page_title: Prostate evolution in monotreme mammals
section_title: Structure of the monotreme prostate
hierarchical_section_title: Prostate evolution in monotreme mammals / Structure of the monotreme prostate
caption_attribution_description: histologic lining of the echidna urethra with transitional epithelium
context_page_description: 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.
context_section_description: The glandular tissue surrounding the monotreme urethra most likely represents a rudimentary prostate. 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.
caption: Figure 2. Spermatogenic echidna prostate
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