Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Urinary and Reproductive Systems of the Opossum (Monodelphis domestica)

Text and photo by Sarah Ogburn and Linda Brogdon

Overview:

Marsupials, including opossums, have two uteri and two vaginae. The root word “didelphys” (the “y” was changed to an “i” by mistake in the original description of Didelphidae—the family to which Monodelphis domestica belongs) literally means two wombs [1].  It is unclear why the genus name Monodelphis was chosen, as members of the genus have the usual paired uteri and vaginae. 

Dissection:

The abdominal cavity was opened and the digestive system removed the week before during the lab on the digestive system.  This exposed the urogenital system.  First, we removed the thick layer of fat that covered the kidneys.  It was extremely difficult to remove; and unfortunately, the thin ureters and abdominal aorta were broken.  The kidneys are bean shaped and large relative to body size and compared to the kidneys of the rat.  The specimen’s right kidney is more anterior than its left.  An adrenal gland is visible lying just medial to the left kidney.  Although very small, it appeared to be larger relative to the other animals’ adrenal glands in the class.  We could not locate the renal arteries or veins.   The postcaval vein runs along the midline, from the diaphragm to the junction of the common iliac veins.  The abdominal aorta lies to the left of the postcaval vein.  Caudal to the kidneys in the pubic area were the paired uteri.  They are tube like and fairly short.  At the anterior ends of the uteri were the extremely coiled uterine tubes.  We were able to locate the extremely small ovaries using a dissecting microscope at a magnification of 10X.  The ovaries are less than a millimeter in diameter.  They are located in a membrane that is attached to the coiled loop of uterine tubes.  We could not see Graffian follicles or corpora lutea due to the extremely small size and delicate nature of the ovaries.  The paired vaginae are just caudal to the uteri, and are white and rounded.  Caudal and medial to the two vaginae is the white, rounded bladder.  The vaginae and bladder open into the urethra, and there is a common opening of the three tubes (vaginae and urethra) into the urogenital sinus, which passes under the pubic symphysis.

Function:

The paired vaginae of marsupials are believed to be a result of the ureters lying medially to the Wolfian and Mullerian ducts.  The paired branches of the Mullerian ducts develop into the uteri and vaginae, and the ureters prevent the branches from fusing into one vagina.  In placentals, the ureters are lateral to these ducts and the vagina fuses. The bifid penis of most male marsupials (and all didelphids) functions to deposit sperm in each of the vaginae.  Marsupials do not give birth through the vaginae, however.  The space between the vaginae forms a medial septum.  This non-muscular septum is ruptured during birth as young actively make their way into the urogenital sinus and out of their mother’s body [2].

The placenta of marsupials differs from that of placental mammals in that it has a yolk sac and does not implant in the endometrium.  Pregnancy does not affect the ovarian cycle [3]. The young are born at an extremely early stage of development after only 14.5 days [4] and use their forelimbs to crawl to their mother’s teats, which are usually located within the marsupium—the pouch.  M. domestica, however lacks a pouch and the young simply cling to the teats.  The teats have slips of muscle at their bases, which may serve to pump milk into the mouths of the young [3]. 

Marsupial reproduction is often mislabeled as being primitive, but it is just different from placental reproduction.  As with most things, there are advantages and disadvantages to it.  Because gestation in marsupials is less than or equal to the length of one estrous cycle, the mother invests very little in fetal young.  Likewise, birth poses none of the risks it does in placental mammals because the young are born so small.  Litters of marsupials are only about .2% of maternal body mass, whereas litters of placentals are often over 50% of maternal mass [5].  Marsupials are particularly advantaged in harsh environments, because in the case of a drought or other unfavorable conditions, the young can be abandoned at virtually no cost to the mother.  Some Australian marsupials have highly specialized reproductive systems that allow them to delay implantation of embryos during times of harsh conditions.  This is known as embryonic diapause.  Once conditions improve, the blastocysts come out of this suspended animation, and the mother becomes pregnant. 

Some scientists have proposed that gestation is more efficient than lactation, and placentals are therefore more reproductively efficient because they spend more time gestating and less time lactating than marsupials.  Recent findings, however, have shown that lactation is always more efficient than gestation because milk composition can be changed during the course of lactation to meet changing nutritional needs and more of the energy is converted to neonatal tissue [5]. 

However, there are disadvantages to marsupial reproduction.  The extremely altricial young must be suckled in the pouch or on the nipples until they are old enough to separate from the mother.  This long period of maternal care is accompanied by an increased risk of mortality for the mother and pouch young, and puts a constraint on the maximum number of litters per year.  This means that marsupial populations do not undergo explosive growth. Their adaptive responses to competing placental mammals may be disadvantaged by this [5]. Also, because young must crawl from the birth canal to the teats, marsupials are constrained in their morphology.  They must have grasping hands, so marsupial whales, bats, or hooved marsupials have not evolved. 

References:

1. Hunsaker, D. 1977. Introduction. pp. xiii-xv in Hunsaker, D. (ed.) The Biology of Marsupials.

2. Barbour, R. 1977.Anatomy of marsupials. pp. 237-272 in Stonehouse, B. and Gilmore, D. (ed.s.) The Biology of Marsupials.

3. Barnes, R. 1977. The special anatomy of Marmosa robinsoni. pp. 387-413 in Hunsaker, D. (ed.) The Biology of Marsupials.

4. The Smith Lab. 2002. “Possum Page”. Accessed 2004 at http://www.duke.edu/~kksmith/index.html.

5. Hsu, M.; Garton, D.; and Harder, J. 1999. Energetics of offspring production: a comparison of a marsupial (Monodelphis domestica) and a eutherian (Mesocricetus auratus). Journal of Comparative Physiology: 169: 67-76.