Estrogen-Induced Synthesis of Yolk Proteins in Roosters

The metabolic cost of avian egg formation: possible impact of yolk precursor production?

“…This is consistent with previous findings showing that roosters treated with E2 responded by synthesizing VTG as rapidly as 3-4·h after estradiol injection (Bergink et al, 1974). Our birds also responded to the treatment by increasing VLDL production, which is in accordance with previously reported increases in plasma lipoproteins or liver fat content following exogenous estradiol administration (Yu and Maquardt, 1973a,b;Bergink et al, 1974;Harms et al, 1977;Rosebrough et al, 1982;Williams and Martyniuk, 2000). The rapid decline in VTG after the last injection, however, contradicts previous research where VTG synthesis has been shown to increase linearly for several days (Bergink et al, 1974) or even weeks (Robinson and Gibbins, 1984) after E2 treatment.…”

“…within 24·h) to exogenous E2 Metabolic costs of yolk precursor production administration in the form of a gradual increase in plasma VTG and that this response did not differ from that of non-breeding females undergoing the same treatment. This is consistent with previous findings showing that roosters treated with E2 responded by synthesizing VTG as rapidly as 3-4·h after estradiol injection (Bergink et al, 1974). Our birds also responded to the treatment by increasing VLDL production, which is in accordance with previously reported increases in plasma lipoproteins or liver fat content following exogenous estradiol administration (Yu and Maquardt, 1973a,b;Bergink et al, 1974;Harms et al, 1977;Rosebrough et al, 1982;Williams and Martyniuk, 2000).…”

“…Because the maintenance and activity costs of the oviduct have previously been shown to be related to metabolic rate (Chappell et al, 1999;Vézina and Williams, 2003), we avoided any confounding effects in our results by using male zebra finches as models for this experiment. Males react to E2 injections by producing yolk precursors in the same fashion as females (Bergink et al, 1974;Follett and Redshaw, 1974). Various E2 doses were tested (F. Vézina, K. G. Salvante and T. D. Williams, unpublished results) in order to generate a response in plasma yolk precursor levels comparable with normal female breeding values.…”

“…During the process of egg formation, the hypothalamus initiates a hormonal cascade by releasing gonadotropin-releasing hormone (GnRH), which induces the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland (Williams, 1998;Scanes, 2000). These hormones stimulate the ovary to produce estrogens (Williams, 1998), which then trigger the production of the egg-yolk precursors, vitellogenin (VTG) and yolk-targeted very-lowdensity lipoprotein (VLDLy), by the liver (Bergink et al, 1974;Deeley et al, 1975;Wallace, 1985;Walzem, 1996;Williams, 1998), which are then secreted into the blood. During rapid yolk development, plasma VTG and VLDLy are taken up by the ovary and are processed within the follicles into yolk, the nutrient and energy source for the developing avian embryo Little is known about the energy costs of egg production in birds.…”

The E-SCREEN Assay as a Tool to Identify Estrogens: An Update on Estrogenic Environmental Pollutants

“…Exposure to environmental estrogens, singly or in combination, may be easily assessed in male fish, reptiles, or birds used as sentinels by measuring their vitellogenin plasma levels. Instead, exposure of females to xenoestrogens is more difficult to ascertain through a marker such as vitellogenin because the serum levels of this protein are high in animals laying eggs (30,31). In addition, there is no comparable marker to ascertain exposure in mammals.…”