ORCID Profile
0000-0003-3798-2004
Current Organisation
Coastal Carolina University
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Publisher: Wiley
Date: 16-08-2010
DOI: 10.1002/JMOR.10877
Abstract: The eggshell of lizards is a complex structure composed of organic and inorganic molecules secreted by the oviduct, which protects the embryo by providing a barrier to the external environment and also allows the exchange of respiratory gases and water for life support. Calcium deposited on the surface of the eggshell provides an important nutrient source for the embryo. Variation in physical conditions encountered by eggs results in a tradeoff among these functions and influences eggshell structure. Evolution of prolonged uterine egg retention results in a significant change in the incubation environment, notably reduction in efficiency of gas exchange, and selection should favor a concomitant reduction in eggshell thickness. This model is supported by studies that demonstrate an inverse correlation between eggshell thickness and length of uterine egg retention. One mechanism leading to thinning of the eggshell is reduction in size of uterine shell glands. Saiphos equalis is an Australian scincid lizard with an unusual pattern of geographic variation in reproductive mode. All populations retain eggs in the uterus beyond the embryonic stage at oviposition typical for lizards, and some are viviparous. We compared structure and histochemistry of the uterus and eggshell of two populations of S. equalis, prolonged egg retention, and viviparous to test the hypotheses: 1) eggshell thickness is inversely correlated with length of egg retention and 2) eggshell thickness is positively correlated with size of shell glands. We found support for the first hypothesis but also found that eggshells of both populations are surprisingly thick compared with other lizards. Our histochemical data support prior conclusions that uterine shell glands are the source of protein fiber matrix of the eggshell, but we did not find a correlation between size of shell glands and eggshell thickness. Eggshell thickness is likely determined by density of uterine shell glands in this species.
Publisher: Wiley
Date: 23-04-2010
DOI: 10.1002/AR.21052
Abstract: The evolution of viviparity requires modifications to multiple integrated physiological features to support embryonic development during pregnancy. Embryonic growth during pregnancy is dependent upon the capacity of the uterine vascular system to satisfy increasing embryonic oxygen demand throughout gestation. We tested the hypothesis that total surface area of uterine blood vessels increases in concert with embryonic growth, and hence its oxygen demand, during gestation. We used immunofluorescence and laser-scanning confocal microscopy to quantify uterine microvascular density and morphology during gestation in the oviparous skink Ctenotus taeniolatus and in Saiphos equalis, a skink species with prolonged egg retention. For C. taeniolatus, vessel density (Nv) and vessel length-density (Lv) in the embryonic hemisphere of the uterus is 23% and 17% less, respectively, than that of S. equalis and vascular surface-area does not differ as a function of embryo stage. For S. equalis, overall Nv, Lv, and vessel diameter (Dv), does not change during the first half of gestation but increases by 36% (Nv), 44% (Lv), and 60% (Dv) by near-term embryo stages late in gestation. The chorioallantoic membrane of S. equalis increases in absolute size but vascular density does not differ as a function of embryo stage. The marked increase in uterine vascular density during late gestation coincides with the phase of rapid growth in embryo mass and concomitant increase in metabolic rate. Expansion of the uterine vascular bed in concert with embryo size and metabolism is likely to be an important transitional step in the evolution of viviparity.
Publisher: Springer Science and Business Media LLC
Date: 09-2009
DOI: 10.1007/S00360-009-0400-2
Abstract: A prominent scenario for the evolution of viviparity and placentation in reptiles predicts a step-wise pattern with an initial phase of prolonged oviductal egg retention accompanied by progressive reduction in eggshell thickness culminating in viviparity calcium placentotrophy evolves secondarily to viviparity. Saiphos equalis is an Australian scincid lizard with a reproductive mode that is uncommon for squamates because eggs are retained in the oviduct until late developmental stages, and the embryonic stage at oviposition varies geographically. We studied calcium mobilization by embryos in two populations with different oviductal egg retention patterns to test the hypothesis that the pattern of nutritional provision of calcium is independent of the embryonic stage at oviposition. Females from one population are viviparous and oviposit eggs containing fully formed embryos, whereas embryos in oviposited eggs of the second population are morphologically less mature, and these eggs hatch several days later. The reproductive mode of this population is denoted as prolonged oviductal egg retention. Yolk provided the highest proportion of calcium to hatchlings in both populations. Eggs of both populations were enclosed in calcified eggshells, but shells of the population with prolonged egg retention had twice the calcium content of the viviparous population and embryos recovered calcium from these eggshells. Placental transfer accounted for a substantial amount of calcium in hatchlings in both populations. Hatchling calcium concentration was higher in the population with prolonged egg retention because these embryos mobilized calcium from yolk, the eggshell and the placenta. This pattern of embryonic calcium provision in which both a calcified eggshell and placentotrophy contribute to embryonic nutrition is novel. The reproductive pattern of S. equalis illustrates that calcified eggshells are compatible with prolonged oviductal egg retention and that viviparity is not requisite to calcium placentotrophy.
Publisher: Wiley
Date: 03-2012
DOI: 10.1002/JEZ.B.22003
Abstract: Structural and functional changes to the uterus associated with maintenance of pregnancy are controlled primarily by steroid hormones such as progesterone. We tested the hypothesis that progesterone regulates uterine structural changes during pregnancy in the viviparous skink, Pseudemoia entrecasteauxii, by treating pregnant females with the progesterone receptor antagonist mifepristone at different stages of pregnancy. Expression and distribution of progesterone receptor was determined using Western blot and immunohistochemistry. During early pregnancy, mifepristone treatment resulted in altered uterine epithelial cell surface morphology and high embryo mortality, but did not affect females at mid and late stages of pregnancy. Females treated with mifepristone in early pregnancy exhibited abnormal uterine epithelial cell morphology such as lateral blebbing and presence of wide gaps between cells indicating loss of intercellular attachment. Chorioallantoic membranes of the embryo were not affected by mifepristone treatment. Two isoforms (55 kDa and 100 kDa) of progesterone receptor were identified using immunoblots and both isoforms were localized to the nucleus of uterine epithelial cells. The 55 kDa isoform was expressed throughout pregnancy, whereas the 100 kDa isoform was expressed during mid and especially late pregnancy. In P. entrecasteauxii, mifepristone may prevent successful embryo attachment in early pregnancy through its effects on uterine epithelial cells but may have little effect on pregnancy once the maternal-embryo structural relationship is established.
Publisher: Elsevier BV
Date: 2008
DOI: 10.1016/J.CBPA.2007.09.014
Abstract: Calcium ATPase (Ca2+-ATPase) is a key enzyme that participates in the translocation of calcium in the uterus of oviparous amniotes during eggshell formation. We used Western blot and indirect immunofluorescence microscopy to determine expression and localisation of uterine Ca2+-ATPase during the reproductive cycle of king quail and zebra finch. The pattern of Ca2+-ATPase expression and localisation during the reproductive cycle was similar for both species. Immunoblots of uterine extracts from quail and finch indicated that Ca2+-ATPase expression is reduced in non-reproductive compared to reproductive females. Similarly, in non-reproductive females, weak apical immunofluorescent staining of Ca2+-ATPase is localised to epithelial cells in a small number of uterine tubular glands. A large increase in apical immunofluorescent staining of tubular gland epithelia occurs in both vitellogenic and reproductive females. The presence of Ca2+-ATPase on the apical surface of tubular gland epithelial cells suggests that the enzyme is involved in the translocation of calcium out of the tubular gland epithelia and into the concentrated fluid of the uterine lumen. Presence of Ca2+-ATPase in vitellogenic females indicates that the enzyme is expressed prior to the time of ovulation and eggshell calcification.
No related grants have been discovered for Scott Parker.