Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote.[1][2] Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.[3]
In many species, testicular or ovarian differentiation begins with appearance of Sertoli cells in males and granulosa cells in females.[4] [citation needed]
As male and female individuals develop from embryos into mature adults, sex differences at many levels develop, such as genes, chromosomes, gonads, hormones, anatomy, and psyche. Beginning with determination of sex by genetic and/or environmental factors, humans and other organisms proceed down different pathways of differentiation as they grow and develop.
Humans, many mammals, insects and other animals have an XY sex-determination system. Humans have forty-six chromosomes, including two sex chromosomes, XX in females and XY in males. The Y chromosome must carry at least one essential gene which determines testicular formation (originally termed TDF).[5] In transgenic XX mice (and some human XX males), SRY alone is sufficient to induce male differentiation.[6]
Other chromosomal systems exist in other taxa, such as the ZW sex-determination system in birds[7] and the XO system in insects.[8]
Environmental sex determination refers to the determination (and then differentiation) of sex via non-genetic cues like social factors, temperature, and available nutrients. In some species, such as the hermaphroditic clownfish, sex differentiation can occur more than once as a response to different environmental cues,[9] offering an example of how sex differentiation does not always follow a typical linear path.