Ch 2: Conception and Pre Natal Development

Conception

Occurs when sperm penetrates an ovum

Gametes

Cells that have only 23 chromosomes

(Most cells have 23 pairs of chromosomes)

At conception, the 23 chromosomes of the sperm unite with the 23 chromosomes of the ovum so that the fertilized egg has 23 pairs of chromosomes in an entirely new cell called a

Zygote

Cells in female

Contain only X chromsomes, all her ovaries carry X chromosomes

Cells in males

Half of the sperm contain X chromosomes, half contain Y chromosomes

The sex of the new individual is determined by

The sex chromosome in the sperm

Genes

A segment of DNA that contains the instructions for making proteins that regulate the structure and function of the body

Humans have 20 000 - 25 000

Genome

The complete sequence of an organism’s DNA

We share 99.9% with all other humans

0.01% contributes to physical/ behavioural differences between individuals

The composition of genes that we inherent constitutes our …

Genotype (not observable)

Allele

A variant of a gene

Influence how our genes are expressed outwardly

Phenotype

Used to describe the set of characteristics that are observable

If an allele is dominant

You only need to carry one copy of it for a dominant trait to be observed

If an allele is recessive

You require two copies (one from either parent) in order for it to be observed

Most of our physical, behavioural and health traits are governed by many genes. These are referred to as

Polygenic traits

When gene expression is contingent upon environmental AND genetic factors then it is likely the result of

Multifactorial inheritance

Single-gene disorder

Occurs in a single gene

Individual inherits a pair of recessive alleles responsible for a disorder

Mutations to the DNA sequence can result spontaneously from errors during cell division or from exposure to environmental toxins

Ex: PKU, Tay Sach, cystic fibrosis, sickle cell

Chromosomal disorder

Occurs when there were disruptions in the cell division process, resulting in structural/ numerical abnormalities

Structural abnormalities

When parts within a chromosome are deleted, duplicated, or inverted

Numerical abnormalities

Happen when one of the 23 chromosome pairs contain only 1 chromosome or an extra chromosome (ex down syndrome)

Multifactorial inheritance-related disorder

Occurs when genetic and environmental effects come together to contribute to phenotype expression (ex type 2 diabetes, heart disease, some cancers and psychological disorders)

Phenylketonuria (PKU)

Range of intellectual disability, eczema, lighter skin due to pigment defects

Treatmented by limiting foods containing phenylalanine; nutritional supplements

Tay Sachs disease

Fatal progressive neurological disorder, muscle degeneration, blindness

No current treatments

Cystic fibrosis

Thick, sticky mucus that clogs the lungs; vulnerability to pulmonary infection

Treatments to manage symptoms/ complications

Sickle cell anemia

Sickle shaped red blood cells; anemia; acute chest pain; pain episodes; organ damage

Treatments to manage symptoms, recently approved cell-based gene therapy

Fragile X syndrome

Mild to moderate learning disabilities; distractibility and impulsivity

Treatment; early intervention, special education, treatment for ADHD

Down syndrome

Range of intellectual disabilities; unique facial features; poor muscle tone; possible heart, digestion or hearing problems

Physical, occupational, speech, and educational therapy; medical intervention as needed

All of these disorders are examples of

Inherited and genetic disorders

Genotype x environment interaction

Genes and environment come together to influence a phenotype

Sometimes the expression (phenotype) of a genotype relies on our environment

Fraternal twins (dizygotic)

Conceived when two eggs are fertilized by two separate sperm (2/3 of twins)

Identical twins (monozygotic)

Conceived when a single fertilized ovum separates into two parts, and each develops into a separate individual (1/3 of twins)

When do couples usually become pregnant

Within one year of trying to conceive without intervention (few within first month)

Male infertility

May be due to hormonal irregularity and blockage within reproductive organs, or, sperm may have low motility (11-12% or makes)

Female infertility

Blocked fallopian tubes, uterine abnormalities, endometriosis and polyendocrine metabolic syndrome (PMOS) (15%)

Treatments to support fertility

Ovulation induction

Intrauterine insemination (IUI): places sperm directly in uterus

Invitro fertilization: fertilized in lab dish, embryo placed in uterus

Surrogacy and adoption

Pregnancy

Physical condition in which a women’s body is nurturing a developing embryo or fetus

Prenatal development/ gestation

Process that transforms a zygote into a newborn

First trimester

First 12 weeks

Begins when zygote implants in uterus lining

Period stops, breast enlargement, cervix thickens, fatigue and morning sickness may start

Prenatal care essential to prevent birth defects (organs form)

Greatest risk of miscarriage

Second trimester

Week 12 to 24

Morning sickness usually disappears, appetite inc

Gains weight, uterus expands, feels fetal movements

Monitoring mother and babies vital functions

Sex of baby can be determined after 13th week

Woman monitored for gestational diabetes

Third trimester

At 25 weeks

Weight gain, abdominal enlargement, emotionally connected to fetus

Fetus has regulatory periods of activity and rest

Toxemia of pregnancy (life threatening) may occur - signalled by sudden inc in bp

Cephalocaudal pattern

Development that proceeds from the head downward

Proximodistal pattern

Development that proceeds from the middle of the body outward

Germinal stage

The first two weeks of gestation (conception to implantation)

Cells specialize and divide rapidly

Day 5-7 cells become a hollow, fluid filled ball (blastocyst)

By 12th day, blastocyst implants in uterine tissue

Embryonic stage

Begins when implantation complete (end of week 2 to end of week 8)

Neurons form neural tube, develops brain/ spinal cord by week 4

Gonads develop in week 6

Organogenesis (organ development) week 3-8

Fetal stage

End of week 8 until birth

Involves refinement of organ systems, especially lungs and brain

Week 9-12

Fingerprints, grasping reflex, facial expressions, swallowing and rhythmic breathing of amniotic fluid, urination, genitalia

Week 13-16

Hair follicles, responses to mother’s voice and loud noises; 8 to 12 cm long, 25 to 100g

Week 17-29

Fetal movements felt by mother, heartbeat detectable, lanugo (hair) covers body, eyes respond to light, eyebrows, fingernails

Week 21-24

Vernix (oily substance) protects skin, lungs produce surfactant (vital to resp function), viability becomes possible although most born now do not survive

Viability

Ability of fetus to survive outside womb

Week 25-28

Recognition of mother’s voice, regular periods of rest and activity, 35-39 cm, 660-1000g good chance of survival

Week 29-32

Very rapid growth, antibodies acquired from mother, father deposited under skin, 39-43 cm, 1.2-1.7 kg, excellent chance of survival

Week 33-36

Movement to head down position, lungs mature

44-48cm 1.9-2.6 kg, virtually 100% chance survival

Week 37+

Full term status, 49 cm, about 3kg

The fetal brain

Pace of neural formation picks up dramatically between week 10-38 (neural proliferation)

Between week 13-21, newly formed neurons migrate to parts of brain where they will reside for the rest of individual’s life

Once reached final destination, neurons begin to develop connections (synapses)

Prenatal behaviour

By week 32/33, fetuses can distinguish between familiar and novel stimuli

Newborns appear to remember stimuli they were exposed to prenatally (mother’s heartbeats, odour of amniotic fluid, stories/ music)

Stable individual differences in behaviour

Very active fetuses tend to become very active children

Less active fetuses are more likely to show greater fearfulness/ shyness as children

Teratogens

A chemical that increases the occurence of structural/ functional abnormalities in offspring if administered to either parent before conception, to female during pregnancy or directly to developing organism

When is greatest risk to teratogens

During the first eight weeks of gestation because most organ systems are developing rapidly during this period

Viruses

Several viruses are able to pass through the placental filters and attack the embryo/ fetus directly

HIV (causes AIDS) is one of many sexually transmitted organisms that can be passed directly from mother to fetus

Teratogens - medications

Aspirin: high doses during early weeks of pregnancy can increase risk for miscarriage and bleeding

Some antibiotics can result in infant hearing loss and incomplete development of teeth/ bones

Some anti seizure medications can result in developmental delays

Antipsychotic medications inc risk of heart defects

Why are effects of substances (prescription/ non prescription) on prenatal development challenging to determine?

Sometimes mothers are consuming multiple substances and medications

Maternal stress, lack of social support, poverty and poor prenatal care often accompany substance use

Pregnant women who smoke

Higher rates of miscarriage, stillborn babies, premature birth, neonatal death, low-weight babies, and neurobehavioral disorders

Mothers who are heavy consumers of alcohol or who have an alcohol use disorder (AUD)

Significant risk of delivering infants with fetal alcohol syndrome (FAS)

Cannabis

Chemicals found in cannabis can pass through placenta, which can impact fetal growth, executive functioning issues and increase impulsivity

Opiate use (eg heroin)

Can lead to pre-term birth, reduced growth and cognitive/behavioural issues, inc risk of still birth, and neonatal abstinence syndrome (withdrawal symptoms upon birth)

Paternal influence

Fathers pass on roughly 55 genetic mutations to their children (mother’s 14)

High reproductive risk in many professions that expose father to certain toxins that impacts prenatal development

Adverse birth outcomes and congenital abnormalities in offspring have been linked with both younger/ older fathers

Neonate

Term for babies between birth and 1 month

Health assessed using Apgar scale

Standard screening for rare metabolic disorders

Brazelton neonatal behavioural assessment scale tracks a newborn’s development over first 2 weeks after birth to identify significant neurological disorders