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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Zygotic (or Germinal) Stage
Embryonic Stage
Fetal Stage
0-2 weeks
2-8 weeks
9-40 weeks

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Major tasks : Cell division and implantation
Rate of cell division :
36 hours
48 hours
2 cells
8 cells
72 hours
96 hours
32 cells
70 cells
After 4 th day, cells arranged in a hollow sphere, called the blastocyst .
Cells are undifferentiated (not yet specialized for function)

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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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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

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Major Task: Differentiation of all body systems except cerebral cortex and sensory system.
Trends in differentiation of cells
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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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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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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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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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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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2 months
3 months
4 months
5 months
6 months
7 months
8 months
9 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
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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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Metabolic (Diseases)
Chemicals
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Drugs
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Alcohol, Heroin, Narcotics, Nicotine
Maternal malnutrition
Radiation
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Rubella
Syphilis
Toxoplasmosis
Diabetes cardiovascular defects, deafness, blindness, slow growth of fetus deafness, mental retardation, skin & bone lesions, meningitis microcephaly, hydrocephaly, cerebral calcification, mental retardation cardiac and skeletal malformations, central nervous system anomalies; increased risk of stillbirth
Herpes Simplex skin lesions, encephalitis
Mumps spontaneous abortion
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Alcohol growth & mental retardation, microcephaly, facial and trunk malformations
Chemotherapy major anomalies throughout body
Diethylstilbestrol cervical and uterine abnormalities
Lithium
Mercury
Streptomycin hearing anomalies mental retardation, cerebral atrophy, spasticity, blindness hearing loss, auditory nerve damage
Tetracycline
Thalidomine staining of tooth enamel and bones 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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Most common effect of material malnutrition is low birth weight.
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Radiation may prevent organs from developing and may cause mutations.