Embryology and Fetal Circulation Review

Early Stages of Embryonic Development

• Gametes:

  • Ovum and sperm involved in fertilization.

  • Resulting zygote has 46 chromosomes (visible to naked eye).

• Development Stages:

  • Morula: 16-32 cell mass surrounded by zona pellucida.

  • Blastocyst: 100-150 cells with fluid cyst formation;

• Stages include:

  • 2-celled stage → 4-celled stage → 8-celled stage → 16-celled stage → Blastocyst.

Week 2: Implantation

• Blastocyst Attachment:

  • Attaches to uterine wall for nutrition.

  • Complete implantation usually by the middle of the second week.

• Human Chorionic Gonadotropin (HCG):

  • Synthesized to sustain progesterone production and support endometrium.

  • Later, the placenta takes over progesterone production (Rehman & Muzio, 2024).

Week 3: Gastrulation

• Gastrula Formation:

  • Development of three primary germ layers:

  • Ectoderm: Skin, nervous system, brain.

  • Mesoderm: Muscles, bones, cartilage, circulatory system, kidneys.

  • Endoderm: Lining of digestive and respiratory tracts, liver, pancreas (Rehman & Muzio, 2024).

Development of the Heart

• Mesoderm Contribution:

  • Forms heart and vascular system; fetal heart development begins in week 3, achieving four chambers by week 7.

Embryonic Heart Development (Week 3)

• Structure from Lateral Plate Mesoderm:

  • Two "blood islands" near cephalic region → formation of cardiogenic field.

  • Endoderm stimulates vasculogenesis,

  • Paired heart tubes form in the pericardial cavity due to embryo folding.

Key Regions of the Heart Tube

• Truncus Arteriosus: Future aorta and pulmonary artery.

• Bulbus Cordis: Future right ventricle.

• Primitive Ventricle: Future left ventricle.

• Primitive Atrium and Sinus Venosus: Part of future right atrium (Mathew & Bordoni, 2023).

Weeks 4-8: Heart Development

• Internal Septa Formation:

  • Growth within heart tube divides primitive atrium and ventricle into left and right chambers.

  • Splitting of the atrioventricular canal forms separate orifices, leading to atrioventricular and semilunar valves.

  • Temporary opening (foramen ovale) allows blood flow from right to left atrium and usually closes after birth (Mathew & Bordoni, 2023).

The Placenta

• Function:

  • Temporary organ connecting fetus to mother, providing oxygen and nutrients.

  • Oxygen primarily supplied from the placenta.

  • Non-functional lungs at this stage.

  • Connected to fetus via the umbilical cord (Mayo Clinic, 2024).

Umbilical Vasculature

• Circulatory Pathways:

  • Typically one vein (carries oxygenated blood from placenta to fetus) and two arteries (carry deoxygenated blood from fetus to placenta).

  • Umbilical vein connects to inferior vena cava (IVC) via ductus venosum.

Fetal Circulation

• Path of Oxygenated Blood:

  • Enters right atrium from IVC → passes through fossa ovale to left atrium → travels through mitral valve to left ventricle → aorta.

• Deoxygenated Blood Pathway:

  • Blood from superior vena cava (SVC) enters right atrium → passes through tricuspid valve → goes to right ventricle → pulmonary artery → ductus arteriosus into the aorta → umbilical arteries return deoxygenated blood to placenta.

Changes Upon First Breath

• At Birth:

  • Lungs open and alveoli expand, decreasing pulmonary resistance.

  • Pressure changes close ductus venosus and ductus arteriosus.

  • Septum primum forced against atrial septum, closing fossa ovale (Mayo Clinic, 2024).