Sympathetic innervation of the supraclavicular brown adipose tissue: A detailed anatomical study

Abstract

Background: The supraclavicular fossa is the dominant location for human brown adipose tissue (BAT). Activation of BAT promotes non-shivering thermogenesis by utilization of glucose and free fatty acids and has been the focus of pharmacological and non-pharmacological approaches for modulation in order to improve body weight and glucose homeostasis. Sympathetic neural control of supraclavicular BAT has received much attention, but its innervation has not been extensively investigated in humans.

Methods: Dissection of the cervical region in human cadavers was performed to find the distribution of sympathetic nerve branches to supraclavicular fat pad. Furthermore, proximal segments of the 4th cervical nerve were evaluated histologically to assess its sympathetic components.

Results: Nerve branches terminating in supraclavicular fat pad were identified in all dissections, including those from the 3rd and 4th cervical nerves and from the cervical sympathetic plexus. Histology of the proximal segments of the 4th cervical nerves confirmed tyrosine hydroxylase positive thin nerve fibers in all fascicles with either a scattered or clustered distribution pattern. The scattered pattern was more predominant than the clustered pattern (80% vs. 20%) across cadavers. These sympathetic nerve fibers occupied only 2.48% of the nerve cross sectional area on average.

Conclusions: Human sympathetic nerves use multiple pathways to innervate the supraclavicular fat pad. The present finding serves as a framework for future clinical approaches to activate human BAT in the supraclavicular region.

Fig 1. Cervical nerve plexus in the left supraclavicular region.

Nerves in the left supraclavicular region are carefully dissected and digitally enhanced with light-yellow (left). Cervical nerve plexus, the sympathetic nerve, and the vagus nerve are shown. Dissection focusing on the innervation of the supraclavicular fat pad is challenging because these nerves can only be visualized after meticulous and thorough removal of the supraclavicular fat pad surrounding these nerves. White arrowheads denote the cut edge of the nerve branch from the 3^rd cervical nerve (C3) initially innervating the removed supraclavicular fat pad. Even in the donated elderly bodies, brown adipose tissue (right upper) can be detected histologically in the removed supraclavicular fat pad. Compared to white adipose tissue region (right lower), brown adipose tissue region shows multilocular and smaller adipocytes. A, artery; M, muscle; N, nerve.

Fig 2. Multiple nerves innervating the right supraclavicular fat pad.

Right supraclavicular fat pad is preserved en bloc as far as possible with its innervation (left). The middle part of the right clavicle is removed for a better field of view. Multiple nerve branches innervate this region, and the right suprascapular artery is running in this region. The cervical nerves, vagus nerve and its branches, cervical sympathetic plexus, and arteries are digitally enhanced in yellow, purple, green, and pink, respectively. Red arrowheads denote direct sympathetic nerve branches originating from the cervical sympathetic plexus, innervating the supraclavicular fat pad. Yellow arrowhead shows the communicating ramus from the cervical sympathetic nerves to the proximal segment of the 4^th cervical nerve (C4). White arrowhead indicates the nerve branch from the C4 innervating the supraclavicular fat pad. A, artery; M, muscle; N, nerve.

Fig 3. Histology of the multiple nerves innervating the right supraclavicular fat pad.

The supraclavicular fat pad in Fig 2 was removed en bloc with its innervating nerves (left). White arrowheads denote the nerve fibers from the 3^rd (C3) and 4^th (C4) cervical nerves innervating the supraclavicular fat pad. Red arrowheads indicate the thin nerve fibers originating from the cervical sympathetic plexus, which directly innervate the supraclavicular fat pad. The middle and right panels show the hematoxylin & eosin and tyrosine hydroxylase stains of these nerves, respectively. Upper and middle panels show middle and distal part of the C4, respectively. Tyrosine hydroxylase stain shows scattered pattern of sympathetic nerve fascicles. Bottom panels show the nerve from the cervical sympathetic plexus, showing a ganglion involved within the nerve. The suprascapular artery is the main artery running through the supraclavicular fat pad. Histology of the suprascapular artery confirms the sympathetic nerve fibers running along this artery (green arrowheads). A, artery.

Fig 4. Histology of the proximal segment of the 4^th cervical nerve.

The proximal segment of the 4^th cervical nerve (C4) just lateral to the longus capitis muscle is cross-sectioned and stained with hematoxylin & eosin and for tyrosine hydroxylase. The nerve involves multiple fascicles (left). Middle panels show representative single fascicles showing scattered (upper) and clustered (lower) patterns. The right panels show magnified images of the black-rimmed box in the middle lower panel, showing the cluster of fibers stained positive for tyrosine hydroxylase located at the rim of the fascicle adjacent to the perineurium. Although specific myelin stain was not carried out, these fibers are likely to be unmyelinated postganglionic nerve fibers from the morphological observation with hematoxylin & eosin stain.

Fig 5. Estimated sympathetic pathways innervating the supraclavicular fat pad.

Left panel shows the real dissection of the human right stellate ganglion. Middle panel shows the representative histology of the supraclavicular fat pad obtained from cervical surgery. Tyrosine hydroxylase stain reveals selective sympathetic innervation to the supraclavicular brown adipose tissue. The right panel shows the schematic illustration of potential pathways (1-4) innervating the supraclavicular fat pad. Those involve nerve branches from the 3^rd cervical nerve (C3) (route 1), 4^th cervical nerve (C4) (route 2), cervical sympathetic plexus (red arrowhead) (route 3), and the stellate ganglion (green arrowhead) along the suprascapular artery (Figs 2 and 3) (route 4). Yellow arrowheads denote communicating rami from the cervical superior cervical ganglion to the proximal segment of the C3 and C4. The direct connection between the sympathetic nerve branch running along with the suprascapular artery and the stellate ganglion itself could not be identified in our dissection. A, artery; N, nerve.

Fig 6. Patient with a cervical tumor and infrared thermography finding.

Volume-rendered image of the computed tomography viewed from the right lateral direction (left) shows cervical tumor (green) adjacent to the right internal jugular vein (blue) and carotid artery (red). The location of the tumor is 3^rd (C3) to 4^th (C4) cervical nerve level. Horizontal section viewed from the inferior direction (middle) confirms compression of the right internal jugular vein (white arrowhead), which is also close to the estimated location of the proximal segment of the C4 (yellow dotted lines) just lateral to the longus capitis muscle. Right panels show the pre- and post-surgical result of the infrared thermography of the right supraclavicular region. The increase in the relative temperature (difference between the sternal and supraclavicular temperatures) in response to the cold pressor (blue bar) increased further after surgery (Δ0.164°C to Δ0.270°C). A, artery; M, muscle.