Thermoregulation of Testis in Animals: Mechanisms, Anatomy Adaptations and Thermography

Thermoregulation of testis (testicular thermoregulation) is the process by which the testis maintains a temperature lower than body temperature using structures like the scrotum, muscles, and blood vessels to support sperm production.

For effective functioning, the testes have to be kept at a temperature of 4-6°C lower than the body temperature.

Thermoregulation Structures

The structures involved in the thermoregulation of the testis include:

1. Scrotal Skin and Sweat Glands
2. Dartos Muscle
3. External Cremaster Muscle
4. Pampiniform Plexus
5. Tunica Albuginea

1. Scrotal Skin and Sweat Glands

The scrotal skin and sweat glands contain temperature receptors. When there is an elevated environmental temperature, these receptors trigger a response by inducing panting and sweating, thereby lowering body temperature.

The scrotal skin lacks subcutaneous fat and is enriched with large adrenergic sweat glands. Sweating enables the scrotum to cool down through evaporative heat transfer.

2. Dartos Muscle

Dartos muscle is an open, mesh-like smooth muscle layer that lies beneath the scrotal skin. By contracting in cold weather to hold the testes against the body and by relaxing in warm weather, it acts as the principal thermoregulator of the testis.

The contractile characteristics of the dartos are androgen-dependent, and its ability to contract in cold climates is lost in castrated males.

3. External Cremaster Muscle

The external cremaster muscle raises the testis, thereby playing a role in the testicular thermoregulation.

It contracts and relaxes, creating a ‘pumping action’ on the pampiniform plexus, thus facilitating blood flow and enhancing cooling efficiency.

4. Pampiniform Plexus

In the proximal end of testis testicular artery is coiled and is surrounded by network of spermatic vein. This arrangement is called as pampiniform plexus. 

Blood present in vein cools the incoming blood of artery.

5. Tunica Albuginea

Tunica albuginea is richly supplied with network of blood vessels and plays role in regulation of testicular temperature.

In human beings, the difference between body temperature and testicular temperature is 2°C. In bulls, it is 4°C, and in rams, it is 5-7°C.

These anatomical arrangements of the testis and features of the testicular blood vessels maintain the testicular temperature.

Thermography of the Scrotum

• Testicular temperature must be below body temperature for normal spermatogenesis in bulls. Environmental temperatures may suffer testicular heat exchange and consequently have a lower reproductive efficiency.
• High testicular temperatures decrease semen quality in bulls.
• The production of viable spermatozoa depends on testicular physiological mechanisms, especially thermoregulation, since the temperature of the scrotum should be maintained 2 to 6°C below the body temperature.
• The use of non-invasive methods, such as infrared thermography (IRT), is considered an effective tool in the characterization of the scrotal surface temperature in bulls.
• Infrared thermography provides a noncontact imaging technique to determine normal and abnormal thermal patterns in bulls and other animals.
• Thermography utilizes sensitive infrared imaging to produce a color photograph that portrays variations in surface temperature that reflect the temperature of tissues immediately beneath the skin.
• The normal thermogram of the scrotum in all species is a left and right symmetric pattern with a constant decrease in temperature gradient from the base to the apex.
• In bulls a temperature gradient of 4-6°C from base to apex is considered normal.
• A gradual decrease from base to apex with concentric color bands is consistent with normal function of the vascular counter-current heat exchange mechanism of the testes.
• An excessively cool or excessively warm area as evidenced by thermographic color is consistent with testicular disease or injury.
• More random temperature patterns, including a lack of horizontal symmetry and areas of increased scrotal surface temperature, were interpreted as abnormal thermoregulation of the testes or epididymides.
• Infrared thermography is a useful tool for breeding soundness evaluation of bulls, although it does not replace collection and evaluation of semen.