【背景】低地磁気（HGMF；磁界強度 < 200 nT）は宇宙環境の基本要因の一つである。しかし、生物に対するHGMFの影響は明確になっていない。【目的】アフリカツメガエル（Xenopus laevis）の胚発生に対するHGMFの生物学的影響を調べること。結果：HGMF下で成長したツメガエルの胚において、第3卵割（8細胞期）の水平な卵割溝の減少（溝が卵全体を一周しない）や異常な形態形成が観察された。2細胞期のHGMFばく露は第3卵割の幾何学的パターンを変化させたが、4細胞期以降のばく露では変化が起きなかった。α-チュブリンの免疫蛍光染色法により4細胞期の分割球の紡錘体の方向性の変化が示された。この結果から、HGMFへの短時間（2時間）ばく露で卵割段階の胚の発生は十分に妨害されることが示された。また、有糸分裂時の紡錘体は地磁気が無いことを早期に感知するセンサであるかも知れず、これは発生段階中および/または発生終了した胚で観察された形態学的およびその他の変化の根底をなす分子的メカニズムの手がかりを与えている。
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To investigate the effects of exposure to a hypo-geomagnetic field (< 200 nT) on the development of frog embryos in the early cleavage stages.
One of the fundamental environmental factors in space is the low magnetic flux density, while on earth the geomagnetic field is permanently present.
Two experiments were performed. In the first experiment, adult frogs were kept for one week in a magnetically shielded room with a magnetic flux density below 200 nT. After fertilization, embryos were incubated in the same magnetic shielding room (n > 30) or kept under similar conditions in an incubator while exposed to the normal environmental magnetic field (ca. 22 µT) as a control group (n > 10). Embryos were examined in the two-, four- and eight-cell-stage (cleavage furrows) or after 2 or 4 days (malformations, mortality).
In a second experiment, fertilized frog eggs (parents were kept under normal geomagnetic field conditions) were incubated in a hypo-geomagnetic field environment below 150 nT (n = 30), while the control group (n = 30) was incubated on the same platform at about 52 µT. Blastomeres were examined in the two-, four- and eight-cell-stage (cleavage furrows, spindle orientation).
| ばく露 | パラメータ |
|---|---|
ばく露1:
ばく露時間:
parental animals continuous for 7 days; afterwards, embryos continuous for up to 4 days
|
|
ばく露2:
ばく露時間:
continuous for up to 2 hours (up to eight-cell-stage)
|
| 周波数 |
|
|---|---|
| タイプ |
|
| ばく露時間 | parental animals continuous for 7 days; afterwards, embryos continuous for up to 4 days |
| Additional information | hypo-geomagnetic field |
| ばく露の発生源／構造 |
|
|---|---|
| ばく露装置の詳細 | shielding room (2 x 3 x 2 m³) was constructed of six layers of permalloy, temperature 18.8 ± 0.8°C; embryos were placed on a wooden shelf in the center of the shielding room (50 x 50 x 50 cm³) |
| Sham exposure | A sham exposure was conducted. |
| Additional information | embryos from the control group were incubated at 19.1 ± 0.2°C in an incubator, magnetic flux density approx. 22 µT |
| 周波数 |
|
|---|---|
| タイプ |
|
| ばく露時間 | continuous for up to 2 hours (up to eight-cell-stage) |
| Additional information | hypo-geomagnetic field |
| ばく露の発生源／構造 | |
|---|---|
| チャンバの詳細 | temperature in the laboratory 19.0 ± 2.0°C |
| Sham exposure | A sham exposure was conducted. |
| Additional information | controls were incubated on the same platform, seperated by 20 cm, magnetic flux density approx. 52 µT |
In the four-cell-stage of blastomeres incubated under hypo-geomagnetic field conditions, reoriented spindles were found compared to blastomeres incubated under the normal magnetic environment (experiment 2). Additionally, in the eight-cell-stage, the horizontal third cleavage furrows were significantly decreased when incubated in the hypo-geomagnetic field in comparison to incubation in the normal magnetic environment (experiment 1 and 2). No differences between the groups were found in the two- and four-cell-stage.
In the group exposed to the hypo-geomagnetic field, the malformation percentage was significantly higher than in the group kept under normal magnetic field conditions, while no significant difference was found regarding mortality (experiment 1).
The authors conclude that the exposure to hypo-geomagnetic fields (< 200 nT) could affect the development of frog embryos in the early cleavage stages.
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