Development of Indifferent Gonads and External Genitalia

Development of Indifferent Gonads and Urogenital System

• Urogenital system originates from the urogenital sinus, formed by cloacal partitioning.
• Lined internally by endoderm and surrounded by splanchnic mesenchyme.
• The upper section develops into the bladder; the lower section forms external genitalia.
• The urogenital ridge, arising from dorsal mesenchyme, splits into nephrogenic cord and gonadal bridge.

Sex Determination

• Gonadal morphological differentiation begins at 7 weeks; sex is determined at fertilization based on sex chromosomes.
• Male sex determination involves the SRY gene; for females, the WNT4 gene is expressed.
• Gonads arise from the mesothelium of the posterior abdominal wall, mesenchyme, and primordial germ cells.

Gonadal Development

• Undifferentiated gonads emerge at 5 weeks; primary sex cords develop into cortex and medulla.
• In XX embryos, the cortex differentiates into ovaries, while in XY embryos, the medulla develops into testes.
• Primordial germ cells appear at 4 weeks, migrating to gonadal ridges by the 6th week to form sex cords.

Duct Systems

• Two duct systems develop: mesonephric (Wolffian) ducts and paramesonephric (Müllerian) ducts.
• Paramesonephric ducts form a Y-shaped structure in both sexes, influenced by respective hormones.

Testicular Development

• The SRY gene induces the formation of seminiferous cords, which branch and form rete testis.
• Tunica albuginea forms beneath the tunica vaginalis as seminiferous cords lose connection with surface epithelium.
• Testes separate from mesonephros, become suspended by mesorchium (future tunica vaginalis).
• Leydig cells arise from surrounding mesenchyme, producing testosterone from the 8th week, promoting male differentiation.

Male Genital Duct Formation

• MIF/AMH secreted by Sertoli cells suppress Müllerian duct development.
• Mesonephric tubules convert into efferent ductules, leading to the ductus epididymis and eventually ductus deferens.
• Seminal vesicles arise from lateral growth of the mesonephric ducts; the ejaculatory duct connects to the urethra.

External Male Genitalia

• Labioscrotal swellings form the scrotum, fusing at the scrotal raphe; urogenital folds turn into spongy urethra.
• The genital tubercle, ventral to the cloacal membrane, develops into the phallus.
• Default pathway without sex hormones favors female genitalia; testosterone drives development of male features, like the penis.

Genital Canalization

• Ectodermal growth at the tip of the phallus forms the spongy urethra, which canalizes to form external urethral orifice.
• Mesenchyme contributes to the formation of corpora cavernosa and corpus spongiosus.
• Prostate evolves from endodermal growth of the urethra; glandular epithelium derives from urethral endoderm.
• Bulbourethral glands arise from endoderm of the spongy urethra, differentiating smooth muscle and secretory epithelia from neighboring tissues, contributing to semen.

This quiz explores the development process of indifferent gonads, testis, and external genitalia within the urogenital system. It focuses on the embryological origin from the urogenital sinus and the role of endoderm and mesenchyme in the formation of reproductive structures. Test your understanding of these developmental stages and terminology.