Lecture 5 - Prenatal Development Flashcards
Remember
Human development is a continuous process beginning with fertilisation and continuing throughout pregnancy, birth, childhood, adolescence, adulthood and into old age
Conception
- Ovulation: an egg cell from the ovary enters the fallopian tube at 9-16 days of the menstrual cycle
- Fertilisation usually takes place in the upper third of the tube, within 24 hours after ovulation
- 24-36 hour after fertilisation Male (sperm) and female (egg) chromosome material unite
- Egg cell divides for the first time
- 36 hours after fertilisation: 2 cells
- 48 hours after fertilisation: 4 cells
- 3 days: a cluster of 16-32 cells
- 4 days: a hollow ball of about 100 cells
- 4-5 days: zygote enters the uterus
- 6-7 days: zygote begins to attach to the wall of the uterus
- 12-14 days: zygote is completely implanted in the uterine wall
Prenatal development
Conception to birth: 40 weeks
3 stages
1. Zygote (up to week 2)
The cell at conception -> divides repeatedly -> cells specialise
- skin, hair
- sensory organs
- nervous system
- muscles
- digestive, respiratory, circulatory, excretory systems
2. Embryo (weeks 2-8)
Human features develop rapidly -> sexual development -> susceptible to damage -> flexible responses
3. Foetal (from 2 months)
From bone cells to birth -> organs fully develop -> premature birth from approx 27 weeks gives a fir chance of survival (viability), but the rates of foetal development vary
Zygote stage
A zygote is s singled cell embryo
The zygote contains 46 unique chromosomes it’s the entire genetic blueprint of a new individual
Enters a period of rapid cell division (replicating into over 100 cells that begin to specialise)
Develops into an embryo
Embryo stage
From 2 to 8 weeks in the womb
Development occurs at a rapid pace
Critical period of susceptibility to ‘damage’ (teratogens) - most women don’t find out they are pregnant until this stage and some don’t find out until after
Crucial development in terms of organs formation, sexual development
-Organ formation -
Chemical reactions drive the formation of organs:
- only 18 days after fertilisation, the embryo’s heart appears
- only 3 weeks and 1 day after fertilisation - the heart begins to beat
- by 4 weeks, the heart typically beats between 105-121 times a minute
- but the end of this stage (8 weeks) the major features can be defines as: arms, hands, fingers, legs, toes, head, eyes
- brain and spinal cord-
Early emergence
The site of future brain development is first recognisable with the appearance of the neural plate (day 18 of digestion). Cells are differentiated into those that will become
- forebrain - cerebral hemispheres, thalamus and hypothalamus
- midbrain - superior and inferior colliculi, substantia nigra
- hindbrain (medulla oblongata, pons)
3 weeks - neural plate thickens first at the head end of the embryo and folds into the neural tube which will form the brain and the spinal cord
-Brain development-
Cells are continually born (neurogenic is) within the neural tube
These cells migrate to where they will function and specialise
Neurogenesis and migration continue until 6 month of pregnancy - 10,000,000,000 nerve cells in the brain in the tube from a single layer of identical cells in the neural tube
The hierarchy of control systems in the nervous system becomes more organised and complex as prenatal brain development continues
- movement -
Though a pregnant woman does not typically feel movement for at least another 10 weeks, the embryo begins to move between 5-6 weeks
Movements are essential for the normal development of bones and joints
This also mean the embryo can begin to move reflexively (e.g. reactions begin)
- sexual development -
Chromosomes: XX=female, XY=male
A gene on the Y chromosome initiates chemical processes for the development of testes (medulla develops into a testes). If no Y chromosome then cortex develops into an ovary
The Y chromosome initiates development of testes, which in turn begin to secrete sex hormones called androgens (the most important of which is testosterone_
Foetal stage
From 2 to 9 months in the womb
- brain development continues -
In the first 2-3 months gestation cerebral cortex develops a little
Early foetal movements mostly reflexive - often controlled by midbrain
Cortex begins developing around 9 weeks and expands rapidly, covering the whole brain by mid pregnancy
By 6 months, the rapid cell proliferation causes infolding in the cortex
- increases surface area
- development of suicide valleys and gyri ridges
- frontal, parietal and occipital lobes differentiated
By 27 weeks gestation the number of cerebral cells mature
Additional volume comes from increases in:
- cell body size
- proliferation of dentritic spines (synaptogenesis)
- myelination of nerve fibres (beginning in month 6 of gestation, continuing during childhood and even to 30 years of age)
- insights in the foetal world-
The foetus swallows by week 9 and sucks the thumb - full sucking reflex much later
- by 9 weeks the face,palms of the hands and soles of the feet can sense light touchy
- 10 weeks fingernails and toenails are growing
- most of the foetus is touch-sensitive by 13-14 weeks
- by 14 weeks a foetus touched near rhe mouth shows the same rooting reflex newborns use to find food
- by 14 weeks clear gender differences emerge as female foetuses move their jaws more than males
- from about 20 weeks the foetus hears and responds to a growing medley of sounds
- by 24 weeks of gestation foetuses can learn - responding to their environment and habituating to repeated auditory stimuli
- by 25 weeks the foetus responds to taste
~ sweet substances placed in the amniotic fluid increase swallowing
~ bitter substances decrease swallowing and may alter facial expression
-Age of first foetal behaviours-
- just discernible movement = 7 weeks
- startle = 8 weeks
- hiccup = 9 weeks
- Foetal breathing movements = 10 weeks
- hand-face contact = 10 weeks
- yawn = 11 weeks
- sucking and swallowing = 12 weeks
- rooting = 14 weeks
- eye movements = 16 weeks
-later in foetal time-
13 weeks continually moving
34 weeks - distinct patterns of rest and activity
- quiet sleep: 20-30% of timeless motionless, steady heartbeat, rhythmic breathing
- active sleep: lots of different body movements, eyes moving and sometimes open; heart rate and breathing rates irregular and responsive to stimulation
38 weeks (term) - less active sleep, more inhibitory pathways therefore less movement, longer periods of deep sleep, cyclical chnages of rest and activity (linked with mothers physiology)
Studying prenatal development
Ultrasound (ultra-sonography)
- low cost, good availability,superb safety profile
Foetal heart rate (FHR) insights
- if a foetus responds to external stimulus there will be a chnage in FHR
But most of our insight comes from newborns
De Casper et al (1994)
Foetal reactions to recurrent maternal speech
- pregnant women recited a short rhyme (called the target), aloud each day between 30-37 weeks of pregnancy
When newborns heard the target their heart rate was lower than when mother read a control rhyme - more relaxed
Researchers conclude that a level of foetal learning has occured
Hepper (1991)
Foetal reactions to exposure to television theme tunes
- babies of mothers who has heard the sound tracks during pregnancy showed a more calm reponse than those who hadn’t
- newborns exhibited chnages in their heart rate and number of body movements (reduction)
Insights from newborns
Testing newborns may reveal components of development from the prenatal phase (called foetal learning)
For exmaple:
- n if a newborn responds to a stimuli they hear/experienced in the womb could this provide some suggestion that they have ‘learnt’ anout this stimulus?
- this is the notion fo exposure learning
Prenatal sensations
Touch
- first online: lips precursor to rotting, palms grapes, plantar grasp
Chemosensory: taste and smell
- receive substances that the mother has consumed/inhaled/touched direct from her blood to theirs as well as by swallowing and inhaling amniotic fluid n
- before and after birth preference for sweet rather than bitter
- some preferences learned in utero
Vestibular:
- balance and spatial orientation
- sensed by semi-circular canals in inner ear
- righting reflex by 25 weeks
- head down birthing orientation
- sensed by semi-circular annals in inner ear
- important in changing arousal sates prenatally and postnatally
Visual
- prenatal development of strcutre of visual system
- little visual stimualtion: eyelids fused until 5-7 months digestion
- visual pathway: connects rides and cones in eye to the brain
- by 15 weeksoptic nerve fibre crossing over complete
- 7 months; BEGIN OPENING EYES AND BLINKNG, some vision, light/dark differentiation
- 30 weeks: premature infants can see high contrast pattens close up
- following brith is when nthe visual system really develops
Auditory
- prenatal development begins around 6 weeks. By 14 weeks the vestibular system begins working
- cochlear function begins around 24 weeks. But it still immature
- sounds include maternal heartbeat and voice - some evidence of presence for prenatal version of compared with postnatal once baby is born
- preference for maternal language over foreign language
Susceptibility to damage
Despite complexities, most babies are born healthy
However, brith defects are known to occur in approx 3-5% of all newborns
Teratogens = substances which, when ingested by the mother, can adversely affect her unborn child
Sometimes complications occur that are not to due teratogens (e.g. heridatry complciations , miscarriage etc)
Mother’s diet while pregnant
Do you think what the mother beats during pregnancy has consequences for prenatal development?
Exmaple:
In The Gambia, the amount of left green vegetable in the other (and possible the father) are eating around the time of your conception has life long consequences
Up until the age of 15 there’s no discernible difference between the children, but much later on difference in death rates are huge
The Dutch Famine Study
- the Dutch famine birth cohort study - the effects of prenatal exposure to famine on later health
- N = 2414 babies brown alive between 1943-47
- investigated since 1996
Results:
- 2 x more liekly to develop heart disease
- more pone to sz, obesity, diabetes, cancer and stress-related illnesses
- evidence that the effects persisted in to the next generation
Specific types of teratogens
Diseases/infection:
- rubella (German measles)
- paediatric AIDS
- HIV
-n herpes virus
- syphilis
Medicine/drugs:
-legal and illegal drugs
- Asprin
- nicotine
- alcohol
Teratogens: timing of exposure
Critical period = time when a particular organ or othwr body part of the organisms is the most susceptible to teratogenic damage
Zygote: before implantation teratogens rarely have AN IMPACT. IF THEY DO THE TINY MASS OF CELLS USAULLY DIE
Embryonic: time when serious defects are most common because the foundations of all body parts are put in place
Foetal: damage is usually minor. However, organs such as the brain, eye, genitals can still be strongly affected
Effects of smoking on a foetus - Reissland (2015)
Method:
- new technology - 4D scan
- observed 30 scans of 20 foetuses
- at 4 intervals between 24 and 36 weeks
- 2 groups of mothers - smoker (N=4), and non-smoker (N=16)
Results:
- all infants born healthy, however infants from smoking mothers displayed higher rate of mouth movements compared to control gorup
- suggests that foetal central nervous system did not develop at the same rate, therefore behvaiour l;ess mature
- evidence of speech delay in infants pre-exposed to smoke
Foetal alcohol syndrome
A disorder of permanent brith defects that occurs in the offspring of women who drink alcohol during pregnancy
It is unknown whether amount, frequency or timing of alcohol consumption during pregnancy causes a difference in degree of damage
Main effect - central nervous system damage (especially brain damage)
Common features of children with FAS are over activity, impaired motor control, attention deficits, memory problems, language impairment
- FAS facial features -
Discriminating features - short palpebral fissures
- flat mid face
- short nose
- indistinct Philtrum
- thin upper lip
Associated features - epicanthic folds
- low nasal bridge
- minor ear abnormalities
- microgtnathia
Summary
There are genetic, environment and epigeneitc influencers on human development
Pretreat development has periods of qualitative and quantitative growth
Human newborns are born with a survival kit of reflexes and behaviours that form the basis of later cogntive,social and physical developments
They are however immature and need suppport from caregivers for am extensive period of time in order to flourish
