Anatomy‐Histology

Tissues from five human freshly frozen cadavers were examined. None of the individuals died due to a neurological disease. Table ​Table11 provides an overview of the cadavers and the measurements performed on each sample. Certain measurement techniques were not available for all tissue samples.

Overview of the Cadavers Included With Patient Characteristics and the Measurements Performed

SCF, subcutaneous fat; adnexa/sliding fat, ADF, fat close to sciatic nerve; IFAS, sciatic fascicular nerve tissue; NFAS, sciatic non‐fascicular nerve tissue.

As a target region, the sciatic nerve was chosen because of its size. In the upper leg, samples of subcutaneous fat (SCF), fat close to the sciatic nerve (adnexa / sliding fat, ADF), sciatic fascicular nerve (IFAS) tissue, and sciatic non‐fascicular nerve (NFAS) tissue were dissected. Adipose tissue is composed of collagen and adipocytes filled with fat droplets; adipose tissue is referred to as fat in the text below 47. Figure ​Figure11 shows an anatomical drawing of the sciatic nerve, including descriptions of the different structures that make up the nerve bundle. The figure also shows a bifurcated sciatic nerve and the fascicles microscopically dissected from the nerve bundle.

(A) Anatomical drawing of a cross‐section of a nerve: (1) subcutaneous adipose tissue, (2) adnex adipose tissue (sliding fat), (3) non‐fascicular nerve tissue and epineurium, (4) blood vessel, and (5) fascicular nerve tissue and perineurium. (B) Part of a sciatic nerve with bifurcation to the tibial (TN) and peroneal (PN) nerve. (C)Fascicles of a sciatic nerve with surrounding epineurium D: Fascicles without epineurium.

Fat samples were collected macroscopically. Three SCF and ADF samples with a volume of 2 ml per cadaver and a piece for cross‐sectional histology were saved. For the collection of 2 ml of fascicular and non‐fascicular nerve tissue, a microscope was used. Immediately after preparation of the samples, samples were frozen (liquid nitrogen, −196°C and stored at −20°C) or fixed (4% neutral‐buffered formaldehyde for 24 hours).

Two different staining techniques were used to differentiate between the different tissue compositions: Mayer's hematoxylin and eosin stain (H&E stain) to color cell nuclei blue and cytoplasm pink; 49 osmium tetroxide (OsO₄) to color lipids black 50. Parts of the samples containing fat were post‐fixed in 1% OsO₄ in phosphate‐buffered saline (PBS), pH 7.4; the other samples were fixed in 4% neutral‐buffered formaldehyde. After de‐paraffinization, they were stained according to the selected staining protocol 51.

The fixed and post‐fixed tissues were embedded separately in paraffin. Then, 4‐μm‐thick histological sections of the sciatic nerve and IFAS tissue and 7‐μm‐thick sections of the other tissues were cut on a Leica 2245 microtome. The sciatic nerve and IFAS tissues were cut transversely; the ADF, SCF, and NFAS tissues were cut in a direction suitable to obtain a large tissue area.