Prenatal development
Weird science
Scientific research is based on WEIRD (Western, Educated, Industrialized, Rich, Democratic)
science, this is also the case for more basic processes, like brain and motor development.
How we perceive things isn’t biologically determined (Müller-Lyer illusion). We know little
about development in non-WEIRD individuals. We should therefore be careful with
conclusions based on WEIRD research.
What is development?
Development: specific type of change. Development is:
1. Qualitative: refers to changes in what there is (not how much change). For example:
it’s not about how many cells, but what type of cells.
2. Sequential: over development, some changes proceed others. They need to proceed
them, before other development takes place.
3. Cumulative: one developmental stage can build up on other developmental stages.
4. Directional: can be progressive or regressive. This means that during development,
things can be built up, but can also be broken down.
5. Multifactorial: there is not one factor determining the course of development.
6. Individual: developmental pathways are unique for each human being. For example:
milestones are reached at different ages, in different orders, or not at all. These
milestones help people develop.
Prenatal development
Cell division: two types:
- Mitosis normal cell division
Each cell contains a full complement of genetic material
(instruction manual) that tells the cell what to be and what to
do. This information is described in our genes, and these genes
are organized in 23 pairs of chromosomes (total 46).
During mitosis, all the genetic material is copied, and then
separated into two full sets, so when the cells split up, they
each have a full set of genetic information.
- Meiosis special form of cell division that creates sex cells.
During meiosis, after the genetic material is copied, the dad-
mum pairs of chromosomes first exchange genetic information
between each other in a process called “crossing over”, basically
mixing up between them. And only then make a first split after
which they split again ending up with four new cells that each
contain a different combination of genetic code. Each of these
four new cells contain only half of the blueprint instead of the
whole. The other half of the genetic blueprint will be delivered by the other gamete
when the egg cell is fertilized by a sperm cell.
This process is the basis for genetic variation, and therefore the genetic basis for
individual differences. It can help us understand the interactions between nature and
nurture.
Monozygotic twins come from a single zygote (fertilized egg cell), so they share the exact
same genetic material.
,Dizygotic twins developed from different zygotes, so they are not genetically identical.
When an egg cell is fertilized, it starts to develop into a full-sized baby. There are four
developmental processes that are crucial for this development:
1. Normal cell division, mitosis, after which you have two identical cells. After the egg
cell is fertilised the zygote starts to divide at a very rapid pace.
2. Cell migration. All the cells that are created to need to go to their designated
positions in the body. Some of the cells are simply pushed away by the new cells that
are formed during cell division, but other cells actively migrate to new positions. This
is the case for brain cells. They first create a short protrusion which they use to pull
themselves into a new position.
3. Cell differentiation. The fertilised egg cell, the zygote, starts to divide rapidly. Soon
there is a lump of cells called the morula. At first all the cells in this lump are the
same and they can become anything, they are not yet specialised (totipotent). While
the number of cells increase in the morula, the total
size of the lump of cells does not change. This means
that pressure is created and this increasing pressure is
probably the trigger for the first type of cell
differentiation. Some cells become blastocyst cells that
later become part of foetal part of the placenta and the
other cells become the inner cell mass cells. These cells
are no longer totipotent, but they're pluripotent: they
can become any type of tissue in the human body.
what they will become, how they will differentiate
depends on the neurochemical signals they receive
during development and once they are specialised into
their final form they cannot change anymore. Cell
differentiation is extremely important for development
4. Apoptosis (programmed cell death). It is an integral part of development.
There are a lot of influences on the foetus while it develops in the womb. There is a lot of
stimulation from the outside world:
- 10 weeks: sense of pressure (muscles, joints, skin)
- 13 weeks: detection of movements. If the mother moves around especially when the
foetus is still small, it's sloshing around in the uterus.
- 20 weeks: detection of light even before they could open their eyelids.
- 26 weeks: detection of sound.
- 26-28 weeks: detection of smell and taste.
The foetus learns from all these experiences. During development, the foetus is far from
passive:
- 5-6 weeks: bending of head and spine.
- 8-9 weeks: startle like movements (hiccups or burping).
- 10 weeks: variety of limb movements.
- 10-11 weeks: head movements, breathing movements.
- 11-12 weeks: yawn, suck, swallow amnionic fluid.
- 14 weeks: non-random movements.
- 20 weeks: move all parts of their face.
- 25 weeks: opening and closing eyes.
,All this movement often has an important function in development.
Function of foetal movement:
- Swallowing: oropharyngeal cavity (cavity at the back of throat), lungs, digestive
system.
- Body movements: muscles, bones, joints, skin.
Teratogens harmful influences from the environment on the developing foetus (stress,
prescriptive drugs, heroin, nicotine). Timing, duration and amount of exposure are important
determinants of the severity of the effects on development, as well as genetic susceptibility.
The foetus is most vulnerable for the negative effects of teratogens during early
development when the major organs and structures of the body are forming. Each organ of
an embryo has a critical period during which its development may be disrupted. Even when
the child is still developing in the womb, development is affected by the world around the
child (Bronfenbrenner ecological systems).
, Brain development
Brain is made up of 100 billion neurons.
Cell body (soma) contains the nucleus. Most cell
processes take place here.
Nucleus all genetic material is stored here.
Dendrites receiving ends of the neuron.
Axon sending part of neuron, signals are sent from
one neuron to another. It is much longer than dendrites.
Myelin sheath covers the axon. It has two important functions:
1. Keep the neuron healthy.
2. Speed up signal transduction along the axon.
Axon terminals where the signal ends up and where the signal is transmitted to
other cells (e.g. neurons, muscle cells.
Signals are sent along the axon in a few steps:
1. Resting potential: -70 mV door positief en negatief geladen ionen (binnen
meer negatief dan positief), natrium-kaliumpomp (sodium-potassium
pump) zorgt voor transport ionen
2. Threshold: -55 mV is reached.
3. Action potential (depolarization): voltage-
gated sodium channels are activated,
which makes N+ enter, +40 mV is reached.
4. Repolarization: because the voltage is
now +40 mV, voltage-gated potassium
channels are activated, which makes K+
leave, -90 mV is reached.
5. Refractory period: the sodium-potassium
pump makes sure -70 mV is reached
again.
6. Resting potential.
Openings in the myelin sheath (node of Ranvier) make sure ions can float in and out.
Signals ‘hop’ from one opening to another.
When neurotransmitters bind on
the dendrite, they change the polarity of the cell membrane. When -55 mV is reached, the
process of signal sending starts in the axon.
Neurotransmitters that are left are cleaned up by neurotransmitter deactivation (reuptake or
metabolism).
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