Sperm sorting

Sperm sorting is a means of sex selection in which spermatozoa are separated into X- (female) and Y- (male) chromosome bearing populations based on their difference in DNA content. The resultant 'sex-sorted' spermatozoa are then able to be used in conjunction with other assisted reproductive technologies such as artificial insemination or in-vitro fertilization (IVF) to produce offspring of the desired sex.

Methods

Sperm sorting utilises the technique of flow cytometry to analyse and 'sort' spermatozoa. Originally developed by the USDA in conjunction with Lawrence Livermore National Laboratories, 'Beltsfield Sperm Sexing Technology' relies on the DNA difference between the X- and Y- chromosomes [Garner DL, Seidel GE. History of commercializing sexed semen for cattle. Theriogenology 2008;69: 886-895.] . Prior to flow cytometric sorting, semen is labelled with a fluorescent dye called Hoechst 33342 which binds to the DNA of each spermatozoon. As the X chromosome is larger (ie has more DNA) than the Y chromosome, the "female" (X-chromosome bearing) spermatozoa will absorb a greater amount of dye than its male (Y-chromosome bearing) counterpart. As a consequence, when exposed to UV light during flow cytometry, X spermatozoa fluoresce brighter than Y- spermatozoa. As the spermatozoa pass through the flow cytometer in single file, each spermatozoon is encased by a single droplet of fluid and assigned an electric charge corresponding to its chromosome status (eg X-postive charge, Y-negative charge). The stream of X- and Y- droplets is then separated by means of electrostatic deflection and collected into separate collection tubes for subsequent processing [Seidel GE, Jr., Garner DL. Current status of sexing mammalian spermatozoa. Reproduction 2002;124: 733-743.] .

Accuracy

While highly accurate, sperm sorting by flow cytometry will not produce two completely separate populations. That is to say, there will always be some "male" sperm among the "female" sperm and vice versa. The exact percentage purity of each population is dependant on the species being sorted and the 'gates' which the operator places around the total population visible to the machine. In general, the larger the DNA difference between the X and Y chromosome of a species the easier it is to produce a highly pure population. In sheep and cattle, purities for each sex will usually remain above 90% depending on 'gating', while for humans these may be reduced to 90% and 70% for "female" and "male" spermatozoa, respectively. [http://www.microsort.net/ Genetics & IVF Institute: MicroSort Clinical Trial] ]

Ethical concerns

For the most part, sperm sorting shares the ethical concerns implicit to the idea of sex selection. However, the fact that sperm sorting is in effect sex-preselection, with the sex of the offspring decided at conception rather than post-conception (preimplantation genetic diagnosis; PGD), post-implantation (ultrasound) or post-birth, makes the technique decidely more attractive from an ethical standpoint.

Regulation

In the US, for humans, the practice of sperm sorting is tightly regulated by the FDA. In effect it remains an ongoing clinical trial, with the procedure made available to a limited number of participants each month, in addition to fulfilling certain criteria, such as having a disease with sex linkage or having at least one child (for family balancing).

ee also

*Sex selection

References