In vivo versus in vitro fertilization Riccardo Talevi * , Roberto Gualtieri Dipartimento di Biologia Evolutiva e Comparata, Universita ` di Napoli ‘‘Federico II’’, Via Mezzocannone 8, 80134 Napoli, Italy Abstract The complex sequence of biological steps involved in reproduction in vivo is only partially reproduced in current IVF procedures. In fact, events playing a key role in vivo such as male gamete selection can only be partially mimicked in vitro. To understand the role played by the mammalian oviduct in sperm storage and selection several in vitro sperm–oviductal cell co-culture systems have been developed. Particular sperm subpopulations have been reported to be selected by in vitro cultured oviductal cells through cell–cell adhesion, in different species. In the bovine, in vitro selected sperm have been demonstrated to be endowed with a superior zona pellucida binding and fertilization competence. In conclusion, research on in vitro sperm oviduct interaction may provide new basic information about early reproductive events allowing the development of alternative methods for a more physiological sperm selection in assisted reproductive biotechnologies. # 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Female reproductive tract; IVF; Bovine; Sperm; Fertilization 1. Introduction The interaction of gametes and the subsequent fertiliza- tion are highly regulated processes that represent at the same time a starting point in the life of a new individual and an endpoint in the function of gametes. A long and exten- sive series of coordinated events has to take place both during the production and the maturation of gametes to ensure their successful interaction at the site of fertilization. In mammals, although male and female gametes basically undergo the same critical biological steps during their production, the subsequent pathways leading to the devel- opment of their fertilization competence are drastically different. In fact, at the end of differentiation, an extremely limited number of oocytes ready for fertilization are released from the ovary whereas an enormous number of sperm, released within the seminiferous tubules, are still immotile and unable to fertilize an oocyte. Thereafter, testicular sperm acquire the ability to move forward during epididymal maturation and they become fully competent for fertilization only after a series of physiological changes referred to as ‘capacitation’ that occurs during their journey in the female reproductive tract [1–4]. Since only a few sperm are able to reach the ampullar region of the fallopian tube, i.e. the physiological site for fertilization, it is obvious that capacitation in vivo is accompanied by a dramatical sperm selection to ensure a successful fertilization and embryo development. The concept of sperm selection has been one of the focal points during the development of in vitro fertilization techniques. Nowadays, sperm preparation methods based on differential centrifugation or motility allow the recovery of fractions enriched for sperm with normal morphology and/or forward progressive motility for use both in IVF and ICSI. In the light of the recent literature, although such criteria are the only routinely used in the in vitro sperm selection, it is becoming increasingly clear that new parameters should be considered to recover those sperm able to properly support fertilization and embryo development. In the present paper, research knowledge on sperm phy- siology during the ascension of the female genital tract is briefly reviewed in the attempt to stress which parameters may be needed to define a ‘‘fertilizing sperm’’ and how it can be selected in vitro. 2. Sperm storage within the oviduct The oviduct or fallopian tube has long been considered a simple conduit that allows the capture of the ovulated oocyte, the ascension of sperm migrating from the uterus, the fertilization and the descense of embryos toward the uterus for implantation [3]. It is now becoming increasingly clear that the oviduct with its changing microenvironment European Journal of Obstetrics & Gynecology and Reproductive Biology 115S (2004) S68–S71 * Corresponding author. Tel.: þ39-081-2535177; fax: þ39-081-2535035. E-mail address: riccardo.talevi@unina.it (R. Talevi). 0301-2115/$ – see front matter # 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejogrb.2004.01.015