In vitro maturation (IVM) is the technique of letting the contents of ovarian follicles and the oocytes inside mature in vitro. It can be offered to women with infertility problems, combined with In Vitro Fertilization (IVF), offering women pregnancy without ovarian stimulation.
History
In 1935, Pincus & Enzmann did the first experiment on immature rabbit oocyte, showing in vitro spontaneous maturation and fertilization. They showed maturation occurs in isolation from normal follicular environment. In 1965 Edwards then continued IVM studies in mouse, sheep, cow, pig, rhesus monkey and human. By 1991, the first human pregnancy was recorded using IVM followed by IVF, and in 1994 the first birth using IVM oocytes from polycystic ovarian syndrome patients was recorded highlighting that PCOS patient's oocytes are capable of maturation.
Background
Oogenesis takes place during fetal life, in which primordial germ cells undergo mitosis until a few weeks prior to birth, forming oogonia. These then begin meiosis to form the oocyte within the primordial follicle. This follicle consists of the oocyte surrounded by flattened pregranulosa cells. Babies are born with 1-2 million primordial follicles, and by puberty have around 300,000. Of these primordial follicles, only around 400 mature oocytes are released and could be potentially fertilised, with the rest undergoing atresia.
'Maturation' of an oocyte is the process by which an 'oocyte attains the competence to be fertilised and undergo embryogenesis'.
Folliculogenesis is the mechanism by which the ovarian follicles mature. This can take many months in vivo and involves primordial follicle growth and differentiation.
Primordial follicles containing the primary oocyte, arrested at prophase of meiosis I, develop into primary follicle containing cuboidal granulosa cells. A secondary follicle is formed with a few granulosa cell layers, as well as a theca layer. Finally before ovulation, a tertiary follicle is formed containing a follicular-fluid filled antrum. Of these small antral follicles, 1 will become dominant and ovulate (in monoovulatory species). During ovulation, the primary oocyte will resume meiosis in response to signals, arresting in metaphase meiosis II, ready for fertilization. The dominant follicle contains the mature oocyte. Follicular development is directly under gonadotropins control, LH and FSH. These use cAMP as an intracellular second messenger, with growth factors and cytokines also influencing their development in vivo.
Through in vitro maturation, folliculogenesis and latter parts of oogenesis are being mimicked outside of the ovaries- trying to recreate the conditions for these processes.
Techniques
If a follicle has reached the early tertiary or antral stage, IVM can be carried out.
Firstly, the oocytes need to be obtained from the subject. The timing of this is dependent on the stage of the cycle the subject is in, which is usually monitored using ultrasonography. If without the use of priming, oocytes are obtained when the largest follicles are around 10mm in size.
In humans, this can be done with an aspiration needle, using ultrasound to allow accuracy. Depending on whether you are aspirating mature or immature follicles, the protocol differs slightly. In both procedures the aspiration pressure is reduced, but to varying degrees. Additionally, it is more important that the aspirate is filtered when retrieving immature follicles, as the follicles are much smaller and harder to see in the fluid extracted.
Priming is the process by which the oocytes are primed with follicle-stimulating hormone (FSH) or human chorionic gonadotrophin (hCG) before retrieval. hCG is important in women with polycystic ovarian syndrome (PCOS). This results in an expanding or dispersed pattern of the cumulus oophorus around the egg cell, facilitating its identification within follicular fluid. This leads to improved maturation and quality of the oocytes. However, the evidence of a clinical effect of hCG priming is still lacking. When IVM was initially introduced, successful pregnancies were low, leading to the use of ovary priming.
This technique is also used in sheep, pigs and other animals. See In animals.
Oocytes classification
Oocytes are classified depending on their condition, such as number of cumulus cells. The best oocytes are chosen to be matured in the hope of then being implanted using in vitro fertilisation techniques.