Penis histology

The penis is composed of three cylindrical columns of erectile tissue, which are bound together by a dense fibroelastic layer called the tunica albuginea.

In this very low power, transverse cross-section of the penis, the upper two columns are the dorsal corpora cavernosa of the penis, and the lower column is the ventral corpus spongiosum, which also contains the penile urethra.

At the distal end of the penis, the corpus spongiosum expands to become the glans penis. The tunica albuginea is surrounded by a layer of superficial fascia that contains connective tissue, prominent blood vessels, and nerves.

The dorsal side of the penis, which is the top side of this image, typically has the major blood vessels present, whereas the blood vessels within the columns of erectile tissue are significantly smaller.

Surrounding the superficial fascia is the outermost sheath of penile skin, which can only be partially seen in the upper left of this image.

The penile skin moves freely over the underlying tissues due to the loose hypodermis. Unless circumcised, it extends over the glans as the prepuce or foreskin, which acts as a retractable protective fold of skin.

If we take a closer look at the corpora cavernosa at 10x magnification, we can see that the tissue is highly vascular, with a lot of red blood cells visible within the wide and irregularly-shaped vascular sinuses.

These sinuses are surrounded by walls or trabeculae that contain mostly elastic connective tissue and smooth muscle. If we zoom in a little closer, we can see a few cross-sections of smooth muscle that stain darker than the surrounding tissue.

The sinuses are supplied by constricted thick-walled arteries and arterioles called helicine arteries. As their name suggests, the helicine arteries are normally coiled when the penis is flaccid.

But during an erection, following the parasympathetic stimulation, the smooth muscle surrounding the sinuses and arteries relax, causing the arteries to dilate and straighten out.

This will lead to an increase in blood flow to the cavernous spaces, resulting in engorgement of the sinuses, as well as compression of nearby veins, which helps further enlarge and maintain engorgement of the erectile tissue.

Moving to the ventral corpus spongiosum, you can see that the erectile tissue looks very similar to the corpora cavernosa, except it contains the urethra in the middle.

The corpus spongiosum also has a tunica albuginea that’s more flexible or extensible. This allows the corpus spongiosum to expand a little more during an erection, which will help decrease compression of the penile urethra.

Zooming in further, we can see more clearly that the urethra is also surrounded by a number of urethral glands called the mucous glands of littre and they are ducts that release a clear mucus directly into the urethra during an erection.

If we take a closer look at the urethra, we can see the lining of the urethra better. The majority of the urethra is lined with pseudostratified columnar epithelium, which can be seen in this image, but the lining will be different in certain portions of the urethra as well.

For example, the prostatic urethra has transitional epithelium. And distally, at the opening of the urethra or urethral meatus the lining changes to stratified squamous epithelium.

Alright, as a quick recap. The penis contains two dorsal columns of corpora cavernosa and one ventral, periurethral column of corpus spongiosum.

All three columns contain erectile tissue that is comprised of highly vascular cavernous sinuses, smooth muscle, and helicine arteries.

The urethra is primarily lined with pseudostratified columnar epithelium.