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Developmental Hazards


... of autosomal trisomy: trisomy 18; trisomy 13 fatal during infancy. tris. 21 (Down Syndrome) trisomy of sex chromosomes Klinefelter syndrome (XXY): male ... – PowerPoint PPT presentation

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Title: Developmental Hazards

Developmental Hazards
  • lead to abnormal development
  • causes
  • genetic
  • environmental factors
  • bacteria, viruses
  • drugs ingested by mother
  • radiation
  • estimated causes
  • 6 due to chromosomal abnormalities
  • 7 due to environmental factors
  • 8 due to single gene defects
  • 25 due to multifactorial inheritance
  • 54 due to unidentified factors

Screening for birth defects
  • Amniocentesis
  • 16th week of pregnancy
  • women over age 34 (or with history of heritable
    genetic disorders)
  • long needle pass thru abdominal wall to sample
    amniotic fluid
  • cells in fluid are cultured
  • in 4 weeks, cells are examined and screened for
    genetic disorders (40)
  • risks 1.5 miscarriage rate
  • disadvantage waiting period

Screening, contd.
  • Alpha fetoprotein test
  • protein made in fetal liver
  • high conc. in fetal blood, CSF
  • also detected in mothers blood
  • high levels may indicate neural tube defect low
    may indicate trisomy 21
  • Ultrasound
  • sound waves applied to abdomen dense tissues
    reflect waves and are detected by receiver
  • size, dimension assessed
  • heart rate detected
  • presence of twins identified
  • assess organogenesis

Chorionic villus sampling
  • 8-10 weeks of pregnancy
  • catheter inserted thru vagina, cervix to uterus
  • chorionic cells removed from placenta and
  • advantage can be performed earlier than amnio
    results obtained in 4-24 hours
  • disadvantages risk of miscarriage risk of
    improperly formed fingers or toes (6-11 times
    compared to amnio)

Genetic factors numerical abnormalities
  • Monosomy
  • conceptus lacks 1 copy of a particular chromosome
  • if its is an autosome, embryo usually aborts
  • can survive with 1 sex chrom

Turner Syndrome
  • XO
  • ovaries underdeveloped
  • short stature
  • webbed neck
  • broad chest
  • cardio., kidney disorders
  • possible mental retardation

Numerical abnormalities Trisomy
  • conceptus has 3 of 1 type of chromosome instead
    of a pair
  • due to nondisjunction
  • types of autosomal trisomy
  • trisomy 18 trisomy 13 gt fatal during infancy
  • tris. 21 (Down Syndrome)
  • trisomy of sex chromosomes gt Klinefelter syndrome
  • male phenotype
  • small testes (no sperm)
  • long limbs
  • rarely detected until puberty
  • mental retardation

Single gene defects
  • individual has appropriate number
  • one has structural abnormality
  • usually induced by environmental factor
  • phenotype depends on particular chromosome
  • can involve deletion or duplication

Single gene defects, contd.
  • dominantly inherited achondroplasia
  • inheritance of 1 copy of mutant gene results in
  • short limbs
  • large head
  • hump-back protrusion of abdomen

Genetic mutations, contd..
  • autosomal recessive cystic fibrosis
  • both parents are carriers (chrom. 7)
  • mucus in lungs and digestive tract is
    particularly thick and viscous
  • previously death in childhood due to recurrent
    lung infections
  • new hope gene therapy
  • 1 in 20 caucasians is a carrier
  • 1 in 2,000 children affected

Extrinsic disruptions of dev.
  • abnormalities can arise due to exogenous agents
  • agents teratogens
  • chemicals, viruses, radiation
  • often exert effects during critical periods of
  • critical period time of growth, formation
  • different organs have different critical pds.
  • heart gt 3-4 weeks
  • external genitalia gt 8-9 weeks
  • brain, skeleton gt week 3 - end of pregnancy
  • days 15-60 critical for most organ systems

Teratogenic agents Retinoic acid
  • major source of teratogens chemicals in
  • some occur naturally most are artificially
  • excess retinoic acid can act as teratogen
  • reported cases
  • RA taken by women of childbearing age for
    treating severe cystic acne
  • study of 59 fetuses gt 26 normal, 12 aborted
    spontaneously, 21 born with anomalies
  • anomalies absent/defective ears and jaws, cleft
    palate, CNS abnormalities, aortic arch abnorm.

Retinoic acid, contd.
  • anomalies similar to those observed in expts.
    with mice
  • effects probably due to altered expression of Hox
    genes (directs neural crest cell migration)
  • RA binds to cranial neural crest cells
  • arrests proliferation, migration
  • teratogenic effect confined to window period
    (8-10 days in mice 20-35 days in humans)

  • sedative used for morning sickness
  • caused rare syndrome of congenital anomalies
  • phocomelia (long bones of limbs absent or
  • heart defects
  • absence of external ears
  • malformed intestines
  • only one pill necessary to cause limb
  • illustrated limits of animal studies

Thalidomide, contd.
  • targets of thalidomide
  • downregulation of cell adhesion molecules in limb
  • decreased number of neurons in dorsal root

  • most devastating teratogen in our society
  • causes fetal alcohol syndrome
  • characteristics
  • small head size
  • indistinct philtrum, narrow upper lip, low nose
  • small brain
  • affects 1 out of every 500-750 children
  • mouse model developed w/ similar abnormalities

Alcohol, contd.
  • L1 mutations in humans gt mental retardation
  • induces cell death in frontonasal process,
    impairs neural crest migration
  • prevents L1 (CAM) from holding cells together

Environmental estrogens?
  • xenoestrogensgt weak estrogens androgen
  • xenoestrogens can interfere with development
  • examples
  • DDT gt 90 decline in birthrate of alligators in
    Florida lake decrease in masculiniz. of ext.
  • dioxin gt reduces sperm counts in mice smaller
  • role in breast cancer? decreased sperm count?
    other birth defects?
  • others polystyrene plastics household cleaners