The
100 trillion cells in the adult human body are divided
into 300 different types of cells, each of which expresses
only a subset of the genome specific to its type. Amazingly,
only 1.1% of the DNA in our genes represents instructions
for manufacturing proteins. So how does the human body
manufacture the tens of thousands of proteins that it contains,
if it has only around 30 000 genes with which to do it?
Obviously, a “one gene, one protein” model
will not provide the answer.
The answer lies in what are known as post-transcriptional processes.
For example, the “gene for insulin”, the hormone
needed to metabolize sugar, actually encodes for the manufacturing
of a very large protein that will subsequently be broken down
into smaller pieces by enzymes. And it is the combination (or “splicing”)
of two of these pieces that yields insulin. In all, this single
gene is used to produce four different proteins and three different
hormones.
When the
embryo becomes deeply embedded in the uterine wall, openings
form in the layer of external cells and fill with the mother’s
blood. This is the embryo’s first contact with the
source that will nourish it throughout its development. The placenta is
thus composed both of trophoblasts from the embryo and of
endometrial cells from the mother’s uterus.
Following implantation, some major hormonal adjustments occur.
The hormone human chorionic gonadotropin (hCG)
is secreted by the embryo’s trophoblasts and stimulates
the production of progesterone and estrogen by
the ovaries (hCG is the hormone that is detected in pregnancy
tests). From the 10th to the 14th day, the amniotic
cavity (which protects the fetus from external shocks)
and the placenta (where exchanges take place between the fetus
and the mother) begin to form. At this stage, the mother has
probably just noticed that she has missed her period.
Implantation normally
takes place around the 10th day, in the upper third of the
uterus, along the median line. When the embryo becomes implanted
somewhere other than the uterus, it is usually in one of
the fallopian tubes. This abnormal condition is called an ectopic
pregnancy.
FROM FERTILIZATION
TO EMBRYO
Sexual reproduction in human beings
is made possible by the fusion of two reproductive cells, or “gametes”—the ovum
from the mother and the sperm cell from the father. This fusion
results in the formation of a zygote with 46 chromosomes:
half from the mother and half from the father. The zygote thus
acquires a unique genetic identity, so that it is far more accurate
to speak of the sex act as one of procreation (the creation of
a new being) rather than one of reproduction, which would imply
simply producing the same thing over again.
One day after fertilization,
the zygote begins a series of mitotic
divisions in which the considerable volume of cytoplasm from
the ovum is divided among the multiple daughter cells. Hence, during
these first cell divisions, the zygote, now called an embryo, does
not grow in size but does move successively through the stages
where it is composed of 2, 4, 8, and then 16 cells, called blastomeres.
This first phase of development, in which the embryo undergoes
a series of rapid divisions in close succession, is called segmentation.
In humans, the embryo reaches the 16-cell
stage, also called the morula, on the third day
after fertilization. At this stage, the embryo is a compact ball
of cells that descends through the fallopian tube toward the uterus.
The cell divisions still have not caused the embryo to grow any
larger than the original ovum.
At the blastula stage, starting at 128 cells,
the ball of cells develops an inner cavity called the blastocoele.
This stage begins around the fourth day and continues until the
embryo has become implanted in the wall of the uterus, around the
sixth or seventh day. During this stage, the protective envelope
(or pellucid area) around the ball of cells gradually
decays, precisely so that the embryo can attach itself to the uterine
wall. It is also at this stage that the embryo differentiates into
two kinds of cells: outer cells called trophoblasts, which
will contribute to the placenta, and inner cells called embryoblasts,
which will form the embryo as such.
Implantation (or nidation) in
the uterine wall begins around the seventh day, when the trophoblasts,
freed from the pellucid area, secrete an enzyme that lets the embryo
burrow into the uterine wall. As the trophoblasts proliferate,
they form two distinct layers of cells: one that continues to surround
the embryo, and another whose membranes fuse, forming a multinucleate
mass called the syncytium, which is responsible
for penetrating the uterine wall.
The third week of development begins with a major cellular reorganization
called gastrulation. In gastrulation, a portion
of the cells on the blastula’s surface invaginates (penetrates
inward), thus forming the endoderm, while the cells remaining on
the outside of this sphere form the ectoderm. The cells inside
the sphere then divide into two sheets of cells forming two superimposed
discs. The upper disc will become the embryo, while the lower one
will turn into a yolk sac that provides nutrients
to the embryo until a functional circulatory system develops.
At the start of this phase, a narrow row
of cells traces a furrow along the embryonic disc and thus defines
the general axis around which the body’s entire bilateral
structure will develop. We can say that the embryo as such officially
begins to exist starting at this stage, when a third layer of cells,
called the mesoderm, interposes itself between
the two that are already present, the ectoderm and the endoderm.
All of the cells in these layers have the same genetic material, but some of
them start to express certain genes rather than others so as to develop particular
organs. The cells in the innermost layer, the endoderm, will
produce such organs as the intestines, the lungs, and the liver. The middle layer,
the mesoderm, will produce the kidneys, the reproductive organs,
the bones, the muscles, and the vascular system. And the outer layer, the ectoderm,
will be the source of both the epidermis and the entire central and peripheral
nervous systems.
Source: adapted from BrainConnection.com
The co-ordinated movements that make gastrulation possible are
called morphogenetic movements, and they involve
the entire embryo. The cells adopt new positions and consequently
acquire new neighbours. Through gastrulation, the cells are thus
assembled into subgroups that act on one another through induction
phenomena.
To sum up, gastrulation follows the rule
of threes: in the 3rd week, 3 cell layers form that are
the origin of 3 important structures: the primitive streak,
which defines the plane of bilateral symmetry of the future embryo,
and the dorsal cord (or notochord),
which will induce the formation of the neural plate in
the following stage, neurulation.
From six to eight
weeks after fertilization, the cerebral hemispheres begin
to form. Around the seventh week, nerves make connections
with some muscles that enable the embryo to make spontaneous
movements.
At the end of the eighth week, all of the body’s essential
internal and external structures are present. The second
and third trimesters of pregnancy are devoted essentially
to growth of these structures that have already been put
in place.
In the epithelium
of the neural tube, proliferation takes
place at specific locations called germinal zones. The
germinal zones from which most parts of the nervous system
will develop are located near the surface of one of the
cavities that will become the ventricular
system of the brain.
HOW THE NERVOUS
SYSTEM BEGINS
About three weeks
after conception, the human brain is nothing more than
a single layer of flattened cells located in the ectoderm and
known as the neural plate. Next, a furrow
forms that extends from the rostral portion to the caudal
portion of this plate. The sides of this neural
furrow then form the neural groove. The
sides of this groove then close over, starting from the
middle of the groove and moving outward rostrally and caudally,
to form the neural tube. Certain cells
in the dorsal portion of the neural tube will become the neural
crest, the structure that is the origin of the neurons
of the peripheral nervous system.
The part of the neural plate located just above the notochord differentiates
into the floor plate. The inductive
signals from this floor plate induce the development of the spinal
motor neurons and the motor neurons of the medulla and the pons from the
most ventral cells of the neural tube. The most dorsal cells will give rise to
the sensory neurons.
The process of formation of the neural tube, which often
begins before the mother even knows she is pregnant, is called neurulation.
It is from this tube that the brain and the spinal cord will
develop. At this stage they will be the largest organs in
the embryo, resulting in its characteristic curved form.
At the end of the third week, the eyes and
the ears will also have begun to form.
After the segmentation and gastrulation phases
are completed, organogenesis begins. In
this phase, the groups of cells are laid down that will become
the various organs of the human body. Organogenesis starts
with a process called metamerization, in
which the mesoderm divides into a series of identical segments
called metameres along the embryo’s longitudinal axis.
At this stage, the mesoderm develops masses called somites on
either side of the neural tube. It is from these somites
that the 33 vertebrae of the spinal column and the corresponding
skeletal muscles will develop.
Source: Dr. K. Tosney, University
of Michigan
At the start of the 4th week after fertilization, the neural
tube closes entirely, completing the first stage of the development
of the brain and the spinal cord. The next stage, histogenesis,
in which the stem
cells differentiate to form the various nerve tissues,
can now begin in earnest. At the same time, the
major subdivisions of the brain will form, and the cell
populations will be rearranged accordingly.
HOW THE MAJOR
SUBDIVISIONS OF THE BRAIN ARE FORMED
The telencephalon
is the most rostral of the secondary vesicles. Two buds
emerge from either side of its rostral portion to form
the two telencephalic vesicles. These two vesicles grow
rapidly to form the two cerebral hemispheres. First they
grow back over the diencephalon, then they grow down to
cover its sides.
General diagram of a sagittal
section of the brain (applicable to all mammals)
Another pair of vesicles
will also sprout from the ventral surface of these cerebral
hemispheres to become the olfactory bulbs and
other structures that contribute to the sense of smell. Various
structures will then emerge from the walls of the telencephalon
while the white matter that connects these structures develops
as well. The neurons of the telencephalon wall proliferate
to form three distinct regions—the cerebral
cortex, the basal telencephalon, and
the olfactory bulb.
The axons of these
neurons will also gradually elongate to make connections
with the other parts of the nervous system. Some of these
axons will constitute the cortical white matter that
arises from and projects to neurons in the cortex. Others
will form the corpus callosum, the band
of nerve fibres that connects the two hemispheres of the
brain. Still others—those of the internal
capsule—will connect the cortical white
matter to the brain stem, generally by way of the thalamus.
For example, the axons
arising from the motor cortex will pass through the
internal capsule to connect to the motor neurons in the
spinal cord.
In the the remaining space between the telencephalon and
the diencephalon on either side, the two cerebral
ventricles (also known as the lateral ventricles or the
first and second ventricle) form, while the third ventricle
forms in the space
at the centre of the diencephalon.
The diencephalon also differentiates into distinct areas:
the thalamus and the hypothalamus.
On either side of
the diencephalon, two secondary vesicles also develop—the optic
vesicles. The optic vesicles lengthen and fold
inward to form the optic peduncles and optic cups, which
will give rise to the retinas and
the optic
nerves. The retinas and the optic nerves are therefore
not part of the peripheral nervous system, but rather they
are integral parts of the brain!
Compared with the prosencephalon (telencephalon and
diencephalon), the mesencephalon undergoes far less transformation.
Its dorsal surface forms the tectum,
while its floor forms the tegmentum.
While these structures are differentiating, the cavity
that separates them shrinks to a narrow channel called
the cerebral aqueduct. The rostral portion of this aqueduct
opens into the third ventricle of the diencephalon.
The mesencephalon serves as the passageway for the bundles of
fibres that connect the cortex to the spinal cord—both
those that arise from the sensory system and those that descend
to participate in movement
control.
The tectum differentiates into two structures. One, the superior
colliculus, receives information directly
from the eye and controls eye movements. The other, the inferior colliculus,
receives information from the ear and serves as an important
relay in the auditory pathways.
The tegmentum is one of the most colourful areas of the brain.
It contains the substantia nigra (“black
matter”) and the red
nucleus, two structures that are involved
in controlling voluntary movement. Other groups of cells in the
mesencephalon project
their axons diffusely into large areas of the brain and influence
a wide variety of functions, such as consciousness, mood, pleasure
and pain.
Caudal to
the mesencephalon lies the metencephalon, which is the rostral
portion of the hindbrain and differentiates into two major
structures: the cerebellum and
the pons. The cerebellum arises
from the thickening of the tissue covering the lateral walls
of the neural tube at this location. The two masses thus
formed ultimately fuse dorsally to form the cerebellum. During
this time, a swelling develops on the ventral side of the
metencephalon and forms the pons. This structure is an important
information pathway between the brain, the cerebellum, and
the spinal cord.
In the the myelencephalon (the caudal portion of the hindbrain)
the changes are less spectacular. The ventral and lateral
regions of this structure swell to form the medulla
oblongata. Along the ventral aspect of the medulla,
the two medullary
pyramids will also develop, formed by the passage of
the corticospinal bundles responsible for voluntary movement.
Lastly, the central canal, which persists while the medulla
is forming, becomes the fourth ventricle.
The entire portion of the
neural tube that lies caudal to the five secondary vesicles
becomes the spinal
cord through a fairly direct process of differentiation
consisting in the thickening of the tube walls. This thickening
gradually reduces the diameter of the neural tube until it
becomes the very narrow spinal canal .
As the cross-section shown here illustrates, the cell bodies
of the neurons in the spinal cord are concentrated in the grey
matter at the centre (the butterfly-shaped area),
while the white matter at the periphery is
composed of bundles of axons.
The grey matter of the spinal cord is in turn divided into the dorsal
horn, which receives sensory inputs, and the ventral
horn, whose neurons innervate the skeletal muscles.
Likewise, within the white matter, there develop dorsal
columns composed of sensory axons that ascend to the
brain and lateral columns composed of corticospinal
axons that descend to transmit signals for controlling movement.
Between the dorsal and ventral horns, a large number of interneurons
also develop that are involved in various types of reflexes as
well as in establishing networks that perform initial processing
of the information received in the spinal cord.
