Glans penis

Structure

The glans penis is a body of spongy erectile tissue molded on the rounded ends of the two corpora cavernosa, extending farther on the upper than on the lower surfaces. It is the expanded cap of the corpus spongiosum, a sponge-like region that surrounds the male urethra and maintains it as a viable channel for ejaculation. The glans is covered by stratified squamous epithelium and a dense layer of connective tissue equivalent to the dermis of typical skin, with the papillary layer blending into the tunica albuginea of the corpus spongiosum behind the glans.

During erection, increased arterial flow fills the erectile tissue with blood, causing the glans to grow in size and sensitivity, though it remains slightly softer than the rigid penile shaft. This soft cushiony texture absorbs impact during copulation. The proportional size of the glans varies among males; while typically acorn-like, it may be wider in circumference than the shaft, giving a mushroom-like appearance, or narrower and more probe-like.

The reason for its shape is uncertain, with some researchers suggesting it evolved to remove semen from previous partners, though this theory is not supported by primate mating behaviors. At the summit is the slit-like vertical external urethral orifice, the urinary meatus, through which urine, semen, and pre-ejaculatory fluid exit. The base forms the corona glandis, a rounded projecting border overhanging the coronal sulcus, and behind the corona is the neck of the penis.

Ventrally, the two glans wings merge on the midline forming the septum glandis, and the frenulum, a highly vascularized elastic band, connects the foreskin to the glans, allowing retraction and return of the foreskin.

Development

The glans develops as the terminal end of the genital tubercle, a phallic structure that forms in the embryo regardless of sex during early pregnancy. Initially undifferentiated, the tubercle develops into a penis depending on exposure to male hormones such as androgens. Sexual differentiation is determined by the sperm carrying either an X or Y chromosome; the Y chromosome contains the sex-determining gene SRY, which encodes a transcription factor for the protein TDF, triggering testosterone creation for male development.

Although sex is determined at conception, complete external differentiation of organs begins about eight or nine weeks after conception, with some sources stating completion by the twelfth week and others by the thirteenth or sixteenth week. Both the penis and clitoris develop from the same tissues, making them homologous. In the female fetus, absence of testosterone stops phallus growth, causing the tubercle to shrink and form the clitoris. In the male fetus, the Y chromosome leads to testis development, which secretes androgens that masculinize the indifferent organs.

Testosterone causes the genital tubercle to elongate into the penis, the urogenital folds fuse to form the spongy urethra, and the labioscrotal swellings unite to form the scrotum. Testosterone secretion during this phase plays a decisive role in final shaping of the penis, and after birth, testosterone levels drop significantly until puberty.

Clinical significance

The epithelium of the glans penis consists of mucosal tissue, and excessive washing with soap may dry the mucous membrane, causing non-specific dermatitis, an inflammation often due to an irritating substance or contact allergy. Sensitivity to chemicals in certain products can cause allergic reactions including irritation, itching, and rash. Inflammation of the glans penis is known as balanitis, a treatable condition occurring in about 3–11% of males, and up to 35% of diabetic males.

It is generally more common in males with poor hygiene or who have not been circumcised, with causes including irritation or infection from a wide variety of pathogens. Symptoms may appear suddenly or gradually and include pain, irritation, redness, or red patches on the glans. Careful identification of the cause through patient history, physical examination, swabs, cultures, and biopsy is essential for proper treatment. The urethral meatus at the tip of the glans may develop meatal stenosis, mostly seen as a late complication of circumcision, occurring in about 2–20% of circumcised boys and rarely in uncircumcised men, characterized by narrowing that may cause sudden or frequent urges to urinate and burning during urination.

Some individuals may experience difficulty achieving full glanular engorgement, diagnosed as soft glans syndrome, often undiagnosed due to lack of standardized nomenclature.

Other animals

The glans penis takes different forms across species. Male cats and other felids can direct urine backward by bending the tip of the glans, and in domestic cats the glans is covered with small spines. Similar penile spines appear on the glans of both male and female spotted hyenas. In male dogs, the glans is smooth and has two sections: the bulbus glandis and the pars longa glandis.

The fossa has a glans that extends about halfway down the penis shaft and is spiny except at the tip. Felids generally have a short, spiny glans, while viverrids have a smooth, long one. The marsh rice rat possesses a long, sturdy glans measuring about 7.3 millimeters in length and 4.6 millimeters in width. Winkelmann's mouse is most easily told apart from close relatives by its partially corrugated glans.

In Thomasomys ucucha, the glans is rounded, short, and small, with a trough dividing it into left and right halves on top and a ridge below; most of the surface is spiny except near the tip. The male cape ground squirrel has a large glans with a noticeable baculum. When a horse is erect, its glans expands to three or four times its flaccid size, and the urethra opens into a small pouch called the urethral fossa at the glans tip. Unlike in humans, the horse’s glans extends backward along the shaft.

Among marsupials, glans shape varies widely: most species have a divided glans, but male macropods have an undivided one. The glans is also split into two parts in platypuses and echidnas. Finally, male Racey’s pipistrelle bats have a narrow, egg-shaped glans.