•the study of malformations or serious deviations fr

om the normal type in organisms

• the branch of science concerned with the producti

on, development, anatomy, and classification of ma

lformed 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.

 

Teratology

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

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 MethylationMethyl 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

Genomic ImprintingGenomic Imprinting

Certain genes are active or inactive during early develCertain genes are active or inactive during early developmentopment

•Depending on whether they are paternal or materDepending on whether they are paternal or maternal genesnal genes

•Eg – IGF-2 is only active in the gene from the malEg – IGF-2 is only active in the gene from the male parente parent

•The gene is The gene is imprintedimprinted according to parental origin according to parental origin

Mammalian genome has > 100 imprinted genes in cMammalian genome has > 100 imprinted genes in clusterslusters

Imprinted due to selective methylation of one of the Imprinted due to selective methylation of one of the allelesalleles

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

BlastocystThe developing embryo becomes ahollow ball of cells and is called a blastocyst.

The cells around the ICM become theextraembryonic membranes

role in implantationsupports embryo’s growth

Group of cells within the hollow space forms the inner cell mass (ICM).

develops into the embryo.

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

Anotia, facial nerve paralysis 35-38

Ear malformations 38-46

Absence of arms 38-45

Phocomelia of arms 39-46

Cardiac malformations 39-45

Absence of legs 41-44

Malformation* Timing of Exposure(Days Since Last Menstrual Period)

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 toxicology1.   mutation 突變somatic mutation in the early embryo, ex.mutagen2.   chromosomal abnormalities 染色體異常ex. advanced maternal age, viral infection, irradiation, and chemical agents3.   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 drugs5.   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 

Attribution of threshold1. high restorative growth potential of mammalian

embryo2. cellular homeostatic mechanisms3. 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 seda

tive 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.

Thalidomide Susceptible period- Susceptible period- 20-36 days20-36 days after fertilization after fertilization Proposed mechanisms (>24)Proposed mechanisms (>24) Embryonal DNA oxidation (PBN can prevent)Embryonal DNA oxidation (PBN can prevent) Misregulation of the expression of genes critical for outgrowth Misregulation of the expression of genes critical for outgrowth

of limbof limb The inability of NF-kappaB, a redox-sensitive tranThe inability of NF-kappaB, a redox-sensitive tran

scription factor, to bind to its DNA promoter results scription factor, to bind to its DNA promoter results in the failure of limb cells to express fibroblast groin the failure of limb cells to express fibroblast growth factor (FGF)-10 and twist in the limb progress wth factor (FGF)-10 and twist in the limb progress zone mesenchyme, which in turn attenuates exprezone mesenchyme, which in turn attenuates expression of FGF-8 in the apical ectodermal ridgession of FGF-8 in the apical ectodermal ridge ..

DiethylstilbesterolDES 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.

MechanismA proposed mechanism is that biologically active reti

noic 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.

Hormonal Targeting of Nuclear Complexes to Chromatin

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.

Tobacco smoke • Spontaneous abortionsSpontaneous abortions• Perinatal deathsPerinatal deaths• Lower birth weightLower birth weight• Increased risk of Increased risk of

• Sudden infant death syndromeSudden infant death syndrome• Behavioral attention disordersBehavioral attention disorders• Orofacial cleft (particular xenobiotic gene polymorphisms)Orofacial cleft (particular xenobiotic gene polymorphisms)• Gastroschisis (with variant alleles N053, ICAM1, NPPA)Gastroschisis (with variant alleles N053, ICAM1, NPPA)• Branching morphogenesis and maturation of the lungBranching morphogenesis and maturation of the lung

• Nicotine-related adverse nerodevelopmental outcomesNicotine-related adverse nerodevelopmental outcomes

Cocaine

•At risk for premature labor, spontaneous abortion, increased perinatal mortality and fetal death.•intrauterine growth retardation, microcephaly, altered presencephalic development, decreased birth weight, a neonatal neurologic syndrome of abnormal sleep, tremor, poor feeding, irritability, and occasional seizures.•Genitaouinary tract malformation•Impaired auditory process

Valproic Acid

Valproic acid was released in 1967 in Europe

and in 1978 in the United States to treat epilepsy. A

pproximately 11,500 epileptic women become preg

nant each year, many of which use valproic acid. B

y 1980, publications began linking malformed childr

en to in utero exposure to valproic acid (greater tha

n 500 mg/day). 

Valproic Acid (antiepilepsy)

spina bifida withspina bifida with menigomyelocele or menimenigomyelocele or menigocelegocele

The proposed mechanism of action is that The proposed mechanism of action is that valproic acid valproic acid influences folate metabolisminfluences folate metabolism

Congenital Minamata DiseaseMethylmercury was used in the past as a fungicide on wheat and grains. Cases have been documented in Iraq (1971-1972), 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 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

Mercury:Declining Threshold of Harm

20001990198019700.01

0.1

1

10

100

YEAR

Level associated withharmful effectRegulatory standard (maximum safe exposure or highend exposure from allowed fishcontamination)

FDA WHO

EPA

ATSDRDA

ILY

IN

TA

KE

(mic

rogr

ams/

kg/

day

Hg)

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

Mercury

ADHD LD OTHER

hyperactivity reading, math fine motorimpulsivity spelling visual motordistractibility pattern recognition aggressivedif. w. instructs word recognition antisocial

conduct problems off-task executive function

attention/vigilancesocial skills

Effects of Lead on Cognitive and Behavioral Traits

Lead

The Significance of Small Effects: EFFECTS OF A SMALL SHIFT IN IQ DISTRIBUTION IN A

POPULATION OF 260 MILLION

160140120100806040

70 130I.Q.

mean 100

6.0 million "gifted"

6.0 million"mentally retarded"

5 Point Decrease in Mean IQ

160140120100806040

mean 95

70 130

2.4 million"gifted"

9.4 million"mentally retarded"

57% INCREASEIN

"MentallyRetarded”Population

I.Q.

PCBs: PERVASIVE

DEVELOPMENTAL EFFECTS

Infant• Birth weight

• Head circumference

• Gestational age

• Performance on Brazelton Neonatal Behavioral Assessment (BNBA) - motor immaturity, poorlability, startle

PCBs

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

Prenatal Exposure to Polychlorinated Biphenyls (PCBs) ug/g of fat

Full-

Scal

e IQ

PCBs

PCB Effects on Thyroid Hormone

• Altered thyroid hormoneMothers: Thyroid Hormone, Thyroid Stimulating

Hormone (TSH)

Infants: Thyroid Hormone, TSH

Seals and Rats: Thyroid Hormone

• Developmental ImplicationsElevated 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

PCBs

Therapeutic Drugs Teratogenic to Humans AnticonvulsantsAnticonvulsants

PhenytoinPhenytoin, , primidoneprimidone, , trimethadionetrimethadione, , valproic acidvalproic acid, , carbamazepinecarbamazepine

Anticancer agentsAnticancer agents

Alkylating agentsAlkylating agents –busulfan, cyclophosphamide –busulfan, cyclophosphamide, , chlorambucilchlorambucil, , mechlorethaminemechlorethamine

AntimetabolitesAntimetabolites-aminopterin, methotrexate-aminopterin, methotrexate, , cytarabinecytarabine

Androgenic hormonesAndrogenic hormones--danazoldanazol

Coumarin anticoagulantsCoumarin anticoagulants--

RetinoiRetinoids-accutane, isotretinoin, etretinate, acitretinds-accutane, isotretinoin, etretinate, acitretin

Other drugsOther drugs--DiethystilbestrolDiethystilbestrol, , thalidomidethalidomide, , penicillaminepenicillamine, , lithiumlithium, , fluconazolefluconazole, ,

misoprostolmisoprostol, statin, statin

Pregnancy Risk Categories