Development from Zygote to Fetus

Introduction

• Humans start as a zygote, about 2mm in size.
• Grow to many times the original size (average human is 1.7 meters tall).

Initial Growth

• Begins with a sperm and egg forming a zygote.
• Growth involves hormonal signals interpreted by cells:
  • Instructions: Divide, Differentiate, Develop.

Cleavage Stage

• Dividing: Cells split into 16 cells (blastomeres) without growing in size.
• Morula Formation: Berry-shaped cluster of cells called a morula forms.
• Blastocyst Development: Morula becomes a blastocyst with an outer trophoblast layer and an inner cell mass.

Journey to Implantation

• Zygote travels down the fallopian tube to the uterus.
• Blastocyst floats in the uterus, looking for a place to implant.

Hormonal Influence and Implantation

• Hormones (estrogen and progesterone) prepare the endometrium for implantation.
• hCG Hormone: Secreted by trophoblasts to maintain hormone levels, preventing menstruation.
• Implantation completes around 12 days post-ovulation.

Development of the Placenta

• Role of Placenta: Forms from maternal and embryonic tissues to support the embryo.
• Provides nutrients, hormones, and waste transfer.

Embryonic Stage

• Differentiation into various cell types.
• Formation of an embryo with an amniotic sac and placenta.

Maternal Anatomical and Physiological Changes

• Breast and Uterus Growth: Increase in size to accommodate the fetus.
• Hormones:
  • Relaxin loosens joints.
  • Human placental lactogen (hPL) prepares for lactation and glucose storage.
• Cardiovascular Changes:
  • Blood volume increases by 40%.
  • Affects gums, vision, and veins.

Labor and Birth

• Hormonal Prep for Labor:
  • Progesterone decreases; estrogen increases as birth nears.
  • Oxytocin and prostaglandins stimulate contractions.
• Labor Stages:
  • Dilation Stage: Cervix dilates to 10 cm.
  • Expulsion Stage: Baby is delivered.
  • Placental Stage: Placenta expelled as afterbirth.

Conclusion

• Pregnancy is a complex physiological process regulated by hormones.
• The mother's body undergoes significant changes to support and deliver a baby.

Acknowledgments

• Crash Course team contributions and Patreon supporters enable educational content creation.