Leydig cells within the testes produce testosterone to induce the mesonephric (Wolffian) duct to develop and ultimately form the male internal reproductive organs, including the seminal vesicles, epididymides, ejaculatory ducts, and ductus deferens.
Female is the default phenotypic differentiation, with degradation of the mesonephric duct and unhindered development of the paramesoneprhic duct. For male development, the SRY gene on the Y chromosome produces the testis-determining factor (TDF) protein, which acts to induce differentiation of the gonads into testes over ovaries. The testes contain both Sertoli and Leydig cells. Sertoli cells produce Mullerian inhibitory factor (MIF), which prevents the development of the female internal genitalia by suppressing the development of the paramesonephric ducts. The Leydig cells produce testosterone, which stimulates the development of the mesonephric duct and formation of the male internal reproductive organs. Some testosterone is transformed into dihydrotestosterone (DHT), which is responsible for the development of the external male reproductive organs.
Lewis discusses a comprehensive newborn examination, focusing on assessment of genital abnormalities. In females, signs of ambiguous genitalia include either clitoromegaly or fused labia. For male infants, signs of ambiguous genitalia can include bilateral undescended testes, a micropenis, or bifid scrotum.
Svechnikov et al. review the origin, development, and regulation of Leydig cells. Leydig cells that are either absent or dysfunctional result in the incomplete 'masculinization' of the male fetus. For 2-3 months after birth, Leydig cells increase in number. This early period of increased androgens may assist certain cell types throughout the body in responding to androgen stimulation later in life. For the majority of childhood, Leydig cells are dormant until the LH surge at puberty stimulates their increased androgen production.
Illustration A summarizes the pathways for genital embryology; note the roles of TDF, testosterone, and MIF. Illustration B shows the role of the Sertoli and Leydig cells in this process.
Answer 1: The TDF is a protein encoded by the SRY gene on the Y chromosome; it is NOT produced by Sertoli cells. Rather, TDF is responsible for gonadal differentiation into testes, which then contain Sertoli and Leydig cells.
Answer 2: Leydig cells do not excrete TDF; TDF is a genetically encoded protein that leads to the differentiation of the gonads into testes.
Answer 3: Sertoli cells do not produce testosterone; instead, they produce MIF, which suppresses the development of the female internal sex organs.
Answer 5: MIF, produced by Sertoli cells, suppresses the default development of the female internal reproductive organs.
Lewis ML. A comprehensive newborn exam: part II. Skin, trunk, extremities, neurologic. Am Fam Physician. 2014 Sep 1;90(5):297-302.
PMID:25251089 (Link to Abstract)
Svechnikov K, Landreh L, Weisser J, Izzo G, Colón E, Svechnikova I, Söder O. Origin, development and regulation of human Leydig cells. Horm Res Paediatr. 2010;73(2):93-101. doi: 10.1159/000277141. Epub 2010 Feb 9.
PMID:20190545 (Link to Abstract)