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.

2 of 30 Stages of Prenatal Development Zygotic (or Germinal) Stage0-2 weeks Embryonic Stage2-8 weeks Fetal Stage9-40 weeks

Part 1: Zygotic Stage

4 of 30 Zygotic Stage Major tasks: Cell division and implantation Rate of cell division: 36 hours2 cells 48 hours8 cells 72 hours32 cells 96 hours70 cells After 4 th day, cells arranged in a hollow sphere, called the blastocyst. Cells are undifferentiated (not yet specialized for function)

5 of 30 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, sense organs, nervous tissue

6 of 30 Zygotic Stage: Week 2 Come to class to see slide!

7 of 30 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 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

8 of 30 Critical Periods of Various Embryonic Structures

Part 2: Embryonic Stage

10 of 30 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

11 of 30 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; 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; pre- gonadal tissue present; intestine begins looping; circulatory system extends to head and limbs

12 of 30 Embryo: Week 4 Come to class to see slide!

13 of 30 Embryo: Week 4

14 of 30 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 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

15 of 30 Embryo: Week 8 Come to class to see slide!

16 of 30 Embryo: Week 9

17 of 30 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).

Part 3: Fetal Stage

19 of 30 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 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.

20 of 30 Brain Development

21 of 30 Brain Development, continued

22 of 30 Fetal Growth 2 months 1” long1/13 ounce 3 months3” long1 ounce 4 months9” long6 ounces 5 months12” long14 ounces 6 months14” long20 ounces 7 months16” long3-4 pounds 8 months18” long5-6 pounds 9 months20” long7.5 pounds

Part 5: Congenital Defects

24 of 30 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.

25 of 30 Categories of Teratogens Metabolic (Diseases) Chemicals Drugs Alcohol, Heroin, Narcotics, Nicotine Maternal malnutrition Radiation

26 of 30 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 Simplexskin lesions, encephalitis Mumpsspontaneous abortion

27 of 30 Chemical Teratogens Alcohol growth & mental retardation, microcephaly, facial and trunk malformations Chemotherapymajor anomalies throughout body Diethylstilbestrolcervical and uterine abnormalities Lithiumhearing anomalies Mercury mental retardation, cerebral atrophy, spasticity, blindness Streptomycinhearing loss, auditory nerve damage Tetracyclinestaining of tooth enamel and bones Thalidominelimb defects, cardiovascular anomalies

28 of 30 Alcohol, Nicotine & Other Addictive Substances 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 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.

29 of 30 Child with FAS

30 of 30 Malnutrition & Radiation Most common effect of material malnutrition is low birth weight. Radiation may prevent organs from developing and may cause mutations.