Organogenesis is the process by which the three germ layers (ectoderm, mesoderm, endoderm) develop into internal organs through cell proliferation, migration, aggregation and differentiation. It occurs primarily during weeks 3-8 of pregnancy, when all essential external and internal structures are formed. This embryonic stage represents a critical period of development, as the environment can have its greatest impact on development through disrupting processes like cell division and gene expression. Exposure to teratogens during this period increases the risk of abnormalities, while the pre-embryonic stage is more resistant and the fetal stage impacts functioning but less often causes defects.

1. Organogenesis
Critical period of Organogenesis
Guided by : Dr. Priyanka Wadhwa
By: Nikita Kumari
BAMS (2020 Batch)

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)

3. 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.

8. ECTODERM MESODERM ENDODERM
Epidermis of skin and its
derivatives(including
sweat glands, hair
follicles)
Notochord
Skeletal system
Muscular system
Notochord
Skeletal system
Muscular system
Epithelial lining of mouth
and anus
Muscular layer of
stomach and intestine
Muscular layer of
stomach and intestine
Cornea and lens of eye Excretory system Excretory system
Nervous system Circulatory and
Iymphatic systems
Circulatory and
lymphatic systems
Sensory receptors in
epidermis
Reproductive system
(except germ cells)
Reproductive system
(except germ cells)
Adrenal medulla Dermis of skin Dermis of skin
Tooth enamel
Epithelium of pineal
Lining of body cavity
Adrenal cortex
Lining of body cavity
Adrenal cortex

9. 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.

10. • 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.

11. Critical periods of development
• It is a specific time during which the environment has its greatest
impact on an individual's development.

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

13. During pre-embryonic 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 pre-embryonic stage could
also result in normal development if the teratogenic substance is
stopped before the embryonic stage of development.

14. 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.

15. THANK YOU