Lecture Notes: Human Development from Zygote to Birth

Introduction

• Starting Point: Human development begins from a zygote, approximately the size of a pinhead (2 millimeters).
• Humans grow to be approximately 850 times taller than their initial size as a zygote.
• Focus of the lecture: the process of human development from a single cell to a fully-formed body.

Initial Development Stages

• Hormonal Influence: Hormones play a crucial role in pregnancy and development.
• Zygote Formation: Sperm and egg unite to form a zygote.
• Cell Division and Cleavage Phase:
  • Begins 24 hours post-fertilization, converting from 1 cell into 16 cells (blastomeres).
  • Cells divide rapidly without growing, increasing surface area for nutrient absorption.
  • Formation of a morula (Latin for mulberry).

From Morula to Blastocyst

• Developing into a Blastocyst:
  • Cells form a hollow sphere with an outer trophoblast layer and an inner cell mass.
  • Trophoblasts will form the placenta and uterine blood vessels.
• Cells move down the fallopian tube towards the uterus.

Implantation and Hormonal Control

• Implantation:
  • Occurs when the blastocyst attaches to the endometrial layer of the uterus.
  • Requires estrogen and progesterone from the corpus luteum.
  • Trophoblasts secrete HCG to maintain hormone production.

Development of the Placenta and Embryo

• Placenta Formation:
  • A new organ formed from maternal and embryonic tissues.
  • Facilitates nutrient and waste exchange between mother and embryo.
• Embryonic Stage: Transition from blastocyst to embryo.

Maternal Adaptations and Fetal Development

• Anatomical Changes in the Mother:
  • Enlargement of breasts and uterus.
  • Increased blood volume by as much as 40%.
  • Secretion of hormones like relaxin and HPL.
• Fetal Development: Organs and bones form, transforming the embryo into a fetus.

Birth Process and Labor

• Hormonal Changes:
  • Decrease in progesterone, increase in estrogen.
  • Cortisol from the fetus triggers more estrogen.
  • Oxytocin and prostaglandins initiate contractions.
• Stages of Labor:
  • Dilation: Cervix dilates to 10 cm.
  • Expulsion: Infant is delivered.
  • Placental Stage: Delivery of the placenta.

Conclusion

• The cycle of human life continues as new generations are born.
• Biological and anatomical transformations during pregnancy and delivery showcase the complexity of human development.