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Role of vitamin D in the development of the chick embryo
Insights
The active form of vitamin D, 1,25(OH)2D3, primarily targets the chorionic epithelium in 15-day-old chick embryos, influencing calcium and phosphate levels. This suggests the chorionic epithelium is key for vitamin D
Area of Science:
- Endocrinology
- Developmental Biology
- Calcium Metabolism
Background:
- 1,25(OH)2D3 (active vitamin D) plays a crucial role in calcium and phosphate homeostasis.
- The specific target tissues and mechanisms of action of 1,25(OH)2D3 during embryonic development are not fully understood.
- Chick embryos offer a model to study vitamin D's effects on calcium resorption and organ development.
Purpose of the Study:
- To investigate the age-dependent responsiveness of chick embryos to 1,25(OH)2D3.
- To identify the primary target tissue for 1,25(OH)2D3 in chick embryos.
- To explore potential immunomodulatory effects of 1,25(OH)2D3 in developing chicks.
Main Methods:
- Injection of varying doses of 1,25(OH)2D3 into chick embryos of different ages and newly hatched chicks.
- Measurement of blood calcium (Ca) and inorganic phosphate (Pi) concentrations 24 hours post-injection.
- Autoradiographic studies using tritiated 1,25(OH)2D3 to identify tissue distribution.
- Histological examination of various organs following high-dose 1,25(OH)2D3 administration.
Main Results:
- 15-day-old chick embryos exhibited maximal sensitivity to 1,25(OH)2D3, showing hypercalcemia and hypophosphatemia at lower doses compared to younger embryos.
- Newly hatched chicks showed minimal response to high doses of 1,25(OH)2D3.
- Autoradiography revealed rapid nuclear uptake of 1,25(OH)2D3 in chorionic epithelium ('villus-cavity' cells), suggesting it as the main target tissue.
- High doses of 1,25(OH)2D3 induced eosinophilic granulocyte infiltration in the spleen, liver, and bursa of Fabricius.
Conclusions:
- The chorionic epithelium is the principal target for 1,25(OH)2D3 in chick embryos, mediating calcium resorption from the eggshell.
- The age-dependent response suggests a critical window of sensitivity during embryonic development.
- 1,25(OH)2D3 may influence leukocyte proliferation and differentiation, warranting further investigation in the chick embryo model.
Abstract:
Chick embryos of various ages and 1-day-old chicks were injected with different doses of 1,25(OH)2D3 and the concentrations of Ca and Pi in their blood were determined 24 hr after. It was confirmed that the dose required to induce hypercalcemia and hypophosphatemia was much lower in the case of 15-day-old embryos than in younger ones. The age of maximal response corresponds to the period when the chorionic epithelium is fully differentiated. Newly hatched chicks did not respond even with doses 20 times higher than those used in 15-day-old embryos. These findings support the idea that the humoral changes produced by 1,25(OH)2D3 result from increased resorption of calcium from the shell and that the chorionic epithelium is instrumental in this phenomenon. The autoradiographic study of tissues from embryos injected with tritiated 1,25(OH)2D3 showed that the exposure time required to detect nuclear concentration of radioactivity was much shorter in the case of "villus-cavity" cells than in any other tissue (kidney, intestine, bone, parathyroid glands) in which target cells were recognized. The above results are interpreted as indicating that the chorionic epithelium is the main target for 1,25(OH)2D3. The histological study of various organs from embryos injected with large doses of 1,25(OH)2D3 showed generalized infiltration of spleen, liver, and bursa of Fabricius with eosinophilic granulocytes. It has been suggested that 1,25(OH)2D3 influences the proliferation and differentiation of leukocytes; the chick embryo might be a good model for further exploration of this problem.