Human Oocytes and Their Chromosomes: An Atlas

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A zygote's morphological characteristics are accepted to be an inherent indicator of both gamete quality and subsequent embryo implantation potential Alpha Scientists in Reproductive medicine and ESHRE Special Interest Group of Embryology, Many studies have underlined the predictive value of zygote morphological assessment through correlations with chromosome constitution and the incidence of zygotic arrest Gianaroli et al. Recent strategies in embryo selection include sequential morphology assessment where PN scoring has been shown to play an important role as an indicator of gamete constitution as well as a prognostic tool for embryo competence.

Scoring of PNs has also proved to be useful in countries where restrictive legislation mandates selection at the zygote stage for embryo transfer and consecutive elimination or cryopreservation of sibling zygotes Senn et al. Although numerous studies have associated positive clinical results with the implementation of PN scoring, other reports have questioned the predictive value of PN scoring systems and see no benefits or improvement in the outcome Nicoli et al.

NDL India: Human Oocytes and Their Chromosomes: An Atlas

The Istanbul consensus workshop comprised a worldwide panel of experts who just recently evaluated the current practice of oocyte to embryo scoring and established common criteria for assessment Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology, It must be noted, however, that the processes associated with fertilization by conventional insemination lags 1h behind fertilization using ICSI Nagy et al. A more complete elucidation of events during the zygote stage, however, can be expected with the application of continuous monitoring through the introduction of time-lapse imaging instead of the traditional isolated observations using light microscopy Montag et al.

Chromosomes and Karyotypes

Normal fertilization is assessed by the presence of two centrally positioned, juxtaposed PNs with clearly defined membranes and two polar bodies. If an abnormal PN number is observed whether it be 1, or 3 or more, PNs, a low viable pregnancy is to be expected thus the transfer of these zygotes is to be avoided Reichman et al. Panel experts also agreed and strongly advised that the assessment and elimination of dysmorphic zygotes with smooth endoplasmic reticulum SER discs see Chapter One due to the association with severely adverse clinical outcomes Otsuki et al.

Correct alignment of PNs onto the polar axis is considered a fundamental feature for the completion of the first cleavage division and normal sequential development Gardner, , , ; Edwards and Beard, , ; Payne et al. Cell cycle-related dynamics of PN events have led many authors to investigate and correlate the presence, pattern and number of NPBs to embryo developmental potential.

As shown for mitotic cells, the inequality in number, size or distribution of NPBs is correlated with abnormal development Pedersen, Category 1 includes zygotes presenting with equal numbers and size of NPBs, either aligned at the junction between PNs or scattered in both PNs. Category 2, non-symmetrical, comprises all other patterns including peripherally localized PNs. The latter were found to be correlated with imprinting errors and delayed onset of functional NPBs and nucleoli formation in animal models Svarcova et al. Reports on early cleavage checks have been demonstrated to be a beneficial tool in selecting embryos with high implantation potential and decreased chromosomal anomalies Sakkas et al.

Sequential morphology assessment through time-lapse cinematography will certainly shed light on the discrepancies in the literature with respect to PN scoring and an in-depth analysis and correlation with the clinical background behind the gametes forming the zygote will possibly reveal even more reliable prognostic tools to improve clinical outcomes.

B. Oocyte maturation stage

The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting. Hum Reprod ; Rhythmic actomyosin-driven contractions induced by sperm entry predict mammalian embryo viability. Nat Commun ; CrossRef Medline Google Scholar. Pronuclear morphology predicts embryo development and chromosome constitution.

Reprod Biomed Online ; Medline Web of Science Google Scholar. Prognosis of oocytes showing aggregation of smooth endoplasmic reticulum.

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Association of pronuclear Z score with rates of aneuploidy in in-vitro fertilised embryos. Reprod Fertil Dev ; Oocyte polarity and cell determination in early mammalian embryos. Mol Hum Reprod ; Hypothesis: sex determination and germline formation are committed at the pronuclear stage in mammalian embryos. Gardner RL. Can developmentally significant spatial patterning of the egg be discounted in mammals?

Hum Reprod Update ; Scrambled or bissected mouse eggs and the basis of patterning in mammals. BioEssays ; Specification of embryonic axes begins before cleavage in normal mouse development. Development ; Pronuclear orientation, polar body placement, and embryo quality after intracytoplasmic sperm injection and in vitro fertilization: further evidence for polarity in human oocytes?

A Scrambled Mess

Pronuclear morphology and chromosomal abnormalities as scoring criteria for embryo selection. Fertil Steril ; Early embryo cleavage is a strong indicator of embryo quality in human IVF. Restricting protein synthesis to the cytoplasm helps ensure that newly made RNA molecules do not become involved in translation before processing is complete.

Typically the largest structure within a cell, it consists of a nuclear envelope containing chromatin , the mass of DNA and its associated proteins, with one or more specialized regions of chromatin called nucleoli. In specific tissues, the size and shape of nuclei normally tend to be uniform.

Nuclei of large, active cells. Liver cells have large, central nuclei. One or more highly basophilic nucleoli are visible within each nucleus, indicating intense protein synthesis by these cells.


Most of the chromatin is light staining or euchromatic, with small areas of more darkly stained heterochromatin scattered throughout the nucleus and just inside the nuclear envelope. This superficial heterochromatin allows the boundary of the organelle to be seen more easily by light microscopy.

Cells year 7

One cell here has two nuclei, which is fairly common in the liver. X; Pararosaniline—toluidine blue. The nuclear envelope forms a selectively permeable barrier between the nuclear and cytoplasmic compartments. Electron microscopy reveals that the envelope has two concentric membranes separated by a narrow nm perinuclear space Figures 3—2 and 3—3. This space and the outer nuclear membrane are continuous with the extensive cytoplasmic network of the rough endoplasmic reticulum RER. Closely associated with the inner nuclear membrane is a highly organized meshwork of proteins called the nuclear lamina Figure 3—4 , which stabilizes the nuclear envelope.

Major components of this layer are the class of intermediate filament proteins called lamins that bind to membrane proteins and associate with chromatin in nondividing cells. Three-dimensional representation of a cell nucleus shows a single large nucleolus and the distribution of the nuclear pores in the nuclear envelope. The outer membrane of the nuclear envelope is continuous with the RER. Human Anatomy.


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