Organ Development


The heart begins to develop from Day 18 and continues until the end of the eighth week. It develops in the cardiogenic area from cardiogenic mesoderm, which is formed from mesenchymal cells migrating from the primitive streak during development of the notochord. This mesoderm differentiates into endocardium which lines the inside of the heart and myocardium which is the muscle of the heart.

The precursor mesoderm fuses to form a single heart tube with five prominent bulges: the truncus arteriosus (aorta and pulmonary trunk), bulbus cordis (part of ventricles), primitive ventricle (part of ventricles), primitive atrium (atria), and sinus venosus (SA node and coronary sinus). The heart then twists and loops in on itself by 90 degrees in the 4th week of development. Soon after, the valves form and the heart divides into four chambers, and the heart is fully formed by the beginning of the 7th week. The heart begins to beat on approximately Day 22, providing the developing foetus with a circulation it can obtain nutrients from.

Heart Development

A = embryonic heart on Day 24, B = embryonic heart on Day 35

The embryonic circulation possesses three shunts which bypass organs. Firstly the ductus arteriosus is a duct which allows blood from the right ventricle to pass directly into the aorta. This bypasses the non-functioning foetal lungs which are full of fluid, and after birth the duct degenerates but leaves a remnant; the ligamentum arteriosum. Secondly the ductus venosus is a duct which allows half of the oxygenated blood from the umbilical vein to pass into the inferior vena cava. This bypasses the liver of the foetus, and soon after birth the duct degenerates and leaves a remnant called the ligamentum venosum. Finally, the foramen ovale is an opening between the left and right atria, allowing blood to bypass the right ventricle. This foramen closes at birth and leaves a shallow pit in the wall of the right atrium called the fossa ovalis, but in some cases can persist as a Patent Foramen Ovale.


The respiratory system begins to develop during the fourth week and continues until after birth. The lungs develop together with the trachea and bronchi from mesoderm, starting from a single respiratory bud which divides into two buds called bronchial buds. The buds grow and soon start to segment into bronchi, from the main right and left bronchi down to respiratory bronchioles. The lung mesenchyme (tissue) develops around the expanding bronchial system, and once the foundations of the lungs have been laid they begin to mature.
Lung maturation occurs in four stages:

  1. Pseudoglandular Period (Weeks 6 – 16):
  2. At the beginning of this period the developing lungs resemble an exocrine gland histologically. By the end of this period most of the major elements have formed with the exception of tissue involved in gas exchange, hence the foetus will be unable to survive if born during this period.

  3. Canalicular Period (Weeks 16 – 26):
  4. The lung mesenchyme becomes highly vascular and the alveolar ducts begin to develop at approximately Week 24. If born during the end of this period the foetus may survive if kept in intensive care.

  5. Terminal Sac Stage (Week 26 – birth)
  6. Terminal sacs, which are primordial alveoli, develop and the epithelium covering them becomes very thin to aid gas exchange. The blood-air barrier is established, and specialised epithelia develop: Type I alveolar cells which gas exchange occurs across, and Type II alveolar cells which secrete pulmonary surfactant (a mixture of proteins and phospholipids) from Week 20 which is crucial for preventing the lungs from collapsing after birth. The presence of surfactant is used as a marker for lung maturation.

  7. Alveolar Period (birth – 8 years)

The number of alveoli and other structures greatly increases, as well as increase in size. On birth the fluid clears from the lungs and the baby takes its first breath, with a well-established pulmonary circulation in place.

Lung Development

Stages of foetal lung development