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.