Estrogen levels influence medullary bone quantity and density in female house finches and pine siskins

Abstract

Medullary bone, a non-structural osseous tissue, serves as a temporary storage site for calcium that is needed for eggshell production in a number of avian species. Previous research focusing primarily on domesticated species belonging to the Anseriformes, Galliformes, and Columbiformes has indicated that rising estrogen levels are a key signal stimulating medullary bone formation; Passeriformes (which constitute over half of extant bird species and are generally small) have received little attention. In the current study, we examined the influence of estrogen on medullary bone and cortical bone in two species of Passeriformes: the Pine Siskin (Spinus pinus) and the House Finch (Haemorhous mexicanus). Females of these species received either an estradiol implant or were untreated as a control. After 4.5-5months, reproductive condition was assessed and leg (femora) and wing (humeri) bones were collected for analysis using high-resolution (10μm) micro-computed tomography scanning. We found that in both species estradiol-treated females had significantly greater medullary bone quantity in comparison to untreated females, but we found no differences in cortical bone quantity or microarchitecture. We were also able to examine medullary bone density in the pine siskins and found that estradiol treatment significantly increased medullary bone density. Furthermore, beyond the effect of the estradiol treatment, we observed a relationship between medullary bone quantity and ovarian condition that suggests that the timing of medullary bone formation may be related to the onset of yolk deposition in these species. Further research is needed to better understand the precise timing and endocrine regulation of medullary bone formation in Passerines and to determine the extent to which female Passerines rely on medullary bone calcium during the formation of calcified eggshells.

Keywords: Estradiol treatment; Medullary bone; Microcomputed tomography.

Fig. 1

Phylogeny of avian orders based on Prum et al. (2015). Heavy lines indicate orders in which medullary bone has been observed. Light lines indicate orders that have not yet been studied. Filled circles indicate orders in which estrogen-stimulated medullary bone deposition has been documented. The open circle for Passeriformes indicates that medullary bone is likely estrogen-induced (see text for details). References are listed by species in Table 1.

Fig. 2

Medullary BV/TV in the a) femora and b) humeri from female Pine Siskins receiving a high dose of E₂, a low dose of E₂, or no exogenous estradiol (control, CTR) reported as a percentage of the control. Letters indicate groups with significant differences in medullary BV/TV (mean ± SD, p < 0.05).

Fig. 3

Two dimensional cross sectional images of the mid-diaphysis of the humerus of each female Pine Siskin studied showing the variation in medullary BV/TV across individuals from the a) High E₂ (n = 4), b) Low E₂ (n = 4), and c) Control, CTR (n = 8) groups.

Fig. 4

Medullary TMD in the a) femora and b) humeri from female Pine Siskins receiving a high dose of E₂, a low dose of E₂, or no exogenous estradiol (control, CTR) normalized to the control. * indicate groups with significant differences in medullary TMD (mean ± SD, p < 0.05).