Corpus: Perineum

The perineum is the area of tissue located between the anus and the external genitalia. In males, it is bounded by the anus and the base of the scrotum, while in females, it is bordered by the anus and the posterior commissure of the labia majora.

Topographically, the perineum is the central part of the perineal region, primarily composed of muscles that belong to the pelvic floor musculature. The overlying skin and subcutaneous tissue are richly innervated, highlighting its significance as an erogenous zone. Sensory innervation is provided by branches of the pudendal nerve.

2.1. Arterial supply

The perineum and pelvic floor receive blood supply from branches of the internal iliac artery:

• Iliolumbar artery: Runs laterally behind the psoas major muscle into the iliac fossa.
• Lateral sacral arteries: Supply the sacral bone, contents of the sacral canal, and the piriformis muscle.
• Internal pudendal artery: Exits the pelvis medially alongside the inferior gluteal artery and sciatic nerve via the infrapiriform foramen. It curves around the ischial spine into the ischioanal fossa, continuing forward in the pudendal canal, and supplies the pelvic and external genital organs, pelvic floor muscles, and branches into the profunda penis or clitorid artery.

2.2. Venous outflow

The veins accompanying the arteries of the perineum run in the opposite direction. The branches of the internal iliac artery are situated between two veins, facilitating venous drainage.

2.3. Lymphatic drainage

Lymphatic drainage of the pelvic floor generally follows the path of the blood vessels:

• Nodi lymphatici iliaci interni: Surround the internal iliac artery and drain the pelvic organs, pelvic floor, and ischioanal fossa.
• Nodi lymphatici iliaci externi: Drain lymph from the more cranial areas of the pelvic wall.
• Nodi lymphatici iliaci communes: Channel lymph from the iliac tracts to the lumbar lymph nodes, which then direct it to the cisterna chyli and thoracic duct via the lumbar trunks.

2.4. Innervation

• Motor Innervation: The pelvic floor muscles receive motor innervation via short branches directly from the sacral plexus or via the pudendal nerve.
• Sensory Innervation: The skin of the perineal region is innervated by:
  • Anococcygeal nerves: From the coccygeal plexus, supplying the area between the anus and coccyx.
  • Pudendal nerve: Extends ventrally to join the anococcygeal innervated area.
  • Inferior cluniac nerve: Innervates the dorsal part of the lateral perineal region.
  • Posterior femoral cutaneous nerve: Originates from the sacral plexus and innervates the ventral part of the lateral perineal region.
  • Ilioinguinal nerve or genital branch of the genitofemoral nerve (from the lumbar plexus): Innervates the ventral area of the scrotum or labia majora.

During the fifth week of embryonic development, the urorectal septum forms within the cloaca, dividing it into a ventral urogenital sinus and a dorsal anorectal canal. This septum grows downward onto the cloacal membrane, splitting it into a ventral urogenital membrane and a dorsal anal membrane, both of which eventually dissolve to form the urogenital and intestinal openings. The primitive perineum forms at the fusion point between the urogenital septum and the cloacal membrane.

During childbirth, the strong stretching of the perineal tissue can result in a perineal tear. In some cases, a perineal incision (episiotomy) may be performed prophylactically to prevent uncontrolled tearing and facilitate delivery.

• Ulfig, Brand Saberi: Short textbook on embryology, © 2017 Georg Thieme Verlag KG
• embryology.ch - 20.4 Lower urinary tract, subdivision of the cloaca, retrieved on 15/11/2023