FACTORS INFLUENCING NORMAL AND ABNORMAL GONADAL DIFFERENTIATION
Genetic Determinants
Gonadal differentiation (testis vs. ovary) is determined by genetic information on the X and Y chromosomes.
Testis Determination: Presence of Y chromosome genes (e.g., SRY) directs the primitive gonad to develop as a testis, even if extra X chromosomes are present.
Ovary Determination: Requires two X chromosomes. Individuals with a single X (45,XO, Turner syndrome) typically develop only rudimentary gonads.
Chromosomal Abnormalities
Meiotic Nondisjunction: Common cause of karyotypic abnormalities in defective gonads.
Chromosomal Mosaicism: Different cell lines with varying chromosome makeup. Can arise from mitotic nondisjunction or chromosomal loss post-fertilization.
Deletions or Translocations: Rearrangements that disrupt sex-determining genes, leading to severe gonadal abnormalities.
Gene Mutations: Specific enzyme errors or defective gonadal structure/hormone secretion due to mutations in sex-determining genes.
STAGES IN GONADAL DIFFERENTIATION
Undifferentiated Stage (Up to ~6 weeks)
Primitive Gonad: A genital ridge along dorsal mesentery.
Cortical Region: Cloak of coelomic epithelial cells; potential to form ovarian (cortical) structures.
Contains primordial germ cells capable of developing into oogonia or spermatogonia.
Medullary Region: Mesenchyme with primary sex cords; potential to differentiate into testicular (medullary) tissue.
Testicular Differentiation
Key Determinants:
SRY gene on Y chromosome + SOX9 (autosomal).
SRY → upregulates SOX9 → drives testis formation (including antimüllerian hormone [AMH] production by Sertoli precursors).
AMH causes müllerian duct regression.
Primary Sex Cords:
Inner portions connect seminiferous tubules to mesonephric (wolffian) duct.
Peripheral portions combine with coelomic epithelial ingrowths (containing germ cells) → form seminiferous tubules.
Most cortex → tunica albuginea + tunica vaginalis (only cortical remnants in the mature testis).
Likely activation of ovary-inducing genes (e.g., WNT4, NR0B1 [DAX1]) and repression of testis genes (SOX9).
Cortical Proliferation:
Occurs later than testicular differentiation.
Secondary sex cords push inward, carrying germ cells to form primordial follicles.
Primary sex cords regress to the hilum (forming rete ovarii remnants).
Proliferation of the cortex ceases at about 6 months.
By that time, the outline of follicular structures is set.
DIFFERENTIATION OF GENITAL DUCTS
Embryonic Duct Systems:
Müllerian ducts: Potential to form fallopian tubes, uterus, upper vagina.
Mesonephric (Wolffian) ducts: Potential to form vas deferens, seminal vesicles, epididymis.
Normal Fate:
Testis Present: Secretes antimüllerian hormone (AMH) from Sertoli cells → müllerian duct regression. Also produces testosterone from Leydig cells → wolffian duct development into male structures.
Ovary or Absent Gonads: Without AMH or testosterone, wolffian ducts regress, and müllerian ducts develop into female internal structures (uterus, tubes, upper vagina).
Female: No SRY → no AMH → müllerian ducts persist, wolffian ducts regress.
Clinical Note:
Some individuals with 46,XY gonadal dysgenesis or 46,XX males show that more factors than SRY alone influence gonadal determination (e.g., additional autosomal or X-linked genes).
DIFFERENTIATION OF EXTERNAL GENITALIA
Undifferentiated Stage (before ~9 weeks)
Both sexes have genital tubercle, urethral groove (flanked by urethral folds), and labioscrotal swellings.
A urogenital sinus beneath these structures is partitioned from the cloacal opening.
The early embryo’s external genitalia can follow male or female pathways.
Male Development
Testosterone from fetal Leydig cells (converted locally to DHT in some tissues) → masculinization.
Critical timing: If androgen exposure is adequate before ~12th week, the urethral folds fuse → penile urethra, and the labioscrotal swellings fuse → scrotum.
Vagina in male is minimal (prostatic utricle) because müllerian ducts regress.
Late or insufficient androgens → incomplete masculinization (various forms of hypospadias, ambiguous genitalia).
Female Development
In absence of significant fetal androgens, the external genitalia develop along intrinsic female lines.
