Clinical meaning
Puberty is initiated by reactivation of the hypothalamic GnRH pulse generator after a quiescent childhood period. Kisspeptin neurons in the arcuate nucleus (KNDy neurons - Kisspeptin/Neurokinin B/Dynorphin) serve as the master regulators of GnRH pulsatility. Kisspeptin binds to GPR54 (KISS1R) on GnRH neurons, stimulating pulsatile GnRH secretion into the hypophyseal portal system. GnRH activates pituitary gonadotrophs to release LH and FSH, which drive gonadal steroidogenesis and gametogenesis.
Delayed puberty is defined as the absence of breast development (thelarche) by age 13 in females or testicular enlargement (>4 mL) by age 14 in males. Constitutional delay of growth and puberty (CDGP) is the most common cause, representing a functional delay in GnRH pulse generator reactivation with an intact HPG axis. Pathological causes include hypogonadotropic hypogonadism (central deficiency) and hypergonadotropic hypogonadism (primary gonadal failure).
Hypogonadotropic hypogonadism results from GnRH deficiency due to genetic mutations in KAL1 (Kallmann syndrome with anosmia - disrupted olfactory bulb and GnRH neuron migration), FGFR1, PROKR2, GnRHR, or KISS1R genes. Acquired causes include hypothalamic-pituitary tumors (craniopharyngioma, prolactinoma), infiltrative disorders, radiation injury, and functional suppression from chronic illness, malnutrition, or excessive exercise (functional hypothalamic amenorrhea).
Hypergonadotropic hypogonadism reflects primary gonadal failure with elevated FSH/LH due to loss of negative feedback. Causes include Turner syndrome (45,X - streak gonads), Klinefelter syndrome (47,XXY - testicular dysgenesis), autoimmune oophoritis, gonadal dysgenesis, and gonadotoxic chemotherapy/radiation.