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).