Progress, challenges and perspectives on fish gamete cryopreservation and embryo research

Progress, challenges and perspectives on fish gamete Cryopreservation and Embryo
research

1. What is Cryopreservation?

· Cryopreservation is a technique to preserve the biological living cells and tissues under very low temperature for an extended period of time. Cryopreservation is to preserve and store the viable biological samples in a frozen at (‘196’ C) state over extended periods of time.
·With the help of cryopreservation technique indefinite storage of the desirable gene pools and ensures the availability of cryopreserved semen for artificial insemination/breeding is preserved.
·Polge et al., (1949) were successful by preserving sperm some 6 decades back that leads a revolution in veterinary sciences.
· Germplasm cryopreservation includes storage of embryos, sperm which thus contributes directly to animal breeding programs.
· Germplasm cryopreservation also helps in preserving the genomes of endangered species. In present days fish gametes especially semen of marine fish is also been conducted worldwide with a good success.
·Hiemstra et al., (2005) have reported that milt of over 200 species of freshwater and marine fish have been cryopreserved till date.

2. Why Cryopreservation is needed?

In
aquaculture, gametes with high survivability during incubation and growth are
always in demand. High-quality gametes produce viable offspring’s which
survives better and attain maturity and therefore better commercialization (Bonnet et al., 2007; Bobe and Labbe, 2010). Bobe and Labbe (2010) have termed that proper
management and selection of broodstock is the key to produce quality gametes.
Several factors are responsible for quality and fertilization potential among
the fish individuals, notable among these factors are nutrition, feeding,
broodstock, genetics, age of the fish. Suitable environmental conditions also
play an important role in fertilization. In addition to these factors chemical
composition of eggs (Giménez et al., 2006),
the dimensions of the perivitelline space (Finn,
2007) and semen quality (Kjorsvik et al.,
1990) is also an important contributor for healthy broodstock. Brooks et
al. (1997) stated that environmental conditions is directly related to gonadal
growth, egg and sperm, any change in temperature, humidity, pH, nutrition has adverse effects on egg and sperm viability (Mansour
et al., 2006). Low feeding reduces the fecundity, delay in maturation
which results in a decrease in the production of gametes. To supply with gametes
continuously is important for the aquaculture production but most of the fishes
spawn once in a year, so there is a dearth of the gametes during non-spawning
period, so to cryopreserve the embryos, sperm and eggs is a product for the
aquaculture industry.

3.Basic Steps in
Cryopreservation of semen

The
complete steps of the procedure involved in the conservation of the obtained
biological samples are the following:

1.    
Selection of material:

Some
important criteria must be followed when selecting biological materials such
as:
· Volume
· Density
· pH
· Morphology
· Free damage

2.    
Cryoprotectant addition:

Cryoprotectants such as
glycerol, FBS, salts, sugars, and glycols are added to samples because they lower the freezing point
of the medium and also allow a slower cooling rate, reducing the risk of
crystallization

3.    
Freezing:

Different methods of
freezing are used in this cryopreservation method to protect cells from cell
damage and death from exposure to hot solutions of cryoprotectants

4.    
Liquid nitrogen storage:

Cryopreserved samples
are stored in extreme cold or -80 ° C in a freezer for at least 5 to 24 hours
before being transferred to storage containers.

5.    
Thawing:

Heating process of biological
samples to control the cooling rate and avoid cell damage caused by
crystallization.

3.1 Progress related to cryopreservation

Cryopreservation
in fish gametes has received great attention over the last decades because of
its applications. The important application related to cryopreservation is its
use in aquaculture purposes, it helps in allowing the improvement of broodstock
management at hatcheries, preserving the quality and disease-resistant strains
which results in genetic improvement programs (Asturiano
et al., 2004). It finds great importance in the fishes which produce
low sperm production (Cabrita et al., 2006).
Another important application of cryopreservation is to preserve genetic
material from natural populations, this results in maintaining the original wild
genotypes in the future (Vandeputte, 2011).
In the case of endangered species, genetic material can be cryopreserved (Martínez-Páramo et al., 2009, 2016).

4. Embryo cryopreservation

· During infertility treatment, hormones are used to stimulate the development of the eggs. Eggs are collected and fertilized in the laboratory.
· More embryos can be created to transfer to the woman’s uterus.
·These embryos can be cryopreserved and transferred later.
·With this, the female can obtain an additional embryo transfer in the future without spending on another IVF cycle.

5. Oocyte
cryopreservation

· In the vitrification method, the eggs freeze quickly, so that ice crystals form shortly.
·New cryoprotectants are used at this very high concentration of antifreeze products.
·The oocyte is first placed in a bath containing low concentration antifreeze, such as a cryoprotectant.
·Sucrose is added to help extract water from the egg.
·The egg is then moved to a high concentration antifreeze cryoprotectant for a few seconds, then immediately transferred to liquid nitrogen.
·When the egg is to be transplanted into women, the egg is thawed and used.

6. Cryopreservation
of sperm

·The semen sample is mixed with a special solution to provide protection during freezing and thawing.
·The samples are then transferred to plastic bottles and frozen in liquid nitrogen.
·This process guarantees the possibilities of conception in the future.
·Semen can also be deposited, frozen and stored in cryobanks for less than a year.
·These sperm can be used for certain infertility treatment procedures.

7. Benefits of Cryopreservation

·Cryopreservation technology applied to the preservation of fish gametes “eggs and sperm” in   aquaculture plays an important role in seed production, genetic management of broodstock and   conservation of aquatic resources.
·Cryopreservation has been successfully carried in a number of fresh water fish, but now much   attention is been given with related to marine fishes.

11. Conclusion of Cryopreservation