The outer aspects of the testicle and epididymis are covered by several tissue layers that are formed during the descent of the testicles by the exaggeration of the peritoneum and the elements of the abdominal wall that are pushed through the inguinal canal into the scrotum. The most superficial layer is the skin.

From deep to superficial, each testicle is covered by the following layers:

The tunica albuginea is a tough dense connective tissue layer that covers the outside of the testicle.

The tunica vaginalis is a serous layer that partially encloses the testicle and is found immediately above the tunica albuginea. It is derived from the peritoneum and embryonic vaginal process. It has two sublayers - visceral and parietal. Between both layers is a small space called the cavity of tunica vaginalis. It contains a small amount of serous fluid that acts as a lubricant, preventing friction and allowing the testicle to move freely within the scrotum.

The visceral layer of the tunica vaginalis is the internal layer. It covers the testicle, the head of the epididymis, and the inferior portion of the ductus deferens.
The visceral layer changes into the external or parietal layer at the posterior margin of the epididymis. It is larger, and at first, it covers the distal portion of the spermatic cord, then it continues over the visceral layer and covers the duct of the epididymis. Finally, it blends with the visceral layer.

The internal spermatic fascia is a thin connective tissue layer that merges with the parietal layer of the tunica vaginalis. It is derived from the transversalis fascia.

The cremaster muscle covers the testicle and spermatic cord. This muscle regulates the distance between the body and the testicle. When the cremaster muscle contracts, it pulls the testicle superiorly towards the inguinal canal, protecting it from various harmful environmental factors such as extreme temperature changes. As mentioned before, a certain temperature is crucial for spermatogenesis.

The cremaster muscle is composed of striated and smooth muscle cells and is derived from the internal abdominal oblique muscle, its aponeurosis and fibers of the inguinal ligament. Some authors state that it is also derived from the transversus abdominis muscle. Sometimes this layer is simply called a cremasteric fascia, as the muscle fibers of the cremaster unite with the surrounding loose connective tissue.

The external spermatic fascia is a thin membranous layer that covers the previous layer. It originates from the external abdominal oblique muscle and its aponeurosis.

The final two layers - the dartos fascia (dartos muscle) and the skin - are layers of the scrotum. The dartos fascia contains smooth muscle fibers, which is why it is also called the dartos muscle or simply dartos. These muscle fibers participate in the thermoregulation of the testicles, and they contract when it gets cold, resulting in wrinkling of the skin covering the scrotum. This action brings both testicles closer to the body and reduces heat loss.

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Covering layers of testicles