For most mammals, egg cells start as such - round, egg-like masses. After being fertilized, it starts developing the creature's body axis from head to tail, front to back, left to right. While the orientation has been theorized before, it has never been actually observed - until now.
Researchers from the Marine Biological Laboratory (MBL) have successfully imaged how these developments and arrangements in the cell begin, which could answer important biological questions. Details of their work are published in the journal Molecular Biology of the Cell.
Left: Ciona egg before and after fertilization (Movie of Total Internal Reflection Fluorescence Microscopy [TIRFM] images; F-actin probe is AF488-phalloidin). Right: Transient changes of F-actin alignments from the time before fertilization to the first cell division of Ciona egg. Positions of AF488-phalloidin particles bound to F-actin are shown as yellow dots, and the orientation of F-actin is shown as yellow bars.
Development of Body Axes in Animals, and Humans
"The most interesting and mysterious part of developmental biology is the origin of the body axis in animals," shared researcher Tomomi Tani, an MBL scientist with the Eugene Bell Center at the time of the study and now with the Japanese National Institute of Advanced Industrial Science and Technology.
The new study shows how both parents contribute to the offspring's body orientation. In their research, they used sea squirts, further observing how the mother helps determine the back to belly axis orientation of the offspring while the father is involved with the development of the head to tail axis.
"Both the maternal and the paternal cues are required to establish the body plan of the developing animal embryo," Tani adds in a press release from MBL.
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With the new effort, researchers aim to address fundamental questions in developmental biology, offering insights on how body development in offsprings could go wrong. Understanding this aspect better could have implications in different life sciences - from medicine to genetics to agriculture.
The prevailing theory on the development of the body axis suggests that actin filaments within the egg - linear polymers occurring as microfilaments in the cell generally involved in contraction and motion - also dictates the rearrangement of cytoplasmic material in the egg after fertilization. However, direct observation to support this theory has not been done before since this process kicks off pretty rapidly and in close proximity within other living cells.
Imaging the First Body Plan Development in the Embryo
To conduct the first imaging of these processes, researchers used a fluorescence polarization microscope, a novel imaging technology also developed at MBL by Tani, together with Chan Zuckerberg Biohub's Shalin Mehta, and senior scientist Rudolf Oldenbourg, in collaboration with other researchers. With this microscope, researchers can now capture events occurring in the context of nanometers - thousands of times thinner compared to the diameter of a human hair - such as the development of the body axis in sea squirts.
"Using polarized light for looking at dynamics of molecular order is a tradition of MBL imaging," Tani added, being familiar with the process going back with live-cell studies by Japanese scientist Shinya Inoue in the 1950s.
In a fluorescence polarization microscope, light waves are "polarized" and causes it to oscillate in only one direction. Paired with a polarizing filter, only light of the correct oscillation direction will pass through while all others are blocked. Researchers attached fluorescent probe molecules to the actin found in sea squirt eggs.
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