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How the mind develops
From Embryo to Ethics

Help Link : comment se développe un embryon ? Link : Human Reproduction and Development Link : Advanced Fertility Center of Chicago
Link : Fertilization: Sperm/Egg Recognition and Contact Link: Preimplantation

The term embryo refers to the earliest stage of development of a human being, corresponding roughly to the first two months of pregnancy. After that, until the pregnancy ends, the future human being is called a fetus. The fetus has all of the organs of the human body, in rudimentary form.

From a medical standpoint, the duration of a pregnancy is calculated from the first day of the last menstrual period. Actual fertilization does not take place until 14 days later.

Link : Développement de l'embryon et du fœtus Link :  Périodes de development

Human development is remarkable in that it is the product of a highly sophisticated genetic program and a particular environmental setting. It is the combination of these two factors that makes each of us different from everyone else, and that includes identical twins.

The first stage in the development of a human being, the embryonic stage, begins with fertilization. Fertilization generally occurs in the first third of the fallopian tube, the canal that connects each ovary to the uterus. In fertilization, once one of the spermatazoa (sperm cells) has penetrated the ovum, or oocyte, it becomes impenetrable to all the other sperm cells.

Once fertilization has occurred, the primordial cell, called the zygote, migrates to the lining of the uterus. While doing so, this cell undergoes successive divisions, soon forming an embryo with 2 cells, then 4, then 8, and so on.

a) 2-cell stage; b) 4-cell stage; c) 8-cell stage; d) and e) morula stage

The first three-dimensional structure that emerges from these cell divisions is a sphere of cells. The term morula is used to designate the ensuing stages of embryonic development (16, 32,and 64 cells). The morula is thus the product of the first cell cleavages, which result in practically no growth, because the daughter cells become smaller and smaller.

The morula is like a solid ball. But after the 64-cell stage, this ball develops an inner cavity, called the blastocoele, thus becoming a blastula. The blastocoele is bound by a single layer of cells. It is during the blastula stage, about 7 to 8 days after fertilization, that the embryo becomes implanted in the uterine wall.

Some cells of the blastula soon being moving toward the interior of the blastocoele to form distinct layers that will be redistributed as the blastula continues to invaginate (fold inward) in the next developmental stage, known as gastrulation.

The blastula becomes the gastrula when the invaginated cells have formed the ectoderm and the endoderm.

The stage that follows gastrulation is called neurulation, and it is the process of neurulation that initiates the formation of the entire nervous system.

In pregnancy, the placenta develops from the membrane surrounding the fetus and the uterine lining. Attached to the wall of the uterus, the placenta is like a spongy cake that supplies the fetus with nutrients and oxygen through the umbilical cord. The placenta also enables the fetus to eliminate its metabolic waste products and pass then out into its mother’s bloodstream.

In addition, the placenta secretes a number of hormones, including progesterone, estrogens, lactation-promoting hormones, and a hormone called chorionic gonadotropin that is found in the urine of pregnant women and that is the basis for pregnancy tests.

But even as it enables all these intimate exchanges between the mother and the fetus, the placenta also prevents their blood from mixing. The placenta thus acts like a sort of border patrol, preventing most types of germs from crossing over from the mother to the fetus. But the mother’s antibodies, and any drugs that she may take during pregnancy, do cross the placenta. If the drug is a medication such as an antibiotic, it may be helpful, protecting the fetus from infection. But if the drug is alcohol or some kind of street drug, it could have negative effects on the baby’s development.

Lien: Dis-moi, quelle vie mène le fœtus ? Lien :  Infected with Insanity


Link: The trilaminar germ disk (3rd week) Link: Développement précoce du système nerveux: différenciation du tube neural et des crêtes neurales Link : La différenciation des cellules de la crête neurale

The closing of the neural tube is a crucial event in the development of the nervous system. This event in turn depends on a sequence of events that affect the position of the cells and the processes of adhesion between them. When the neural tube fails to close correctly, serious birth defects can result.

One of the best known of these is spina bifida, which occurs in about 1 of every 1000 births. It is caused by a malformation of the caudal portion of the neural tube. This malformation in turn results in a malformation of the lower vertebrae that often leaves the spinal cord exposed, makes it vulnerable to injury, and limits use of the legs and feet.

Spina bifida appears to be associated with a deficiency of folic acid. This vitamin should be available in sufficient quantities in the pregnant mother’s food, but if her diet is poor or imbalanced, the resulting shortage of folic acid can be serious enough to interfere with the formation of the neural tube.

In the opposite condition from spina bifida, the upper portion of the neural tube remains open. The result is a birth defect called anencephaly, which is quite serious too. In this condition, the organization of the major structures of the brain is greatly disturbed.

Link : Spina Bifida Link : Definitions and Descriptions : Spina Bifida Link : Spina Bifida Link : Information On Spina Bifida


Just as studying the evolutionary origins of the human brain can teach us much about its anatomy, studying how the nervous system develops over an individual’s lifetime gives us a better understanding of how this system is organized.

The formation of the nervous system occurs fairly early in embryonic development and is referred to as neurulation. An important structure that appears at the end of the preceding stage (the gastrulation stage) is the dorsal cord, or notochord. This cylinder of cells in the mesoderm defines the embryo’s rostral-caudal axis and extends along its entire length.

Around the third week of gestation, the notochord sends a molecular signal that causes the cells of the ectoderm just above it to thicken into an individualized epithelial column, the neural plate. After this “neural induction”, the neural plate begins to invaginate to form the neural groove, which then rises from the embryo’s surface and closes to form the neural tube.

On the dorsal side of the neural tube, another special population of cells is distinguished where the neural tube protrudes, whence its name, the neural crest. These cells will eventually migrate along specific pathways that will expose them once again to various inductive molecules. Ultimately, these cells will differentiate to form structures such as the spinal and vegetative ganglia.

On either side of the neural tube, the mesoderm thickens and divides into structures called somites. These are the precursors of the axial musculature and the skeleton. The part of the neural tube in the vicinity of the somites will form the future spinal cord. The rostral end of the neural tube will close and continue to grow to form the various structures of the brain.


Link : Stages of Brain Development Link: Embryonic period
Original modules
Tool Module: The Connection between Ontogeny and Phylogeny The Connection between Ontogeny and Phylogeny

The body’s first movements begin during fetal life. They consist essentially of reflex movements such as sucking and grasping and spontaneous movements such as stretching. The reason is that the first parts of the brain to become functional are mainly subcortical structures. These structures developed earlier in the course of evolution and are responsible for stereotyped movements such as the reflexes.


The stage in which the more elaborate structures of the brain develop from the neural tube is called differentiation.

The first structure to appear is an embryonic brain composed of three primary vesicles. During the seventh week of development, two of these vesicles themselves divide in two, so that there are then a total of five secondary vesicles.

The rostral part of the prosencephalon produces two lateral buds that grow into the telencephalon—two large vesicles that will ultimately become the cerebral hemispheres. The posterior part of the prosencephalon forms the diencephalon, which will comprise the thalamus, hypothalamus, pituitary gland, pineal gland, and retina.

The middle member of the three primary vesicles, the mesencephalon, does not subdivide. It evolves more slowly and ultimately forms such structures as the tegmentum and the superior and inferior colliculi.

The most caudal of the three primary vesicles, the rhombencephalon, elongates rapidly. As a result, it must bend ventrally, forming the pontine flexure. This flexure divides the rhombencephalon into a rostral portion, the metencephalon, which will become the pons and cerebellum, and a caudal portion called the myencephalon, which will become the medulla oblongata.

These five subdivisions constitute the precursors of the primary functional divisions of the brain that will develop subsequently.

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