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The
Scientific Facts:
According to embryology, the coccyx (tailbone) is a
small bone associated with the last bone of the spine
and usually consists of four fused vertebrae and is
considered to be the base of the primitive streak that
precedes the formation of the whole fetus, especially
the nervous system. After primitive streak disappears,
only a small bone known as the coccyx remains.
The formation of the primitive streak:
On the fourteenth day, the endoderm
(innermost layers of the embryo) and the ectoderm
(outermost layers of the embryo) grow until they become
pear shaped. The front part will be wider than the back.
At the back part, the ectoderm will actively formulate
the primitive streak which appears for the first time on
the fifteenth day after conception.
Rapid
growth and cell division in the primitive streak begins
and cells start to move rightwards and leftwards between
the endoderm and the ectoderm forming a third layer
called the mesoderm.
After
the appearance of the primitive streak, the formation of
the nervous system and the notochord begins and the
bodily organs of the fetus start to take shape. If the
primitive streak does not take place, bodily organs will
not start their formation.
Due
to its importance, Warnock British Commission (a
commission specialized in human conception and embryos)
has pointed out that the existence of the primitive
streak is the point at which doctors and researchers
have stopped conducting experiments on early fetuses
produced by in vitro fertilization.
At
its first appearance and due to its extensive activity,
the primitive streak is followed by the following:
1-
At the point when the neural tube closes, the otic
placode and the lens placode starts to appear. The human
brain starts to take shape at the upper two thirds of
the neural tube while the spinal cord is created at the
lower third which is below the fourth-fifth somite, as
the fist four somites are part of the base of the skull.
2-
The mesoderm - which starts to take shape - grows
extensively around the fetus cord creating somites which
later turn into muscles and the backbone. Later, the
early ends of the upper and lower limbs - which
formulate the skeleton and the muscles, as well as the
urinary and procreative system - will stem from these
somites. In this mesoderm, the peritoneum, pleura, and
tumor membranes in addition to blood vessels, the heart,
and the digestive system muscles, take form.
In
this way, the formation of the primitive streak is an
important indicator that the bodily organs and tissue of
the fetus are being created. In fact, the stage known as
organogensis does not start except after the formation
of the thickening of cell layers, the neural groove, and
the somites. It extends from the beginning of the fourth
week to the end of the eighth week. At the end of this
period, the fetus is equipped with all basic systems and
only tiny details and growth will follow.
What happens to the primitive streak?
Having fulfilled the purpose of its existence at
the fourth week, the primitive streak starts to shrink
and its remains stay in the sacrococcygeal
region. These very tiny remains of the primitive
streak stay there forever. |
