gonadal ridge

**Origin and formation** - Arises from intermediate mesoderm as part of the urogenital ridge (which also gives rise to the mesonephros/mesonephric duct). The gonadal ridge forms on the medial side of the mesonephros around week 4-5, as proliferating coelomic epithelium and underlying mesenchyme. - Key transcription factors for formation: WT1, SF1 (NR5A1), LHX9. WT1 mutations: WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, Intellectual disability), Denys-Drash (early nephropathy, Wilms tumor, gonadal dysgenesis), and Frasier syndrome. SF1 mutations: gonadal dysgenesis and adrenal insufficiency (adrenals share a common precursor).

**Germ cell migration** - Primordial germ cells (PGCs) arise from the epiblast, are first seen in the yolk sac wall near the allantois at ~week 3, and migrate along the dorsal mesentery of the hindgut into the gonadal ridge during weeks 4-6. - Primary sex cords (proliferating coelomic epithelium) grow into the underlying mesenchyme and surround the PGCs. Without PGCs, the gonad fails to develop normally (streak gonad). Failed migration can leave extragonadal germ cell tumors along the midline route (e.g., sacrococcygeal teratoma, mediastinal, pineal).

**Bipotential (indifferent) stage, weeks 5-6** - The gonad is morphologically identical in XX and XY embryos until about week 7. - **XY**: the SRY gene (Yp, sex-determining region) encodes testis-determining factor, which upregulates SOX9. This leads to primary sex cords becoming seminiferous tubules (Sertoli cells), with Leydig cells in the mesenchyme. Sertoli cells secrete AMH (MIS), which causes paramesonephric (Müllerian) duct regression. Leydig cells secrete testosterone, which maintains the mesonephric (Wolffian) duct (epididymis, vas deferens, seminal vesicles). DHT (via 5α-reductase) forms male external genitalia. - **XX**: absent SRY leads to ovarian development (WNT4, RSPO1, FOXL2). Primary sex cords degenerate and secondary (cortical) sex cords form, surrounding the PGCs (oogonia) to form primordial follicles. Granulosa cells come from the sex cords and theca cells from the mesenchyme. No AMH means the paramesonephric ducts persist (fallopian tubes, uterus, upper vagina), and the lack of testosterone means the mesonephric ducts regress.

**Descent** - Gonads initially lie at the lumbar level and then descend. The gubernaculum (a mesenchymal band) guides descent: in males it becomes the gubernaculum testis, and in females it becomes the ovarian ligament and round ligament of the uterus. Testes descend through the inguinal canal, carrying the processus vaginalis (becomes the tunica vaginalis). Cryptorchidism is a risk factor for infertility and testicular cancer.

**Clinical correlations (high-yield)** - **Swyer syndrome (46,XY complete gonadal dysgenesis)**: SRY mutation leads to streak gonads, a female phenotype, and a uterus and tubes present (no AMH, but also no testosterone). Gonadoblastoma risk makes gonadectomy necessary. - **Turner syndrome (45,X)**: streak ovaries (gonadal dysgenesis), primary amenorrhea, elevated FSH/LH. - **46,XX testicular DSD**: SRY translocation to the X chromosome or an autosome. - **True hermaphroditism (ovotesticular DSD)**: both ovarian and testicular tissue. - **Androgen insensitivity syndrome (46,XY)**: normal testes and AMH, so there is no uterus. The phenotype is female with a blind-ending vagina and undescended testes. - **5α-reductase deficiency**: ambiguous genitalia at birth, with virilization at puberty. - **Kallmann syndrome**: failure of GnRH neuron migration, not a gonadal ridge defect.

**Mnemonic**: SRY → SOX9 → Sertoli (AMH) and Leydig (testosterone). "Sertoli kills Müllerian; Leydig keeps Wolffian."