Embryo Development and Assessment of Viability potx

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Embryo Development and Assessment

of Viability

Thomas Ebner

IVF-Unit, Women’s General Hospital, Linz, Austria

Germ cell wastage is a universal phenomenon throughout reproductive life

in mammals, including humans. Before puberty and adult life, the vast

majority of oocytes become atretic at various stages of follicular develop￾ment and, of those actually managing to ovulate, only a limited number

are capable of repeating the life cycle.

Compared to the natural cycle, the situation in controlled ovarian hyper￾stimulation is substantially aggravated because accidental maturation and

ovulation of germ cells of reduced developmental potential may occur (1).

In other words, the actual implantation potential may be overestimated

although oocyte morphology, fertilization, and cleavage rate may appear in￾conspicuous at first glance. On the other hand, even embryos of worst quality

may sometimes turn out to be viable, e.g., giving birth to healthy babies.

Taken together, viability of individual embryos is strongly correlated

to optimal maturational steps in the ovary, adequate fertilization, progress￾ive development through all pre-implantation stages, as well as subsequent

implantation in the endometrium. Combining cytogenetical analysis—

morphological evaluation throughout preimplantation development (2),

and embryo metabolism (3)—the ability to select the most competent

embryo out of a pool of concepti will further improve and definitely help

to reach the ultimate goal in assisted reproduction, namely a healthy single￾ton delivery.

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THE FOLLICLE

It is well accepted that the developmental fate of an embryo is largely dic￾tated by the quality of the oocyte, which in turn reflects the follicular milieu.

Most likely, affected gametes are derived from follicles with reduced

blood supply since various reports suggest a close relationship between fol￾licular blood flow and developmental competence of the corresponding

oocyte or embryo (4,5). If vascularization in ovaries is underdeveloped,

some follicles will be confronted with hypoxia which in turn causes a change

in energy metabolism by switching from oxidative phosphorylation to gly￾colysis. As a consequence, adenosine triphosphate (ATP) production in

the affected follicle will decrease dramatically, since glycolysis generates

only two molecules of ATP compared with oxidative phosphorylation (38

molecules). In addition, ATP depletion is increased since the vast majority

of ATP is used for remodeling the vascular network via angiogenesis which

is triggered by chronical underoxygenation (6). Since vascular endothelial

growth factor (VEGF) is a potent mediator of angiogenesis, it can be

expected that it is produced by granulosa and theca cells in response to

hypoxia. In fact, a significant correlation between elevated levels of VEGF

in follicles and a reduced viability of the corresponding embryo has been

described (7).

Since conventional parameters, such as follicle size or fluid volume, are

not considered to be adequate predictors of developmental potential of har￾vested oocytes and arising embryos, pulsed color Doppler ultrasound may

be the first-line indirect technique for screening for competent oocytes which

might serve as a basis for viable embryos or blastocysts, followed by follicu￾lar fluid analysis for oxygen, ATP, and/or VEGF.

THE OOCYTE

It is still unknown how follicular underoxygenation affects normal cellular

and genetic development of the human oocyte; however, there is evidence

that gametes with a reduced internal cytoplasmic pH and ATP content

may arise if oxygen saturation falls below a certain threshold of less than

or equal to 1% (8).

Nuclear Component

According to Gaulden (9), hypoxia is responsible for a reduction in meta￾bolic activity as well as for a change in internal pH both of which are likely

to affect organization and integrity of the meiotic metaphase spindle. This is

supported by data from pre-antral follicle culture indicating that in vitro

maturation at 5% oxygen tension (instead of 20%) resulted in a significant

reduction of gametes finishing nuclear maturation (10), e.g., characterized

by a complete spindle absence. More interestingly, the rate of unaligned

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