What is the Difference Between Septum Primum and Septum Secundum

The key difference between septum primum and septum secundum is that septum primum is thin and located in the left side of the heart on the left atrium, while septum secundum is thicker and located in the right side of the heart on the right atrium.

The formation of the heart of the embryo begins after around 18 to 19 days of fertilization. The heart starts to develop near the head of the embryo and near the cardiogenic area. At the beginning of the fourth week, the developing heart starts to beat and pump, circulating blood. The main walls of the heart are usually formed between 27 to 37 days after the developing embryo. At the end of the fourth week, the left side of the heart with the atrium starts to form with the septum primum. When the right atrium starts to expand, a new fold called the septum secundum is formed at the end of the fifth week or the start of the sixth week.

CONTENTS

1. Overview and Key Difference
2. What is Septum Primum 
3. What is Septum Secundum
4. Similarities –  Septum Primum and Septum Secundum
5. Septum Primum vs Septum Secundum in Tabular Form
6. Summary –  Septum Primum vs Septum Secundum

What is Septum Primum?

Septum primum is a muscular tissue that divides the left and right sides of the atrium on the heart of a human embryo. During the development of the human embryo, the septum primum usually grows downward into the single atrium.  The septum primum is a thin ridge that comprises a crescent-shaped membrane growing down from the top layer of the primitive atrium towards the developing endocardial cushions. The septum eventually forms a large opening as it continues to grow. This opening is called the foramen primum, and it lies between the septum primum and the endocardial cushion. This opening allows oxygenated blood to pass from right to left atrium.

Septum primum grows further, fusing with the endocardial cushions, and this destroys the foramen primum. Once the foramen primum disappears, the septum primum completes, forming the primordial atrioventricular septum. However, as a result of programmed cell death, tiny perforations start to appear in the central part of the septum primum. This occurs just before the foramen primum disappears. After the septum primum fuses with the endocardial cushions, the perforations become larger; as a result, another foramen is formed. This is called the foramen secundum and is similar to foramen primum. This also passes oxygenated blood from right to left atrium.

What is Septum Secundum?

Septum secundum is a muscular flap-like structure that is important in the development of the heart. It is semilunar in shape and grows downwards from the upper wall of the atrium to the right side of the septum primum and ostium secundum. Generally, the growth of the septum secundum starts once the septum primum fuses to the endocardial cushion. Septum secundum grows from the ventrocranial wall of the atrium, which is to the right of the septum primum.

Septum secundum grows at the end of the fifth week or the beginning of the sixth week of development. It grows to the left of the septum primum from the upper wall of the primitive atrium. The septum secundum is essential in the closure of the foramen ovale in the fetus after birth. Before the birth, it does not fuse with the septum intermedium but leaves a gap for the foramen ovale to be formed. Just after birth, it fuses with the septum primum and forms the interatrial septum, and the foramen ovale is closed.

What are the Similarities Between Septum Primum and Septum Secundum?

• Septum primum and septum secundum are associated with fetal heart development.
• Both septum primum and secundum contain a foramen
• They start to develop during the embryonic stage

What is the Difference Between Septum Primum and Septum Secundum?

Septum primum is a flap-like structure that divides the left and right of the primitive atrium of the embryo. Septum secundum is a fold that is important in the development of the embryonic heart since it fuses with the septum primum and forms the interatrial septum. Thus, this is the key difference between septum primum and septum secundum. Moreover, the septum primum develops at the end of the fourth week, while the septum secundum develops at the end of the fifth week or the beginning of the sixth week.

The below infographic presents the differences between septum primum and septum secundum in tabular form for side by side comparison.

Summary – Septum Primum vs Septum Secundum

Septum primum is the structure that divides the left and right sides of the atrium on the heart of a human embryo. This septum is thin and comprises a crescent-shaped membrane that grows down from the top layer of the primitive atrium towards the developing endocardial cushions. Septum primum is the structure that allows the blood to flow from the right side to the left side of the heart initially through an opening called foramen primum and later foramen secundum. Septum secundum is a muscular flap-like structure that is important in the closure of the foramen ovale after birth. So, this summarizes the difference between septum primum and septum secundum. Both septum primum and septum secundum are important in the development of a fetus.

Reference:

1. Naqvi, Nitha, et al. “Anatomy of the Atrial Septum and Interatrial Communications.” Journal of Thoracic Disease, AME Publishing Company, Sept. 2018.

Image Courtesy:

1. “Septum primum and Septum secundum – early 7th week” By Yahia.Mokhtar – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “FO-Anatomie” By Pezard – Own work (CC BY-SA 3.0) via Commons Wikimedia