Innervation of dog ciliary ganglion

Summary There are species differences with regard to the composition of the ciliary ganglion. For instance, in rabbits and cats it consists solely of oculomotor nerves and has no sympathetic or sensory innervation. The purpose of this study is to clarify the participation of these nerves in the ciliary ganglion of the dog by histochemical methods. Cholinesterase (ChE) activity was studied by Karnovsky's method and catecholamine fluorescence by the glyoxylic acid method. Furthermore, the origins of the respective nerves were investigated by a serial preparation method, involving unilateral cervical sympathectomy and tracer dye injection in the ganglion. The results obtained were: (1) Ciliary ganglion cells showed intense ChE activity. Oculomotor nerve fibers leading to the ganglion showed moderate ChE activity, while the reaction in the short ciliary nerves was strong. (2) Aminergic nerves were present in the intercellular space of the ciliary ganglion, and bilateral or central innervation was suggested by the results of cervical sympathectomy. (3) Connection between the ciliary and trigeminal ganglia was proved by the dye tracer study. The results show that the ciliary ganglion in dogs is composed of oculomotor, trigeminal and sympathetic nerves.     

Neuropeptide Y: presence in sympathetic and parasympathetic innervation of the nasal mucosa

Summary The occurrence of neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) in the sympathetic and parasympathetic innervation of the nasal mucosa was studied in various species including man. A dense network of NPY-immunoreactive (IR) fibres was present around arteries and arterioles in the nasal mucosa of all species studied. NPY was also located in nerves around seromucous glands in pig and guinea-pig, but not in rat, cat and man. The NPY-IR glandular innervation corresponded to about 20% of the NPY content of the nasal mucosa as revealed by remaining NPY content determined by radioimmunoassay after sympathectomy. These periglandular NPY-positive fibres had a distribution similar to the VIP-IR and PHI-IR nerves but not to the noradrenergic markers tyrosine hydroxylase (TH) or dopamine--hydroxylase (DBH). The NPY nerves around glands and some perivascular fibres were not influenced by sympathectomy and probably originated in the sphenopalatine ganglion where NPY-IR and VIP-IR ganglion cells were present. The venous sinusoids were innervated by NPY-positive fibres in all species except the cat. Dense NPY and DBH-positive innervation was seen around thick-walled vessels in the pig nasal mucosa; the latter may represent arterio-venous shunts. Double-labelling experiments using TH and DBH, and surgical sympathectomy revealed that the majority of NPY-IR fibres around blood vessels were probably noradrenergic. The NPY-positive perivascular nerves that remained after sympathectomy in the pig nasal mucosa also contained VIP/PHI-IR. The major nasal blood vessels, i.e. sphenopalatine artery and vein, were also densely innervated by NPY-IR fibres of sympathetic origin. Perivascular VIP-IR fibres were present around small arteries, arterioles, venous sinusoids and arterio-venous shunt vessels of the nasal mucosa whereas major nasal vessels received only single VIP-positive nerves. The trigeminal ganglion of the species studied contained only single TH-IR or VIP-IR but no NPY-positive ganglion cells. It is concluded that NPY in the nasal mucosa is mainly present in perivascular nerves of sympathetic origin. In some species, such as pig, glandular and perivascular parasympathetic nerves, probably of VIP/PHI nature, also contain NPY.     

Histochemical demonstration of cholinesterase activity in the cornea of the rat and the effect of various denervations on the corneal nerves

Summary Cholinesterase (ChE) activities of the rat cornea were demonstrated histochemically by using both light and electron microscopes. Acetylcholinesterase (AChE) reaction was localized in the axolemma of the nerves in the corneal stroma. The epithelial cell membranes and the intraepithelial nerve endings also showed AChE reaction.Non-specific cholinesterase (NsChE) activity was observed only in the endothelial cell membranes.Cervical sympathectomy, ciliary ganglionectomy and stereotactic coagulation of the ophthalmic division of the trigeminal nerve were performed in order to study the routes of the AChE-containing nerves to the cornea. The disappearence of AChE-containing nerves was observed only after ophthalmic neurotomy. It is suggested that the AChE-containing nerves are distributed to the rat cornea exclusively via the ophthalmic nerve. They seem to be sensory nerves.     

Functional and anatomical variability of canine cardiac sympathetic efferent pathways: implications for regional denervation of the left ventricle

To further elucidate the functional anatomy of canine cardiac innervation as well as to assess the feasibility of producing regional left ventricular sympathetic denervation, the chronotropic and (or) regional left ventricular inotropic responses produced by stellate or middle cervical ganglion stimulation were investigated in 22 dogs before and after sectioning of individual major cardiopulmonary or cardiac nerves. Sectioning the right or left subclavian ansae abolished all cardiac responses produced by ipsilateral stellate ganglion stimulation. Sectioning a major sympathetic cardiopulmonary nerve, other than the right interganglionic nerve, usually reduced, but seldom abolished, regional inotropic responses elicited by ipsilateral middle cervical ganglion stimulation. Sectioning the dorsal mediastinal cardiac nerves consistently abolished the left ventricular inotropic responses elicited by right middle cervical ganglion stimulation but minimally affected those elicited by left middle cervical ganglion stimulation. In contrast, cutting the left lateral cardiac nerve decreased the inotropic responses in lateral and posterior left ventricular segments elicited by left middle cervical ganglion stimulation but had little effect on the inotropic responses produced by right middle cervical ganglion stimulation. In addition, the ventral mediastinal cardiac nerve was found to be a significant sympathetic efferent pathway from the left-sided ganglia to the left ventricle. These results indicate that the stellate ganglia project axons to the heart via the subclavian ansae and thus effective sympathetic decentralization can be produced by cutting the subclavian ansae; the right-sided cardiac sympathetic efferent innervation of the left ventricle converges intrapericardially in the dorsal mediastinal cardiac nerves; and the left-sided cardiac sympathetic efferent innervation of the left ventricle diverges to innervate the left ventricle by a number of nerves including the dorsal mediastinal, ventral mediastinal, and left lateral cardiac nerves. Thus consistent denervation of a region of the left ventricle can not be accomplished by sectioning an individual cardiopulmonary or cardiac nerve because of the functional and anatomical variability of the neural components in each nerve, as well as the fact that overlapping regions of the left ventricle are innervated by these different nerves.     

Effect of preganglionic sympathectomy on the cholinesterase activity in the superior cervical ganglia of rats and hamsters

Summary By employing biochemical assay and histochemical enzyme techniques the effect of preganglionic sympathectomy on the cholinesterase (ChE) activity in the superior cervical ganglia of rats and hamsters was investigated. Biochemical assays indicate that the ChE activity in the superior cervical ganglia of adult rats and hamsters is 57.19 and 28.63 respectively (expressed in u moles acetylcholine hydrolyzed per min per g of tissue); two weeks after preganglionic denervation, about 50% and 60% of ChE activity are lost respectively. Histochemical enzyme examination reveals that in the rat superior cervical ganglion, the majority of the neurons are adrenergic with weak to moderate acetylcholinesterase (AChE) reaction and the minority of the neurons are cholinergic with strong AChE activity, while only one type of adrenergic neurons exhibits a weak AChE activity in the hamster superior cervical ganglion. The AChE activity is localized in the perinuclear area, in the cisternae of the rough surfaced endoplasmic reticulum, in the Golgi complex and on the plasma membrane of the hamster's neurons; it is mainly localized in the cisternae of the rough surfaced endoplasmic reticulum of the rat's neurons. AChE reaction product is also detected on the axolemmal membranes of the preganglionic nerve fibers in the sympathetic ganglia of rats and hamsters.After preganglionic sympathectomy, the AChE activity in the adrenergic neurons and in the preganglionic unmyelinated nerve fibers is markedly reduced, whereas the cholinergic neurons and preganglionic myelinated nerve fibers remain unchanged. On the basis of these results two conclusions have been reached: (1) The fact that strong AChE activity localized in the cholinergic neurons and preganglionic myelinated fibers is not influenced by denervation, suggests that these structures are able to produce AChE. (2) The reduction of AChE activity in the rat and hamster superior cervical ganglia two weeks after preganglionic denervation, observed by histochemical examination, can be correlated with a concomitant measurable reduction determined by biochemical assays.Supported in part by a grant from the National Science Council, Republic of China. The author wishes to express his gratitude to the Department of Pharmacology, College of Medicine, National Taiwan University, for the use of its equipment for biochemical assays     

Both neural crest and placode contribute to the ciliary ganglion and oculomotor nerve

The chick ciliary ganglion is a neural crest-derived parasympathetic ganglion that innervates the eye. Here, we examine its axial level of origin and developmental relationship to other ganglia and nerves of the head. Using small, focal injections of DiI, we show that neural crest cells arising from both the caudal half of the midbrain and the rostral hindbrain contribute to the ciliary as well as the trigeminal ganglion. Precursors to both ganglia have overlapping migration patterns, moving first ventrolaterally and then rostrally toward the optic vesicle. At the level of the midbrain/forebrain junction, precursors to the ciliary ganglion separate from the main migratory stream, turn ventromedially, and condense in the vicinity of the rostral aorta and Rathke's pouch. Ciliary neuroblasts first exit the cell cycle at early E2, prior to and during ganglionic condensation, and neurogenesis continues through E5.5. By E3, markers of neuronal differentiation begin to appear in this population. By labeling the ectoderm with DiI, we discovered a new placode, caudal to the eye and possibly contiguous to the trigeminal placode, that contributes a few early differentiating neurons to the ciliary ganglion, oculomotor nerve, and connecting branches to the ophthalmic nerve. These results suggest for the first time a dual neural crest and placodal contribution to the ciliary ganglion and associated nerves.     

An immunohistochemical study on the innervation of SP-IR nerves in the cerebral arteries of the bent-winged bat

Summary The overall distribution of substance P (SP) immunoreactive (IR) nerves surrounding the cerebral arteries of the bent-winged bat were investigated immunohistochemically. In this microchiropteran species, the walls from the vertebral artery to the caudal part of the basilar artery have considerably well-developed plexuses of SP-IR nerves, whereas no demonstrable SP-IR fibers were found in the crostral part of the basilar artery, and in more rostrally located arteries the nerve supply was very sparse or occasionally lacking. This innervation pattern has not yet been established for the cerebral arterial systems of other mammals that have been studied under normal conditions, but it is very similar to the pattern of SP-IR innervation observed in the guinea pig and cat of which the trigeminal ganglia have been destroyed. From the combination of this and other immunohistochemical findings, it is suggested that SP-IR nerves innervating the vertebral and basilar arteries of the bent-winged bat originate from the upper cervical dorsal root ganglia (DRG) and enter the cranial cavity along the vertebral artery and through the meninges.Abbreviations BA basilar artery - CSN cervical spinal nerves - ICS internal carotid system - SCG superior cervical ganglion - SNB sympathetic nerve bundle - VA vertebral artery - VBS vertebro-basilar system     

Are there cholinergic through-fibers in the superior cervical ganglion of the mouse?

Choline acetyltransferase immunocytochemistry was used to detect the presence and distribution of cholinergic through-fibers in the superior cervical ganglion of adult mice. The results revealed a great number of choline acetyltransferase-positive axons in the cervical sympathetic trunk and the varicose terminal axons involved in the innervation of the principal ganglionic cells within the ganglion. Immunostained axons were scarce or absent in the external or internal carotid nerves. The immunocytochemical results argue against the physiological importance of cholinergic through-fibers in the postganglionic nerves.     

On the origin and distribution of vasoactive intestinal polypeptide-, peptide HI-, and cholecystokinin-like-immunoreactive nerve fibers in the rat iris

Summary The presence and distribution of nerve fibers expressing immunoreactivity to the neuropeptides vasoactive intestinal polypeptide, peptide HI and cholecystokinin was examined in stretch-prepared rat iris whole mounts. By use of antiserum to vasoactive intestinal polypeptide an irregular, relatively sparse network of varicose, intensely fluorescent fibers was observed innervating both the dilator plate and the sphincter area. Positive fibers were present also in the ciliary body and the choroid membrane. Surprisingly, a large variation in the amount of vasoactive intestinal polypeptide-positive nerves was seen among irides. Furthermore, an uneven distribution of fluorescent nerve fibers was observed within individual irides. Thus, some areas had a relatively dense innervation, whereas others were devoid of immunoreactive nerve fibers. A similar fiber system was detected using antiserum to peptide HI. In all probability, vasoactive intestinal polypeptide and peptide HI coexist within the same nerve population. A denser and more regular network of cholecystokinin-positive fibers was found in normal rat irides. Such fibers were also present in the sphincter area and in high density in the choroid membrane. Neither extirpation of the superior cervical nor the ciliary ganglion caused any detectable decrease in amount of either vasoactive intestinal polypeptide/peptide HI- or cholecystokinin-positive fibers. However, capsaicin, which in the iris causes permanent disappearance of substance-P fibers, had a similar effect on cholecystokinin-positive fibers, whereas no effect was noted on the vasoactive intestinal polypeptide/peptide HI fiber network. It is concluded that the rat iris contains a network of vasoactive intestinal polypeptide/peptide HI-positive nerves that does not originate in either the superior cervical or the ciliary ganglion, and most probably also not in the trigeminal ganglion, and a cholecystokinin-positive network that probably originates in the trigeminal ganglion.     

Embryonic origin of avian corneal sensory nerves.

Sensory nerves play a vital role in maintaining corneal transparency. They originate in the trigeminal ganglion, which is derived from two embryonic cell populations (cranial neural crest and ectodermal placode). Nonetheless, it is unclear whether corneal nerves arise from neural crest, from placode, or from both. Quail-chick chimeras and species-specific antibodies allowed tracing quail-derived neural crest or placode cells during trigeminal ganglion and corneal development, and after ablation of either neural crest or placode. Neural crest chimeras showed quail nuclei in the proximal part of the trigeminal ganglion, and quail nerves in the pericorneal nerve ring and in the cornea. In sharp contrast, placode chimeras showed quail nuclei in the distal part of the trigeminal ganglion, but no quail nerves in the cornea or in the pericorneal nerve ring. Quail placode-derived nerves were present, however, in the eyelids. Neural crest ablation between stages 8 and 9 resulted in diminished trigeminal ganglia and absence of corneal innervation. Ablation of placode after stage 11 resulted in loss of the ophthalmic branch of the trigeminal ganglion and reduced corneal innervation. Noninnervated corneas still became transparent. These results indicate for the first time that although both neural crest and placode contribute to the trigeminal ganglion, corneal innervation is entirely neural crest-derived. Nonetheless, proper corneal innervation requires presence of both cell types in the embryonic trigeminal ganglion. Also, complete lack of innervation has no discernible effect on development of corneal transparency or cell densities.     

Are there cholinergic through-fibers in the superior cervical ganglion of the mouse?

Summary Choline acetyltransferase immunocytochemistry was used to detect the presence and distribution of cholinergic through-fibers in the superior cervical ganglion of adult mice. The results revealed a great number of choline acetyltransferase-positive axons in the cervical sympathetic trunk and the varicose terminal axons involved in the innervation of the principal ganglionic cells within the ganglion. Immunostained axons were scarce or absent in the external or internal carotid nerves. The immunocytochemical results argue against the physiological importance of cholinergic through-fibers in the postganglionic nerves.     

选择性切除交感神经对大鼠心内神经节中去甲肾上腺素、乙酰胆碱和NPY的影响

为了探讨心内神经节中去甲肾上腺素（ＮＡ）、乙酰胆碱（ＡＣｈ）和ＮＰＹ的相互作用,本实验应用６－ＯＨ－ＤＡ选择性切除大鼠心脏交感神经纤维,然后应用荧光和酶组织化学法、免疫组织化学结合图像分析法观察了大鼠心内神经节ＮＡ、ＡＣｈＥ活性和ＮＰＹ的变化。结果显示：实验组大鼠心内神经节中儿茶酚胺荧光反应阳性和ＮＰＹ免疫反应（ＮＰＹ－ＩＲ）阳性的神经纤维明显减少,ＡＣｈＥ阳性神经纤维明显增多,ＡＣｈＥ反应性神经元积分光密度增加,而儿茶酚胺荧光反应和ＮＰＹ－ＩＲ阳性神经元变化不明显。结果提示：１．交感神经化学切除后大鼠心内神经节中ＮＡ、ＡＣｈＥ活性和ＮＰＹ出现不同的变化,体现了心脏交感神经和副交感神经的相互抑制作用;２．心内神经节可能含有两种性质的ＮＰＹ,即交感性和非交感性ＮＰＹ。     

Distribution of substance P immunoreactive cell bodies and fibers in cranial sensory and autonomic ganglia of the chick

Substance P-immunoreactive neurons were demonstrated in chick embryonic and adult trigeminal ganglion and jugular-superior ganglionic complex using FITC-immunohistochemical methods. Both small-size and large ganglion cells exhibited SP immunoreactivity, without apparent changes during embryonic and post-hatching development. SP-positive fibers could be detected in a good number in the sympathetic cranial cervical ganglion, either during embryonic development or in adult chick. No immunoreactive perikarya were observed in this ganglion. In the ciliary ganglion, both choroidal and ciliary neurons were SP-negative, whereas SP immunoreactive fibers surrounded the perikarya of both cell populations.     

Innervation of the carotid body. An experimental quantitative study

The innervation of the carotid body in the cat was studied by means of light- and electron-microscopic techniques. Sinus nerve resection, glossopharyngeal resection, bilateral cervical sympathectomy, excisions of two nerves, and injection of 6-hydroxydopamine (6-OH-DA) were performed in different groups of animals. It was found that resection of the sinus nerve produces a rapid phase of degeneration of intralobular fibers and synaptic boutons, followed by a reinnervation with a progressive reappearance of these elements. This reinnervation is retarded by sympathectomy and prevented by 6-OH-DA. It is therefore concluded that reinnervation is due to collateral regeneration of nearby sympathetic fibers. Resection of the sinus nerve produces an increase in the number of argentaffin cells and dense-cored vesicles in the cytoplasm of principal cells. These findings suggest the existence of efferent synaptic contacts between this nerve and principal cells. Part of the intralobular fibers and synaptic boutons degenerate after bilateral sympathectomy demonstrating that sympathetic axons connect synaptically to the principal cells. Sympathetic fibers reach the carotid body, not only from branches of the cervical plexuses but also from fibers running in the adventitia of the common carotid artery, and via glossopharyngeal and sinus nerves. The vagus nerve contributes a few fibers to the parenchymal lobules of the carotid body.     

Effects of infantile/prepubertal chronic estrogen treatment and chemical sympathectomy with guanethidine on developing cholinergic nerves of the rat uterus.

The innervation of the uterus is remarkable in that it exhibits physiological changes in response to altered levels in the circulating levels of sex hormones. Previous studies by our group showed that chronic administration of estrogen to rats during the infantile/prepubertal period provoked, at 28 days of age, an almost complete loss of norepinephrine-labeled sympathetic nerves, similar to that observed in late pregnancy. It is not known, however, whether early exposure to estrogen affects uterine cholinergic nerves. Similarly, it is not known to what extent development and estrogen-induced responses in the uterine cholinergic innervation are affected by the absence of sympathetic nerves. To address this question, in this study we analyzed the effects of infantile/prepubertal chronic estrogen treatment, chronic chemical sympathectomy with guanethidine, and combined sympathectomy and chronic estrogen treatment on developing cholinergic nerves of the rat uterus. Cholinergic nerves were visualized using a combination of acetylcholinesterase histochemistry and the immunohistochemical demonstration of the vesicular acetylcholine transporter (VAChT). After chronic estrogen treatment, a well-developed plexus of cholinergic nerves was observed in the uterus. Quantitative studies showed that chronic exposure to estrogen induced contrasting responses in uterine cholinergic nerves, increasing the density of large and medium-sized nerve bundles and reducing the intercept density of fine fibers providing myometrial and perivascular innervation. Estrogen-induced changes in the uterine cholinergic innervation did not appear to result from the absence/impairment of sympathetic nerves, because sympathectomy did not mimic the effects produced by estrogen. Estrogen-induced responses in parasympathetic nerves are discussed, considering the direct effects of estrogen on neurons and on changes in neuron-target interactions.     

Sprouting of aberrant neuropeptide Y-immunoreactive sympathetic nerves into neonatally denervated smooth muscle.

Sympathetic nerves normally project ipsilaterally to lateral cranial targets. Following unilateral superior cervical ganglionectomy in neonatal rats, however, neurons from the contralateral superior cervical ganglion sprout into the denervated region. In the present study we examined neuropeptide Y immunoreactivity (NPY-ir) of neurons comprising ipsilateral (control) and denervation-induced contralateral pathways to the superior tarsal smooth muscle of the eyelid. Fluoro-Gold injection of the control muscle retrogradely labelled 133 +/- 18 neurons in the ipsilateral superior cervical ganglion; of these, 21 +/- 3% displayed detectable NPY-ir. Fluoro-Gold injections of the reinnervated muscle labelled 20 +/- 4 neurons in the contralateral superior cervical ganglion, of which 85 +/- 3% contained detectable NPY-ir. Examination of the control tarsal muscle revealed DBH-ir noradrenergic nerves throughout the muscle and vasculature, while NPY-ir nerves were present primarily around blood vessels. In the reinnervated preparation, NPY-ir fibers innervated both blood vessels and tarsal muscle in a pattern similar to that of DBH-ir innervation. Acute excision of the remaining superior cervical ganglion eliminated all DBH-ir fibers bilaterally; NPY-ir was reduced markedly in the reinnervated preparations, though some fibers remained. We conclude that, following neonatal denervation, the tarsal muscle is reinnervated by a subpopulation of sympathetic neurons that differs in neuropeptide phenotype from that of the normal ipsilateral innervation.     

The distribution and origin of nerve fibers immunoreactive for substance P and neurokinin A in the anterior buccal gland of the rat

The distribution and origin of substance P (SP) and neurokinin A (NKA) were studied in rat in the anterior buccal glands, which are minor mucous salivary glands. Indirect immunofluorescence staining showed moderate SP and NKA innervation of salivary acini and interlobular ducts, whereas blood vessels were more sparsely innervated, and there were few nerve fibers in the stroma and around the intralobular ducts. About 10%–20% of the trigeminal ganglion cells showed equally strong immunoreactivity to both SP and NKA. Unilateral denervation of the branches of the trigeminal nerve caused complete disappearance of the stromal fibers and greatly reduced the number of all other SP-immunoreactive and NKA-immunoreactive nerve fibers. In the superior cervical ganglia, SP and NKA immunoreactivity was restricted to small intensely fluorescent cells; SP and NKA immunoreactivity was absent from principal ganglionic cells, and thus sympathectomy had no any effect on the number or distribution of fibers immunoreactive for SP and NKA in the anterior buccal glands. The fibers remaining after sensory denervation could have been of parasympathetic origin, indicating a dual origin of nerves immunoreactive for SP and NKA in these glands. The present data demonstrate that the major part of the glandular SP and NKA innervation in the minor salivary glands derives from the trigeminal ganglia. The distribution of the peripheral nerve fibers indicates that they may play a role in the delivery of potent neuropeptides involved in the vascular, secretory, and motor (myoepithelial cells) functions of salivary glands.     

A description of the upper thoracic autonomic nervous system in the rhesus monkey (Macaca mulatta)

The purpose of this study was to describe the autonomic innervation of the carotid sinus and heart in the rhesus monkey. Nine male rhesus monkeys (Macaca mulatta) and one male crab-eating macaque (M. fascicularis) were carefully dissected from the origin of the vagus nerves and superior cervical ganglia to the level of the fourth thoracic ganglion. The specimens were either freshly killed or obtained no later than 24 hours post mortem. The macaque monkeys were found to possess an innervation pattern that displayed features common to dog (connections between the vagus nerves and middle cervical ganglia), baboon (distinct cervical sympathetic and cervical vagal nerve trunks), and man (nerves projecting from the middle cervical and stellate ganglia to the heart). Distinct inferior cervical and first thoracic ganglia were never seen, but rather, large and well defined stellate ganglia were found. The macaque innervation pattern, when considered as a whole, most closely resembled the baboon.     

Lymphopoiesis in the nude fetal thymus following sympathectomy

This study was carried out to examine the innervation of the nude fetal thymus during ontogeny and to see if lymphopoietic activity would occur within these thymic lobes in the absence of sympathetic neuronal input. Fetal thymic rudiments from nu/nu mice were removed and examined for galoxylic acid-induced histofluorescence to detect the catecholaminergic nerves. Some of these lobes were organ cultured for 5 to 7 days in the presence of deoxyguanosine to eliminate any existing lymphoid cells within the rudiments. Such "nonlymphoid" thymic rudiments were implanted into the anterior eye chambers of syngenic BALB/c mice (heterozygous) from which cervical sympathetic ganglia and part of the sympathetic chain had been surgically removed (right side) one week earlier. The left side was only sham operated. The thymic implants were allowed to grow for up to 21 days on both sides; they were then removed and examined by histofluorescence, immunofluorescence, and light microscopy. The results indicate for the first time that the nude fetal thymus is innervated by sympathetic nerves and that following sympathectomy the nude thymus is able to sustain lymphopoietic activity and generate lymphoid cells which have characteristics present on thymocytes during in vivo development in normal mice, such as binding to peanut agglutinin and expression of Thy-1 antigen. The relationship between the presence of sympathetic inhibitory influence and the thymic atrophy seen in the nude mice during ontogeny, is being investigated.