastocyst transition.

Aneuploidy Rate in Human Preimplantation Embryos

In human embryos obtained in vitro, aneuploidies, mosaics of diploid and aneuploid cells, polyploidy and haploidy have been registered [10,14]. For cleavage stage embryos, it has been observed that the level of chromosomal errors is about three times higher in embryos with morphological defects - 37% vs. 12% for morphologically normal embryos [14]. In the late 1980s, the rates of embryonic aneuploidy reported by different authors varied widely. This could be explained by the small number of embryos studied and by the low efficiency of classical karyotyping used at that time. The mean percentage of cleavage stage embryos with chromosomal errors found till then was 23% [24]. As with oocytes, it was observed that chromosomal anomalies are more frequent for human early embryos than for those of other mammals. For example, in mice this rate varied between 3% and 21%, and in investigated domestic animals it was between 7% and 10%. After introduction of FISH for cytogenetic testing of blastomeres, data for human embryos varied between 15% and over 85%. This great discrepancy was due to differences in embryo cohorts included in the studies - embryos with or without morphological defects, with progressing or arrested divisions, and originating from different groups of patients [8,25,26]. The data can be summarized as follows:

Citation: Delimitreva S, Zhivkova R and Markova M. Natural Causes for Low Human Fertility. Austin J In Vitro Fertili. 2014;1(2): 5. ISSN:2471-0628