Structure and function of the oviduct in gekkonid lizards

Abstract

Although the structure of the oviduct has been analysed in several reptilian species using light microscopy, little ultrastructural information is available, particularly folloWing hormonal manipulation. Here, oviductal structure and ultrastructure from wild females, and females surgically and hormonally manipulated in the laboratory, was analysed using light (LM), scanning electron and transmission electron (TEM) microscopy in three species from the lizard family Gekkonidae: Hoplodactylus maculatus (viviparous), Saltuarius wyberba (oviparous, prodUcing a soft, parchment-like eggshell) and Hemidactylus turcicus (oviparous, producing a hard, calcareous eggshell).

The oviduct of the viviparous gecko H. maculatus exhibited seasonal changes. Maximal epithelial cell height was measured during vitellogenesis. The uterus exhibited the most obvious changes, due to the presence of a large, yolky egg following ovulation. Ultrastructural analysis identified features, such as bleb cells, apical protrusions and secretory granules, which were not visible at the LM level.

The uterus also exhibited the most distinct differences in a comparison of oviductal structure among vitellogenic females from the three gekkonid species. Viviparous H. maculatus had few uterine mucosal glands, whereas the oviparous species S. wyberba and H. turcicus had numerous glands (which secrete the eggshell membrane). The difference in gland density between the viviparous and oviparous species is consistent with other groups of reptiles in which viviparity has independently evolved. The number of secretory granules, and the staining properties of gland cells, differed between S. wyberba and H. turcicus. The differences may relate to the different types of eggshells produced by these species. Other differences among species, which could not be directly related to parity mode or eggshell type, may be related to the time period until ovulation for the individuals examined.

The literature shows that estradiol, secreted by the ovary during vitellogenesis, causes seasonal oviductal development in reptilian species; that ovariectomy, which removes the natural source of estradiol, causes oviductal regression; and that administration of estradiol to ovariectomised females causes oviductal development. The above patterns were confirmed in this study for H. turcicus. This is the first study to analyse the effects of exogenous estradiol on reptilian oviductal structure using TEM and to observe changes during the period of estradiol treatment. Changes observed included differentiation of the epithelium into ciliated and non-ciliated cells and increased numbers of secretory granules in non-ciliated cells.

Contrary to expectation, oviducts in ovariectomised H. maculatus were fully differentiated and secretory, resembling those of naturally vitellogenic females. No differences in oviduct structure at the LM and TEM level were observed among ovariectomised H. maculatus treated with vehicle solution only, estradiol, or estradiol followed by progesterone. Plasma estradiol concentrations were similar among groups. These results question the traditional view of estradiol-mediated oviductal development and suggest an extra-ovarian source of estradiol or that other hormones may maintain oviductal hypertrophy following ovariectomy in H. maculatus.

Immunocytochemistry was explored (ultimately unsuccessful) to try and determine the distribution of insulin-like growth factor-I (believed to facilitate the actions of estradiol) in the gekkonid oviduct.

Current literature and the findings of this study are incorporated into a review of oviductal structure and function in reptiles. Overall, this thesis provides comprehensive ultrastructural information including the identification of features not visible at the LM level and changes following hormonal manipulation.