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How the mind develops

Help Embryological Development of the Human Brain Développement embryonnaire de l'homme Les déterminants cytoplasmiques
CLIVAGE - BLASTULATION - GASTRULATION Le comptable de l’évolution, entretien avec André Langaney

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.

Link : PROTEIN SEQUENCES: NOT SO PREDICTABLE AFTER ALL Link : Comment Les Protéines s'Epissent-Elles ? Link : New T-cell epitope math Link : Predicted proteins are found in rat brain
Link : Organisation de l'ADN Tool : Les gènes homéotiques

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.

Link : Ectopic pregnancy Link : La grossesse ectopique

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.

Link : Fertilization Link : First Cell Division, Blastomeres

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.

Link : Early Blastocyst

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.

Link : Implantation Begins Link : La fixation de l'embryon Link : Implantation de l'embryon: faits nouveaux Link : Implantation Complete, Placental Circulation Link : Implantation

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

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.

Link : GASTRULATION Link : Gastrulation, Chorionic Villi Formation Link : Neurulation and Notochordal Process Link : La reorganisation cellulaire: le troisième feuillet


Evolution de l'embryon L'organogenèse ( mise en place des organes) Les Etapes du Développement d'un Amphibien Gastrulation and 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.

Link : Stage 14 Link : Stage 15 Link : Stage 16 Link : Neural Folds Begin to Fuse, Heart Tube fuses

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.


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.

Link : Primitive Pit, Notochordal Canal and Neurenteric Canals Link : La neurulation Link : Formation de la notochorde

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.

Link : Neural Folds Begin to Fuse, Heart Tube fuses Link : Neural Development

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.

Link : Appearance of Somites Link : Thirteen to Twenty Somite Pairs


Stages of Brain Development Développement général du système nerveux central: formation des vésicules et des courbures cérébrales
Original modules
Tool Module: The Connection between Ontogeny and Phylogeny The Connection between Ontogeny and Phylogeny

The encephalon begins to form when the neural tube swells and subdivides, first into the three primary vesicles (the prosencephalon, mesencephalon and rhombencephalon), and then into the five secondary vesicles (the telencephalon, diencephalon, mesencephalon, metencephalon and myelencephalon).

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.

Link : brain development (week 5-9)


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.

Link : brain development (week 9-16)



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!

Link: Embryologie de l'oeil et de ses annexes


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.

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