Follicular growth and development

 

What are the stages of follicular development its diameter and acquisition of selectable stage?    Ans: The visible follicle by ultrasound means is the result of a sophisticated journey of an oocyte through the reproductive life of a female. A follicle is an oocyte surrounded by granulosa cells. The follicle grows by a small increase in the oocyte volume, a significant proliferation of the surrounding granulosa cells and an expansion of the antral cavity. There are three stages of follicles : primodial,early growing and antral.
The antral follicle count (AFC)is a direct quantitative marker of ovarian reserve and responsiveness. The follicular number shows an annual loss of antral follicles per year following the reproductive ageing of woman. There is a high correlation between AFC and reproductive age which is widely applicable in assisted reproduction treatment.

Primordial follicles have a very small size of less than 0.05 mm and are not visible. Early growing follicles are less than 2 mm and comprise of large primary, secondary preantral, early antral and small antral follicles.

Several months are required for a new growing follicle to reach the preantral stage and 70 additional days to reach the size of 2 mm.

Early growing follicles growth in unaffected by cyclic hormonal fluctuations and is regulated by subtle interactions between FSH and local factors produced by these granulosa cells as well as the oocyte. 
Only a small number of preantral follicles process to antral stage which are more than 2mm and become selectable during the late follicle phase.  From the time they enter the selectable stage during the late luteal phase.  Follicles become sensitive to cycles changes of FSH in terms of granulosa cell proliferation. These are the follicles that contribute to the hormonal cyclic profile depicted in the classic diagram of the menstrual cycle .Indeed as the follicle develops in responsiveness to Gonadotrophins progressively increases under the control of local factors acting in an autocrine /paracrine fashion.
The number of these selectable follicles especially the small antral is belived to reflect the number of remaining primordial follicles and this the ovarian reserve. Their number is also strongly correlated with serum AMH levels. The longer follicles are totally Gonadropin dependet and one of them will evolve to dominant during the next follicular phase while the rest will become atretic. So in this phase all other healthy follicles with granulosa cell activity tend not to exceed 6mm., suggesting that all larger follicles are possibly atretic and do not reflect the actual reproductive capability of the woman .Exogenous gonadoteopin administration during IVF rescues these small antral follicles from atresia and promotes their growth. Eventually the retrieved oocytes by follicle aspiration come from this cohort of visible follicle. 

PCOS is related to an excess in small antral follicles. Although the pool of growing primary and secondary follicles in women with the syndrome is two to threefold that of normal ovaries, the pool of primordial follicles is normal .This excess is drastically involved in the follicular arrest of PCOS presumably through an auto- inhibiting effect that could involve AMH. Still the 6-9 mm follicles also appear to be affected by the unfavourable environment of the syndrome.
The follicle number using two dimensional ultrasound is estimated both in longitudinal and anteroposterior cross sections of the ovaries as the performer slowly moves the transducer from one side of the ovary to the other. After the identification of the ovary, a scout sweep is performed in the two planes and the largest follicle is localized.Then the counting is performed starting from the outer ovarian margin to the opposite. The procedure is repeated with the contralateral ovary.It has been observed that the number of follicles counted by 2D is overestimated compared to oocytes retrieved and even more in ovaries with many follicles as the polycystic when they are stimulated possibly because of double counting and inclusion of atretic follicles.
The size of follicles in 2D ultrasound is expressed as the mean of the diameters measured on the two aforementioned sections. However in clinical practice three techniques are applied. The first includes a single measurement of the maximal diameter in the longitudinal plane the second includes an additional measurement of a diameter first.and the third is expanded to the measurement of a perpendicular to the previous two diameter in the transverse plane after manual rotation of the transducer.In the latter two cases the diameter is the mean of the two or three diameters respeçtively. 
This matter has been addressed by the three dimensional calculation of follicular volume which vab5 be assessed by two ways: manually and automatically.The manual measurement is performed more often by the program virtual organ computer sided analysis .Infertility the data is acquired by an automatic mechanical sweep of the region ensuring that the entire ovary is included. The process is repeated for the contralateral ovary and the data are saved. The data are then processed using VOCAL. Each follicle is delineated manually by tracing around its perimeter and the column of interest is calculated automatically
The automatic technique is performed by the program Automatic Volume Calculation . The data are captured as described above and then processed by aonoAVC after right positioñing. This program identified every single follicle with a specific Color and then automatically calculated the mean diameter the maximum dimension and the follicle volumn. This later method is highly valid and provides more accurate values than those estimated from 2D measurements and automated measurements of follicular diameter as well as calculated using VOCAL.
Antral follicle count can also be performed by 3D ultrasound. Data are acquired as described above. There are three waves to count the follicles .In the first the observer counts manually the follicles In a multiplanar view that is using all three perpendicular planes simultaneously in order to enhance the spatial awareness. In the second way the ovary is defined by VOCAL inversion mode is applied and the follicle are displayed without the surrounding ovarian tissue and finally the counting is performed in multiplanar view .In the last way sonoAVC displays every single follicle in a specific Color in an inversion mode again without the ovarian tissue .SonoAVC can distinguish follicle of diameter and provides the option of post procession where manually the observer picks any missed follicles or excludes any that has been included incorrectly.Post procession seems necessary since sonoAVC missed follicles of random sizes that are easily recognized in the multiplanar view due to their specific Color.

Women with PCOS have a larger ovarian volume . The ovarian volume declines with ages as the follicle both in women with PCOS and controls but this decline for a not correlate so well with age as the follicle number does . The pattern of the ovarian volume falling in women with PCOS is different because declined less markedlybthan of controls despite the similar decline in follicle number. This fact suggests that the stroma plays a significant role and also the size of the follicle because the decrease with age affects mainly the number of small follicles but not of bigger follicle5in women with PCOS . Alsamarai at al demonstrated a linear decline in ovarian volume and concluded that age dependent criteria for the diagnosis of PCOS are necessary . This point could be of value in assisted reproduction field as the patients are very often more than 40 years old but still danger for OHSS.
The calculation of the ovarian volume is performed either using the formula for a primate ellipsis or automatically by the software of the ultrasound equipment just outlining the ovary.
Again 3D ultrasound provides a not reliable accurate and reproducible assessment of ovarian volume than the 2D based methods with better spatial irregularities .3D ultrasound also confirmed the greater ovarian volume of women with PCOS. There two ways to calculate the ovarian volume : the conventional full planar technique and the VOCAL program.During the conventional method the observer scrolls through one plane of the multiplanar display and simultaneously delineates the ovary in a different plane .with VOCAL program the observer while the dataset is rotated through . Raise fanning at al compared the 
two techniques and found that measurements with VOCAL program are superior to conventional though comparable.

Despite the fact the increased stromal area and echogenicity are not included to the diagnostic criteria of PCOS they are still characteristic ultrasonographic features of the syndrome .Patients with PCOS present higher stromal area and volume .Stromal hypertrophy is a common and specific indicator of ovarian hyperandrogenism . The hypertrophic theca cells in the stroma of women with PCOS produce higher amounts of androgens.Indeed ovarian stromal area was found to correlate with androgen levels and free androgen index .In clinical practice the measurement of ovarian volume is a good surrogate for the stromal volume because increased stromal volume is the main cause of ovarian enlargement in PCOS , except for patients taking contraceptive pills. 
Another marker of stromal hypertrophy is the stromal area to total ovarian area ratio is the stromal area defined by the periphery of the hyperechoic stroma divided by the total ovarian area defined by the perimeter of the ovary in the maximum plane section . Woman with PCOS have a higher value when compared to women with polycystic ovarian morphology or controls whereas the last two groupsndo not differ significantly .Futhermore S/A ratio in women with PCOS correlates well with androstenedione testosterone and insulin levels ratio could be the most efficient ultrasound performance for hyperandrogenism. In this line a cutoff value of S/A is the best predictor of elevated androstenedione and testosterone levels. This cutoff value could be used in everyday clinical practice and even included in the diagnostic criteria of the syndrome.
2D ultrasound measurement of stromal area can by performed by two ways: the manual and the semiautomatic. In the first method the area is calculated using the formula for an ellipse. In the second method the stromal area is defined by delineating its perimeter and is then calculated automatically by the ultrasound machine.3D measurement of stromal volume is achieved either after the calculation and subtraction of the total follicular volume from the total ovarian volume or using VOCAL program and by determining a limit area which determining the stromal and follicular area .Thus above and below the limit are respeçtively. 
Stromal echogenicity a had been a key feature for many years until the first more objective assessments showed that there was no significant difference in stromal echogenicity between women with PCOS and control .2D ultrasound measurement of stromal echogenicity can be either a subjective operator assessment an object calculation derived by the intensity level of the ultrasound pixels within the stroma displyednon the sonographic image . The difference found with the first subjective measurement was attributed to increased volume of ovarian stroma in relation to the lower mean echodensity of the ovary due to the higher number of follicles. Another marker of echogenicity is the stromal echogenicity to the mean ovarian echogenicity. Stromal index was found hogherbin PCOS but this was not confirmed.
3D ultrasound assessment of stromal echogenicity were in accordance with the 2D objective calculation which shows no difference between women with PCOS and controls. The 3D assessment of echogenicity is performed by the mean grey value represents the mean tissue density of a defined area and is calculated by the mean signal intensity of the gray scale voxels. 3D ultrasound is considered more appropriate for the quantification of the stromal echogenicity especially for research purpose.