Sperm sorting

Sperm sorting is a means of choosing what type of sperm cell is to fertilize the egg cell. It can be used to sort out sperm that are most healthy, as well as determination of more specific traits, such as 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.

For general health

DNA damage in sperm cells may be detected by using Raman spectroscopy.[1] It is not specific enough to detect individual traits, however.[1]

The sperm cells having least DNA damage may subsequently be injected into the egg cell by intracytoplasmic sperm injection (ICSI).[1]

For sex selection

Methods

Sperm sorting utilizes the technique of flow cytometry to analyze and 'sort' spermatozoa. During the early to mid-1980s, Dr. Glenn Spaulding was the first to sort viable whole human and animal spermatozoa using a flow cytometer, and utilized the sorted motile rabbit sperm for artificial insemination. Subsequently, the first patent application disclosing the method to sort "two viable subpopulations enriched for x- or y- sperm" [2] was filed in April 1987 as US Application Serial Number 35,986 and later became part of US Patent 5,021,244; and the patent included the discovery of haploid expression (sex-associated membrane proteins, or SAM proteins) and the development of monoclonal antibodies to those proteins. Additional applications and methods were added, including antibodies, from 1987 through 1997.[3] At the time of the patent filing, both Lawrence Livermore National Laboratories and the USDA were only sorting fixed sperm nuclei,[4] after the Application Serial Number 35,986 patent filing a new technique was utilized by the USDA where "sperm were briefly sonicated to remove tails".[5] USDA in conjunction with Lawrence Livermore National Laboratories, 'Beltsfield Sperm Sexing Technology' relies on the DNA difference between the X- and Y- chromosomes.[6] Prior to flow cytometric sorting, semen is labeled with a fluorescent dye called Hoechst 33342 which binds to the DNA of each spermatozoon. As the X chromosome is larger (i.e. 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 (e.g. X-positive 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.[7]

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 dependent 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.[8]

Ethical concerns

Sperm sorting raises 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), post-implantation (ultrasound) or post-birth, makes the technique decidedly 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).[8]

See also

References

  1. 1 2 3 Ranking sperm cells could improve the odds of in vitro fertilization. By Courtney Humphries. TechnologyReview. Wednesday, January 21, 2009
  2. ↑ US Patent 5,021,244, column 9, Sorting Sperm; http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=5021244.PN.&OS=PN/5021244&RS=PN/5021244
  3. ↑ US Patent 5,021,244; 5,346,990; 5,369,012; 5,439,362; 5,496,722; 5,648,468; 5,660,997; PCT/US1989/002069
  4. ↑ L. A. Johnson, J. P. Flook, M. V. Look, D. Pinkel, Flow sorting of X and Y chromosome-bearing spermatozoa into two populations. Gamete Research Volume 16, Issue1, pages 1-9, January 1987; http://onlinelibrary.wiley.com/doi/10.1002/mrd.1120160102/abstract
  5. ↑ L. A. Johnson, J. P. Flook and M. V. Look, Flow cytometry of X and Y chromosome-bearing sperm for DNA using an improved preparation method and staining with Hoechst 33342. Gamete Research Volume 17, Issue 3, July 1987, Pages: 203–212; http://onlinelibrary.wiley.com/doi/10.1002/mrd.1120170303/abstract
  6. ↑ Garner DL, Seidel GE. History of commercializing sexed semen for cattle. Theriogenology 2008;69: 886-895.
  7. ↑ Seidel GE, Jr., Garner DL. Current status of sexing mammalian spermatozoa. Reproduction 2002;124: 733-743.
  8. 1 2 Genetics & IVF Institute: MicroSort Clinical Trial
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