Penis

The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis, which are the extremities of the corpora cavernosa. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum, which sits beneath the corpora cavernosa and through which the urethra passes. The glans is ensheathed by the prepuce or foreskin. The penis is homologous with the clitoris in females.

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Gross Anatomy

Overview

The penis is attached to the pubic bone by the crura penis. This attachment is the root of the penis, and it is located just below the prostate gland. The urethra enters the penis at the root and travels within the corpus spongiosum on the dorsal aspect of the penis.

Illustration of the penis

Cross-sectional anatomy (sagittal) of the male genitalia

Image: “Illustration of the penis” by US Federal Government/National Cancer Institute. License: Public Domain, edited by Lecturio.

Structure

The penis consists of a root, body, and glans.

Root:

  • Internal structure, attaches in the perineum
  • 3 masses of erectile tissue originate from the urogenital triangle:
    • 2 crura (proximal ends of the corpora cavernosa)
    • 1 bulb (proximal end of the corpus spongiosum)

Body:

  • Extending from the 2 crura and 1 bulb are 3 erectile columns that run throughout the shaft:
    • Paired corpora cavernosa
    • Single corpus spongiosum
  • Erectile tissue is contained within the tunica albuginea (a dense fibroelastic sheath of connective tissue).
  • Bundles of smooth muscle form cavernous sinuses, lined by endothelium.
    • Sinuses engorge with blood during sexual arousal → erection
    • The oblique angle of the venous outflow channels become constricted due to the blood volume in the cavernous sinuses during arousal, maintaining an erection.
  • Urethra:
    • Contained within the corpus spongiosum
    • Maintains patency for outflow in the form of micturition and ejaculation

Glans:

  • Composed of the corpus spongiosum as it enlarges and assumes a bulbous shape at the head of the penis
  • Corona separates the glans from the shaft.
  • The glans penis is covered by the prepuce (foreskin).
Anatomical illustration of the penis structure

Erectile tissues of the penis

Image: “Anatomical illustration of the penis structure” by Grant, John Charles Boileau. License: Public Domain

Skin

  • The dartos fascia and underlying Buck’s fascia are loosely attached → highly mobile skin
  • General absence of glandular elements and hair follicles
  • Retractable prepuce covers the glans penis and is attached at the frenulum.
Cross section of the penis

Cross-sectional anatomy (frontal) of the penis
a.: artery
v.: vein
n.: nerve

Image: “Cross section of the penis” by Mcstrother. License: CC BY 3.0

Microscopic Anatomy

  • Corpora cavernosa:
    • Network of cylindrical sinuses
    • Lined by endothelium
    • Sinuses are separated by smooth muscle.
  • Corpus spongiosum:
    • Network of cylindrical sinuses
    • Similar in appearance to corpora cavernosa
  • Tunica albuginea has 2 layers of elastic fiber surrounding the corpora:
    • Inner circular layer
    • Outer longitudinal layer

Neurovasculature and Lymphatic Drainage

Arterial supply

  • Internal iliac artery → internal pudendal artery → perineal artery and common penile artery
  • Common penile artery has 3 branches:
    • Bulbourethral artery: supplies the bulb, urethra, corpus spongiosum, and glans
    • Dorsal penile artery: supplies glans penis, corpus spongiosum, and urethra
    • Cavernous artery: travels near the center of each corporal body, giving off projections directly to the cavernous sinuses
Arterial supply of the penis

Arterial circulation of the penis

Image: “Arterial supply of the penis” by Gray’s anatomy. License: Public Domain

Venous supply

Blood leaving the penis is drained by 1 of 3 venous systems:

  • Superficial:
    • Within dartos fascia
    • Drains from skin and prepuce
  • Intermediate:
    • Under Buck’s fascia
    • Drains from distal penis including glans
  • Deep
    • Drains proximal penis

Lymphatic drainage

  • Skin → superficial inguinal nodes
  • Glans → deep inguinal and external iliac nodes
  • Body → internal iliac nodes

Innervation

  • The glans penis is highly innervated with sensory nerves from the dorsal nerve of the penis, which is a branch of the pudendal nerve.
  • The corpora cavernosa receive sympathetic and parasympathetic innervation from the cavernous nerve, which is a branch of the pelvic plexus.

Erection and Ejaculation

Neurovascular process of erection

  • Sexual stimulation → release of neurotransmitters (notably nitric oxide), which act on cGMP → smooth muscle relaxation → influx of blood from helicine arteries and consequential “veno-occlusion” → penile erection
  • Under parasympathetic control

Emission and ejaculation

  • Emission:
    • Transmission of seminal fluid from vas deferens, prostate, and seminal vesicles into the prostatic urethra  
    • Under sympathetic control
  • Ejaculation:
    • Expulsion of seminal fluid from the prostatic urethra
    • Under both autonomic and somatic control

Clinical Relevance

  • Disorders of sexual development: Sex chromosomes have a pivotal role in sex determination. Aberrations of sex chromosomes in diverse ways at different stages of development of an individual result in disorders of sexual development (DSD). Penile disorders may include hypospadias, epispadias, phimosis, paraphimosis, micropenis, and a variety of others.
  • Erectile dysfunction: defined as the inability to achieve or maintain a penile erection that would result in the inability to perform vaginal sexual intercourse. Systemic, neurologic, and local diseases can cause erectile dysfunction. Treatment includes lifestyle modifications, vacuum-assisted erection devices, and phosphodiesterase inhibitors.
  • Kallmann syndrome: a form of reproductive failures due to severe hypogonadism that in males results in small testes, micropenis, undescended testes, and a lack of secondary sexual characteristics. Kallmann syndrome is also associated with complete loss of the sense of smell. Hormone replacement therapy is helpful for most individuals.
  • Penile cancer: The most common penile cancer is squamous cell carcinoma. The cancer type accounts for about 95% of all penile cancer. The other malignancies that affect the penis include basal cell carcinoma, mesenchymal tumors, malignant melanoma, and metastasis. Age, promiscuous sexual behavior (HPV positivity), tobacco use, and poor hygiene are all risk factors for developing penile cancer.

References

  1. Comperat, E. Moguelet, P. (2021) Lesions of the penis, the anatomy, epidemiology and carcinogenesis. Annales de Pathologie. Retrieved August 22, 2021, from https://europepmc.org/article/med/33820663
  2. Drake, R. L., Vogl, A. W., & Mitchell, A. W. M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
  3. Sam, P., LaGrange C. A. (2020). Anatomy, Abdomen and Pelvis, Penis. StatPearls. StatPearls Publishing. Retrieved August 22, 2021, from https://www.statpearls.com/ArticleLibrary/viewarticle/26904

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