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Human Embryo Development
Introduction
- Human development is a fascinating subject that offers critical insights into fertility, reproduction, and the body’s functions.
- The process begins when an ovum is fertilized by sperm, leading to continuous cell division and differentiation, ultimately forming a multicellular human.
- Though growth continues postnatally, the formation of the body’s organs and systems happens between fertilization and birth during the prenatal period.
- Embryogenesis describes the early development stages, progressing from the zygote to the fetus.
Historic Embryology
- 1400 BC: Egyptians recognized the placenta’s role as vital for life.
- Hippocrates (460–370 BC): Proposed that the embryo develops by drawing moisture and nutrients from the mother.
- Aristotle (384–322 BC): Studied embryonic development, noting that young embryos of various species share universal traits.
- Galen (150–180 AD): Asserted the umbilical cord’s necessity for respiration.
- Leonardo da Vinci: Pioneered embryonic studies through dissection and noted the chronological growth of embryos.
- William Harvey (1578–1667 AD): Identified the blastoderm as the origin point for the embryo and discussed the importance of amniotic fluid.
- Karl Ernst von Baer (1827): Discovered the mammalian ovum and proposed the germ layer theory.
First Trimester
From Week 1 to 4
- Pregnancy starts from the first day of the last period.
- By the third week, the zygote (fertilized egg) forms, with genetic traits already determined.
- The embryo implants in the womb, and the placenta begins to develop.
- Neural tube formation starts, marking the beginnings of the brain, spinal cord, and backbone.
From Week 5 to 8
- By week 5, the heart begins to beat.
- The embryo develops three layers: ectoderm, endoderm, and mesoderm.
- By week 6, brain hemispheres start forming, and brain waves are detectable.
- Distinct facial features and rapid brain growth occur during week 7.
- By week 8, the embryo reaches about 1.1 cm, with developing ears, eyelids, and nose tips.
From Week 9 to 12
- Reproductive organs and other organs like the pancreas and gallbladder start forming.
- By week 10, the baby transitions from an embryo to a fetus.
- Fingers, toes, and bones develop, and by week 12, the fetus measures about 4.4 cm, moving slightly.
Week 13
- The fetus’s arms grow proportionate to the body, and the fetus begins thumb-sucking.
- By now, all essential organs and systems have formed.
Second Trimester
From Week 14 to 17
- Week 14: Prostate forms in males, and ovaries descend in females.
- Week 15: Lanugo (downy hair) forms, helping with body heat retention.
- Week 16: Skeleton begins to harden, and the umbilical cord fully matures.
- Week 17: Fat is deposited under the skin, and sweat glands develop.
From Week 18 to 21
- Week 18: Ears develop enough for the fetus to hear internal sounds.
- Week 19: Vernix caseosa, a protective coating, forms on the skin.
- Week 20: The fetus is about 6 inches long, and quickening (fetal movements) may be felt.
From Week 22 to 25
- Week 22: The baby’s skin becomes opaque, and brain cells multiply rapidly.
- Week 24: The inner ear is fully developed, and the baby can sense its position.
- Week 25: Meconium (baby’s first bowel movement) forms in the intestines.
From Week 26 to 27
- The baby establishes a sleep-wake pattern.
- Hiccups may be felt as involuntary diaphragm movements, though silent due to fluid-filled lungs.
Third Trimester
From Week 28 to 30
- Week 28: The baby opens its eyes, and iris pigmentation develops.
- The baby weighs about 2.5 pounds, and if born now, survival is likely.
- Week 30: Weight gain accelerates as the baby adds protective fat.
From Week 31 to 34
- Week 31: In males, testicles descend into the scrotum, while females develop the clitoris.
- Week 32: Major organs, except the lungs, are fully functioning.
- Week 34: The bones in the skull remain unfused, allowing flexibility for birth.
From Week 35 to 38
- The baby positions its head down in preparation for birth.
- Body fat accumulates, helping with temperature regulation.
- Week 37: The umbilical cord prepares to pass nutrients to the baby, and hearing and vision continue improving.
From Week 39 to 42
- By week 39, the baby reaches full term, weighing 6-9 pounds.
- The placenta continues providing nutrients until delivery.
Some Recent Findings
- Fetal Skeletal Development: Fetal kicks generate mechanical forces that stimulate bone growth.
- Kidney Location: The left kidney is higher than the right during gestation, with changes corresponding to vertebral levels.
- Maternal Obesity: Exposure to an altered intra-uterine environment affects fetal cardiovascular development.
- Estetrol (E4): A natural estrogen produced by the fetal liver with potential anti-thrombotic properties.
Conclusion
- Human embryonic development has long captivated scientists, offering a logical foundation for understanding the human body.
- Despite the many advances, much remains to be discovered regarding gamete formation, fertilization, and early embryonic development.
Bibliography
- Google: www.google.co.in
- Embryo Project Encyclopedia: https://embryo.asu.edu
- UNSW Embryology: https://embryology.med.unsw.edu.au
- Parents.com: https://www.parents.com/pregnancy
- OnHealth: https://www.onhealth.com
- ScienceDirect: www.sciencedirect.com
