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How the mind develops

From Embryo to Ethics

Help Link : La fabrication des spermatozoïdes Link : PHYSIOLOGIE DE LA REPRODUCTION Link : Fetal development : first trimester
Link : Fetal development : second trimester Link : Fetal development : third trimester Link : La gamétogénèse Link : La Fécondation
Link : L'appareil reproducteur mâle et femelle Link : Toute l'histoire de l'Evolution en accéléré... Link : La fécondation Link : Sex and Death
Link : The Ark of Life: The Germ Line Link: Spermatogenesis Link: Oogenesis Link: Fertilization
Original modules
Tool Module: Sexual Selection and the Theory of Parental Investment Sexual Selection and the Theory of Parental Investment

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.


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.

Next, the zygote divides into 2 cells, then 4, then 8, then 16, and so on, until a spherical mass of cells has formed. This first stage of development is often called segmentation. Gradually, a cavity develops at the centre of this ball as it exits the fallopian tube to implant itself in the wall of the uterus. Only then do the cells form into the three layers from which all of the body’s organs and systems, including the nervous system, will develop.

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.

Link : Mieux comprendre les jumeaux Link : Twins FAQ Link : Twins - identical and fraternal Link : Differential gene expression and development


Link : Brain Development

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.


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.

Defects in the closing of the neural tube at this stage can have dramatic consequences for the baby once it is born.

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.

Inside the neural tube, the cells continue to proliferate at a rate that varies along the length of the tube, depending on what future brain structure is forming at any particular point; the cortex, for example, develops later than some other structures.


Original modules
Tool Module: The Connection between Ontogeny and Phylogeny The Connection between Ontogeny and Phylogeny

The structure of the forebrain that has developed the most in the course of human evolution and that also displays the most impressive growth during gestation is unquestionably the cerebral cortex. Its neurons are the seat of language, conscious sensory perception (vision, hearing, etc.), and voluntary movement.

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.

Link : What Is Brain Plasticity?


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.

Once these three primitive brain regions have been laid down, two of them will begin to subdivide to form the five major subdivisions of the brain.

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