Testicular Heat Stress and Sperm Quality 8
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Testicular Heat Stress and Sperm Quality 8 Damayanthi Durairajanayagam , Rakesh K. Sharma , Stefan S. du Plessis , and Ashok Agarwal (such as elephants and rhinoceroses) and aquatic Introduction mammals (such as whales and dolphins) have intra-abdominal testes throughout their lifespan. In the male, exposure to heat has a deleterious The abdomen is metabolically active and it effect on fertility and is considered a signifi cant therefore generates a lot of heat. However, risk factor for male infertility [ 1 ]. Testicular tem- spermatogenesis functions optimally in these peratures should ideally be hypothermic compared mammals despite the proximity of their testes to to the core body temperature of 36.9 °C. This is the abdomen. essential for maintaining normal spermatogenesis Humans, on the other hand, have intra-scrotal and ideal sperm characteristics. A crucial feature testes that develop within the abdomen and, that contributes towards this is the anatomical posi- towards the end of the gestation period, begins its tion of the human testes, which is located outside descent through the inguinal canals into the scro- the body. Homeothermic animals have the ability tum. In humans, normal testicular function is to maintain a stable core body temperature despite temperature dependent and the extra-abdominal fl uctuating environmental temperatures. This is testes are maintained at temperatures below that achieved by regulating heat production and loss by of core body temperature [ 2 ]. Under normal means of adjusting the body’s metabolism. healthy environmental conditions, testicular ther- In most homeothermic birds and mammals, moregulation maintains scrotal hypothermy to including humans, testicular function depends on ensure optimal testicular function [ 1 ]. temperature. Temperatures that either fall below or above the physiological range required for optimal testicular function could potentially dis- Testicular Thermoregulation rupt spermatogenesis. Certain land mammals The normal physiological temperature of the human testis ranges between 32 and 35 °C [3 ]. Thermoregulation in the testis occurs via two D. Durairajanayagam , PhD • R. K. Sharma , PhD • A. Agarwal , PhD (*) mechanisms: the physiological properties of the Center for Reproductive Medicine, Cleveland Clinic , scrotum and the counter-current mechanism. Cleveland , OH , USA The scrotum is a loose sac-like structure that e-mail: [email protected] houses each testicle. The main function of the S. S. du Plessis , BSc (Hons), MSc, MBA, PhD (Stell) scrotum in most mammals is to prevent heat from Division of Medical Physiology, Department of reaching at the testis by means of adjusting to Biomedical Sciences , Faculty of Medicine and Health Sciences, Stellenbosch University , heat stress [ 4 ]. The scrotum has features that Tygerberg , Western Cape , South Africa allow free dissipation of heat through passive S.S. du Plessis et al. (eds.), Male Infertility: A Complete Guide to Lifestyle and Environmental Factors, 105 DOI 10.1007/978-1-4939-1040-3_8, © Springer Science+Business Media New York 2014 106 D. Durairajanayagam et al. convection and radiation. These include a large loss. Conversely, when ambient temperatures total skin surface area that changes according to increase, the dartos and cremasteric muscles the surrounding temperature, a large number of relax causing the testes to lower away from the sweat glands, minimal subcutaneous fat, and body and the scrotal skin to become looser around sparse hair. When external temperatures rise and the testes, aiding heat loss. cause the scrotal temperature to increase beyond a threshold value, cutaneous receptors on the scrotal skin are activated, initiating secretions of Mechanism of Heat Stress: the scrotal sweat glands and active heat loss Testicular and Germ Cell Changes occurs through the evaporation of sweat [4 , 5 ]. Vasodilation of the scrotal vessels, the very thin Germ cells have high mitotic activity, which scrotal skin and the near-absence of surface hair makes them more susceptible to heat stress [ 8 ]. further contribute to heat dissipation. The type of germ cells that is most sensitive to The spermatic cord is made up of the testicu- heat is the pachytene and diplotene spermatocytes lar artery, veins, cremaster muscle, and vas defer- and early round spermatids in both the rat [ 9 , 10 ] ens. The testicular artery is greatly coiled while and in humans [ 11 ]. In fact, the spermatogenic the veins have thin walls and poor musculariza- process, particularly the differentiation and matu- tion. The bulk of the spermatic cord is composed ration of spermatocytes and spermatids, is tem- of numerous testicular veins that anastomose and perature dependent and occurs ideally at a drain into the convoluted pampiniform plexus temperature of at least 1–2 °C below core body [ 6 ]. The testicular arterial and venous blood ves- temperature [ 1 , 10 ]. As such, raising the scrotal sels are intimately associated with each other, temperature causes testicular germinal epithelial facilitating the transfer of heat between the atrophy and spermatogenic arrest [ 12 ], leading to infl owing arterial blood to the outfl owing venous lower sperm counts. The supportive role of Sertoli blood in the spermatic cord. Thus, the arterial [ 13 ] and Leydig [ 14 ] cells towards germ cell blood arriving at the testis is effectively cooled development are also impacted by heat stress. while the venous blood disperses this heat Levels of a biochemical marker of spermatogene- through the scrotal skin [ 7 ]. In a normal individ- sis, inhibin B [ 15 ], decrease along with sperm ual, this counter-current heat exchange regulates concentration when scrotal temperatures are high the temperature of the arterial blood supply to the [ 16 ]. Irreversible testicular weight loss follows testis and epididymis at 2–4 °C below rectal tem- shortly after heat exposure [ 17 ]. Histopathological perature [ 7 ]. changes in the testis following heat exposure Thermoregulation of the testis is further aided include degeneration of the mitochondria, dilata- by two muscles: the cremasteric and dartos mus- tion of the smooth endoplasmic reticulum, and cles. The cremaster muscle is skeletal-type mus- wider intercellular spaces in both Sertoli and sper- cle that is associated with the spermatic cord and matid cells [ 18 ]. testis. A refl ex contraction of the cremasteric The fundamental mechanism by which loss of muscle can be produced by gently stroking the germ cells occurs in response to heat stress is due skin on the medial side of the thigh (cremasteric to apoptosis [ 9 , 19 ]. The intensity of heat stress refl ex). The dartos muscle is a layer of smooth and duration of heat exposure infl uence germ cell muscle fi bers that surround the testis subcutane- apoptosis. For example, 2 days after a single ously. When the ambient temperature falls, both exposure to heat (43 °C for 15 min), late pachy- the cremaster and the dartos muscles contract tene and early spermatids degenerate [ 20 ]. involuntarily, raising the testes and bringing them However, shorter heat exposure of the rat testes closer to the warmer body. The scrotal skin wrin- (43 °C for 10 min) does not result in apoptotic kles with the contraction of these muscles, reduc- germ cells whereas a longer heat exposure (43 °C ing the exposed surface area to avoid further heat for 30 min) intensifi es germ cell apoptosis [21 ]. 8 Testicular Heat Stress and Sperm Quality 107 Fig. 8.1 Schematic highlighting various mechanisms by which testicular heat stress causes germ cell apoptosis, DNA damage in mature and immature sperm and male infertility Similarly, higher heat exposure (45 °C for mentally cryptorchid rats, heat stress (due to 15 min) causes generalized, nonspecifi c damage increased scrotal temperatures) increases genera- to many different germ cell types in adult rats. tion of reactive oxygen species leading to oxida- Besides apoptosis, heat stress also causes tive stress [23 , 24 ]. Moreover, in adult rats, the defects in DNA synapsis and DNA strand breaks effects of scrotal hyperthermia (43 °C for 30 min in pachytene spermatocytes and induces DNA once daily for 6 consecutive days) include damage in mature spermatozoa [ 20 ]. Sperm DNA decreased levels of glutathione, superoxide dis- damage that occurs in the heat-stressed testis is mutase, and glutathione peroxidase and increased likely due to excessive generation of reactive lipid peroxidation in the testes [ 18 ]. Further, gene oxygen species (which causes the sperm cell to expression for DNA repair and cellular antioxi- be in a state of oxidative stress) as well impaired dants are suppressed during testicular heat stress DNA repair in the germ cells [20 , 22 ]. In experi- [ 25 ] (Fig. 8.1 ). 108 D. Durairajanayagam et al. In summary, heat-induced changes due to also suppressed in the hyperthermic testis [ 32 ]. increased scrotal temperatures in the testes lead Exposure to high temperature causes deteriora- to apoptosis of germ cells and sperm DNA dam- tion in sperm morphology and impairs motility as age, which subsequently suppresses spermato- well as sperm production, all of which have a del- genesis [ 18 , 20 ]. eterious effect on male fertility [ 33 , 34 ]. Impact of Failed Thermoregulation Pathological Failure on Semen Parameters of Thermoregulation Semen analysis is carried out as a routine labora- Increased testicular temperatures due to either tory assessment of the infertile male. Fundamental endogenous or exogenous stimuli decrease sperm sperm parameters evaluated