The Internal Anatomy of the Thylacine - A Historical Perspective

    The reproductive tract of the female thylacine, which is much like that of dasyurids, was first described by Pearson & de Bavay (1953), and again by Moeller (1997) and more recently by R. Leon Hughes (2000). 

    Marsupial and placental mammals differ markedly in their reproductive anatomy.  In female marsupials, the reproductive tract is duplicated.  The right and left vaginae are separate structures, and are not fused as is the case in placental mammals.  The right and left uteri are also not fused.  The ovary is enveloped by the delicate membranous fimbria (finger-like process), which are an extension of the infundibulum of the oviduct (funnel-shaped opening of the fallopian tube adjacent to the ovary), into which the eggs are shed.  The eggs pass into the ampulla, which is an expanded region of the oviduct (Fallopian tube), and this is where fertilisation occurs.  The ampulla leads into the isthmus, a convoluted section of the oviduct, which in turn leads into the uterus.

    Joseph Pearson and J. M. de Bavay (1953) published an account of the urogenital system of the Dasyurinae and Thylacininae in the Papers & Proceedings of the Royal Society of Tasmania, in which they examined a series of transverse stained sections of the urogenital system from two female thylacine pup specimens in the Australian Museum and Museum Victoria collections.  They noted that as their interpretation was based on immature material, it lost much of its value with respect to determining the precise arrangement of the urogenital system in a mature female of the species.  In the thylacine, the uteri have separate openings into a combined vaginal cul-de-sac.  The lateral and medial vaginae join at the beginning of the urogenital sinus, into which the urethra from the bladder also empties.  Birth occurs through a central, pseudo-vaginal canal.  This short-cut to the outside forms from the cul-de-sac, where each lateral vagina loops around a ureter at the base of the uteri.  In most marsupials, the pseudo-vaginal canal opens and closes with each birth.  The culs-de-sac of the thylacine are noteworthy in that they form a fifth of the anterior-posterior length of the vaginal complex.  Pearson & de Bavay (1953) state: 

    "It is important to note that even in these extremely immature specimens the culs-de-sac have reached a higher stage of development than that found in mature specimens of Antechinus".

mammalian reproductive/excretory systems - (image - C. Campbell, after R. Neave)
A comparison of the monotreme, marsupial and placental reproductive/excretory systems.

    In marsupials, the outlets of the female reproductive and excretory systems are somewhat less widely spaced than in many placental mammals, and are contained within a single external vent.  However, it is not like the true cloaca of the monotremes (e.g. echidna and platypus), in that the urethral and rectal outlets are separate.  In monotreme mammals, the passages of the uterus, bladder and rectum all open into a cloaca, one of a number of reptilian characteristics that they have retained.  The term monotreme means "one opening" in Greek.

    R. Leon Hughes (2000), in a paper entitled "Structure of the female reproductive tract of an adult parous Tasmanian tiger, Thylacinus cynocephalus" published in the Australian Journal of Zoology, completed the pioneering work initiated by Pearson & de Bavay in describing the reproductive tract of Hill's adult female specimen, then located in the Hubrecht laboratory in the Netherlands.  Hughes notes: 

    "As in other marsupials, the reproductive tract was characterised by the presence of a uterus duplex and a vaginal complex where the ureters passed dorsally over each lateral vaginal canal to enter the bladder.  The lateral vaginal canals each entered a urogenital sinus that terminated in a shallow cloaca.  The gross dimensions of the reproductive tract of the thylacine were greater than those of any extant dasyurid marsupial.  The distance from the rostral pole of the ovaries to the most caudal extremity of the urogenital sinus measured 25 cm.  The distinctive aspects of the reproductive tract included a disproportionate enlargement of the corpus uteri that is without

reproductive tract of the female thylacine - (image - S. Sleightholme)
Reproductive tract of the female thylacine - drawn from the "wet" specimen in the collection of the Museum für Naturkunde (Berlin).
parallel in any other marsupial species.  The bodies of the right and left uteri measured 10.4 cm 1.2 cm 0.9 cm and 9.1 cm 0.8 cm 0.7 cm respectively.  The rostro-caudal length of the right and left cervices measured 2.7 cm and 1.7 cm respectively.  The cervical canals entered the vaginal complex by way of a thick median vaginal septum.  The elongated caudal component of the vaginal culs-de-sac lacked a median vaginal septum.  As in other dasyurid marsupials, the lateral vaginae and associated vaginal complex were of diminutive proportions in relation to the typical marsupial pattern".

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