Innervation of supraclavicular adipose tissue: A human cadaveric study

Abstract

Functional brown adipose tissue (BAT) was identified in adult humans only in 2007 with the use of fluorodeoxyglucose positron emission tomography imaging. Previous studies have demonstrated a negative correlation between obesity and BAT presence in humans. It is proposed that BAT possesses the capacity to increase metabolism and aid weight loss. In rodents it is well established that BAT is stimulated by the sympathetic nervous system with the interscapular BAT being innervated via branches of intercostal nerves. Whilst there is evidence to suggest that BAT possesses beta-3 adrenoceptors, no studies have identified the specific nerve branch that carries sympathetic innervation to BAT in humans. The aim of this study was to identify and trace the peripheral nerve or nerves that innervate human BAT in the supraclavicular region. The posterior triangle region of the neck of cadaveric specimens were dissected in order to identify any peripheral nerve branches piercing and/or terminating in supraclavicular BAT. A previously undescribed branch of the cervical plexus terminating in a supraclavicular adipose depot was identified in all specimens. This was typically an independent branch of the plexus, from the third cervical spinal nerve, but in one specimen was a branch of the supraclavicular nerve. Histological analysis revealed the supraclavicular adipose depot contained tyrosine hydroxylase immunoreactive structures, which likely represent sympathetic axons. This is the first study that identifies a nerve branch to supraclavicular BAT-like tissue. This finding opens new avenues for the investigation of neural regulation of fat metabolism in humans.

Fig 1. Diagram outlining the posterior triangle of the neck.

The posterior triangle of the neck is shown by the blue outline. The adipose depot of the supraclavicular region is located in the inferior (lower) portion of the posterior triangle. The origin of the cervical plexus and its branches mainly lie in the superior (upper) part of the posterior triangle.

Fig 2. Lateral view of left side of dissected specimens.

Dissected specimens showing depot of supraclavicular adipose tissue and surrounding nerves in the posterior triangle of the neck. (A) Specimen number 2. Innervation to adipose tissue as direct branch from cervical plexus. (B) Specimen number 1. Innervation to adipose tissue as branch of supraclavicular nerve. 1 = sternocleidomastoid muscle; 2 = clavicle (cut in B); 3 = omohyoid muscle (A); 4 = trapezius muscle; 5 = supraclavicular depot of adipose tissue; 6 = nerve innervating supraclavicular adipose tissue; 7 = dorsal scapular nerve (A); 8 = subcutaneous white adipose tissue (A); 9 = Branches of cervical plexus marked by the box; 10 = transverse cutaneous nerve of the neck (A); 11 = supraclavicular nerve; A = anterior; P = posterior; S = superior; I = inferior.

Fig 3. Comparison of adipocyte morphology.

(A) Example of white adipocytes from axilla stained with hematoxylin and eosin. (B) Example of the darker BAT-like adipocytes from supraclavicular region stained with hematoxylin and eosin. (C) Adipocyte mean cross sectional area (± SEM) of adipose from axilla (Ax) (n = 75 adipocytes from one specimen) versus adipocytes from supraclavicular region (Sc) (n = 75 adipocytes from the same specimen). **** indicates p<0.001 by two-tailed Student’s t-test.

Fig 4. Tyrosine hydroxylase immunoreactivity in adipose tissue.

BAT-like adipocytes taken from the supraclavicular region imaged with (A) brightfield microscopy, (C) fluorescence microscopy, (E) brightfield and fluorescence merged, (G) brightfield and fluorescence merged and zoomed in. White adipocytes taken from the axilla imaged with (B) brightfield microscopy, (D) fluorescence microscopy, (F) brightfield and fluorescence merged, (H) brightfield and fluorescence merged and zoomed in. D, F and H were captured with 10x longer exposure time than C, E and G.