Oocyte Nuclear Maturation

Question 1

Oocyte nuclear maturation is defined as

Answer 1

“nuclear alterations that take place during the resumption of meiosis producing a haploid chromosomal complement from the previous diploid state.”

Question 2

Primary oocytes arrested at prophase I of meiosis and are characterized,

Answer 2

at the light microscope level, by a visible nucleus, also referred to as a germinal vesicle (GV). Within three GV is a nucleolus.

Question 3

There is a very narrow perivitelline space between the

Answer 3

oocyte plasma membrane (oolemma) and the zona pellucida.

Question 4

Oocytes isolated from their follicular environment at this stage have

Answer 4

cumulus oophorus cells packed tightly around them.

Question 5

The chromosomes are decondensed and the chromatin is

Answer 5

transcriptionally active throughout follicle growth.

Question 6

Transcriptional activity ceases towards the end of folliculogenesis when

Answer 6

the oocyte is fully-grown and the chromatin in the GV becomes condensed.

Question 7

Thus, the final stages of oocyte maturation as well as fertilization and early embryo development occur in

Answer 7

the absence of transcription and rely on stored maternal messenger RNAs (mRNAs) the translation of which is highly regulated.

Question 8

The developmental competence of mature oocytes depends

Answer 8

therefore exclusively on post-transcriptional events.

Question 9

Resumption of meiosis is triggered by

Answer 9

a mid-cycle surge in the gonadotropins and is characterized by dissolution of the nuclear envelope or germinal vesicle breakdown (GVBD).

Question 10

Once GVBD is initiated, chromatin within the nucleus

Answer 10

condenses into discrete bivalents (chromosome pairs) that align on the meiotic spindle at metaphase of meiosis I.

Question 11

During anaphase and telophase of meiosis I,

Answer 11

the bivalents separate.

Question 12

There is no prophase of meiosis Il (and hence no DNA replication); thus oocytes progress directly to

Answer 12

metaphase Il and have reached the secondary oocyte stage.

Question 13

Secondary oocytes are recognizable at the light microscope level by

Answer 13

the presence of the first polar body (PB1) within the perivitelline space, an expanded cumulus oophorus, a distinct corona radiate and are observed routinely during laboratory IVF procedures.

Question 14

They await the signal to resume meiosis Il after

Answer 14

sperm penetration.

Question 15

The meiotic spindle apparatus is comprised of

Answer 15

microtubule filaments of polymerized tubulin.

Question 16

Maintenance of the precise arrangement and function of the spindle structure is critical

Answer 16

to the correct separation of the homologous chromosomes and sister chromatids during meioses I and II, respectively.

Question 17

Microtubules are particularly temperature sensitive and will depolymerize rapidly at

Answer 17

temperatures below 37°C.

Question 18

Thus, oocyte handling protocols in the laboratory should include strict

Answer 18

temperature regulation to avoid spindle damage.

Question 19

Chromosome segregation (disjunction) of homologous chromosomes (bivalents) at meiosis I, one set of chromosomes remains in the oocyte and the second set is

Answer 19

extruded in the first polar body.

Question 20

Sister chromatids during meiosis II (one chromatid from each chromosome remains in the oocyte and the sister chromatid is

Answer 20

extruded in the second polar body).

Question 21

This segregation must occur correctly to avoid

Answer 21

aneuploidy (chromosome loss or gain).

Question 22

Chromosome disjunction is triggered by

Answer 22

the protein separase following release from its inhibitory chaperone, securin.

Question 23

Appreciation of the consequences of abnormal meiotic chromosome separation is very important for

Answer 23

reproductive biologists and practitioners.

Question 24

Abnormal chromatin separation during meiosis is most often due to meiotic nondisjunction which results in

Answer 24

dosage imbalance of whole chromosomes (aneuploidy) typically incompatible with subsequent embryo/fetal viability.

Question 25

In most organisms, meiotic nondisjunction is a very rare occurrence. The human is a notable exception to this rule. An estimated

Answer 25

10%-30% of fertilized human eggs have the incorrect number of chromosomes with most being either trisomic (one extra chromosome) or monosomic (lacking one chromosome).

Question 26

Meiotic nondisjunction has profound clinical consequences;

Answer 26

approximately 1/3 of all miscarriages are aneuploid.

Question 27

When viability is not compromised aneuploidy is the leading

Answer 27

genetic cause of developmental and mental disabilities.

Question 28

However, despite the high frequency and clinical importance of human oocyte-derived aneuploidy, very little is known about factors that

Answer 28

contribute to meiotic nondisjunction.

Question 29

One incontrovertibly linked factor is

Answer 29

increasing maternal age.

Question 30

Increased DNA damage and repair deficiency in granulosa cells have been associated

Answer 30

with ovarian aging in the Rhesus monkey

Question 31

Thus, in addition to decreasing oocyte and follicle number, the microenvironment within follicles and ovaries could also deteriorate with

Answer 31

age and contribute to loss of oocyte integrity.

Question 32

Among women

Answer 32

are trisomic.

Question 33

This value approaches 35% among women over 40 years of age. The elevated incidence of aneuploidy reported in embryos generated by assisted reproductive technology (ART) is likely a result of

Answer 33

inappropriate chromatin segregation during oocyte meiosis.

Question 34

Genomic imprinting is another important nuclear change leading to gene activation or silencing specific manner via specific modifications in DNA methylation during gamete and embryo and to the

Answer 34

reprogramming of cellular DNA expression by imposition of imprints in a gender-manner via specific modifications in DNA methylation during gamete and embryo development.

Question 35

Correct genomic imprinting, or epigenetic modification (genetic events determined by factors outside the genome), has been established as

Answer 35

an absolute pre-requisite for normal post-fertilization embryonic and fetal development.

Question 36

Deletion or errors of imprinting become lethal or give rise to

Answer 36

abnormalities and genetic diseases such as Prader-Willi syndrome, Angelman syndrome, and several types of cancer.

Question 37

Modifications also occur to the nuclear histones in the form of

Answer 37

methylation, acetylation, ubiquitination and phosphorylation that in turn determine gene activity.

Question 38

During spermatogenesis, genes are regulated via the

Answer 38

number and location of methyl groups on sperm-specific histones.

Question 39

Histone methylation is crucial for embryo

Answer 39

development in the mouse oocyte and lack of imprinting is one of the reasons why parthenotes (non-fertilized activated oocytes) fail to develop.

Question 40

Indeed, histone and DNA methylation may act as a means of

Answer 40

stabilizing epigenetic modifications during critical developmental transitions.

Question 41

Our knowledge of these modifications during oocyte maturation and early embryogenesis has

Answer 41

increased greatly in recent years but a complete understanding of their impact has yet to be determined