Transcript 4 Prenatal Development

Prenatal Development
In which we examine the three stages of
development from conception to birth,
and the causes of congenital defects
that can occur during that time.
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Stages of Prenatal Development
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Zygotic (or Germinal) Stage
Embryonic Stage
Fetal Stage
0-2 weeks
2-8 weeks
9-40 weeks
Part 1: Zygotic Stage
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Zygotic Stage
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Major tasks: Cell division and implantation
Rate of cell division:
36 hours
2 cells
48 hours
8 cells
72 hours
32 cells
96 hours
70 cells
After 4th day, cells arranged in a hollow sphere, called
the blastocyst.
Cells are undifferentiated (not yet specialized for
function)
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Zygotic Stage, continued
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Zygote develops from an area of blastocyst called the
embryonic disk
At 2 weeks, the embryonic disk as 3 layers:
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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, sense
organs, nervous tissue
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Zygotic Stage: Week 2
Come to class to see slide!
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Critical Periods
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Critical Period: specific time when a given event, or its
absence, has the greatest impact on development
Development = differentiation, then growth
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Differentiation: the process in embryonic development
during which unspecialized cells or tissues become
specialized for particular functions. (Ex: pre-gonadal tissue
differentiates into pre-ovarian or pre-testicular tissue)
Growth: Once the cells have differentiated, the structure
grows
The critical period for prenatal defects is during
differentiation
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Critical Periods of Various Embryonic Structures
Part 2: Embryonic Stage
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Embryonic Stage
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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
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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; lung
bud appears; esophagus, stomach and intestine are
single tube; nerves begin to form; optic cup appears
5 weeks: nasal pits appear; jaws begin to form;
premuscle masses appear; genital buds appear; pregonadal 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
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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 in
3 layers; taste buds appear; lymph system
developing; testes and ovaries identifiable; skeletal
systems begins to ossify; brain attains general
structure, 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
Development of
Three Species
Early stages in the
development of a guinea
pig (left column), a
monkey (middle column),
and a human embryo
(right column).
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Part 3: Fetal Stage
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Fetal Stage
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Major Tasks: Differentiation of neocortex, sensory
systems, and overall growth
Neocortex and sensory systems
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The old brain develops early in the embryonic stage.
Needed to control circulatory system.
The midbrain develops later in embryonic stage.
The neocortex doesn’t develop until fetal stage. All
structures not present until after first year of life.
Sensory systems are intricately connected to neocortex
and so develop at same time.
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Brain Development
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Brain Development, continued
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Fetal Growth
2
3
4
5
6
7
8
9
months
months
months
months
months
months
months
months
1” long
3” long
9” long
12” long
14” long
16” long
18” long
20” long
1/13 ounce
1 ounce
6 ounces
14 ounces
20 ounces
3-4 pounds
5-6 pounds
7.5 pounds
Part 5: Congenital Defects
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Teratogens & Congenital Defects
Teratogen: a chemical or physical agent which can
lead to malformations in the fetus
Congenital Defect: a defect present at birth caused
by a teratogen.
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Categories of Teratogens
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Metabolic (Diseases)
Chemicals
 Drugs
 Alcohol, Heroin, Narcotics, Nicotine
Maternal malnutrition
Radiation
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Metabolic Teratogens
Rubella
cardiovascular defects, deafness, blindness,
slow growth of fetus
Syphilis
deafness, mental retardation, skin & bone
lesions, meningitis
Toxoplasmosis
microcephaly, hydrocephaly, cerebral
calcification, mental retardation
Diabetes
cardiac and skeletal malformations, central
nervous system anomalies; increased risk of
stillbirth
Herpes Simplex
skin lesions, encephalitis
Mumps
spontaneous abortion
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Chemical Teratogens
Alcohol
growth & mental retardation, microcephaly,
facial and trunk malformations
Chemotherapy
major anomalies throughout body
Diethylstilbestrol
cervical and uterine abnormalities
Lithium
hearing anomalies
Mercury
mental retardation, cerebral atrophy, spasticity,
blindness
Streptomycin
hearing loss, auditory nerve damage
Tetracycline
staining of tooth enamel and bones
Thalidomine
limb defects, cardiovascular anomalies
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Alcohol, Nicotine & Other Addictive Substances
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The most common defect of addictive substances,
including nicotine, is low birth weight
Infants born to addicted women will also be addicted.
Fetal Alcohol Syndrome
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Growth deficiencies
Skeletal and facial deformities
Organ deformities: heart defects; genital malformations;
kidney and urinary defects.
Central nervous system handicaps: small brain; mental
retardation learning disabilities; hyperactivity, poor
coordination.
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Child with FAS
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Malnutrition & Radiation
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Most common effect of material malnutrition is low
birth weight.
Radiation may prevent organs from developing and
may cause mutations.