SpOvum® Blog

Smooth Endoplasmic Reticulum (SER) role in oocyte

SER’s are translucent vacuoles observed occasionally during ICSI in the cytoplasm of the egg. [2] testing

The endoplasmic reticulum (ER) is a network of membranes found throughout the cell and connected to the nucleus. ER functions as a manufacturing and packaging system to make products such as hormones and lipids. [2]

There are two types of ER, Rough ER, and Smooth ER

SER’s are easily distinguishable from fluid-filled vacuoles because they are not separated from the rest of the ooplasmic volume by a membrane, and are seen as translucent vacuoles. [3]

SER helps in steroid hormones and fat metabolism and production. It is smooth due to association with smooth slippery fats and is not studded with ribosomes. SER ‘s pivotal role is to store and release calcium, which will affect the calcium balance in SER-positive oocytes. .[2]

The mechanism of formation of SER’s are due to some functional and structural alterations of the SER during oocyte maturation, such as the increase in the sensitivity of the IP3 receptor for calcium, increase storage of calcium that is released during oscillation. In human oocytes, the localization of mobilizable calcium ions was detected in the small vesicles beneath the plasma membrane of SER. [3]

According to transmission electron microscopic analysis, there are three forms of SER’s, large (18 µM); medium (10-17 µM) which can be classified by light microscopy and small (2-9 µM) which are not visible under clinical embryology laboratory conditions. [3]

Oocytes consist of SER due to the presence of high estradiol level. In many cases, serum estradiol levels on the day of hCG administration were significantly higher in SER-positive cycles. [2]

It has been observed that the occurrence of SER’s is significantly related with longer duration and higher dosage of the stimulation. [3]

Pregnancies in women with affected gametes were accompanied by a higher obstetric problem leading to non-significant trends towards earlier delivery and significantly reduced birth weights. It is strongly recommended to avoid the transfer of embryos/blastocysts derived from SER-cluster positive gametes. It is known that even transfer of sibling oocytes without these anomalies involves high risk and detrimental outcome.

It is showed that the results of transfer of SER-positive embryos results in a high rate of miscarriage and the women tend to deliver earlier at 36.4 weeks of gestation which leads to lower birth rate 

References:

1. https://www.rbmojournal.com/article/S1472-6483(10)60563-9/pdf
2. https://www.slideshare.net/malpani/eggs-showing-smooth-endoplasmic-reticulum-clusters-produce-outcomes-similar-to-normal-eggs
3. https://books.google.co.in/books?id=Kp5_AwAAQBAJ&pg=PA83&lpg=PA83&dq=refractile+bodies+in+oocyte&source=bl&ots=VCxnK7vkwu&sig=Akh6mi0lSgdMs36GtIulV5Zrgfo&hl=en&sa=X&ved=0ahUKEwi5mrDgspvcAhXIT30KHZkqAtI4ChDoAQhMMAc#v=onepage&q=refractile%20bodies%20in%20oocyte&f=false

Vacuoles role in oocyte

One of the most common oocyte dysmorphism is cytoplasmic vacuolization. Vacuoles are membrane-bound cytoplasmic inclusions filled with fluid that is virtually identical with perivitelline fluid. They vary in size as well as in number. They arise spontaneously or by fusion of preexisting vesicles derived from Golgi apparatus/SER. [1]

It has been shown that vacuolized oocytes have significantly reduced fertilization rates and developmental ability. Vacuoles of size 5-10 µM in diameter don’t show any biological consequences. A vacuole >14 µM in diameter can completely block fertilization. Single or multiple large vacuoles may displace the meiotic spindle from its polar position or disturb the cytoskeleton resulting in fertilization failure. [1 and 2]

Two types of the vacuole in oocytoplasm are seen. Type 1 vacuole is related to apoptosis. The formation of type 1 vacuoles is one of the morphological characteristics of apoptosis. The mechanism responsible for type 2 vacuoles is unknown. Type 1 vacuole can be seen very clearly and they look like lunar craters. Type 2 vacuoles are not obvious as type 1 vacuoles and are flat and more like a bulge than a crater. Type 2 vacuoles are common in MII oocytes. [1] 

References:

1. https://www.rbmojournal.com/article/S1472-6483(11)00349-X/fulltext
2. https://www.researchgate.net/publication/229326639_The_oocyte?_sg=hXmowSf9g-nhMoxNcXfHz3khl6F7c6fBTyrlIkTAeAcVyFJR1kzxL4RsmxYM_31frJPpJ8sq8w

Refractile bodies role in oocyte

Refractile bodies are cytoplasmic inclusions that can be dark incorporations, fragments, spots, dense granules, lipid droplets, and lipofuscin. TEM studies and Schmorl staining have shown the refractile bodies >5 µM in diameter showed the conventional morphology of lipofuscin inclusions that consisted of a mixture of lipids and dense materials. Lipofuscin bodies in human oocytes can be detected throughout meiotic maturation (GV, MI, and MII). Accumulation of lipofuscin occurs during the growth phase of the oocyte when dominant follicles are being recruited into the preovulatory pathway. The occurrence of large lipofuscin bodies in normal aging may also be related to conditions of the developing ovarian follicles, such as perifollicular blood circulation and follicular fluid composition. [1]

The average diameter of a recognizable refractile body under bright-field microscopy is approximately 10 µM. According to studies lipofuscin inclusions are associated with reduced fertilization and unfavorable blastocyst development only when their diameter is >5 µM. [1]

References:

1. https://books.google.co.in/books?id=Kp5_AwAAQBAJ&pg=PA83&lpg=PA83&dq=refractile+bodies+in+oocyte&source=bl&ots=VCxnK7vkwu&sig=Akh6mi0lSgdMs36GtIulV5Zrgfo&hl=en&sa=X&ved=0ahUKEwi5mrDgspvcAhXIT30KHZkqAtI4ChDoAQhMMAc#v=onepage&q=refractile%20bodies%20in%20oocyte&f=false

ZP role in oocyte

The zona pellucid (ZP) is the specialized ECM layer that directly surrounds the oocyte. The ZP is an extracellular translucent matrix composed of long, cross-linked filaments, which first appears during oocyte growth and increases in thickness as oocytes increase in diameter. ZP represents the interface between the oocyte and its enclosing cumulus cells. ZP morphological abnormalities reported were centered in shape or thickness variety. ZP plays a critical role in fertilization by acting as a “docking site” for binding of spermatozoa followed by induction of the acrosome reaction in the zona bound sperm and an adequate block to polyspermy. Any disturbance in ZP morphology or texture may lead to abnormal fertilization result. Narrow PVS and heterogeneous ZP were always concurrences with the abnormal oocytes in many studies reported. [1] The ZP, acting as a protective and selective barrier, actually mediates the metabolic exchanges between the oocyte/embryo and the surrounding microenvironment [2]

A large number of ZP variants (appearance, thickness, irregularities, composition, and organization) have been described with the advent of ICSI. Thicker ZPs are associated with decreased fertilization rates, implantation and pregnancy rates. The ZP also plays a pivotal role in pre-implantation embryos; for instance, abnormalities in oocyte (and thus ZP) shape are associated with irregular cleavage patterns, compromised cell-cell contacts, and subsequent difficulties in the developmental progression. The importance of the ZP continues until the blastocyst stage, a time when the embryo needs to hatch out of the zona prior to implanting into the uterine epithelium. [2]

References:

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3947078/
2. https://books.google.co.in/books?id=zlB4T2ER4msC&pg=PA197&lpg=PA197&dq=PVS+in+oocyte&source=bl&ots=fw0PfSP80u&sig=e9QGojBuh79bRbFDr2fdrpBxCFI&hl=en&sa=X&ved=2ahUKEwjU9fyN7qzcAhXTXisKHaW1DHoQ6AEwCHoECAkQAQ#v=onepage&q=PVS%20in%20oocyte&f=false

PVS role in oocyte

The Perivitelline space (PVS) represents the acellular compartment in between the plasma membrane of the oocyte and its ZP. [2] The PVS of mammalian oocytes is made up of hyaluronan-rich extracellular matrix prior to fertilization. [1] It becomes clearly visible in a mature oocyte with the extruded polar body located in its most prominent portion. An indistinguishable PVS typically corresponds to immature oocytes while a distinct space to mature oocytes.

It has been proved that large PVS may result in disrupted or compromised communication between the cumulus cells and the oocyte, particularly via gap junctions and transzonal projections. Presence of large PVS will be seen due to over-mature eggs, where such eggs have shrunk in relation with ZP presenting a large gap between the oocyte and surrounding zona. [3] Large PVS is seen when the large portion of the cytoplasm is extruded together with the haploid chromosomal set during PB I formation. [2]   Oocytes with large PVS during ART treatment were usually reported with lower fertilization rate. [1] Granularity in the PVS has been associated with over-maturity of oocytes. Coarse granulation in the PVS is a morphological abnormality occasionally seen after stripping of the oocyte in preparation for ICSI and the presence of coarse granules in PVS is associated with lower pregnancy and implantation rates. [4 and 5]

References:

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3947078/
2. https://books.google.co.in/books?id=zlB4T2ER4msC&pg=PA197&lpg=PA197&dq=PVS+in+oocyte&source=bl&ots=fw0PfSP80u&sig=e9QGojBuh79bRbFDr2fdrpBxCFI&hl=en&sa=X&ved=2ahUKEwjU9fyN7qzcAhXTXisKHaW1DHoQ6AEwCHoECAkQAQ#v=onepage&q=PVS%20in%20oocyte&f=false
3. https://www.researchgate.net/publication/229326639_The_oocyte?_sg=hXmowSf9g-nhMoxNcXfHz3khl6F7c6fBTyrlIkTAeAcVyFJR1kzxL4RsmxYM_31frJPpJ8sq8w
4. https://link.springer.com/article/10.1023/A:1021243530358
5. https://www.slideshare.net/Yasminmagdi/oocyte-morphology-assessment

Polar body role in oocyte

A polar body is a small haploid cell that is formed concomitantly as an egg cell during oogenesis, which generally does not have the ability to be fertilized. When certain diploid cells in animals undergo cytokinesis after meiosis to produce egg cells, they sometimes divide unevenly. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg or ovum, while the smaller polar bodies only get a small amount of cytoplasm. [1]

Polar bodies eliminate one half of the diploid chromosome set produced by meiotic division in the egg, leaving behind a haploid cell. To produce the polar bodies, the cell divides asymmetrically, which later leads to furrowing (formation of a trench) near a point on the cell membrane. The presence of chromosomes induces the formation of an actomyosin cortical cap, a myosin II ring structure and a set of spindle fibers, the rotation of which promotes invagination at the edge of the cell membrane and splits the polar body away from the oocyte. [1]

Meiotic errors can lead to aneuploidy in the polar bodies, which, in most the cases, produces an aneuploid zygote. Errors can occur during either of the two meiotic divisions that produce each polar body but are more pronounced if they occur during the formation of the first polar body because the formation of the first polar body influences the chromosomal makeup of the second. For example, during the pre-division (the separation of chromatids before anaphase) in the first polar body can induce the formation of an aneuploid polar body. Therefore, the formation of the first polar body is an especially important factor in forming a healthy zygote. [1]

Oocytes showing an intact PB I give rise to higher rates of implantation and pregnancy, probably due to an increase in blastocyst formation. Recent research has shown that some polar body abnormalities may be an artifact of oocyte handling or aging. Abnormal morphology of PB1 in MII oocytes attributes to chromosomal aneuploidy of the oocyte. As PB 1 extrusion is directly related to spindle formation and sister chromatid exchange, it could be the most likely point of non-disjunction or aneuploidy formation. Fragmentation rates of the PB1 depend on the time elapsing between retrieval, denudation and ICSI performance. [2]

References:

1. https://en.wikipedia.org/wiki/Polar_body
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3663962/

Oocyte Granularity 

Granularity in ooplasm of the oocyte refers to the presence of “heterogeneous area”. Granularity is correlated with localization of mitochondria within the cytoplasm of the cell.

It also represents a domain of high ATP request which is very much necessary for normal development of embryos. A healthy MII oocyte should contain a clear and moderately granular cytoplasm. Top quality oocytes have the granularity homogeneously distributed at the sides of the oocyte cytoplasm and not at the center.

Oocyte with centrally located granular cytoplasm (CLGC) is considered as a dysmorphic oocyte. These kinds of oocyte show high chromosomal abnormalities like aneuploidy (presence of an abnormal number of chromosomes).

Also, the oocytes which lack granularity are considered as a bad oocyte. The severity of granulation is based on the diameter of the granular area, depth of lesion and crater-like appearance. [1,2 and 3]

References:

1. https://www.researchgate.net/publication/229326639_The_oocyte?_sg=hXmowSf9g-nhMoxNcXfHz3khl6F7c6fBTyrlIkTAeAcVyFJR1kzxL4RsmxYM_31frJPpJ8sq8w
2. http://atlas.eshre.eu/es/14611418225805670
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3455008/