On what day heart morphogenesis starts in human embryo

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I now have comprehensive, authoritative information from both Langman's Medical Embryology and Robbins Pathology. Here is the full answer:

Heart Morphogenesis in the Human Embryo

Heart morphogenesis begins on approximately day 18-19 of embryonic development, making the heart the first organ to develop and function in the human embryo.

Timeline of Key Events

Day	Event
~Day 16-18	Mesodermal cells in the lateral plate mesoderm form the primary heart field (PHF); cardiac progenitor cells migrate to the midline in two waves (first and second heart fields)
Day 17-18	The cardiogenic region forms a horseshoe-shaped endothelial-lined tube surrounded by myoblasts - this is the start of morphogenesis
Day 18-19	The cardiogenic area is anterior to the oropharyngeal membrane; paired cardiac primordia begin forming
Day 20	The nascent heart has become a beating tube (primitive heart tube begins contracting)
Day 21-22	With lateral folding of the embryo, the paired endocardial tubes fuse in the midline to form a single continuous heart tube; the heart moves toward the thorax via cephalic folding
Day 22-28	The heart tube loops to the right (D-looping), creating the precursors of distinct cardiac chambers
~Day 28	Basic heart chambers begin to form (~8 days after the beating tube is established)
Day 50	Septation of ventricles, atria, and atrioventricular valves produces a full four-chambered heart

What Triggers Morphogenesis

The underlying pharyngeal endoderm induces cells of the PHF to form cardiac myoblasts and blood islands. These islands fuse into the horseshoe-shaped cardiogenic region, which overlies what will become the pericardial cavity.

Two populations of progenitor cells contribute:

First heart field - forms the left ventricle
Second heart field (SHF) - forms the outflow tract, right ventricle, and most of the atria

Key signaling pathways involved include Wnt, Hedgehog, VEGF, BMP, TGF-β, FGF, and Notch.

Clinical Significance

Because cardiac morphogenesis begins at day 18-19 - often before a woman knows she is pregnant - teratogens (e.g., alcohol, SSRIs, rubella, gestational diabetes) during this window can disrupt laterality signaling and heart field specification, leading to congenital heart defects (CHDs). Folate supplementation during early pregnancy reduces CHD incidence by supporting proper morphogenesis.

Sources:

Langman's Medical Embryology (Chapter 13 - "Formation and position of the heart tube"), which states: "Initially, the cardiogenic area and the pericardial cavity are in front of the oropharyngeal membrane. A. 18 days. B. 20 days. C. 21 days. D. 22 days."

Robbins, Cotran & Kumar Pathologic Basis of Disease: "By day 20 of development, the nascent heart has become a beating tube, which begins to form the basic heart chambers roughly 8 days later."

In short: Cardiac morphogenesis begins at day 18-19, and the heart is beating by day 20.

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