The Internal Anatomy of the Thylacine
- A Historical Perspective
(continued)
.
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 pseudovaginal 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
pseudovaginal canal opens and closes with each birth.
The culs-de-sac of the thylacine are of note, as they are a fifth of the
anterior-posterior length of the vaginal complex. Pearson & de
Bavay (1953) write:
"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".
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.
.
| A
comparison of the monotreme, marsupial and placental reproductive/excretory
systems. |
R. Leon Hughes, in a
paper published in the Australian Journal of Zoology (2000), completed
the pioneering work initiated by Pearson & De Bavay in describing the
reproductive tract of Hill's adult female specimen 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 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".
.
| A
photographic diagram of the preserved reproductive tract of a female thylacine
(Hill's specimen). |
Thanks to the foresight
of the early collectors and in particular that of Sir Colin MacKenzie we
are fortunate to have “wet” specimens of all of the thylacine's internal
organs together with the eviscerated carcases of four adults preserved
in museum and university collections around the world. These specimens
are invaluable in that they allow us a unique opportunity to study in detail
the internal anatomy of a species that is on the
brink of extinction. With the advent of new diagnostic and
imaging techniques there is still much to be learnt about the anatomy of
the thylacine. It is somewhat ironic that we have learnt far more
about the thylacine from its physical remains than we have from the living
animal itself. |
