Sperm cells surround an embryo 311.
(photo credit: Debbi Morello/Detroit Free Press/MCT)
Biology students, when they begin to study embryos, are usually presented with
an image that looks like the inside of a melon, representing a disk-like cluster
of cells. But it has been a mystery to mankind how, within the first month of a
human pregnancy, this structure turns into an elongated embryo. Now Hebrew
University of Jerusalem scientists have explained for the first time how the
human embryo acquires this shape: “organizer” cells.
significant step towards understanding the issue was made nearly a century ago
in experiments conducted by German embryologists Hans Spemann and Hilde Mangold,
who, using early newt embryos, identified a group of cells within them which
upon transplantation formed a two-headed tadpole. They concluded that the
transplanted cells were organizing cells in their vicinity into a typical
embryonic shape, and dubbed them “organizer” cells. The two-headed newt embryos
possessed both their own organizers and the transplanted ones, both of which
organized nearby cells to form a head structure.
University researchers managed to generate human organizer cells using human
embryonic stem cells. The research was conducted by Nadav Sharon, a graduate
student under the supervision of Prof. Nissim Benvenisty at the Alexander
Silberman Institute of Life Sciences, in collaboration with genetics Prof.
Abraham Fainsod at the HU-Hadassah Medical Faculty, and was published in a
recent issue of the journal Stem Cells
Based on the similarity that
dominates the initial developmental processes of all vertebrates, the group
raised the human cells in conditions that recreated those of early amphibian
embryogenesis. Within two days, the human cells started expressing genes
characteristic of the organizer cells. To verify that these cells possesed true
organizing ability, the researchers repeated Spemann and Mangold’s experiments.
But this time human cells rather than those of amphibians were transplanted,
into frog embryos.
The midline of an amphibian embryo is marked by a
neural tube – a tissue destined to form the embryo’s central nervous system. To
the researcher’s complete surprise, some of the frog embryos that were implanted
with the human cells were later found to possess not one but two neural tubes –
the second also composed of frog cells, proving that the injected human cells
organized the cells in their vicinity to acquire a tubular shape.
determination during human embryonic development is a very important process;
any error could lead to miscarriage or the birth of a severely defective baby.
The identification of the human organizer should allow better understanding of
this process, the researchers suggest. In addition, they said, the ability of
the human organizer cells to shape a frog neural tube may assist in forming
human neural tubes in culture, from which neural cells could be obtained for
transplantation into people with spinal damage, though much further research
would be required to reach that stage.INVENTION TO PREVENT CHILDREN FROM
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