Evolution of descended testes in mammals

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Mammals are the only animals in which the testes descend from their point of origin into a scrotum. Concurrently, mammals are the only class of vertebrates to evolve a prostate gland starting with prostate evolution in monotreme mammals.

Testicular descent occurs to a variable degree in various mammals, ranging from virtually no change of position from the abdominal cavity (monotremes, elephants, and hyraxes); through migration to the caudal end of the abdominal cavity (armadillos, whales, and dolphins); migration just through the abdominal wall (hedgehogs, moles, seals); formation of a sub-anal swelling (pigs, rodents); to the development of pronounced scrota (primates, dogs, ruminants) in mammals. [1]

Since the descent of the testes into a scrotal pouch subjects the animal to enhanced risk of accidental damage and/or vulnerability from predators and rivals, presumably there must be some evolutionary adaptive advantage to testicular descent. It has been proposed that the scrotum may act as a form of sexual decoration. [2] A scrotal location also exposes the testes to a reduced temperature below that of the body, [3] which has been suggested to reduce the spontaneous rate of germ cell mutations. [4]

Mechanism for the sperm storage region of the epididymis promoting testicular descent

An alternative proposal is that testicular descent was preceded, and possibly influenced, by migration of the sperm storage region of the epididymis to a cooler extra-abdominal location. [5] [6] [7] The evolutionary adaptive advantage of testicular descent into an extra-abdominal position may be related more to the enhanced sperm storage capacity of the epididymis at lower extra-abdominal temperatures than to the testis itself. [7] Greater sperm storage capacity in the epididymis has been associated with enhanced fertility. In this context, the proportion (26% of total) of mature sperm stored intra-abdominally in the monotreme epididymis [8] is considerably less than the proportion of mature sperm stored in the epididymis of many eutherian mammals (50-75% of total) with descended testes. [9] [10] [11] Moreover, this increase in scrotal storage of sperm corresponds with epididymis evolution from reptiles to mammals.

The mechanism by which sperm storage in the epididymis is enhanced at lower extra-abdominal temperatures has been shown to be a consequence of the biophysics of oxygen availability and sperm oxidative respiration. [12] The cauda epididymis, where sperm are stored, can be up to 7 °C below abdominal temperatures. For a reduction in temperature of 7 °C the respiration rate of sperm declines by one half, and the solubility of oxygen in solution increases by approximately 10%. [12] Hence for a reduction in temperature of 7 °C the availability of oxygen is doubled, and hence twice as many sperm can be stored per unit volume of epididymal duct. [12] This increased sperm reserve at lower extra-abdominal temperatures has been related to enhanced fertility which provides an evolutionary advantage to the survival of the species. [7]

In conclusion, the evolution of descended testes was promoted by the lower extra-abdominal temperature of the cauda epididymis which increased oxygen availability to sustain and store more sperm. [12]

Related Research Articles

<span class="mw-page-title-main">Testicle</span> Internal organ in the male reproductive system

A testicle or testis is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testosterone. Testosterone release is controlled by the anterior pituitary luteinizing hormone, whereas sperm production is controlled both by the anterior pituitary follicle-stimulating hormone and gonadal testosterone.

<span class="mw-page-title-main">Epididymis</span> Tube that connects a testicle to a vas deferens

The epididymis is an elongated tubular structure attached to the posterior side of each one of the two male reproductive glands, the testicles. It is a single, narrow, tightly coiled tube in adult humans, 6 to 7 centimetres in length; uncoiled the tube would be approximately 6 m long. It connects the testicle to the vas deferens in the male reproductive system. The epididymis serves as an interconnection between the multiple efferent ducts at the rear of a testicle (proximally), and the vas deferens (distally). Its primary function is the storage, maturation and transport of sperm cells.

<span class="mw-page-title-main">Vas deferens</span> Part of the male reproductive system of many vertebrates

The vas deferens, with the more modern name ductus deferens, is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube which exits the abdominal cavity through the inguinal canal.

<span class="mw-page-title-main">Cryptorchidism</span> Medical condition

Cryptorchidism, also known as undescended testis, is the failure of one or both testes to descend into the scrotum. The word is from Greek κρυπτός 'hidden' and ὄρχις 'testicle'. It is the most common birth defect of the male genital tract. About 3% of full-term and 30% of premature infant boys are born with at least one undescended testis. However, about 80% of cryptorchid testes descend by the first year of life, making the true incidence of cryptorchidism around 1% overall. Cryptorchidism may develop after infancy, sometimes as late as young adulthood, but that is exceptional.

<span class="mw-page-title-main">Spermatogenesis</span> Production of sperm

Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules. These cells are called spermatogonial stem cells. The mitotic division of these produces two types of cells. Type A cells replenish the stem cells, and type B cells differentiate into primary spermatocytes. The primary spermatocyte divides meiotically into two secondary spermatocytes; each secondary spermatocyte divides into two equal haploid spermatids by Meiosis II. The spermatids are transformed into spermatozoa (sperm) by the process of spermiogenesis. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa and four haploid cells.

<span class="mw-page-title-main">Spermatocele</span> Medical condition

Spermatocele is a fluid-filled cyst that develops in the epididymis. The fluid is usually a clear or milky white color and may contain sperm. Spermatoceles are typically filled with spermatozoa and they can vary in size from several millimeters to many centimeters. Small spermatoceles are relatively common, occurring in an estimated 30 percent of males. They are generally not painful. However, some people may experience discomfort such as a dull pain in the scrotum from larger spermatoceles. They are not cancerous, nor do they cause an increased risk of testicular cancer. Additionally, unlike varicoceles, they do not reduce fertility.

<span class="mw-page-title-main">Gubernaculum</span> Embryonic structure

The paired gubernacula also called the caudal genital ligament, are embryonic structures which begin as undifferentiated mesenchyme attaching to the caudal end of the gonads.

<span class="mw-page-title-main">Male reproductive system</span> Reproductive system of the human male

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body, and within the pelvis.

An alternative male contraceptive method involves heating the testicles so that they cannot produce sperm. Sperm are best produced at a temperature slightly below body temperature. The muscles around a male's scrotum involuntarily tighten if the man's body temperature drops, and they loosen, allowing the testes to hang, if the body temperature rises. This is the body's way of keeping the sperm at an ideal temperature. This means that sperm production can be disrupted with increased temperature. Some suggest exposure to high temperatures can affect fertility for months.

<span class="mw-page-title-main">Testicular sperm extraction</span> Surgical procedure

Testicular sperm extraction (TESE) is a surgical procedure in which a small portion of tissue is removed from the testicle and any viable sperm cells from that tissue are extracted for use in further procedures, most commonly intracytoplasmic sperm injection (ICSI) as part of in vitro fertilisation (IVF). TESE is often recommended to patients who cannot produce sperm by ejaculation due to azoospermia.

<span class="mw-page-title-main">Sperm granuloma</span> Lump of extravasated sperm found in some vasectomized men

A sperm granuloma is a lump of leaked sperm that appears along the vasa deferentia or epididymides in vasectomized individuals. While the majority of sperm granulomas are present along the vas deferens, the rest of them form at the epididymis. Sperm granulomas range in size, from one millimeter to one centimeter. They consist of a central mass of degenerating sperm surrounded by tissue containing blood vessels and immune system cells. Sperm granulomas may also have a yellow, white, or cream colored center when cut open. While some sperm granulomas can be painful, most of them are painless and asymptomatic. Sperm granulomas can appear as a result of surgery, trauma, or an infection. They can appear as early as four days after surgery and fully formed ones can appear as late as 208 days later.

<span class="mw-page-title-main">Scrotum</span> Sac of skin that protects the testicles

In most terrestrial mammals, the scrotum or scrotal sac is a part of the external male genitalia located at the base of the penis that consists of a suspended dual-chambered sac of skin and smooth muscle. The scrotum contains the external spermatic fascia, testicles, epididymis, and ductus deferens. It is a distention of the perineum and carries some abdominal tissues into its cavity including the testicular artery, testicular vein, and pampiniform plexus. The perineal raphe is a small, vertical, slightly raised ridge of scrotal skin under which is found the scrotal septum. It appears as a thin longitudinal line that runs front to back over the entire scrotum. In humans, the scrotum becomes covered with pubic hair at puberty. The scrotum will usually tighten during penile erection and when exposed to cold temperatures. One testis is typically lower than the other to avoid compression in the event of an impact.

The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.

Vasectomy reversal is a term used for surgical procedures that reconnect the male reproductive tract after interruption by a vasectomy. Two procedures are possible at the time of vasectomy reversal: vasovasostomy and vasoepididymostomy. Although vasectomy is considered a permanent form of contraception, advances in microsurgery have improved the success of vasectomy reversal procedures. The procedures remain technically demanding and may not restore the pre-vasectomy condition.

<span class="mw-page-title-main">Scrotal ultrasound</span> Medical ultrasound examination of the scrotum.

Scrotalultrasound is a medical ultrasound examination of the scrotum. It is used in the evaluation of testicular pain, and can help identify solid masses.

<span class="mw-page-title-main">Mammalian reproduction</span> Most mammals are viviparous, giving birth to live young

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.

Male genital examination is a physical examination of the genital in males to detect ailments and to assess sexual development, and is normally a component of an annual physical examination. The examination includes checking the penis, scrotum, and urethral meatus. A comprehensive assessment of the male genitals assesses the pubic hair based on Sexual Maturity Rating and the size of the testicles and penis. The exam can also be conducted to verify a person's age and biological sex. The genitourinary system can also be assessed as part of the male genital examination. During a genital examination, the doctor can detect any of the following: structural abnormalities, urethral opening abnormalities, problems related to not being circumcised, lumps, tumors, redness, excoriation, edema, lesions, swelling, cancer, hair-related issues, and many others. In some instances where a physical examination of the male genitals is not sufficient to diagnose an individual, then an internal genital examination using imaging or ultrasounds will be needed for further evaluation.

The monotremes represent the order of extant mammals most distantly related to humans. The platypus is indigenous to eastern Australia; the short-beaked echidna is indigenous to Australia and Papua New Guinea; whereas the long-beaked echidna is restricted to Papua New Guinea and Irian Jaya. Since monotremes exhibit characteristics common with both reptiles and therian mammals, they are of great interest for the study of mammalian evolution.

The epididymis, which is a tube that connects a testicle to a vas deferens in the male reproductive system, evolved by retention of the mesonephric duct during regression and replacement of the mesonephros with the metanephric kidney. Similarly, during embryological involution of the paired mesonephric kidneys, each mesonephric duct is retained to become the epididymis, vas deferens, seminal vesicle and ejaculatory duct. In reptiles and birds both the testes and excurrent ducts occur in an intra-abdominal location (testicond). Primitive mammals, such as the monotremes (prototheria), also are testicond. Marsupial (metatheria) and placental (eutheria) mammals exhibit differing degrees of testicular descent into an extra-abdominal scrotum. In scrotal mammals the epididymis is attached to the testes in an extra-abdominal position where the cauda epididymis extends beyond the lowest extremity of the testis. Hence, the cauda epididymis is exposed to the coolest of temperatures compared to all other reproductive structures.

<span class="mw-page-title-main">Daniel Djakiew</span>

Daniel Djakiew is a scholar, researcher, teacher, and tenured full professor in the Department of Biochemistry and Molecular & Cellular Biology, School of Medicine, Georgetown University, Washington DC.

References

  1. ↑ Setchell B.P. (1978). The Mammalian Testis. Cornell University Press, Ithaca, New York.
  2. ↑ Portman, A. (1952). Animal Forms and Patterns, Faber & Faber, London.
  3. ↑ Moore C.R. (1923). On the relationship of the germinal epithelium to the position of the testis. Anatomical Record 34: 337-358.
  4. ↑ Ehrenberg L., von Ehrenstein G., Hedgram A. (1957). Gonad temperature and spontaneous mutation-rate in man. Nature 180: 1433-1434.
  5. ↑ Heller, R.E. (1929). New evidence for the function of the scrotum. Physiology Zoology 2: 9-17.
  6. ↑ Glover T.D. (1973). Aspects of sperm production in some East African mammals. Journal of Reproduction and Fertility 35: 45-53.
  7. 1 2 3 Bedford H.M. (1978). Anatomical evidence for the epididymis as the prime mover in the evolution of the scrotum. American Journal of Anatomy 152: 483-508.
  8. ↑ Djakiew D. & Jones J.C. (1981). Structural differentiation of the male genital ducts of the echidna (Tachyglossus aculeatus). Journal of Anatomy 132: 187-202.
  9. ↑ Dott H.M. & Skinner J.D. (1967). A reassessment of extragonadal spermatozoa reserves in Suffolk rams. Journal of Agricultural Sciences 69: 293-295.
  10. ↑ Orgebin-Crist M.C. (1968). Gonadal and epididymal sperm reserves in the rabbit; Estimation of the daily sperm production. Journal of Reproduction and Fertility 15: 15-25.
  11. ↑ Amann R.P., Johnson L., Thompson D.L. & Pickett B.W. (1976). Daily spermatozoal production, epididymal spermatozoal reserves and transit time of spermatozoa through the epididymis of the rhesus monkey. Biology of Reproduction 15: 586-592.
  12. 1 2 3 4 Djakiew D. & Cardullo R. (1986). Lower temperature of the cauda epididymidis facilitates the storage of sperm by enhancing oxygen availability. Gamete Research 15: 237-245. Reprint available from the author.
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