Blackburn, D.G. (1998a).
Structure, function, and evolution of the oviducts of squamate reptiles,
with special reference to viviparity and placentation. Journal of Experimental Zoology 282:
560-617.
Abstract: In
lizards and snakes, the oviducts function in fertilization, sperm storage, egg
transport, eggshell deposition, maintenance of the early embryo, and expulsion
of the egg or fetus. In viviparous
forms they also contribute to placentae responsible for gas exchange and
nutrient provision to the fetus.
Dissections of species of 30 genera coupled with data from the
literature indicate that squamate
oviducts vary interspecifically in seven macroscopic features, including the
extent and nature of regional differentiation, vascular supply, topographic
asymmetry, number of oviducts, vaginal pouches, and relationship to the
cloaca.
The uterus, infundibulum,
and vagina differ histologically in their epithelia, glands, and myometrial
layers. Seasonal cyclicity occurs in
all three oviductal regions, most prominently in the uterus, and is under endocrinological
control. Regional and cytological
specializations reflect the diverse functions performed by the oviduct. Definitive evidence for oviductal albumen
production and egg resorption is lacking.
In viviparous squamates, three uterine specializations may
facilitate maternal-fetal gas exchange: an attenuated epithelium, reduced
uterine glands (and a reduced shell membrane), and increased
vascularization. Contrary to previous
reports, pregnant uteri show no epithelial erosion or capillary exposure. Specializations for nutrient provision to
the fetus include mucosal hypertrophy, enlarged glandular epithelia, and
multicellular glands whose secretions are absorbed by the chorioallantois.
Comparisons with other amniotes indicate that squamates inherited
the oviduct as an organ with capabilities for egg uptake and transport,
fertilization, eggshell deposition, and oviposition. Other features have evolved convergently among squamates:
infundibular sperm receptacles, unilateral oviduct loss, uterine gestation,
placentation, and specializations for placentotrophy. Cladistic analysis indicates that oviductal features associated
with deposition of tertiary egg investments in reptiles reflect evolutionary
convergence as well as secondary simplification, rather than a unidirectional
trend towards increased specialization.
Reprints: daniel.blackburn@mail.trincoll.edu