The fertilized ovum
divides first into two cells, then into four, then into eight,
and so on. This type of cell division, called mitosis,
will therefore exponentially increase the number of cells
in the embryo. Mitosis is the predominant process at the
very start of the development of all multicelled organisms.
As the result of mitosis, an adult human will have 100 trillion (100 000
billion) cells! This figure is astronomical compared with
the mere 30 000 human
genes that provide the plans for creating and
positioning all these cells.
The embryonic development that takes place over the 9
months of human gestation is thus a very precisely choreographed
process in which every member of the 300 different classes
of cells in the human body is placed exactly where it belongs.
The development process continues after the human baby
is born, for example, with the enhancement of its immune
system and with fine tuning of its nervous system.
FROM FERTILIZATION TO EMBRYO
It is very hard to imagine
how a human being, with all the complex intellectual capabilities
conferred by the human brain, can develop from an embryo, much
less from the single cell with which each embryo begins.
But to understand how the human
nervous system develops, that is what we must do: go back
to the very first cell in the human body, or rather the first
two cells that create it. In human beings, as in all other species
that reproduce sexually, half of the individual’s genetic
material comes from its mother and half from its father. Each
parent produces a special cell called a gamete, and when the
two gametes merge, they form the first cell that has all the
information needed to build a new individual.
In human beings, the female and male gametes are very different. A woman produces
one large ovum (egg) per month, while a man produces 200 million to 300 million
tiny spermatozoa (sperm cells) in a single day! When the woman ovulates, the
ovum is ejected from the ovary and carried through the fallopian tubes toward
the uterus.
The sperm cells are produced in the
man’s testicles and
released into the woman’s vagina by ejaculation. The sperm
cells then use their mobile flagallae (tails) to swim up through
the uterus and into the fallopian tubes. The sperm cells are attracted
by the ovum but must break down its thick membrane in order to
penetrate it. As soon as one sperm cell penetrates the ovum, it
triggers chemical changes that prevent any more sperm cells from
entering.
Though the amounts of genetic material contributed by the ovum
and the sperm cell are equivalent, the amount of cytoplasm that
the sperm cell contributes is negligible compared with the ovum.
In mammals, fertilization is completed about
12 hours later, when the nucleus of the ovum merges with that of
the sperm cell to form the first cell that contains all the genetic
material of the new organism.
This cell is called the zygote. It is about one-fifth the size
of the period at the end of this sentence. Every human being began
life in this same way, with a single ovum being fertilized by a
single sperm cell to form a zygote.
A cell’s development
potential means the number of different types
of cells that it can produce. Thus, neural stem
cells are described as “pluripotent”,
meaning that they can differentiate into several different
types of neurons and glial cells. But only the zygote and
the first two or four cells are that it produces when it
divides are “totipotent”(able to form absolutely
all of the cells in the body).
It is the premature separation of totipotent cells from each
other that produces identical twins. In contrast, non-identical
twins share half of their genes like any other brothers
or sisters, because they are produced by the fertilization
of two ova by two different sperm cells.
The earliest stages
of embryonic development are crucial for the formation
of the nervous system. They require perfect co-ordination
that can be disturbed, especially during the first trimester
of pregnancy, if the mother eats poorly, or runs a fever,
or takes alcohol or
other drugs.
That said, every hour some 9 000 babies are born worldwide,
and the vast majority of them are healthy, with perfectly
developed brains, even though the conditions that their mothers
experience during pregnancy are sometimes quite difficult.
HOW THE NERVOUS SYSTEM BEGINS
The human
nervous system starts to form very early in the embryo’s
development. At the end of the gastrulation phase,
an elongated structure, the notochord, is laid down. The
embryo thereby changes from a circular organization to an
axial one—a critical step in the development of its
nervous system.
Next, the notochord sends out a signal to the layer of cells
just above it (the ectoderm),
which causes certain of these cells to form the first structure
from which the nervous system originates: the neural
plate. This is the start of the development of the
human nervous system, a process also known as neurulation.
In the next step of neurulation, the edges of the neural plate
begin to fold inward, forming the neural groove.
This groove soon closes completely to form the neural
tube,
from which the entire brain and spinal
cord will develop.
The cells that form the interior of the neural tube, in addition
to being the origin of the brain and spinal cord, will also
give rise to the neural
crest, another structure that is important for the ensuing
stages in laying down all the components of the nervous system.
The neurons of the cerebral cortex also maintain great plasticity over
the human lifetime. This ability to alter the efficiency
of synaptic connections is much less pronounced in the more
primitive parts of the brain, such as the brainstem.
HOW THE MAJOR SUBDIVISIONS OF THE BRAIN ARE
FORMED
The appearance of
vesicles in the rostral part of the neural
tube is an important step in the development of the
vertebrate nervous system, because it is these vesicles
that will eventually form the brain. (In contrast, invertebrates,
such as insects and molluscs, have no brain in the strict
sense. Instead, they have only ganglia—clusters of
nerve cells—at various points in their bodies. However,
some invertebrates, such as octopi, do have a highly developed
cerebral ganglion.)
In a mammalian embryo, the neural
tube is initially a straight, linear structure. But then, even
before the caudal portion of the neural tube has begun to develop,
the rostral portion undergoes some spectacular changes. At
the start of the 4th week, this end of the neural tube begins
to curve and divides into three bulges, known as the primary (or
primitive) vesicles.
From front to rear, these vesicles are the beginnings of the prosencephalon (forebrain), mesencephalon (midbrain),
and rhombencephalon (hindbrain). Behind the
rhombencephalon, the neural tube continues; this part of the
neural tube will produce the spinal
cord.
