Torri Tarelli L, Petruccioli MG (1971) Studies on the ultrastructure of collagen fibrils: II. Filamentous structure with negative staining. J Submicrosc Cytol 3:153–170.

NA of lenscalculator

As a result of the highly developed tendon fibers (Fig. A1), tendon cells are laterally compressed, and this leads to the formation of thin, winglike cytoplasmic processes (Fig. A2). Tendon cells are fibrocytes which have predominantly adapted to synthesize collagen. The cell body contains an ovoid nucleus with condensed chromatin. The cytoplasm displays the features of a cell highly involved in protein synthesis, i.e., well-developed rough endoplasmic reticulum and great numbers of free ribosomes.

Yamada M (1976) Ultrastructural and cytochemical studies on the calcification of the tendon-bone joint. Arch Histol Jpn 39:347–378.

Numerical aperture formula

Greenlee TK Jr, Ross R (1967) The development of the rat flex or digital tendon, a fine structure study. J Ultrastruct Res 18:354–376.

Numerical apertureofoptical fiber

No, there's really not such a thing as a negative numerical aperture. Understand that NA is about an image forming lens, bringing the incoming light (in the case you mentioned, collimated) to a single focus point. Meaning, a net-convex system.

Trelstad RL, Hayashi K (1979) Tendon collagen fibrillogenesis: intracellular subassemblies and cell surface changes associated with fibril growth. Dev Biol 71:228–242.

I understand that it's tempting to look at an equation such as NA = n sin θ, and wonder what meaning can be inferred if n or θ were negative.

Na of lensformula

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Krstić, R.V. (1985). Regular Dense Connective Tissue. Tendon Cell. In: General Histology of the Mammal. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-70420-8_80