Is it correct to use the terms multinucleate and syncitia interchangeably?

Question

If you see the skeletal muscle,it is having several nuclei and is said to be "multinucleate".Similarly the cardiac muscle is also having several nuclei yet it is specifically termed"syncitial".So my question is: Is there any reason behind this or are the terms interchangeable? I am listing the sources from which the question came up in my mind:

[Source1]

Individual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, cylindrical, multi-nucleated cells. Differentiation into this state is primarily completed before birth.[Source2]

The cardiac syncytium is a network of cardiomyocytes connected to each other by intercalated discs that enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium.[Source3]

Answer 1

No, the terms multinucleate and syncytium are not synonymous in this context. Most cardiomyocytes are not multinuclear. To be clear, syncytium is used to refer to multinuclear cells that originate from fusion of uninuclear cells, including the skeletal muscle, but that isn't what is referred to in the context of cardiac muscle.

The Wikipedia page on syncytium summarizes this nicely:

Cardiac tissue is therefore described as a functional syncytium, as opposed to the true syncytium of skeletal muscle.

The "intercalated disks" of cardiac muscle are filled with gap junctions and desmosomes. The desmosomes hold the cells together physically, while gap junctions allow ions (and small molecules) to pass directly between cells, forming "electrical synapses". This allows electrical signals to propagate between the myocytes to coordinate contraction.

Gap junctions are formed by proteins, rather than fusions of the plasma membrane, so they are relatively small. This means that not everything in the cytoplasm can pass through them, mostly just ions and some small molecules. Proteins, for example, would not pass through gap junctions. Therefore, cells that are connected via gap junctions, like cardiomyocytes, are still somewhat independent, unlike fully multinuclear cells.

There are also types of neurons, including some in the CNS, that also form gap junctions and act at least somewhat in concert in this way.

Answer 2

Just to add some information related to the biological terminology as a whole, not only to the cardiac muscle (which was already correctly addressed in the accepted answer):

Multinucleate and syncytium are not synonyms and cannot be used interchangeably.

As the name clearly shows, multinucleate refers to a (single) cell containing several nuclei.

However, according to the mechanism of formation of such cell, it can be classified as:

• Coenocyte: when a single cell with a single nucleus performs several cell divisions without dividing the cytoplasm (cytokinesis);
• Syncytium: when several uninucleate cells aggregate, forming a single multinucleate cell.

Therefore, every syncytium is a multinucleate cell, but not every multinucleate cell is a syncytium.