title: Sperm selection in natural conception: what can we learn from Mother Nature to improve assisted reproduction outcomes?
abstract: BACKGROUND
  In natural conception only a few sperm cells reach the ampulla or the site of fertilization. This population is a selected group of cells since only motile cells can pass through cervical mucus and gain initial entry into the female reproductive tract. In animals, some studies indicate that the sperm selected by the reproductive tract and recovered from the uterus and the oviducts have higher fertilization rates but this is not a universal finding. Some species show less discrimination in sperm selection and abnormal sperm do arrive at the oviduct. In contrast, assisted reproductive technologies (ART) utilize a more random sperm population. In this review we contrast the journey of the spermatozoon in vivo and in vitro and discuss this in the context of developing new sperm preparation and selection techniques for ART.
  
  
  METHODS
  A review of the literature examining characteristics of the spermatozoa selected in vivo is compared with recent developments in in vitro selection and preparation methods. Contrasts and similarities are presented.
  
  
  CONCLUSIONS
  New technologies are being developed to aid in the diagnosis, preparation and selection of spermatozoa in ART. To date progress has been frustrating and these methods have provided variable benefits in improving outcomes after ART. It is more likely that examining the mechanisms enforced by nature will provide valuable information in regard to sperm selection and preparation techniques in vitro. Identifying the properties of those spermatozoa which do reach the oviduct will also be important for the development of more effective tests of semen quality. In this review we examine the value of sperm selection to see how much guidance for ART can be gleaned from the natural selection processes in vivo.
authors.affiliation: Boston IVF, 130 Second Ave, Waltham, MA 02451, USA dsakkas@bostonivf.com.
authors.forename: Denny
authors.initials: D
authors.lastname: Sakkas
authors.affiliation: Reproductive and Developmental Biology, Medical School, Ninewells Hospital, University of Dundee, Dundee DD19SY, UK.
authors.forename: Mythili
authors.initials: M
authors.lastname: Ramalingam
authors.affiliation: IVI Valencia, Guadassuar 1 Bajo, 46015 Valencia, Spain.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Garrido
authors.affiliation: Reproductive and Developmental Biology, Medical School, Ninewells Hospital, University of Dundee, Dundee DD19SY, UK.
authors.forename: Christopher L R
authors.initials: CL
authors.lastname: Barratt
mesh_terms: D000818:Animals; D005260:Female; D005306:Fertilization / Q000502:physiology*; D020869:Gene Expression Profiling; D006801:Humans; D008297:Male; D055432:Metabolomics; D040901:Proteomics; D027724:Reproductive Techniques, Assisted; D055101:Semen Analysis / Q000379:methods*; D013079:Sperm Maturation; D013081:Sperm Motility; D013084:Sperm-Ovum Interactions; D013094:Spermatozoa / Q000502:physiology*; D016896:Treatment Outcome
keywords: Fallopian tube; sperm selection; spermatozoa; uterine tube; uterus; vagina