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Prenatal Development
In which we examine the three stages ofdevelopment from conception to birth,and the causes of congenital defectsthat can occur during that time.
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Stages of Prenatal Development Zygotic (or Germinal) Stage 0-2 weeks
Embryonic Stage 2-8 weeks
Fetal Stage 9-40 weeks
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Part 1: Zygotic Stage
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Zygotic Stage Major tasks: Cell division and implantation
Rate of cell division:
36 hours 2 cells48 hours 8 cells
72 hours 32 cells
96 hours 70 cells
After 4th day, cells arranged in a hollow sphere, calledthe blastocyst.
Cells are undifferentiated (not yet specialized forfunction)
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Zygotic Stage, continued Zygote develops from an area of blastocyst called the
embryonic disk
At 2 weeks, the embryonic disk as 3 layers: Entoderm: from which develops pharynx, tonsils,
thyroid, trachea, lungs, digestive system, bladder,urethra
Mesoderm: from which develops muscles, bones,
circulatory system, lymph system, kidneys, gonads
Ectoderm: from which develops skin, hair, nails, senseorgans, nervous tissue
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Zygotic Stage: Week 2
Come to class to see slide!
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Critical Periods Critical Period: specific time when a given event, or its
absence, has the greatest impact on development
Development = differentiation, then growth Differentiation: the process in embryonic development
during which unspecialized cells or tissues becomespecialized for particular functions. (Ex: pre-gonadal tissuedifferentiates into pre-ovarian or pre-testicular tissue)
Growth: Once the cells have differentiated, the structuregrows
The critical period for prenatal defects is duringdifferentiation
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Critical Periods of Various Embryonic Structures
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Part 2: Embryonic Stage
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Embryonic Stage Major Task: Differentiation of all body systems except
cerebral cortex and sensory system.
Trends in differentiation of cells Size and structure: from uniformity to diversity;
from simplicity to complexity
Shape: from irregular to regular; from vagueness
to definiteness Adaptability: from plasticity to rigidity
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Embryonic Stage, continued 3 weeks: neural groove closes; thyroid differentiates;
liver differentiates; urinary duct begins to form;primitive heart begins beating
4 weeks: limb buds appear; tongue bud appears; lungbud appears; esophagus, stomach and intestine aresingle tube; nerves begin to form; optic cup appears
5 weeks: nasal pits appear; jaws begin to form;premuscle masses appear; genital buds appear; pre-gonadal tissue present; intestine begins looping;circulatory system extends to head and limbs
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Embryo: Week 4
Come to class to see slide!
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Embryo: Week 4
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Embryonic Stage, continued 6 weeks: external ear appears; limbs recognizable;
lobes of lungs appear; bronchi dividing
7 weeks: back straightens; tail begins to disappear;larynx developing; muscles begin to differentiate
8 weeks: head elevating; digits formed; epidermis in3 layers; taste buds appear; lymph systemdeveloping; testes and ovaries identifiable; skeletalsystems begins to ossify; brain attains generalstructure, with lower brain more developed
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Embryo: Week 8
Come to class to see slide!
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Embryo: Week 9
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Development of
Three Species
Early stages in the
development of a guineapig (left column), amonkey (middle column),and a human embryo(right column).
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Part 3: Fetal Stage
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Fetal Stage Major Tasks: Differentiation of neocortex, sensory
systems, and overall growth
Neocortex and sensory systems
The old brain develops early in the embryonic stage.Needed to control circulatory system.
The midbrain develops later in embryonic stage.
The neocortexdoesnt develop until fetal stage. All
structures not present until after first year of life.
Sensory systems are intricately connected to neocortexand so develop at same time.
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Brain Development
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Brain Development, continued
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Fetal Growth2 months 1 long 1/13 ounce
3 months 3 long 1 ounce
4 months 9 long 6 ounces5 months 12 long 14 ounces
6 months 14 long 20 ounces
7 months 16 long 3-4 pounds
8 months 18 long 5-6 pounds9 months 20 long 7.5 pounds
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Part 5: Congenital Defects
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Teratogens & Congenital DefectsTeratogen: achemical or physical agent which canlead to malformations in the fetus
Congenital Defect: a defect present at birth causedby a teratogen.
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Categories of Teratogens Metabolic (Diseases)
Chemicals
Drugs
Alcohol, Heroin, Narcotics, Nicotine
Maternal malnutrition
Radiation
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Metabolic TeratogensRubella
cardiovascular defects, deafness, blindness,slow growth of fetus
Syphilis
deafness, mental retardation, skin & bone
lesions, meningitis
Toxoplasmosismicrocephaly, hydrocephaly, cerebralcalcification, mental retardation
Diabetescardiac and skeletal malformations, centralnervous system anomalies; increased risk ofstillbirth
Herpes Simplex skin lesions, encephalitis
Mumps spontaneous abortion
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Chemical TeratogensAlcohol
growth & mental retardation, microcephaly,facial and trunk malformations
Chemotherapy major anomalies throughout body
Diethylstilbestrol cervical and uterine abnormalities
Lithium hearing anomalies
Mercurymental retardation, cerebral atrophy, spasticity,
blindnessStreptomycin hearing loss, auditory nerve damage
Tetracycline staining of tooth enamel and bones
Thalidomine limb defects, cardiovascular anomalies
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Child with FAS
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Malnutrition & Radiation Most common effect of material malnutrition is low
birth weight.
Radiation may prevent organs from developing andmay cause mutations.