page_title: Epididymis evolution from reptiles to mammals
section_title: Structural differentiation of the epididymis in reptiles
hierarchical_section_title: Epididymis evolution from reptiles to mammals / Structural differentiation of the epididymis in reptiles
caption_attribution_description: English: Drawing of epididymis of reptile, monotreme and scrotal mammals
context_page_description: 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. Primitive mammals, such as the monotremes, also are testicond. Marsupial and placental 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.
  Whereas testicond reptiles contain an excurrent duct system, they lack male reproductive glands.
context_section_description: The reptilian testis and epididymis typically undergo seasonal recrudescence coupled to the breeding season. All reptiles retain their testes and excurrent ducts within the abdomen (testicond). Generally, the reptilian epididymis does not exhibit the same degree of anatomical regionalization compared to scrotal mammals (Figure 1). Indeed, the anatomical appearance of the epididymis of many reptiles appears much more similar to the epididymis of monotremes than scrotal mammals (Figure 1). Anatomically, the gross morphologic features of the reptilian epididymis can vary between species, with some species of reptiles exhibiting just two anatomical regions whereas others (snakes) may exhibit no observable regionalization of the epididymis.
  A reptilian histologic initial segment of the epididymis has been extensively documented in several species homologous to the initial segment of mammals. The initial segment of the epididymis, first described in the guinea pig epididymis, is a histologically distinct region of tall pseudostratified columnar epithelium that receives spermatozoa from the ductuli efferentes (Figure 1).
  The epididymis is the primary sperm storage organ in male reptiles. In all reptiles and mammals the sperm storage region of the epididymis can objectively be identified as that distal extremity of the epididymis that exhibits a widened diameter of duct which contains additional layers of circumferential smooth muscle capable of contraction during ejaculation in direct continuity with the vas deferens (Figure 1). This sperm storage region has been described as the anatomical cauda epididymis or the histologic terminal segment of the epididymis. The caudal region of the reptilian epididymis, where sperm are stored, is an anatomical extension that narrows into a conical shape before forming the vas deferens.  The coiled epididymal duct within the cauda epididymis does not appear to be particularly long, and so may be limited in its capacity to store sperm in comparison to scrotal mammals. Limited sperm storage in the reptilian epididymis may be circumvented by the ability of female reptiles to store viable spermatozoa within their reproductive tract for utilization months or years after insemination. A competing reproductive strategy to long-term sperm storage that explains the production of offspring after prolonged periods in the absence of males is facultative parthenogenesis. In either case, these female reproductive strategies may have evolved to counter limited sperm storage in the reptilian male epididymis.
caption: Figure 1: Schematic diagrams of the epididymis of a reptile (A), monotreme mammal (B) and scrotal mammal (C) showing the anatomical caput, corpus and cauda, and the histologic initial segment, middle segment and terminal segment.
exif: {"Image Artist": "David M Klemm", "Image ExifOffset": "2136", "Image XPAuthor": "David M Klemm", "Image Padding": "[]", "EXIF Padding": "[]"}