Calcium metabolism and its control--a review

Area of Science:

• Avian physiology
• Nutritional biochemistry
• Reproductive endocrinology

Background:

• Avians utilize cholecalciferol, converted to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], for calcium metabolism.
• Hens fed only 1,25-(OH)2D3 exhibit poor egg hatchability due to abnormal embryonic beak calcification.
• Inadequate transport of 1,25-(OH)2D3 into eggs, contrasted with sufficient precursor 25-hydroxyvitamin D3 (25-OH-D3), underlies this issue.

Purpose of the Study:

• To investigate the role of vitamin D metabolites in avian reproduction and embryonic development.
• To understand the transport mechanisms and utilization of vitamin D forms in egg formation.
• To explore hormonal influences on vitamin D metabolism in laying hens.

Main Methods:

• Analysis of vitamin D metabolite concentrations in plasma and eggs.
• Correlation of vitamin D levels with reproductive parameters like shell quality and hatchability.
• Investigation of hormonal stimulation (estradiol, prolactin, parathyroid hormone) on kidney hydroxylase activity.

Main Results:

• Embryos utilize 1,25-(OH)2D starting by Day 10 of incubation.
• Plasma 1,25-(OH)2D3 concentrations are significantly higher during active egg production and positively correlate with eggshell thickness.
• Estradiol effectively stimulates the kidney hydroxylase responsible for 1,25-(OH)2D3 synthesis.

Conclusions:

• 25-hydroxyvitamin D3 is preferentially transported into eggs, supporting embryonic development.
• Hormonal regulation, particularly by estradiol, is crucial for maintaining adequate 1,25-(OH)2D3 levels for eggshell formation and hatchability.
• Optimal vitamin D metabolism and hormonal balance are essential for successful avian reproduction.