Gonadal sex determination represents a unique model for studying cell fate decisions. However, a complete understanding of the different cell lineages forming the developing testis and ovary remains elusive. The widespread adoption of advanced sequencing technologies, such as scRNA-seq, has provided the field of developmental biology with an opportunity to discover previously unrecognized cell types, such as short-lived progenitors or rare cell lineages. By combining single cell transcriptomic analyses during the critical period of sex determination with in vivo lineage tracing, we will describe the specification and differentiation of several previously neglected gonadal cell lineages that give rise to multiple cell types such as rete testis cells, peritubular myoid cells, as well as fetal and adult Leydig cells.