Mitochondria are dynamic organelle that changes its shape, size as well as it moves with in the cell. Under stress conditions like hypoxia, mitochondria undergoes fission (fragmented and/or round) and under some specific nutrient starvation condition it undergoes fusion. The fused form of mitochondria appears to be tubular at low resolution which I guess you were referring to as fibrils. The tubular or fragmented form is dependent upon the cell type, stress, age and many other factors like lipids. In a normal unstressed cell, you will predominantly observe tubular form.

Kalimera Thanasis, most cordial hello from Germany - I am missing times of CLUSTOXDNA!

I do not know at what magnification are you working. My first guess is that your are observing whole mitochondria as fluorescent fibers (mitochondria often have a fiber-like morphology). In this, I agree entirely with Abhishek.

My best wishes to you,

Andriy

Hi Andriy! Good to hear from you, you're so right, clustoxdna was a good network, I still remember the good people and collaborations!

I've send you a personal e-mail. Let me know if it hasn't reached you in case I have the wrong e-mail.

With my best regards,

Thanasis

@Andriy Khobta