Neuroendocrinology and neurobiology of sebaceous glands.

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

New aspects in acne inflammation.

There is ample clinical evidence suggesting that the nervous system such as emotional stress can influence the course of acne. We examined possible participation of cutaneous neurogenic factors including neuropeptides, neuropeptide-degrading enzymes and neurotrophic factors, in association with inflammation in the pathogenesis of acne. Immunohistochemical studies revealed that substance P (SP)-immunoreactive nerve fibers were in close apposition to the sebaceous glands, and that neutral endopeptidase (NEP) was expressed in the germinative cells of the sebaceous glands in the skin from acne patients. Nerve growth factor showed immunoreactivity only within the germinative cells. In addition, an increase in the number of mast cells and a strong expression of endothelial leukocyte adhesion molecule-1 on the postcapillary venules were observed in adjacent areas to the sebaceous glands. In vitro, the levels and the expression of stem cell factor by fibroblasts were upregulated by SP. When organ-cultured normal skin specimens were exposed to SP, we observed significant increases in the sizes of the sebaceous glands and in the number of sebum vacuoles in sebaceous cells. Furthermore, supplementation of SP to organ-cultured skin induced expression of NEP, and we demonstrated the subcellular localization of NEP in the endoplasmic reticulum and the Golgi apparatus within the sebaceous germinative cells using preembedding immunoelectron microscopy. These findings suggest that SP may stimulate lipogenesis of the sebaceous glands which may be followed by proliferation of Propionibacterium acnes, and may yield a potent influence on the sebaceous glands by provocation of inflammatory reactions via mast cells. Thus, cutaneous neurogenic factors should contribute to onset and/or exacerbation of acne inflammation.

Neuropeptides and sebaceous glands.

This review provides a new insight into the participation of neuropeptides, notably substance P (SP), in the pathophysiology of acne. We show morphological alterations of sebaceous glands elicited by SP and differences in expression of various neurogenic factors in association with sebaceous glands in acne-prone versus normal facial skin. In vitro studies reveal that SP promotes both the proliferation and the differentiation of sebaceous glands. SP induces the expression of neutral endopeptidase, a potent neuropeptide-degrading enzyme, in sebaceous germinative cells and of E-selectin by perisebaceous venules. Facial skin from acne patients is characterized by rich innervation, by increased numbers of SP-containing nerves and mast cells, and by strong expression of neutral endopeptidase in sebaceous glands and E-selectin in venules around sebaceous glands, compared with normal skin. Mast cell-derived IL-6 and TNF-alpha, followed by SP-stimulated degranulation, have the potential to induce nerve growth factor expression by sebaceous cells which results in the promotion of innervation and in the expression of E-selectin, respectively. SP enhances mast cell proliferation through up-regulation of stem cell factor expression in fibroblasts. These findings suggest the involvement of neurogenic factors, such as neuropeptides, in the disease process of acne and explain the possible mechanism of the exacerbation of acne from a neurological point of view.

Acetylcholinesterase-positive and paraformaldehyde-induced-fluorescence-positive innervation in the upper eyelid of the sheep (Ovis aries).

This is the first study which describes the innervation of some eyelid structures, such as the glands of Moll and the glands of Zeiss. It is also the first to investigate the innervation pattern of the eyelid as a whole. We have studied the acetylcholinesterase-positive and paraformaldehyde-induced-fluorescence-positive (FIF+) innervation pattern of the different structures that constitute the upper eyelid of the sheep. There is widespread acetylcholinesterase-positive innervation in the epithelium, but not such an abundant FIF+ innervation. Both types of innervation are represented in the connective tissue by trunks or fibers that are distributed towards the different structures immersed within them. In the glands of Zeiss, cholinesterase-positive innervation is much more widespread than FIF innervation. On the contrary, the glands of Moll present denser FIF+ innervation than acetylcholinesterase-positive innervation. The Meibomian glands and the lachrymal glands show a rich acetylcholinesterase-positive and FIF+ innervation. Eyelid muscle innervation is mainly acetylcholinesterase-positive. In the conjunctive membrane there is no acetylcholinesterase-positive innervation, and only scarce FIF+ fibers can be demonstrated.

Light and electron microscopic study of the distribution of substance P-immunoreactive fibers and neurokinin-1 receptors in the skin of the rat lower lip.

Cutaneous antidromic vasodilatation and plasma extravasation, two phenomena that occur in neurogenic inflammation, are partially blocked by substance P (SP) receptor antagonists and are known to be mediated in part by mast cell-released substances, such as histamine, serotonin, and nitric oxide. In an attempt to provide a morphological substrate for the above phenomena, we applied light and electron microscopic immunocytochemistry to investigate the pattern of SP innervation of blood vessels and its relationship to mast cells in the skin of the rat lower lip. Furthermore, we examined the distribution of SP (neurokinin-1) receptors and their relationship to SP-immunoreactive (IR) fibers. Our results confirmed that SP-IR fibers are found in cutaneous nerves and that terminal branches are observed around blood vessels and penetrating the epidermis. SP-IR fibers also innervated hair follicles and sebaceous glands. At the ultrastructural level, SP-IR varicosities were observed adjacent to arterioles, capillaries, venules, and mast cells. The varicosities possessed both dense core vesicles and agranular synaptic vesicles. We quantified the distance between SP-IR varicosities and blood vessel endothelial cells. SP-IR terminals were located within 0.23-5.99 microm from the endothelial cell layer in 82.7% of arterioles, in 90.2% of capillaries, and in 86.9% of venules. Although there was a trend for SP-IR fibers to be located closer to the endothelium of venules, this difference was not significant. Neurokinin-1 receptor (NK-1r) immunoreactivity was most abundant in the upper dermis and was associated with the wall of blood vessels. NK-1r were located in equal amounts on the walls of arterioles, capillaries, and venules that were innervated by SP-IR fibers. The present results favor the concept of a participation of SP in cutaneous neurogenic vasodilatation and plasma extravasation both by an action on blood vessels after binding to the NK-1r and by causing the release of substances from mast cells after diffusion through the connective tissue.

Innervation of the digit on the forepaw of the raccoon.

The innervation of the digits on the raccoon forepaw was examined by using immunochemistry for protein gene product 9.5, calcitonin-gene related peptide, substance P, neuropeptide-Y, tyrosine hydroxylase, and neurofilament protein. The larger-caliber axons in the ventral glabrous skin terminate as Pacinian corpuscles deep in the dermis, small corpuscles and Merkel endings around the base of dermal papillae, and Merkel endings on rete pegs in dermal papillae. Extensive fine-caliber innervation terminates in the epidermis and on the microvasculature. The innervation is more dense in the distal than in the proximal volar pads. Pacinian endings are also concentrated in the transverse crease separating the distal and proximal pads. In the dorsal hairy skin, hair follicles are well innervated with piloneural complexes. Merkel innervation is located under slight epidermal elevations and in some large Merkel rete pegs located at the apex of transverse skin folds just proximal to the claw. No cutaneous Ruffini corpuscles were found anywhere on the digit. The claw is affiliated with dense medial and lateral beds of Pacinian endings, bouquets of highly branched Ruffini-like endings at the transition from the distal phalanx and unmyelinated innervation in the skin around the perimeter. Encapsulated endings are located at the lateral edge of the articular surface of the distal phalanx. Extensive fine-caliber innervation is affiliated with sweat glands and with the vasculature and is especially dense at presumptive arteriovenous sphincters. Virtually all of the sweat gland and vascular innervation is peptidergic, whereas most of the unmyelinated epidermal innervation is nonpeptidergic.

Structure, vascularization, and innervation of the mystacial pad of the rat as revealed by the lectin Griffonia simplicifolia.

The mystacial pad of the rat is endowed with rows of vibrissal follicle-sinus complexes (F-SCs) that receive a dense and rich variety of innervation, much of which is C fibers. Each F-SC consists of a follicle at the core of a spindle-shaped, encapsulated vascular sinus. Previous studies have shown that the B subunit of the lectin Griffonia simplicifolia (GSA I-B4) binds selectively to a subset of small neurons in the trigeminal ganglion and to a subset of C fibers preferentially distributed to inner lamina II and outer lamina III of nucleus caudalis in the brainstem trigeminal complex in the rat. These laminae are also a major site of termination for afferents in superficial vibrissal nerves (SVNs) that innervate the upper portion of F-SCs. To determine the peripheral distribution of the afferents that bind GSA I-B4, mystacial pads from rats were prepared for fluorescence microscopy with GSA I-B4 conjugated to rhodamine. At the neck of each F-SC, numerous circumferentially oriented bundles of fine-caliber axonal profiles were labeled in the inner conical body, which receives nearly all of its innervation from the SVNs. A sparse, random distribution of fine-caliber profiles from deep vibrissal nerves was labeled at the level of the cavernous sinus in the deep half of the F-SCs. GSA I-B4 also labeled a variety of nonneural structures. By binding to vascular linings, GSA I-B4 revealed a dense, highly organized capillary system within the mesenchymal sheath that forms the inner lining of the vascular sinuses. Thus each F-SC appears to have a closed capillary system within the open vascular sinus. Trabeculae within the lumen of the cavernous sinus were also revealed to span between the sinus capsule and the mesenchymal sheath only about midway along the length of the follicle instead of the entire deeper half, as was previously believed. in addition, GSA I-B4 bound to the surface of follicular cells preferentially in the superficial half of the F-SCs. Sweat glands within the intervibrissal fur and some cells within sebaceous glands in F-SCs were also labeled as well as their ducts. The potential functional implications of these various features are discussed.

Multiple afferent innervation of primate facial hairs--Henry Head and Max von Frey revisited.

Large guard hairs as well as small vellus hairs are multiple innervated having lanceolate terminals of variable number. Ruffini corpuscles consisting of fine axonal ramifications are arranged circularly and located external to the lanceolate terminals. Free nerve endings (FNE's) can also be identified on some hairs distinct from Ruffini terminals. Ruffini terminals and FNE's are usually innervated by axons from the superficial dermal nerve net whereas lanceolate terminals are innervated by axons from the deeper portions of the dermal nerve net. All guard hairs have both types of terminals (lanceolate and Ruffini) confirming Hoggan and Hoggan, Retzius and Symonowicz, and most guard hairs have presumptive FNE's. Many vellus hairs have only small Ruffini endings or FNE's. The diameter of axons supplying Ruffini terminals is 1-2 micrometer and those to lanceolate terminals is 2-4 micrometers. Axons innervating lanceolate and Ruffini terminals branch rarely as correlated with small punctate receptive fields. FNE's branch widely and are correlated with large receptive fields of known nociceptors. The multiplicity of anatomically defined terminals is consistent with the known diversity of physiologically defined hair mechanoreceptive afferents as well as perceptual complexity of human hairy skin. The concept of multiple innervation of hairs confirms Head's prediction and could provide the anatomical basis of Head's basic thesis of altered sensibilities in nerve regeneration (i.e. epicritic and protopathic responses). Head's concept of two separate nervous systems, however, is an over-simplification in the light of current knowledge.

Innervation of the skin of camel (Camelus dromedarius) as revealed by cholinesterase technique.

Skin samples from 4 body sites were taken from 10 camels and histochemically treated for the localization of AChE and BuChE enzymes. The sebaceous and sewat glands were active site for both enzymes. The weat gland were innervated by a plexus of AChE-positive nerve fibers. In the papillary layer, the nerve breaks to form a plexus supplying the blood vessels, from this plexus fibers end in the deep interface of the epidermis. End bulbs and free intraepidermal nerve ending reactive for AChE were demonstrated.

Innervation of the rat preputial gland.

Acetylcholinesterase, butyrylcholinesterase and monoamine oxidase activities were observed in the neuronal and non-neuronal components of the preputial gland. Histological, histochemical and experimental studies clearly indicate that the preputial gland of the rat is innervated by cholinergic as well as adrenergic nerves.

Central nervous system control of mucous gland secretion from frog skin.

Surface electrical stimulation of the diencephalic roof of the frog. Rana berlandieri forreri, brain initiates integumental mucous gland secretion which is abolished by pithing.

[The morphogenesis and functional control of sebaceous glands with reference to free sebaceous glands]. / Zur Genese und Funktionssteuerung der Talgdrüsen unter Berücksichtigung freier Talgdrüsen

Not very much was known until now about the physiology of sebaceous glands--especially about the regulation of their function. The author divides primary stimuli for the function of sebaceous glands during lifetime from secondary stimuli for the common supply of sebum. Other primary stimuli are a hypophysial sebotropic factor and other hormones of the gonads and adrenal glands. Capillary attraction and the autonomous system (in a low degree) belong to the secondary stimuli. Following the opinion of renowned authors sebaceous glands and production of sebum are just a special secretoric function of the epidermis. The author voices the hypothesis that androgens can stimulate epidermal basal cells to form sebaceous glands and to produce sebum. This is only possible under certain circumstances, above all only in non-keratinised epidermis.

[Experimental studies on seborheric alopecia. III. Localization of testosterone receptors in human hairy follicles]. / Estudios experimentales sobre la alopecia seborréica. III. Localización de los receptores de la testosterona en folículos pilosos humanos

The author studies the location of the testosterone receptors in huamn hair follicles, by means of the isolation of the two most outstanding fractions of the pilosebaceous follicle, i.e., hair follicle and sebaceous gland. The two fractions were obtained by micro-dissection, homogenisation by separate, and isolation of the correspondig cytosol fraction. It was demonstrated that the two fractions possess the same binding activity against the andreogen tested.