DEVELOPMENT OF THE NERVOUS SYSTEM:Teratology studies, Aristotle

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Neurological Basis of Behavior (PSY - 610)

Lesson18

DEVELOPMENT OF THE NERVOUS SYSTEM

Objective

To familiarize the students with:

· Various stages of neuronal development.

· Development of the brain: from the fertilization to the various developmental stages in-utero,

and postnatally.

· Cell migration, (inside-out), cell competition, Cell death, growth Cones, Nerve growth Factor

and its role, Influences in growth and development of the brain

Intrauterine Development of the Brain:

Genes determine the growth and development from fertilization to death. Therefore the programmed

direction of development would continue within the intrauterine development. The Intrauterine

environment is stable and protection is provided to the embryo/fetus, by the placental barrier. However,

if even a minor alteration occurs it can lead to serious defects in the process of the development of the

body and the brain. Serious malformations can take place. The study of the effects of substances which

affect normal development is known as Teratology.

Teratology studies: The effects of Poisons, drugs, X-Rays, radiation, abnormal blood conditions of the

mother, excess/lack of vitamins, proteins deficiency, psychological stress which can affect the

intrauterine environment by crossing the placental barrier, and thereby altering the normal course of

development. The timing of exposure is important. There are critical periods (highly responsive) in

which the embryo/fetus can be affected by growth stimulating influences and at the same time is

vulnerable to disruptive influences.

Some of the deformities are listed below:

· Anencephaly: (lack of brain)

· Harelip, Cleft palate

· Phocomelia (seal like flippers: extremities)

· Reversal or ambiguity of sex

Development of the Nervous System

As has been said before we must remember, that genes determine growth and development throughout

the life span. However, there is an interaction between the genetic programming and the environment.

It appears to happen from the time of birth; however, developmental neurobiological researches have

shown that the intrauterine environment is extremely important. The influence of the immediate

environment is important even during this cushioned state in the womb, and the effects are in some

cases irreversible. Intrauterine environment is important at every phase of intrauterine development

Stages of development and influences:

Immediately after embedding (implantation) of the fertilized egg onto the uterine wall Takes place,

three germinal cell layers are formed from the increasing number of cells that have been dividing. All

three germinal layers are exactly the same. The nervous system develops from one of these layers.

These are known as the Ectodermal, the mesodermal and the endodermal cell layers. The Endodermal

layer eventually forms the internal organs, Ectodermal layer forms the Brain cells and the Nervous

system, and the Mesodermal forms the skeletal muscles etc. How do these layers become different and

develop in different direction would be discussed in the later section

Neurological Basis of Behavior (PSY - 610)

Development of the body and the brain

The development of the organs from the endodermal, brain from the ectodermal and muscles etc from

the mesodermal tissue begins immediately after differentiation takes place. The cells in each layer

multiply and the organs, muscels etc start being visible

At the Embryonic stage which is the 1st two months in gestation, the heart beat starts in 1st month, and

other organs such as kidneys start functioning around 3 months.

At the Fetal stage which is between the 2nd months to gestation to 9 months (immediately before birth)

the skeletal motor functions growth and development is initiated and after 5 months there is actual

movement of the muscles (and connection s being formed with the appropriate brain areas).

Immediately at birth the Neonate has functional lungs but these are filled with embryonic fluid. The first

time the little organism breathes is at birth.

Stages of development (from V. Casagrande 1988)

There have been philosophers, early scientists and thinkers who have tried to unravel the mystery of

birth and the formation of the newborn. This process has now moved form mere speculation to scientific

research and evidence. We would talk about three important milestone names only: Aristotle, Van Baer,

and Haeckel.

Aristotle: studied the development of the chick brain and gave the concept of preformation- animals

were preformed into the shape they were born. The zygote according to Aristotle is a mini individual

that grows from egg into shape and form of the animal.

Van Baer (1792-1876) Pre Darwinian evolutionary theorist provided evidence against preformation.

Since he saw similarities in the various animal embryos he studied, he stated that young stages of higher

animals resemble young stages of lower animals. That is to say that all embryos have the same shape

and form. He further said that all animals higher up on the evolutionary scale go through the various

developmental stages of the animals lower on the evolutionary scale.

Ernst Haeckel gave the famous concept of ontogeny recapitulates phylogeny, appears a logical

progression of Van Baer's premises. During early growth and development (ontogeny) go through the

same series of forms which resemble with the biological and evolutionary ancestry (evolutionary

history phylogeny). If we look at the figures without identifying the embryo we would have difficulty

naming the human embryo in its stages from other embryos.

We also find that Baer's was correct in his assumption that there are no pre-formations.

Further, those general features of all animal embryos appear earlier than special features, which would

explain the common shape and form of all embryos. During the later embryonic periods, these depart

more and more towards specialized development. The similarity a) the embryonic gill pouches in

mammalian as well as fishes, though these are not functional in mammalians, b) brain development is

also similar till specialized development takes place (i.e. specialized development of the cerebral

cortex).

Before we begin to understand the development of the brain, we must know the basic concepts of the

neuronal development.

Neurological Basis of Behavior (PSY - 610)

Concepts in development of neurons/brain

Embryogenesis: is defined as the process of development during embryonic stages: the processes

which take place during embryogenesis are: induction, neurulation, vesicle formation, and neural

proliferation,

Histogenesis: This is the process where neurons (cells) specialize and move to relevant

neuroanatomical sites: cellular differentiation, determination, cellular maturation, cell migration, cell

aggregation, cell competition, cell death

Growth: Axonal growth and synapse formation

Differentiation and Induction. These are key changes and important stages in all vertebrate embryos.

At all stages of the development-(structural, functional or behavioral) there is a complex interaction of

genes and environment. The genes predetermine layers, and shape and form they eventually take.

However, environment is also very important, as it provides the trigger for the genetic program take

place.

As has been said before all three germ cell layers all exactly the same, this is called being, Totipotential

i.e. each layer has the potential to develop/ become any one of the layers, these are the STEM CELLS.

If cell layers rearranged, the top layer would always become brain tissue. Within the three layers rapid

cell division/ multiplication is taking place within the layers, and would continue to do so, if the trigger

for change does not occur (imagine having just three flat layers of membrane growing in size, not

becoming the brain or other organs!). Thus, differentiation takes place and this places restriction on

direction of growth--specialization begins.

In the three germ cell layer sandwich, the Ectodermal tissue always lies on top of Mesodermal tissue

which lies on top of Endodermal layer. The top one would always be the brain. We can shuffle the

layers- and the layer on the top would become the brain cell layer. So far there is one stimulation for the

growth- the position of the cell layer relative to the others. However the next major steps would change

the fate of the cells forever--- Differentiation and induction.

Differentiation takes place when a rod shaped tissue of Mesodermal layer forms under Ectodermal

layer. Attaches itself stretching from head to tail of the embryo, this is the Notochord

Induction: is the process which signals change (induces change), leads to cell determination (cells fates

are determined) leads to differentiation (cells develop in different directions). Thus, this induces

development of 3 cell layers by differentiation.

NOTOCHORD INDUCES ECTODERMAL TO BECOME NEURODERMAL

Notochord is the signal for Ectodermal tissues to start differentiating and to become nervous system,

and initiate development of nervous tissue

Notochord very important, without it the Ectodermal tissue cannot differentiate

If placed elsewhere these tissues will start dividing and become nervous tissue, if nervous tissue is

placed instead of the internal organs, the layer would become internal organs. The location of the cell

layers with reference to notochord is the important key to differentiation. Thus, the Ectodermal tissue

cell layer needs environmental signal and stimulation.

"In all cases it acts to limit or specify the developmental opportunities of one group of cells through

their interaction with another". When the initial division of cells is differentiated, the change is induced

by the notochord, the fate of the cells and their future is determined.

Neurological Basis of Behavior (PSY - 610)

Cells would grow in only the direction determined (by biological and genetic mechanisms) but the

stimulation was needed, and as we shall see in the later sections would continue to be affected by

external environment.

References

Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors

(3rd edition) Worth Publishers New York

Bridgeman, B (1988) the Biology of Behaviour and Mind. John Wiley and Sons New York

Brown,T.S. And Wallace. (1980) P.M Physiological Psychology

Academic Press New York

Note: References #2, 3, and 4 are followed most closely, as they have been used in teaching as well;

further individual references/pages are also given on the power points of each lesson

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