Embryology

A recent case report of sex-discordant monochorionic twins has challenged the notion that monochorionic twins are definitely monozygotic. This phenomenon may be explained by a postulated third type of twinning, referred to as sesquizygosity. Unlike monozygotic and dizygotic cases, the underlying mechanisms of this rare event are less extensively defined. However, triploidy or the presence of three pronuclei is thought to explain sesquizygosity. The proposed mechanisms of triploid embryo formation explaining sesquizygotic twinning are discussed, considering the relevance and incidence of triploid embryos in IVF.

Twinning is a complex phenomena occurring at 1.25% of spontaneous pregnancies, while its incidence in IVF cases rises up to 25%. Three types of twins can be defined according to its zigosity: if they originate from one fertilised egg, monozygotic, if they come from two eggs fertilised by two different sperm, dizygotic, or a postulated intermediate of these events defined as sesquizygotic. Here, the characteristics and main causes of monozygotic and dizygotic twinning will be explained. Also, reasons for the increased frequency of twins in IVF will be provided.

FF components vary under hormonal regulation and may also be affected by pathologies, environmental conditions, stress or contaminants. The study and analysis of FF parameters can be used to distinguish the best oocytes, to develop personalised treatments or to improve gamete culture conditions.

Figure 1. (A) Embryo displaying multi-/micronucleation in both blastomeres; (B) Embryo with binucleation in one of the blastomeres. Arrows point to the position of multiple nuclei (1). INTRODUCTION Selection of high-quality embryos is an important factor for the successful outcome of assisted reproduction technologies (ART). Nowadays, criteria for selection are mainly based on morphological features such as embryo fragmentation, cell number, blastomeres uniformity, etc. (2). The parameters studied so far have been demonstrated to be useful indicators of embryo quality. Their…

Figure 1. Artificial shrinkage of blastocoelic cavity in human expanded blastocysts by hyperosmotic sucrose solutions, before and after the treatment (1). One of the main factors related to embryo selection for transfer is the extension of embryo culture up to blastocyst stage. This approach has been demonstrated to improve clinical outcomes after in vitro fertilization IVF (2). Consequently, in the last couple decades a specific blastocyst grading system has been applied to assess embryo morphology during blastocyst stage (appearance of the inner cell…

Fertilization  involves  the fusion of male and female gametes. However, for this to occur both cells must undergo certain morphological and physiological changes in order to be able to fuse. The sperm cell must undergo a process known as capacitation, which implies a series of changes regarding motility and plasma membrane composition (among others), essential to acquire the ability to fertilize.

Figure 1. Human blastocysts (10). The main goal of in vitro fertilization (IVF) is the birth of a single healthy child. However, the consequences and the effects of assisted reproductive techniques on children’s short- and long-term health have always been a source of discussion. Although IVF techniques and protocols have dramatically improved, the overall success rates are still relatively low, and assisted reproduction units still face the challenge of improving pregnancy rates (1). For this purpose, transfer of a single human embryo at…

Figure 1. Mouse embryo culture from 2-cell stage to blastocyst using KSOM medium (1). INTRODUCTION WHY IS IT IMPORTANT TO TALK ABOUT IVF CULTURE MEDIA? Within the last 40 years, the improvement of assisted reproduction technologies (ART) has experienced rapid and great advances (2). This success is largely due to the important enhancement of the culture conditions, specially IVF media (2,3), since this is where the early embryo develops up to blastocyst stage (Figure 1) before being transferred into the womb. However,…

Figure 1. Illustration showing evidences of endometriosis found in specific locations of the female reproductive organs [1]. Endometriosis is defined as the presence of endometrial tissue outside the uterus, which induces a chronic inflammatory reaction. It is generally assumed that lesions arise through retrograde endometrial tissue loss during menstruation, coelomic metaplasia and lymphatic spread in immunologically and genetically susceptible individuals. Associated symptoms usually have an impact on the general physical, mental and social well-being (2,3,4). ​ This condition is found…

Figure 1. Pre-ovulation human ovary. http://www.glowm.com/section_view/heading/Clinical%20Anatomy%20of% 20the%20Uterus,%20Fallopian%20Tubes,%20and%20Ovaries/item/1 INTRODUCTION Cryopreservation is a basic procedure in the daily work with assisted reproduction techniques. It is routinely and successfully applied to sperm samples, spare embryos from in vitro fertilization (IVF) cycles and oocytes. In the past, slow freezing procedures were carried out with no reliable clinical outcomes, especially in egg freezing programs. After the recent introduction of vitrification it is now possible to cryopreserve embryos and oocytes, and to assure patients that this…

  Figure. 1. Schematic overlapping view of the follicular and luteal phases of the ovary with their corresponding predominant steroid profiles and the proliferative and secretory phases of the endometrium cycle. Modified from (1) INTRODUCTION The menstrual cycle in humans lasts 28 days in regular terms, being the first day of menses considered the first day of the cycle. But in actuality, cycle refers to a series of intermingled events mainly related to three aspects: the ovary, the uterus (or…

Figure 1. Blastocyst. Modified from https://i.ytimg.com/vi/dvTXEGcNFZg/sddefault.jpg INTRODUCTION WHAT IS CRYOPRESERVATION? The first successful in vitro fertilization (IVF) treatment was in 1978. Since that, there have been a remarkable number of advances in assisted reproductive technologies (ART). Initially, all available embryos were transferred in IVF treatments owing to its low success rate. However, improvements on clinical and laboratory aspects led not only to increased pregnancy rates, but also to increased risk of multiple pregnancies. To prevent this, fewer embryos are transferred…

Figure 1. Benchtop incubators in an IVF laboratory (1) Every procedure carried out in an IVF laboratory, from ovarian puncture and semen capacitation to the embryo transfer, must be performed under specific safety conditions. These standards must be followed to avoid a decrease in gamete/embryo viability (2). From the in vitro culture, gametes and embryos are exposed to diverse artificial situations that do not take place in nature. In vivo, both fertilization and embryo development in the female reproductive tract…

Figure 1. Assisted hatching performed on an embryo (1) Implantation is one of the key steps in human reproduction, and hatching of the blastocyst is a critical point in the sequence of physiological events that lead to implantation (2). It has been estimated that only 15-20% of embryo transfers culminate in implantation, and as a consequence clinical pregnancy and live birth rates are quite low (3, 4). The potential of an embryo to implant is related to its own nature, the quality of…

Figure 1. Embryonic development (1) Fertilization is a sequence of coordinated events that results in the metabolic activation of the ootid (nearly mature oocyte) and triggers cleavage of the zygote (2). Nowadays, in assisted reproduction laboratories cleavage can be evaluated in vitro and in real time. Once in vitro fertilization (IVF) has been accomplished, early development of the embryo can be recorded by using time-lapse systems (TLP) (3). This approach makes it possible to evaluate morphology, including dynamic parameters, based on the uninterrupted culture of the embryo, which also allows…

Figure 1: MII oocytes obtained in vitro (1) Some weeks ago, Dr. Hayashi’s group published a study in Nature in which they described how to generate functional mouse eggs from cultured stem cells. To better understand this experiment, it is necessary to explain the natural ovarian cycle. Although the experiment was carried out in mice, we believe these experiments could be of great importance in humans in the near future. Therefore, the following description applies to human ovarian cycle. GONADAL…

Figure 1. Illustration of a fetus (1). Genetic problems in the embryo are one of the most important causes of pregnancy loss and miscarriage. However, identifying embryo mosaicism as the cause of genetic problems during development is not an easy task.  WHAT IS A MOSAIC EMBRYO? The term mosaicism refers to the presence of more than just one cell line, which presents different chromosome count (1). Additionally, the most common situation in these cases is the presence of a mixture…