Development of the Penis and Scrotum Reproductive System > Embryology > Embryology

Home , Cloacal membrane, Genital tubercle, Urogenital sinus, Vaginal process

MALE EXTERNAL GENITALIA Includes the penis, penile urethra, and scrotum (which houses the testes). During gestation, development is heavily influenced by testosterone produced by the Leydig (aka, interstitial) cells of the testes. Weeks 3-4

• Period of undifferentiated primordial genitalia

• The genital tubercle, which is the precursor to the penis, emerges.

• Cloacal membrane is bordered by the primorida of the genital tubercle, and is surrounded by mesenchymal cells.

• Anterior fusion of the primordia forms the definitive genital tubercle.

— Posteriorly, the unfused portion gives rise to the urogenital (aka, genital) folds, which enclose the cloacal membrane. — Lateral to the folds the genital swellings (aka, labioscrotal swellings) arise. Weeks 7-12 The urorectal septum has reached the cloacal membrane and now forms the perineal body; male differentiation begins.

• The perineal body divides the cloacal membrane into urogenital and anal membranes, which then rupture, forming the urogenital sinus and anus.

• The genital tubercle enlarges and is now referred to as the phallus.

• The urogenital folds are now the urethral folds, as they line the urethral groove.

— The roof of the groove comprises an endodermal expansion.

• Distally is the glans, and the glans plate, which is a solid endodermal remnant of the cloacal membrane.

• The genital swellings are now the scrotal swellings.

• Posteriorly, the perineum separates the urethral folds and groove from the anus.

Week 12 The urethral folds and scrotal swellings fuse at the midline.

• Fusion of the urethral folds forms the floor of the spongy urethra within the shaft of the penis.

• Midline raphes reflect fusion of the tissues.

• The surrounding mesenchyme will give rise to the erectile tissues and vasculature of the penis.

• Formation of the glans portion of the urethra occurs separately, via canalization of the glans plate (the exact mechanism is uncertain).

• At the midline, the raphe reflects the fusion of the urethral folds.

1 / 2 9 Months Folds are fully fused, and urethral opening is at the tip of the penis. The scrotal sac houses the testes. Clinical Correlation

• Hypospadia

— Incomplete fusion of the urethra leads to hypospadias, in which the urethral opening is displaced ventrally. Summary: Genital tubercle -> Shaft and glans of the penis Genital folds -> Floor of the spongy urethra Genital swellings -> Scrotum

TESTICULAR DESCENT:&NBSP;

Week 7 The testes are originally located within the abdominal cavity, deep to the peritoneum (aka, retroperitoneal).

• The caudal genital ligament attaches to the superior pole of the testes.

• The gubernaculum anchors the inferior pole to the scrotal swelling, inferiorly.

Week 12 The peritoneum begins to evaginates, forming the vaginal process as it grows towards and into the scrotal swelling. A* s it migrates, the vaginal process creates a cavity as it pushes through the abdominal wall layers, ultimately giving rise to the inguinal canals. 9 Months The testes have traveled through the inguinal canals to reach the scrotal sac; thus, in the mature form, the testes are external to the pelvic cavity.

• Though the caudal genital ligament regresses, the gubernaculum persists to anchor the testis.

- — The gubernaculum directs the testis during its migration through the inguinal canal to the scrotal sac; although the exact mechanisms are uncertain, shortening of the gubernaculum paired with growth of the abdominal organs (and, consequently, increased intra-abdominal pressure) likely facilitate testes descent.

Clinical Correlation

• Cryptorchidism

— Failure of one or both testes to descend into the scrotal sac; if the testes remain undescended too long after birth, surgery may be required. Additional resources:

- Li, Y., Sinclair, A., Cao, et al. Canalization of the Urethral Plate Precedes Fusion of the Urethral Folds during Male Penile Urethral Development: The Double Zipper Hypothesis. Journal of Urology: 193(4): 1353-1360 (2015).

- Hadidi, A., Roessler, J., Coerdt, W. Development of the Human Male Urethra: A Histochemical Study on Human Embryos. Journal of Pediatric Surgery. 49: 1146-1152. (2014).

- Cunha, G.R., Sinclair, A., Risbridger, G., Hutson, J. & Baskin, L.S. Current Understanding of Hypospadias: Relevance of Animal Models. Nature Reviews in Urology.(2015) doi: 10.1038/nrurol.2015.57. UC San Francisco: 898757. Retrieved from: http://escholarship.org/uc/item/60s7k7n0 February 15, 2017.

- Singh, V. Textbook of Clinical Embryology. Elsevier Health Sciences. (2012).