1-4 prenatal dev

7/27/2019 1-4 Prenatal Dev

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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



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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



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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.

1-4 prenatal dev

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