Fundamentals of human embryonic growth in vitro and the selection of high-quality embryos for transfer

Abstract

Knowledge of the nature of embryo growth, and the handling and scoring of quality in human embryos are significant aspects for embryologists in IVF clinics. This review describes the formation, growth and maturation of human oocytes, many aspects of fertilization in vitro, embryonic transcription during preimplantation stages, and the formation of polarities, timing controls, role of mitochondria and functions of endocrine and paracrine systems. Modern concepts are fully discussed, together with their significance in the practice of IVF. This knowledge is essential for the correct clinical care of human embryos growing in vitro, especially in view of their uncharacteristic tendency to vary widely in implantation potential. Underlying causes of such variation have not been identified. Stringent tests must be enforced to ensure human embryos develop under optimal conditions, and are scored for quality using the most advanced techniques. Optimal methods of culture are described, including methods such as co-culture introduced to improve embryo quality but less important today. Detailed attention is given to quality as assessed from embryonic characteristics determined by timers, polarities, disturbed embryo growth and anomalous cell cycles. Methods for classification are described. Approaches to single embryo transfers are described, including the use of sequential media to produce high-quality blastocysts. These approaches, and others involved in surgical methods to remove fragments, transfer ooplasm or utilize newer approaches such as preimplantation diagnosis of chromosomal complements in embryos are covered. New outlooks in this field are summarized.

Respiration rates of individual bovine in vitro-produced embryos measured with a novel, non-invasive and highly sensitive microsensor system

“…We support the general idea (Overströ m 1992, Boiso et al 2002, Abe & Hoshi 2003, Neuber et al 2003, Scott 2003) that a more objective measurement of embryo quality is required and is undoubtedly a key to overcome the shortcomings of morphological evaluations, such as subjectivity. The objectivity of the microsensor technology, combined with the traditional and accepted morphological evaluation, might prove to be a very accurate method for improving embryo classification and subsequent selection.…”

“…Morphological evaluation is the main and usually the only technique used routinely to assess bovine embryo quality and subsequent viability, both in experimental work and in practice (Overströ m 1992, Boiso et al 2002. However, such evaluation is subjective in itself, and consent is often not reached even among specialized and experienced personnel (Farin et al 1995).…”

Insights on blastomere nuclearity

“…Standard laboratory procedures were used [29]. Briefly, oocytes were inseminated or injected 5-6 h post-egg retrieval by standard procedures.…”

The simplified SART embryo scoring system is highly correlated to implantation and live birth in single blastocyst transfers

“…The ability to predict successful ART outcomes based on embryo morphology is key. Numerous studies have evaluated systems devised to grade embryo morphology [1,6]. There are simple systems used to grade cleavage stage embryo based on appearance, which take into account cell symmetry and fragmentation [17].…”