Tree shrew urogenital system
Photo and Text: Terence Mitchell
The digestive tract was removed during a previous dissection, exposing most of the urogenital system. You should be able to locate many of the structures of the UG system with little effort. First, find the kidney, a bean-shaped organ approximately 1 cm caudal to the liver. In Figure 1, note the left renal vein draining the kidney and emptying into the inferior vena cava. The abdominal aorta runs deep to the inferior vena cava and urogenital structures, and cannot be seen in Figure 1. However, the left renal artery can be seen entering the kidney deep to an oval-shaped lymph node. Also note that the left testicular artery branches off of the left renal artery and runs inferiorly to supply the testicle. The right testicular vein empties directly into the inferior vena cava. The ureter leaves the kidney dorsal to the major vessels and runs inferiorly to enter the dorsal wall of the bladder.
You will now need to dissect the testicle and spermatic cord. Use a sharp probe or needle to cut through the skin and muscle layers surrounding the testicle and spermatic cord. At the caudal tip of the left testicle look for a thin protrusion known as the epididymis. Also at the caudal tip of the testicle find the vas deferens. Follow its course superiorly through the spermatic cord, and out the superficial and deep inguinal rings. It then arches over the bladder and joins the urethra with the seminal vesicle dorsal to the prostate gland (not pictured). Find the penis between the two testicles.
There are minor differences in the urogenital systems for Ptilocercus lowii,
Anathana wroughtoni, Dendrogale, and Tupaia described in
the references below. However, there appear to be no references examining features
of the tree shrew urogenital system from a functional or comparative perspective.
Observations of this specimen yield several interesting facts.
First, the medulla of the kidney does not seem well differentiated. In mammals the medulla contains the loops of Henle, which remove sodium and chloride ions. Drier environments require mammals to excrete ions in a very concentration to save water. This requires a large and differentiated medulla. Tree shrews, which live in moist tropical forests, are not under selective pressures to develop such kidneys. Second, the testis of tree shrews are not enlarged relative to their body size. Enlarged testis are expected in polygynous species. Male and female tree shrews have home ranges with extensive overlap. Since males keep out other males and females keep out other females tree shrews are essentially monogamous. They are thus not under selective pressure to evolve testis capable of producing large quantities of semen.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale. Field Mus. Publ. Chicago Zool. 20:383-405.
Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus
lowii). Proceedings of the Zoological Society of London 1926:559-567.
Verma, K. 1965. Notes on the biology and anatomy of the Indian tree shrew, Anathana wroughtoni. Mammalia. 29: 289-330.
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