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Teratology
•the study of malformations or serious deviations
from the normal type in organisms
• the branch of science concerned with the
production, development, anatomy, and
classification of malformed fetuses.
•Teratogens
–A teratogen is a substance, organism, or physical agent
capable of causing abnormal development.
–Teratogens can cause abnormal structure, abnormal
function, growth retardation, or death.
6 Principles of Teratology
1. genetic influences-由於遺傳差異，引起個體對致畸作用的
敏感度不同
2. critical periods-不同時期對致畸作用的敏感度不同
3. initiating mechanism-致畸原以特定的機轉對細胞組織作用
引發一連串的不正常發育
4. access to embryo and fetus-致畸原的特性決定其與胚胎接
近的難易
5. abnormal development-不正常的發育的結果→death,
malformation, growth retardation, and functional
disorder
6. dose-response relationship
Critical (Sensitive) Periods
Time of exposure is a major determinant of toxicological
consequences
• range from no effect to death
• may affect one organ system at one exposure time
and a different organ system at a different time
• may affect one part of a structure (or function) at one
exposure time and a different part (or function) at a
different time
• may require continued exposure throughout
development
• possible that a single acute exposure at the wrong
time could affect development
Human Development Sensitive Periods During Pregnancy
Hamster
Development Sensitive Periods
Critical periods of susceptibility and endpoints of toxicity
Stages of development and toxicity
1. Fertilization 受精
受精後6hr暴露ethylene oxide, ethylmethane
sulfonate→malformed fetus
mechanism:unclear, may be related to imprinting

DNA Methylation
Methyl groups may be attached
to cytosine (C5 position)
• Methyltransferases
 Methyl groups provide a tag
Concentrated in CG-rich domains,
often in promoter regions

Maintains a gene in inactive state rather than initiating
gene repression – Example:
•Inactivation of genes of one X chromosome in female
mammals occurs prior to a wave of methylation
•Implantation – a new wave of methylation occurs
•Early Zygote – most methylation tags removed


DNA Methylation – Genomic Imprinting
Certain genes are active or inactive during early
development

•Depending on whether they are paternal or
maternal genes
•Eg – IGF-2 is only active in the gene from the
male parent
•The gene is imprinted according to parental origin
Mammalian genome has > 100 imprinted genes in
clusters

Imprinted due to selective methylation of one of the
alleles

2.Preimplantation著床前期 (blastocyst)
囊胚形成，細胞分裂到1000個細胞，僅3個細胞將
發育成胎兒，餘發育成胎盤等支持組織，在此期暴
露，理論上不影響或稍微影響胎兒生長，不然就導
致死胎。
DDT, nicotine, methylmethane→body and/or brain
weight deficits and embryo lethality but not
malformation
然而, Methylnitrosourea,
cyproterone→malformation
Early development: ovulation to implantation
Blastocyst
The developing embryo becomes a
hollow ball of cells and is called a
blastocyst.
Group of cells within the hollow space forms
the inner cell mass (ICM).
develops into the embryo.
The cells around the ICM become the
extraembryonic membranes
role in implantation
supports embryo’s growth
3. Implantation 著床 第6-13days
4. Gastrulation-三胚層形成, 第3週
在此期暴露有害物質將造成眼、腦及臉部的畸形
5. Organogenesis 器官形成，第3-8週
為最容易受影響的時期，因為本期Cell proliferation,
cell migration，cell-cell interactions，morphogenetic
tissue remodeling
6. Fetal period胎兒期 第8wk-birth
在此期暴露，影響生長和功能的成熟，需要在出生後仔
細觀察才能察覺。如中樞神經的異常包括行為、智力、
運動的缺失，生殖力降低，以及免疫系統、心臟、肺臟、
腎臟功能受損等。
*若有構造的改變乃是破壞原本正常的構造稱為
deformation，不同於前述malformation
Malformation*
Timing of Exposure
(Days Since Last Menstrual Period)
Anotia, facial nerve paralysis
Ear malformations
38-46
Absence of arms
38-45
Phocomelia of arms
39-46
35-38
Cardiac malformations 39-45
Absence of legs
41-44
Phocomelia of legs
41-47
The type of malformation could be related, approximately, to the time when the pregnant
woman took the drug. This illustrates the concept of critical or sensitive periods during
development. Most of the severe malformations could be attributed to thalidomide ingestion
during a critical period between 35 and 50 days after the last menses.
6 Principles of Teratology
1. genetic influences-由於遺傳差異，引起個體對致畸作用的
敏感度不同
2. critical periods-不同時期對致畸作用的敏感度不同
3. initiating mechanism-致畸原以特定的機轉對細胞組織作用
引發一連串的不正常發育
4. access to embryo and fetus-致畸原的特性決定其與胚胎接
近的難易
5. abnormal development-不正常的發育的結果→death,
malformation, growth retardation, and functional
disorder
6. dose-response relationship-however,一般致畸原存在
threshold level
Mechanisms and pathologenesis of developmental toxicology
1. mutation 突變
somatic mutation in the early embryo, ex.mutagen
2. chromosomal abnormalities 染色體異常
ex. advanced maternal age, viral infection, irradiation, and
chemical agents
3. mitotic interference干擾細胞分裂
slow or arrest DNA synthesis (hydroxyurea or irradiation),
interfere with spindle formation (colchicine, vincristine)
4. interference with nucleic acid function干擾核酸的功能
including replication , transcription, translation ex. antibiotics
and antineoplastic drugs
5. nutritional deficiencies營養缺乏
ex. vitamins , minerals
6. deficient or alter energy supply 缺少或改變能量的供給
ex. inadequate glucose supply (hypoglycemia), interference
with glycolysis (iodoacetate, 6-aminonicotinamide), inhibition
of the citric acid cycle (riboflavin deficiency), blockage of the
terminal electron transport (hypoxia, cyanide)
7. changes in osmolarity滲透壓的改變
ex. hypoxia, hypertonic solutions, adrenal hormone→edema,
hematoma, and blisters
8. changes in cell membranes細胞膜的改變
ex. solvent, vitamin A
9. enzyme inhibition酵素的抑制
抑制代謝酵素,DNA repairing, polymerase
Principles of Teratology
4. access to embryo and fetus-致畸原的特性決定其與胚胎接
近的難易
5. abnormal development-不正常的發育的結果→death,
malformation, growth retardation, and functional
disorder
6. dose-response relationship-however,一般致畸原存在
threshold level
Attribution of threshold
1. high restorative growth potential of mammalian embryo
2. cellular homeostatic mechanisms
3. maternal metabolic defenses
Lack of threshold-even one molecule exposure→point
mutation→abnormal development
Dose-response Patterns and the threshold concept
Thalidomide
Thalidomide was released in 1956 as a mild
sedative used to combat nausea in pregnant women. It
was later (1961) withdrawn from the market once it was
discovered thalidomide was a human teratogen. As little
as one dose could cause a significant birth defect.
Approximately 5,000-7,000 malformed infants were born
to women who ingested thalidomide during pregnancy.
Symptoms:
malformed intestines, hearing defects, absent ears,
and/or ocular and renal anomalies. However, the most
striking phenotype is phocomelia: severe limb
malformations in which the long bones of the limb are
either greatly reduced in length or absent all together.
Teratogenic between 20-36 days after fertilization
•Proposed mechanisms (more than 30)
•Altering cellular redox status
Free radical scavenger (alpha -phenyl-butyl-tert-nitrone PBN)
Reduce embryonal DNA oxidation and teratogenic effect
-Angiogenesis
-Integrin regulation
-growth factor antagonism
Now approved for
Oral ulcer for AIDS, erythema nodosum leprosum
-new anticancer drug? Anti-angiogenesis
Approximately 91 percent of all promoter regions contain the
nucleotide sequence TATA, CCAAT, or both. Only 9 per cent have
neither TATA nor CCAAT sequences, but have the sequence
GGGCGG (called the GC box). The promoter regions for the
alpha v beta 3, FGF-2 and IGF-I genes are all TATA-less and
contain several GC boxes. Furthermore, the promoter regions of
both the FGF-2 and the IGF-I receptor genes contain multiple GC
boxes.
Diethylstilbesterol
DES was prescribed between 1940 and 1970 to prevent
miscarriages in high risk pregnancies. This was
accomplished by DES increasing estrogen and progesterone
synthesis by the placenta. In the mid 1970 cases of vaginal
adenocarcinoma in women ages 16-20 were linked to fetal
exposure through maternal DES ingestion early in the
pregnancy. Approximately 1 in 1000 pregnancies were
exposed, 75% of which resulted in female children with
vaginal and cervical carcinomas as well as uterine
anomalies. Male offspring had abnormal genitalia.
Retinoic Acid
Retinoic acid
Retinoic acid is teratogenic in humans at very low doses.
Exposure to retinoic acid during pregnancy between 3-5 weeks
of pregnancy may result in malformations of the fetus:
craniofacial alterations, cleft palate, neural tube defects,
cardiovascular malformations, thymic aplasia, psychological
impairments, absent or defective ears, small jaw, kidney
alterations. Fifty percent of affected children have an IQ below
85.
Mechanism
A proposed mechanism is that biologically active
retinoic acid binds retinoic acid receptors which in turn bind
DNA enhancer elements such as the retinoic acid response
elements. Several Hox genes (responsible for early patterning
of the embryo) contain this enhancer element in their
promotors. Therefore, Hox signaling may be altered due to
increased retinoic acid concentrations resulting in multiple
birth defects.
Molecular biology
Target cell
RBP–ROH
Receptor
ROH–CRBP
RA–CRABP
RA
RA
Nucleus
RA–RXR
Regulated
gene
transcription
RA
= Retinoic acid
RBP
= Retinol binding
protein
CRBP
= Cellular retinol
binding protein
CRABP
= Cellular retinoic acid
binding protein
RXR, RAR = Nuclear retinoic acid
receptors
Intracellular
Extracellular
S17
= Retinol
RAR–RA
DNA
ROH
ROH
Hormonal Targeting of Nuclear Complexes to Chromatin
SIGMA-ALDRICH.com/rbi
Alcohol (Ethanol)
Ethanol is the causative agent of Fetal
Alcohol Syndrome (FAS). FAS is seen
in approximately 2 in 1000 live births,
depending upon culture and
socioeconomic status. For instance,
there is an occurrences of FAS in
19.5:1000 live births in American
Native Indian culture verses a rate of
1.9:1000 in middle class Caucasian
families. FAS does seem to be dose
dependant in that greater amounts of
alcohol consumed increases the
chances of having an FAS child.
FAS was formally defined in 1970 as containing a combination of
the malformations seen below:
Growth
deficiencies
Maxillary
hypoplasia
Decreased
philtrum size
Microphthalmia
Microcephally
Narrow upper
lip
Cardiovascular
disorders
Short palpebral
fissures
Low nose
bridge
Small brain size
Fetal alcohol effects (FAE)
Neural crest cells are particularly sensitive to
alcohol-induced injury and cell death
Alcohol interfere with development of neurotransmitter Systems
Tobacco
Nicotine restricts uterine blood vessels and restricts blood
flow to the fetus resulting in chronic hypoxia and
malnutrition leading to birth defects. On average, offspring
of smoking women weigh 170-200 g less at birth as
compared to a non smoker’s child. There is a dose
dependence in that the child weight decreases in
proportion to number of cigarettes smoked by the mother.
There is also a reduction in overall fetal length, reduced
head circumference, intrauterine growth retardation as
well as behavioral alterations after birth.
Possible outcomes of smoking during preganancy include:
spontaneous abortion
perinatal deaths
increase risk of sudden infant death syndrome
increased risk of learning, behavioral, and attention
disorders.
Perinatal exposure to tobacco smoke can affect
branching morphogenesis and maturation of the lung.
Smoking during pregnancy increases the risk for premature
delivery, abruption placenta, placenta previa and perinatal
mortality.
Cocaine
Cocaine is an anesthetic and
vasoconstrictor. An estimated 45%
of urban pregnancies and 6% of
suburban pregnancies involve
cocaine exposure.
Exposed fetuses often have intrauterine growth retardation,
microcephaly, cerebral infarction, urogenital anomalies, an
increased risk of sudden infant death syndrome as well as
neuronal and behavioral abnormalities. These pregnancies are
at risk for premature labor, spontaneous abortion, increased
perinatal mortality and fetal death.
Cocaine is thought to induce birth defects by disrupting the
vasculature in the placenta thereby inducing intrauterine
hypoxia and malnutrition.
Valproic Acid
Valproic acid was released in 1967 in Europe and in
1978 in the United States to treat epilepsy. Approximately
11,500 epileptic women become pregnant each year, many
of which use valproic acid. By 1980, publications began
linking malformed children to in utero exposure to valproic
acid (greater than 500 mg/day).
These children were born with lumbosacral spina bifida
with menigomyelocele or menigocele, often accompanied
by midfacial hypoplasia, deficient orbital ridge, prominent
forehead, congenital heart disease and decreased
postnatal growth. The proposed mechanism of action is
that valproic acid influences folate metabolism, thereby
altering the closure of the spinal column resulting in spina
bifida.
Congenital Minamata Disease
Methylmercury was used in the past as a fungicide on wheat
and grains. Cases have been documented in Iraq (19711972), Sweden, Japan and New Mexico of birth defects due
to maternal ingestion of bread made with contaminated
grain. There have also been documented cases in Canada,
New York and Sweden of paper mill contaminants polluting
the water with inorganic mercury. Exposure in utero may
result in sensory and motor impairments, cerebral palsy,
mental retardation and behavioral damage.
Children with Congential Minamata Syndrome seem to be normal
at birth and begin to present symptoms at approximately six
months of age. They have instability of the neck, convulsions,
reduced IQ, microcephaly, malformed limbs, restricted growth and
an altered cerebellum. In utero exposure to methylmercury
induces general brain atrophy and hypoplasia.
Mercury
Mercury
Effects of Higher Dose Prenatal Exposure
•
•
•
•
•
•
•
•
Mental retardation
Seizures
Cerebral palsy
Disturbances of vision, hearing, sensation
Abnormal gait
Abnormal speech
Disturbances of swallowing and sucking
Abnormal reflexes
Mercury:
Declining Threshold of Harm
(micrograms/kg/day Hg)
DAILY INTAKE
100
Level associated with
harmful effect
Regulatory standard
(maximum safe exposure or high
end exposure from allowed fish
contamination)
10
1
FDA
WHO
ATSDR
0.1
EPA
0.01
1970
1980
1990
YEAR
2000
Mercury
Mercury Exposures
Advised Exposure Limit
• EPA Reference Dose (“safe” upper limit) –
0.1 microgram/kilogram/day
• Equivalent consumption limit
• Women: 1.5 oz. swordfish or 7 oz.
tuna/week
• Child: 1 oz. tuna per 20 lb. body
weight/week
Basic Toxicology
Toxicity-related Concepts: specific
processes disrupted by neurodevelopmental toxicants
proliferation
radiation, ethanol, mercury,
cholinesterase inhibitors
migration
radiation, mercury, ethanol
differentiation
ethanol, nicotine, mercury, lead
synaptogenesis
radiation, ethanol, lead, triethyl tin,
parathion, PCBs
gliogenesis &
myelinization
dec. thyroid, ethanol, lead
apoptosis
signaling
ethanol, lead, mercury
ethanol, cholinesterase inhibitors,
mercury, lead, PCBs
Lead
Effects of Lead
on Cognitive and Behavioral Traits
ADHD
LD
OTHER
hyperactivity
reading, math
fine motor
impulsivity
spelling
visual motor
distractibility
pattern recognition aggressive
dif. w. instructs word recognition
antisocial
 conduct problems
off-task
 executive function
attention/vigilance
social skills
The Significance of Small Effects:
EFFECTS OF A SMALL SHIFT IN IQ DISTRIBUTION IN A
POPULATION OF 260 MILLION
mean 100
6.0 million
"gifted"
6.0 million
"mentally retarded"
40
80
60
70
100
120
I.Q.
140
160
130
5 Point Decrease in Mean IQ
mean 95
57% INCREASE
IN
"Mentally
Retarded”
Population
2.4 million
"gifted"
9.4 million
"mentally retarded"
40
80
60
70
100
I.Q.
120
140
130
160
PCBs
PCBs: PERVASIVE
DEVELOPMENTAL EFFECTS
Infant
•
•
•
•
Birth weight
Head circumference
Gestational age
Performance on Brazelton Neonatal Behavioral
Assessment (BNBA) - motor immaturity, poor
lability, startle
PCBs: PERVASIVE
DEVELOPMENTAL EFFECTS
Early Childhood
• Memory, attention, verbal ability, information
processing
• Psychomotor development
• Sustained activity, high level play
• Withdrawn, depressed behavior
• Hyperactivity
Preteen
• Word and reading comprehension
• Full scale and verbal IQ
• Memory and attention
Full-Scale IQ
PCBs
Prenatal Exposure to Polychlorinated Biphenyls
(PCBs) ug/g of fat
PCBs
PCB Effects on Thyroid Hormone
• Altered thyroid hormone
Mothers:
Thyroid Hormone,
Thyroid Stimulating
Hormone (TSH)
Infants:
Thyroid Hormone, TSH
Seals and Rats: Thyroid Hormone
• Developmental Implications
Elevated maternal TSH during pregnancy, with or
without reductions of thyroid hormone,
associated with reduced IQ at age 7-9 yrs.
PCBs
PCB Neurodevelopmental Effects:
Possible Mechanisms
• Altered neurotransmitter levels
• Ah receptor mediated effects (dioxin-like
PCBs)
Disruption of production of growth factors and hormones
including enzyme induction, modulation of growth factors,
hormones
• Interference with thyroid hormone
 metabolism through enzyme induction
 interference with thyroid-hormone-mediated gene
transcription
 displacement of thyroxin from carrier protein
Develpmental toxicity of endocrine-disrupting
chemicals
Definition of endocrine-disrupting chemicals
“Exogenous agent that interferes with the
production, release, transport, metabolism, binding,
action, or elimination of natural hormones
responsible for the maintenance of homeostasis
and the regulation of developmental processes.”
Endocrine-disrupting chemicals
Four modes of action
1. Serving as steroid receptors ligands
2. Modifying steroid hormone metabolizing
enzymes
3. Perturbing hypothalamic-pituitary release of
trophic hormones
4. Uncharacterized proximate modes of action
Impact on screening and testing programs
1. Expansion of the periods of dosing from the end of
organogenesis to the end of pregnancy in order to
include
the urogenital differentiation
2. EDSTAC recommended a high through put screening
(HTPS) cell-based, receptor-mediated gene
transcription assay