Germ cells normally occur inside the gonads (ovary and testis). In vertebrates, ova are produced by female gonads (sexual glands) called ovaries. The ovaries are considered the female gonads. FSH is subject to oestrogen feed-back from the gonads via the hypothalamic pituitary gonadal axis. The natural steroid hormones are generally synthesized from cholesterol in the gonads and adrenal glands. Androstenedione is produced in the adrenal glands and the gonads. The intermediate mesoderm develops into kidneys and gonads. DHEA is produced in the adrenal glands, the gonads, and the brain. Pregnenolone is produced mainly in the adrenal glands, the gonads, and the brain. In medicine, Mars presides over the genitals, the muscular system, the gonads and adrenal glands. They all have gamete-producing organs known as gonads. The developing spleen forms near the urogenital ridge from which the gonads develop. Other reasons include atypical chromosomes, gonads or hormones. The reproductive system consists of the gonads and the internal and external sex organs. Both sexes possess one or two tubular gonads.