Human Fertilisation and Embryology Authority
The Scientific and Clinical Advances Group
Committee:
Scientific and Clinical Advances Group
Meeting Date:
16th September 2008
Agenda Item:
7
Paper Number:
SCAG(09/08)02
Paper Title:
Techniques involving immature gametes
Author:
Helen Richens
For Information or Decision?
Decision
Resource Implications:
Accounted for in the business plan (horizon scanning)
Members are asked:
Recommendation to the
Committee:
-
whether centres are likely to want to freeze immature
testicular tissue?
-
whether centres are likely to want to carry out
spermatogenesis in vitro or in vitro growth of eggs (as
opposed to transplanting the tissue)?
-
if centres want to carry out spermatogenesis in vitro,
what are the likely timescales and safety implications
for using this sperm in treatment?
-
what are the likely timescales and safety implications
for using eggs grown in vitro from ovarian tissue?
Lay summary
Under the Human Fertilisation and Embryology Bill the definition of a gamete will be
expanded to include immature eggs and sperm at all stages of development, not just
mature gametes. This means the HFEA will need to licence the storage and use of
immature gametes and tissue containing immature gametes once the Bill has been
passed.
There have been several recent studies looking at freezing immature testicular tissue
from pre-pubescent boys. This tissue does not contain mature sperm, but does
contain stem cells that will form sperm (spermatogonial stem cells) and early sperm
cells (spermatogonia and spermatocytes). Young boys who are undergoing cancer
treatment that is likely to affect their fertility may be able to freeze their testicular
tissue before treatment to try and preserve their fertility. Clinicians may be able to
transplant this tissue back into the patient when they are older in the hope that this
will allow them to produce sperm naturally. Alternatively researchers may be able to
mature sperm from this tissue in the lab. This could then fertilise an egg in vitro and
be used in fertility treatment.
1. Background
1.1. The Human Fertilisation and Embryology (HFE) Bill defines a gamete as:
Section 1 (4)
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“(a) references to eggs are to live human eggs, including cells of the female
germ line at any stage of maturity…
(b) references to sperm are to live human sperm, including cells of the male
germ line at any stage of maturity,
(c) references to gametes are to be read accordingly.”
1.2. Therefore under the Bill a gamete includes immature eggs and sperm (i.e. all
egg and sperm precursor cells at any stage of development), as well as
mature eggs and sperm. Initial talks with the Department of Health suggest
that this includes spermatogonial stem cells.
1.3. This has implications for how the HFEA license the storage and use of tissue
that contains immature gametes, as this will fall under the HFEA’s remit
when the Bill is passed.
1.4. Centres have already been freezing ovarian tissue for some time. Now
studies are also being carried out on freezing immature testicular tissue.
2. Testicular tissue freezing
2.1. Cancer treatment can often be toxic to the gonads. Men and boys who
produce mature sperm can preserve their fertility by freezing sperm samples.
However this is not an option for pre-pubescent boys who do not produce
mature sperm. These boys only have immature testicular tissue containing
spermatogonia (the earliest sperm cells) and spermatocytes (sperm cells at
the next stage of development). There have been several recent studies that
suggest that immature testicular tissue from pre-pubescent boys could be
frozen and used to restore their fertility, for example by transplanting the
tissue back into the patient at a later date.
2.2. A Swedish group, Keros et al (2007), developed an effective method for
cryopreserving immature human testicular tissue. Their method maintained
spermatogonia, Sertoli cells (cells that support the formation of sperm cells)
and the structure of the tissue during freezing, thawing and tissue culture.
Jahnukainen et al (2007) also managed to maintain spermatogonial stem
cells after freezing immature non-human primate testicular tissue. The stem
cells were able to begin spermatogenesis (the process of sperm formation).
2.3. A Belgian group, Wyns et al (2007), froze pieces of human cryptorchid
testicular tissue and then grafted them onto mice for 21 days. 14.5% of
spermatogonia survived, with 32% of those showing proliferative activity. The
number of Sertoli cells remained unchanged.
2.4. Wyns et al (2008) cryopreserved immature human testicular tissue and then
transplanted it into mice for 6 months. They found that spermatogonia were
able to survive and proliferate. A few spermatocytes and spermatid-like cells
were seen. However normal spermatogenesis was not seen and there was
an increasing loss of spermatogonia over time.
2.5. Though research groups have not managed to restore normal
spermatogenesis after transplantation of human immature testicular tissue,
this has been achieved in mice. A Japanese group (Shinohara et al 2002)
managed to produce mature sperm after transplanting cryopreserved
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immature mouse testicular tissue back into mice. The group fertilised eggs
with the sperm resulting in offspring.
2.6. Freezing whole sections of the tissue (as opposed to cell suspensions)
preserves the microenvironment of the seminiferous tubule and interactions
between the sperm cells and Sertoli cells throughout spermatogenesis.
Therefore the tissue can potentially be transplanted back into the patient at a
later date. Spermatogenesis may then be restored, allowing the patient to
produce their own sperm naturally.
2.7. In some patients transplantation of testicular tissue may not be safe, for
example if they suffer from haematological malignancies that may be retransmitted by transplantation. In these cases in vitro spermatogenesis has
been suggested as a possible option, if this technique is developed in the
future (Keros et al, 2007).
2.8. Research into the use of human immature testicular tissue is still at a very
early stage. Though groups have developed effective cryopreservation
protocols they have not managed to restore normal spermatogenesis
following transplantation. We are not aware of any groups carrying out in
vitro spermatogenesis from immature human testicular tissue.
3. Regulation of immature gametes
3.1. Once the HFE Bill is passed, the HFEA’s remit will be expanded to cover the
storage and use of immature gametes. The HFEA will need to license
centres storing tissue that contains immature gametes as well as mature
gametes.
3.2. Though the Bill applies to both ovarian and testicular tissue storage, it may
have less impact on centres storing ovarian tissue. Currently clinics who
store ovarian tissue only need a licence from the HFEA if mature eggs are
stored. If ovarian tissue is intended for human application (i.e. transplant),
centres need a licence form the Human Tissue Authority (HTA). However,
as it is difficult to determine whether ovarian tissue contains immature or
mature eggs, centres often already have a licence from the HFEA to store
ovarian tissue.
3.3. Although the HFEA will regulate the storage of testicular and ovarian tissue,
the HTA regulates the transplant of both tissue types. Tissue stored under
an HFEA licence, however, will not normally be stored in conditions suitable
for the tissue to be transplanted at a later date. Fewer centres are storing
ovarian tissue now because they do not have the facilities to store tissue in
conditions that make it suitable for transfer.
3.4. It is also possible that centres may want to freeze immature testicular tissue
and then carry out spermatogenesis in vitro. Sperm could then be used in
ICSI. In this case in vitro spermatogenesis would come under the HFEA’s
remit, in a similar way that in vitro growth or in vitro maturation of eggs falls
under the HFEA’s remit. Sperm derived in this way would be permitted
gametes for treatment under the HFE Bill because they would have
originated from the testes (unlike in vitro derived gametes from stem cells).
However there may be safety implications of using sperm derived in this way.
3.5. The following table briefly shows the regulation of immature testicular and
ovarian tissue:
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Tissue
Storage regulated Technique after
by
freezing
Regulated by
Immature testicular HFEA
tissue
Transplant
HTA
Immature testicular HFEA
tissue
In vitro
spermatogenesis
followed by ICSI
HFEA
Ovarian tissue
HFEA
Transplant
HTA
Ovarian tissue
HFEA
In vitro growth or HFEA
maturation followed
by IVF/ICSI
4. Update on in vitro growth of oocytes
4.1 Telfer et al (2008), at the University of Edinburgh, recently reported
successful in vitro growth of human eggs to an advanced stage. Ovarian
biopsies were taken from six women aged between 26 and 40 years, then
ovarian cortical strips were cultured for 6 days, after which pre-antral
(secondary) follicles were dissected from the strips. 74 intact pre-antral
follicles were then further cultured, for 4 days, to attempt to reach the late preantral/early antral stage. Follicles grew to a larger size and out of the follicles
which survived the entire culture period (cultured in the presence of growth
factor activin A) 30% showed normal morphology with intact oocytes and
antral formation.
4.2 The group conclude that this research provides the first encouraging step
towards achieving full in vitro growth of human oocytes and raises the
possibility of bridging the gap between IVG of follicles and IVM of oocytes.
The group suggest that the challenge is now to develop further culture steps
and to optimise timings of exposure to key factors such as activin and FSH to
enable oocyte development. This technique is still in its very early stages and
it still needs to be established whether eggs developed in this way are
genetically normal and can undergo normal maturation and fertilisation.
5. Conclusions
5.1 Members are asked:
-
whether UK centres are likely to want to freeze immature testicular tissue?
-
whether centres are likely to want to carry out spermatogenesis in vitro or in
vitro growth of eggs (as opposed to transplanting the tissue)?
-
if centres want to carry out spermatogenesis in vitro, what are the likely
timescales and safety implications for using this sperm in treatment?
-
what are the likely timescales and safety implications for using eggs grown in
vitro from ovarian tissue?
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6. References
- Jahnukainen K et al (2007) Effect of cold storage and cryopreservation of
immature non-human primate testicular tissue on spermatogonial stem cell
potential in xenografts. Human Reproduction 22(4): 1060-1067.
-
Keros V et al (2007) Methods of cryopreservation of testicular tissue with
viable spermatogonia in pre-pubertal boys undergoing gonadotoxic cancer
treatment. Human Reproduction 22(5): 1384-1395.
-
Shinohara T et al 2002) Birth of offspring following transplantation of
cryopreserved immature testicular pieces and in-vitro microinsemination.
Human Reproduction 17(2): 3039-3045.
-
Telfer E et al (2008) A two-step serum-free culture system supports
development of human oocytes from primordial follicles in the presence of
activin. Human Reproduction 23(5): 1151-1158.
-
Wyns C et al (2007) Spermatogonial survival after cryopreservation and
short-term orthotopic immature human cryptorchid testicular tissue grafting to
immunodeficient mice. Human Reproduction 22(6): 1603-1611.
-
Wyns C et al (2008) Long-term spermatogonial survival in cryopreserved and
xenografted immature human testicular tissue. Human Reproduction July 28
2008 advanced online publication.
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