Our current understanding of the molecular basis of sex determination and gonadal development in humans is mostly an extrapolation of knowledge gained from studies in the mouse. However, the timing of gene expression in the mouse is unusual among mammals, and it is therefore important that data from other models are also available to help elucidate this pivotal process in human development. Here we describe the sequence of molecular and morphological events marking testis differentiation in bovine embryos. The genital ridges first appeared at CRL 12 (day 32). SRY expression began at CRL 18 (day 37) and peaked at CRL 20 (day 39), leading to a cascade of regulatory, signaling, and steroidogenic gene expression at later stages, detected by quantitative real-time RT-PCR and immunohistochemistry. Testis cords were distinguishable at CRL 27 (day 42). We conclude that the timing of gene expression observed in developing human embryos is much more similar to bovine development than it is to the mouse. Therefore, Bos taurus may represent a useful model in which to study gene expression during sex determination, relevant to human development.

Copyright 2009 S. Karger AG, Basel.