Organogenesis is the process by which the three germ layers (ectoderm, endoderm, and mesoderm) develop into internal organs through proliferation, migration, reaggregation and differentiation of cells. This process occurs between weeks 3 to 8 of development in utero. During this critical period, environmental factors can greatly impact development by disturbing key processes like cell division, apoptosis, gene expression and metabolism. Exposure to teratogens during organogenesis raises the risk of birth defects and structural abnormalities, while exposure later in pregnancy is less likely to cause such defects.

1. Organogenesis
Critical period of Organogenesis
By: Gajaan S
Tossif G
512

2. Organogenesis
Organogenesis is the process by which
the ectoderm, endoderm, and mesoderm
develop into the internal organs of the
organism.
Cells of each germ layer
proliferate, migrate, reaggregate and
differentiate into various tissues that form
the organs (organogenesis)

4. How does it occur?
The germ layers in organogenesis differ by three
processes:
• folds,
• splits, and
• Condensation
Cells of each germ layer
proliferate, migrate, reaggregate and differentiate into
various tissues that form the organs (organogenesis)

5. When does it occur?
Internal organs initiate development
in humans within the 3rd to 8th
weeks in utero.

10. Embryonic stage
• Week 3: Beginning development of the brain, heart, blood
cells, circulatory system, spinal cord, and digestive system.
• Week 4: Beginning development of bones, facial structures, and limbs
(presence of arm and leg buds); continuing development of the heart
(which begins to beat), brain, and nervous tissue.
• Week 5: Beginning development of eyes, nose, kidneys, lungs; continuing
development of the heart (formation of valves), brain, nervous tissue, and
digestive tract.
• Week 6: Beginning development of hands, feet, and digits; continuing
development of brain, heart, and circulation system.
• Week 7: Beginning development of hair follicles, nipples, eyelids, and sex
organs (testes or ovaries); first formation of urine in the kidneys and first
evidence of brain waves.
• Week 8: Facial features more distinct, internal organs well developed, the
brain can signal for muscles to move, heart development ends, external
sex organs begin to form.
By the end of the embryonic stage, all essential external and internal
structures have been formed. The embryo is now referred to as a fetus.

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13. CRITICAL PERIODS OF DEVELOPMENT
It is a specific time
during which the
environment has its
greatest impact on an
individual's
development.

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15. Why it is critical period?
Because it will disturb the processes of
developments:
• Control of cell division,
• Apoptosis,
• Gene expression, and
• Cellular metabolism

16. During pre-embronic stage
• The effects of maternal drug use on the
development of the fetus are dependent upon
the stage of embryonic and fetal development, as
well as the dosage of the substance. After
fertilization occurs, the embryo is highly resistant
to birth defects, but the results of a high dose
teratogen could result in death. Intake of
teratogens during this preembryonic stage could
also result in normal development if the
teratogenic substance is stopped before the
embryonic stage of development.

17. During embryonic stage
• During the embryonic stage of development
(weeks 3-8), the effects of teratogens are
increasingly detrimental. Organs begin forming
during the embryonic stage, and harmful
teratogens can result in miscarriage, and structural
abnormalities.
• Once the embryonic stage is complete, the fetal
period begins at week 9 and continues until full
term at week 38. Teratogens taken during this
period can result in improper organ
functioning, delayed growth, but seldom result in
birth defects .