Vicki N. Meyers-Wallen,     Ithaca, New York

Normal Sexual Development

Normal sexual development depends on successful completion of three consecutive steps: (1) establishment of chromosomal sex, (2) development of gonadal sex, and (3) development of phenotypic sex. Chromosomal sex, which corresponds to genetic sex in normal animals, is established at fertilization. The zygote receives either two X chromosomes or an X and a Y chromosome and maintains this chromosomal constitution in all cells by mitotic division. Morphology of early XX and XY embryos is sexually indifferent. Both have a genital ridge, from which the testis or ovary develops. They also have müllerian and wolffian ducts, a urogenital sinus, a genital tubercle, and genital swellings, from which the internal and external genitalia will arise (Figure 217-1). Differentiation of the genital ridge into a testis or an ovary defines gonadal sex and marks the end of the sexually indifferent stage. Although several genes are necessary for normal development through the sexually indifferent stage, genes that determine gonadal sex have a pivotal role in sexual development.

Gonadal sex typically is determined by sex chromosome constitution: presence of the Y chromosome results in testis development, whereas its absence results in ovarian development. The sex-determining region Y gene, SRY, is located normally on the Y chromosome, and the SRY protein is the signal for initiating testis differentiation in the genital ridge (Jakob and Lovell-Badge, 2011). In the absence of the Y chromosome and SRY, the genital ridge normally becomes an ovary. However, ovarian induction is not a passive process: testis-promoting and ovary-promoting signaling pathways are responsible for gonadal sex determination (Quinn and Koopman, 2012).

Phenotypic sex is controlled normally by gonadal sex. If the genital ridges are removed from XX or XY embryos before gonadal differentiation occurs, a female phenotype develops, indicating that the embryo is programmed to develop as a female and must be diverted from this pathway to develop as a male. The critical diverting step is testis development. The testis secretes two substances that act within embryonic critical periods to induce masculinization: (1) müllerian-inhibiting substance/antimüllerian hormone (MIS/AMH), which causes the müllerian ducts to regress, and (2) testosterone, which stimulates formation of the vasa deferentia and epididymides from the wolffian ducts (see Figure 217-1). In the external genitalia, testosterone is converted to dihydrotestosterone (DHT) by the enzyme 5α-reductase. Dihydrotestosterone stimulates formation of the prostate and male urethra, penis, and scrotum from the urogenital sinus, genital tubercle, and genital swellings, respectively (see Figure 217-1). Descent of the testes into the scrotum completes the male external genitalia, but the genetic and hormonal control of this process is incompletely understood. Testosterone and insulin-like 3 factor (INSL3), both secreted by Leydig cells, are required for testis descent, as are their receptors, but other unknown factors also likely are involved. In the absence of testicular secretions, female genitalia develop (see Figure 217-1).

Diagnosis of Disorders of Sexual Development

For the purpose of pursuing a diagnosis, it is useful to identify the initial step at which development differs from normal, either at the level of chromosomal sex, gonadal sex, or phenotypic sex (Table 217-1). A more precise diagnosis defines the disorder according to its etiology, preferably by the specific gene mutation responsible for the defect. To eliminate older, confusing terms such as pseudohermaphrodite and facilitate incorporation of molecular diagnoses, a new nomenclature has been established (Pasterski, Prentice, and Hughes, 2010). With this terminology, intersex individuals are described as having a disorder of sexual development (DSD), a nonspecific term. All DSDs are categorized initially by karyotype, with sex chromosome DSD including all errors at the level of chromosomal sex. Errors occurring at the level of gonadal sex or phenotypic sex now are divided according to karyotype, either XX DSD or XY DSD (see Table 217-1).

Regardless of the nomenclature used, the diagnostic plan for any DSD includes a karyotype (dog 78,XX or 78,XY; cat 38,XX or 38,XY). The presence or absence of the SRY gene in dogs and cats can be tested by polymerase chain reaction (PCR). Gonadal sex is determined by histology and may require serial gonadal sections for identification of ovotestes. A concise description of the internal and external genitalia is necessary to define phenotypic sex. Assays of peripheral hormones may be helpful but are not a substitute for gonadal histology. Gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG) stimulation tests, rather than single peripheral samples, are necessary in many cases, particularly those in which peripheral androgen concentrations are of concern.