The

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s (egg laying mammals) represent the order of extant mammals most distantly related to humans. The

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monotypi ...

(''Ornithorhynchus anatinus'') is indigenous to eastern Australia; the

short-beaked echidna The short-beaked echidna (''Tachyglossus aculeatus''), also called the short-nosed echidna, is one of four living species of echidna, and the only member of the genus ''Tachyglossus'', from Ancient Greek (), meaning "fast", and (), meaning ...

(''Tachyglossus aculeatus'') is indigenous to Australia and Papua New Guinea; whereas the long-beaked echidna (''Zaglossus bruijni'') is restricted to Papua New Guinea and Irian Jaya. Since monotremes exhibit characteristics common with both reptiles (e.g. presence of a cloaca) and

therian mammals Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

(e.g. mammary glands), they are of great interest for the study of mammalian evolution.Griffiths, M. (1968). Echidnas. Pergamon Press.Griffiths, M. (1978). The Biology of the Monotremes. Academic Press.

Monotremes exhibit a combination of reptilian and mammalian characteristics

Male monotremes are testicond (have intraabdominal testes) with the testes undergoing seasonal emergence during winter. The fully developed seminiferous tubules exhibit distinctly small stages of

spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testicle. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of ...

in that more than one stage is often observed in a cross section of the tubule,Djakiew, D. (1984). Reproduction in the Male Echidna (''Tachyglossus aculeatus'') with Particular Emphasis on the Epididymis. Ph.D. Thesis, University of Newcastle, Australia. a characteristic of spermatogenesis that has also been observed in a reptile,Sousa, A.L. Campos-Junior, P.H.A., Costa, G.M.J. and de Franca, L.R. (2014). Spermatogenic cycle length and sperm production the freshwater turtle ''Kinosternon scorpioides''. ''Biology of Reproduction'', 90; 1-10. common in birdsAire, T.A., Olowo-okorun, M.O. , Ayeni, J.S. (1980). The seminiferous epithelium of the Guinea fowl (''Numida meleagris''). ''Cell and Tissue Research'', 205; 319-325.Abdul-Rahman, I.I., Obese, F.Y., Robinson, J.E. (2017). Spermatogenesis and cellular associations in the seminiferous epithelium of Guinea cock (''Numida meleagris''). ''Canadian Journal of Animal Science'', 97; 241-249. and man.Clermont Y. (1963). The cycle of the seminiferous epithelium in man. ''American Journal of Anatomy'', 112; 35-51. The monotreme paired excurrent ducts ( ductuli efferentes,

epididymides The epididymis (; : epididymides or ) is an elongated tubular genital organ 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, in length; ...

Djakiew, D. and Jones, R. (1981)
Structural differentiation of the male genital ducts of the echidna (''Tachyglossus aculeatus'')
''Journal of Anatomy''; 132; 187-202. and

vasa deferentia The vas deferens (: vasa deferentia), ductus deferens (: ductūs deferentes), or sperm duct is part of the male reproductive system of many vertebrates. In mammals, spermatozoa are produced in the seminiferous tubules and flow into the epididyma ...

) empty into a single urethra. Glandular tissue surrounds the urethra into which a pair of

bulbourethral gland The bulbourethral glands or Cowper's glands (named for English anatomist William Cowper) are two small exocrine and accessory glands in the reproductive system of many male mammals. They are homologous to Bartholin's glands in females. The bul ...

s (Cowper's glands) empty at the base of the penis. The intraabdominal testes and excurrent ducts, along with the presence of a cloaca exhibit homology to the reptilian male reproductive tract. The combination of reptilian and mammalian structures within the monotreme reproductive tract has informed the evolution of the male reproductive tract in mammals. For example, the intraabdominal low sperm storage capacity of the echidna epididymis informed the role of the epididymis as a prime mover in the evolution of descended testes in mammals as it relates to lower extragonadal temperatures enhancing epididymal sperm storage in scrotal mammals.Djakiew, D. and Cardullo, R. (1986). Lower temperature of the cauda epididymis facilitates the storage of sperm by enhancing oxygen availability. ''Gamete Research'', 15; 237-254. The glandular designation of periurethral tissue within the monotreme male reproductive tract has been a matter of scientific discussion for approximately two centuries.Saint-Hilaire, M. (1827). Sur les appariels sexuels et urinaires de l’Ornithorhynque. ''Memoires du Museum d’Histoire Naturelle''; 15, 1-48.Oudemans. J.T. (1892). Die accessorishen Geschlechtsdrusen der Saugetthiere – Monotremata. Natuurkundige Verhandelingen van de Hollandsche Maatschappij der. Wetenschap Haarlem Verz. 3, Deel 5, 2 de Stuk; 11-14.Temple-Smith, P. (1973). Season breeding biology of the platypus, ''Ornithorhynchis anatinus'' (Shaw, 1799), with special reference to the male. Ph.D. Thesis, Australian National University.Djakiew, D. (1978). Studies of the Male Reproductive Tract of the Echidna (''Tachyglossus aculeatus'') with particular emphasis on the Epididymis. Honors Thesis, University of Newcastle, Australia. Examination of the monotreme periurethral tissue has been limited by the availability of these protected and relatively rare mammals, hence, the long time line for scientific research of reproductive tissue between studies.

Structure of the monotreme prostate

The glandular tissue surrounding the monotreme urethra most likely represents a rudimentary

prostate The prostate is an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...

. There are no periurethral glands in reptiles. Hence, the evolution of the prostate gland is unique to mammals. Primordial periurethral glands have been described in the platypus as secretory glandular tissue surrounding the length of the urethra. The periurethral tissue exhibits regional swelling, being widest immediately beneath the bladder and progressively reducing in thickness along the length of the urethra. Surrounding the periurethral glands is a urethral muscularis. Observations in the platypus of the periurethral glands were non-committal as to homology with the prostate. Subsequently, the periurethral tissue in the echidna was definitively identified as a rudimentary prostate. This is supported by: # anatomically these glands are placed at their widest at the base of the bladder, which is similar in location for the prostate in other mammals, # histologic lining of the echidna urethra with transitional epithelium (Figures 1 & 2), and not post-prostate urethral lining of pseudostratified or stratified columnar epithelium observed in eutherian mammals, # location of periurethral glands with 9-11 buds per cross section surrounding and exiting into the urethra, # histology of simple columnar secretory cells lining compound alveolar glands, # the glandular tissue is surrounded by a muscularis to facilitate expulsion of secretory products into the urethra, # seasonal changes in echidna glandular development, whereby in aspermatogenic animals the glandular tissue surrounding the urethra is less developed (Figure 1) than during the breeding season when the secretory glands surrounding the urethra of spermatogenic animals appear well developed (Figure 2) coinciding with elevated levels of testosterone and active spermatogenesis. In aggregate, these characteristics of glandular tissue surrounding the urethra identify a rudimentary disseminate prostate in monotremes.

References

Monotremes Evolution of mammals {{Male reproductive system