RESUMEN

The major aim of this project is to provide interactive video computer based courseware that can be used by the medical student and others to supplement his or her learning of this very important aspect of basic biomedical education. Embryology is a science that depends on the ability of the student to visualize dynamic changes in structure which occur in four dimensions--X, Y, Z, and time. Traditional didactic methods, including lectures employing photographic slides and laboratories employing histological sections, are limited to two dimensions--X and Y. The third spatial dimension and the dimension of time cannot be readily illustrated using these methods. Computer based learning, particularly when used in conjunction with interactive video, can be used effectively to illustrate developmental processes in all four dimensions. This methodology can also be used to foster the critical skills of independent learning and problem solving.

Asunto(s)

RESUMEN

Sexual dimorphism of neuron number has been observed in several areas in the central and peripheral nervous system. In many of these areas enhanced neuron survival exists in males during the period of naturally occurring cell death. This has been attributed to high levels of circulating testosterone in the perinatal period. The superior cervical ganglion (SCG) of the rat exhibits this sexual dimorphism. The difference in neuron numbers is established by two weeks postnatally and precedes the differences in body weight and sympathetic target mass between the sexes. At this two week time point, fewer SCG neurons in the female rat must supply neurotransmitter to the same mass of sympathetic target as in the male. The present study examined some of the mechanisms used by neurons in the SCGs of male and female rats to compensate in supplying neurotransmitter to their targets. At birth, the SCGs of male and female rats contain equal numbers of neurons. There is also no sex difference at this time in the content of norepinephrine (NE) in these neurons or in the enzyme activity of tyrosine hydroxylase (TH). However, a sex difference does exist in the expression of TH-mRNA, with SCG neurons in female expressing more TH-mRNA than males. At this time, there is no sex difference in either the total body weight of males and females or in the mass of sympathetic target organs. During the first two postnatal weeks, natural neuron death in the SCG results in the loss of significantly more neurons in females than in males. At the end of the period of cell death, neurons in females continue to express more TH-mRNA, and at this time both TH enzyme activity and NE content per neuron are also higher in females. Since the adult sex difference in body weight and sympathetic target mass has not yet been established, the same amount of target mass is innervated by fewer neurons in females. In the adult, the sex difference in SCG neuron number is maintained. However, both overall body weight and sympathetic target mass are significantly greater in males. At this time expression of TH-mRNA is greater in SCG neurons of males, while both TH enzyme activity and NE content per neuron are equal in males and females. One of the challenges presented to the developing nervous system is to match a population of neurons with its targets.(ABSTRACT TRUNCATED AT 400 WORDS)

Asunto(s)

RESUMEN

Steady-state levels and turnover of the neurotransmitter, norepinephrine (NE), were measured in sympathetic perikarya and in two sympathetic target organs in the rat at various times during postnatal development. NE content in sympathetic perikarya in the superior cervical ganglion (SCG) increases 15-fold from birth to reach adult levels by 60 days. This increase in NE content parallels the increase in total protein in the ganglion. The rate of turnover of NE in the sympathetic perikarya increases slightly from birth to adulthood. Since the perikarya in the SCG project to a variety of different targets in the head and neck, NE metabolism was also examined in two terminal sympathetic plexuses, in the iris and in the submandibular gland (SMG). The terminal noradrenergic plexuses within these target organs do not mature with the same time course. In the iris, levels of NE increase 24-fold from birth until 90 days postnatally. Turnover of NE in sympathetic terminals in the iris at the time of birth is equivalent to that in the adult. In contrast, both the content and turnover of NE in sympathetic terminals in the SMG are very low at birth, and increase dramatically in the first month postnatally. Deafferentation of the SCG at birth impairs the development of normal levels of NE in sympathetic perikarya by approximately 40%, and total ganglionic protein is similarly affected. NE turnover in sympathetic perikarya deafferented at birth is only slight reduced from normal. The response to neonatal deafferentation differs in the two terminal sympathetic plexuses. In neurons that project to the iris, no detectable NE turnover could be measured, although the content of transmitter attains 64% of control values after deafferentation.(ABSTRACT TRUNCATED AT 250 WORDS)

Asunto(s)

RESUMEN

The ultrastructure of synapses in the autonomic nervous system is reviewed. The synaptic organization of the parasympathetic ganglia is relatively simple. Preganglionic axons form synapses either on the soma or on short perikaryal processes of the ganglionic neurons. The presynaptic terminals have a cholinergic morphology and contain mainly small clear vesicles with a few large dense cored vesicles. A few neuropeptides have been localized to the large dense cored vesicles of these terminals. The postganglionic parasympathetic axons ramify within their target tissues where they form close associations, but not true synaptic contacts. Sites of release of transmitter are recognized morphologically as varicosities along the length of the axon that contain clusters of small clear vesicles with a few large dense cored vesicles. The organization of the sympathetic nervous system is somewhat more complex. In addition to acetylcholine, enkephalin also exists in these terminals, probably in the large dense cored vesicles. There are at least three types of ganglion cell neurons in the paravertebral portion of the sympathetic nervous system: those that contain norepinephrine alone, those that contain norepinephrine along with neuropeptide Y, and those that contain acetylcholine and vasoactive intestinal polypeptide. The first type provides innervation to the parenchyma of the target tissues, while the second mainly innervates blood vessels. The third type innervates the sweat glands. In the prevertebral ganglia, a fourth type of neuron exists that contains norepinephrine and somatostatin. This neuron probably innervates the gut. Preganglionic terminals of the cholinergic type form synaptic connections mainly with the dendrites of the sympathetic ganglion neurons. In addition to the types of synapses described for the paravertebral ganglia, neurons in the prevertebral ganglia receive synaptic connections from dorsal root ganglia and from the enteric nervous system. The sympathetic ganglia also contain interneurons that receive preganglionic synapses and form efferent synapses with some of the principal ganglion cells. The interneurons have been shown to contain a variety of transmitters, including norepinephrine, epinephrine, dopamine, serotonin, and a number of neuropeptides. The postganglionic sympathetic axons have a similar morphology to the parasympathetic axons. They form networks in their targets, and the axons display varicosities with concentrations of both small and large vesicles. After appropriate fixation, these vesicles are seen to possess dense cores.(ABSTRACT TRUNCATED AT 400 WORDS)

Asunto(s)

RESUMEN

Adult male rats have more neurons in their superior cervical ganglia than do adult females. This difference arises over the first two postnatal weeks, and is apparently related to perinatal levels of circulating testosterone. Exposure of neonatal rats to testosterone or estradiol during the first postnatal weeks results in an increase in the number of neurons in the superior cervical ganglion seen at 15, 30 or 60 days postnatally. The present studies were undertaken to determine whether this observed increase in neurons is due to an increase in neuronal proliferation or to a decrease in neuronal death. Results of autoradiographic studies show no evidence of enhanced neuronal proliferation following postnatal exposure to estradiol, but do show an increased survival of a prenatally labeled population of cells. Counts of degenerating cells in the superior cervical ganglion show that during the peak period of normal neuronal degeneration, on postnatal day 5, 17-beta-estradiol or testosterone propionate treated animals have significantly fewer degenerating superior cervical ganglion cells than do vehicle-injected littermate controls. In addition, vehicle-injected females have more degenerating cells on day 5 than do vehicle-injected males. Taken together these results provide strong evidence that the increase in superior cervical ganglion neurons seen after neonatal exposure to estradiol results from a reduction in developmental neuron death.

Asunto(s)

RESUMEN

Previous studies from our laboratory have shown that synaptogenesis in the superior cervical sympathetic ganglion (SCG) of the rat occurs predominantly during the first weeks after birth. The purpose of the present study was to examine the normal development of dendrites of the ganglion neurons, and to assess the importance of the afferent input in shaping this development. Two independent methods for examining dendritic morphology were used. One was to label neurons in the SCG by injecting a conjugate of horseradish peroxidase and wheat germ agglutinin (HRP-WGA) into a target of the SCG neurons (the submandibular gland). This procedure results in a 'Golgi-like' filling of the retrogradely labelled cell bodies and their dendrites. The second method was stereologic analysis (point-counting) of electron micrographs of sections of the SCG. At birth, the ganglion neurons give rise to an average of 2.4 primary dendrites and 1.2 secondary branches. No tertiary branches are observed at this age. The total dendritic length is 15.3 microns. Electron microscopic stereology reveals that the mean volume occupied by dendrites in the newborn SCG is 0.0093 mm3. In the adult, there is an average of 5.2 primary, 6.3 secondary, 4.8 tertiary and 1.9 quaternary dendrites. The total dendritic length is increased 23-fold to 347 microns. The mean volume occupied by dendrites is 0.0771 mm3, representing an 8-fold increase. In ganglia from adult rats which were deafferented at birth, an essentially normal dendritic form is attained. There are 4.5 primary, 6.2 secondary, 2.8 tertiary and 2.2 quaternary dendrites, and the total dendritic length is 297 microns. The mean volume of dendrites is 0.0571 micron3.(ABSTRACT TRUNCATED AT 250 WORDS)

Asunto(s)

RESUMEN

In the superior cervical sympathetic ganglion (SCG) of the rat, a significant amount of morphological and biochemical maturation occurs in the first few postnatal weeks. The specific activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of norepinephrine (NE), increases during this time and is subject to transsynaptic regulation by the preganglionic inputs. In the present study, we examined the normal postnatal development of NE stores in sympathetic neurons and the transsynaptic regulation of this development. NE content undergoes an 8-fold increase from the time of birth, and stabilizes at adult levels at one month. Following neonatal deafferentation, there is a temporary stunting of NE accumulation in sympathetic neurons and a permanent reduction in the activity of TH, whether or not regeneration of the afferents occurs. When regeneration is prevented, the turnover of NE is significantly reduced, while NE levels rise to near normal levels. When regeneration is permitted, however, both the stored amount and turnover of NE attain normal levels. These data suggest that there is a critical period during the first two postnatal weeks when transsynaptic influences from afferents are necessary for the induction of TH in sympathetic neurons. Levels and turnover of transmitter do not have this critical period, but appear to depend solely on the functional integrity of the system.

Asunto(s)

RESUMEN

Neonatal rats treated with testosterone propionate or 17-beta-estradiol during the first two postnatal weeks have more neurons and synapses in their superior cervical ganglia (SCGs) at 15 days of age than do vehicle-treated littermates. To determine whether a non-aromatizable androgen would similarly increase the number of SCG synapses, dihydrotestosterone (DHT) was injected into male rats beginning on the day of birth. The animals were sacrificed on postnatal day 15 and the SCGs removed on postnatal day 15. Counts of synapses showed no difference in the number of synapses between control and DHT-treated animals. These results suggest that the actions of testosterone to increase the numbers of SCG synapses may be via aromatization to estradiol. An additional study was done to determine whether the additional synapses formed in SCGs of animals treated with estradiol arise from neurons whose axons are in the cervical sympathetic trunk or from intrinsic neurons, i.e., SIF cells or other principal ganglion neurons. Neonatal males were injected with 17-beta-estradiol or vehicle beginning on the day of birth and continuing until the time of sacrifice on day 15. The number of intrinsic synapses formed under control and estradiol treatments was determined in SCGs of animals whose extrinsic synapses were caused to degenerate by severing the cervical sympathetic trunk bilaterally on postnatal day 13, two days before sacrifice. The total number of synapses (extrinsic plus intrinsic) in the ganglion after vehicle or estradiol treatment was determined in unoperated animals and used to calculate the number of extrinsic synapses.(ABSTRACT TRUNCATED AT 250 WORDS)

Asunto(s)

RESUMEN

There is a normal sexual dimorphism in the number of neurons in the rat superior cervical sympathetic ganglion (SCG), with adult males having more neurons in this ganglion than females. We confirm this finding here, report that this sex difference is not present at birth and that neonatal castration of males reduces the adult sex difference. These results indicate that perinatal levels of circulating testicular hormones play a role in regulating the numbers of neurons in the SCG. Treatment of neonatal rats with testosterone propionate or estradiol significantly increases the number of neurons in the SCG. To determine whether this effect is primary androgenic or estrogenic, the effects of dihydrotestosterone, on SCG neuron number were investigated. Dihydrotestosterone, unlike testosterone, is not aromatized to estradiol intracellularly. There was no difference in the number of neurons between animals injected from birth with vehicle or dihydrotestosterone. This difference in effects between dihydrotestosterone and testosterone suggests that the actions of testosterone may be via aromatization to estradiol, rather than action at an androgen receptor.

Asunto(s)

RESUMEN

In the rat superior cervical sympathetic ganglion (SCG), alpha-bungarotoxin (alpha BT) demonstrates binding that is saturable and inhibited by nicotinic ligands. However, alpha BT does not inhibit the physiological response of ganglionic neurons to preganglionic stimulation or to exogenously applied acetylcholine. Thus the specificity of alpha BT for ganglionic nicotinic cholinergic receptors has been questioned. The present study provides a morphological localization of the binding sites of 125I-labelled alpha BT in the rat SCG using the method of Blackett and Parry on electron microscopic radioautographs. The distribution of grains resulting from specific binding was calculated by subtracting the nonspecific distribution (alpha BT in the presence of D-tubocurarine, a known nicotinic ligand) from the total grain distribution (alpha BT alone). A hypothetical grain distribution was obtained based on the geometrical properties of the tissue sections. A computer minimizing routine was employed to adjust the relative weights of each of the potential sources of hypothetical grains until a 'best-fit' with the real grain distributions occurred. The nonspecific binding of alpha BT was uniform across all tissue components, with the exception of a significant concentration on the membrane of the ganglion cell body. By contrast, the specific binding of alpha BT was highly localized to synaptic membranes, and to a lesser extent, to dendritic membranes.

Asunto(s)

RESUMEN

Counts of neurons of the rat superior cervical ganglion (SCG) were made at two days before birth and at several postnatal ages. There is a significant decline in the number of apparently normal neurons over the first postnatal week, with the number falling from 39 500 at 3 days to 26 500 at 7 days. Cell numbers then remained constant up to day 60 when the number of neurons was 27 500. The incidence of degenerating neurons, identified by light and electron microscopy, was correlated temporally with the loss of normal neurons. The early manifestations of the neuron degeneration were chromatin clumping and the presence of free monoribosomes. Later stages were characterized by increased chromatin clumping, dense aggregations of monoribosomes, numerous intracytoplasmic vacuoles, and only short segments of rough endoplasmic reticulum. The ultrastructure of the majority of these dying neurons is similar to the 'nuclear' types of degeneration described by Pilar & Landmesser (1976) and Chu-Wang & Oppenheim (1978). Based on the presence of degenerating neurons coincident with the reduction in neuron numbers, we conclude that neuron death is an important aspect of early postnatal development in the rat SCG.

Asunto(s)

RESUMEN

Published values for the number of neurons in the superior cervical ganglion of the adult rat range from 13 000 to 45 000. These studies have employed different methods for determining what unit to count (cell body, nucleus, nucleolus), how many sections to count, and how to correct the raw counts for split particles and for profiles that are too small to resolve. The purpose of this study was to examine the extent to which these parameters may influence the calculated value for the total number of neurons, using computer simulations of neuron populations. These simulations permitted us to determine the effects on neuron number of varying the diameter of the neuronal nucleus, the size of the smallest resolvable profile, and the thickness of the section. The data from the simulations were used to test the validity of several methods that are in common use for correction of neuron counts. Our results indicate that most of the methods that are in routine use are unsatisfactory. We propose the use of either one of two methods that consistently result in highly accurate estimates of neuron numbers. These are: (1) a modification of the method proposed by Hendry (1976), using computer analysis; or (2) a modification of the method proposed by Abercrombie (1946), which does not require the use of a computer.

Asunto(s)

RESUMEN

Neonatal treatment with gonadal steroids has been reported to alter morphological as well as functional development in various regions of the brain and spinal cord. Among the observed alterations are changes in numbers of neurons and in the organization and numbers of synapses. These regions have been found to be sexually dimorphic, and the dimorphism dependent upon gender differences in circulating levels of gonadal steroids. Neonatal treatment with testosterone has been shown to produce an increase in the number of neurons in the superior cervical sympathetic ganglion in female rats. The present studies were designed to investigate the possibility of a normally occurring sexual dimorphism in the SCG of the rat, and to characterize the effect of neonatal treatment with testosterone on neurons and synapses in the male rat. We report a sexual dimorphism in the number of neurons but not in the number of preganglionic axons or ganglionic synapses. In addition, neonatal administration of testosterone propionate results in a 40% increase in the number of superior cervical ganglion neurons in treated male rats over the control male number at 15 and 30 days of age. The testosterone propionate treatment results in a 66% increase in the number of synapses in male superior cervical ganglia, without a concomitant increase in the number of preganglionic axons.

Asunto(s)

RESUMEN

The superior cervical sympathetic ganglion of the rat receives its preganglionic afferent innervation through the cervical sympathetic trunk, and sends most of its postganglionic axons through two major nerves, the internal (ICN) and external carotid nerves (ECN). In the present study, the ICN alone or both the ICN and the ECN were cut in neonatal and adult rats. Two months after these lesions, ganglionic neurons, synapses and preganglionic axons were counted and compared with unoperated control values. After cutting the ICN alone in neonatal rats, ganglionic neurons were reduced in number by 70% and synapses were reduced by 50%, but there was no change in the number of preganglionic axons. Cutting both the ICN and ECN in neonates resulted in an 88% reduction of ganglionic neurons and an 83% reduction of synapses. In this case there was a 63% reduction in the number of preganglionic axons. After cutting either the ICN alone or both the ICN and the ECN in neonates, there was a hyperinnervation (increased number of synapses) of the remaining ganglionic neurons. In the adult rat, cutting either the ICN alone or both the ICN and ECN resulted in a smaller loss of ganglionic neurons, and there was no loss of preganglionic axons. There was no hyperinnervation of surviving neurons in adult rats. Thus, the response by preganglionic axons to a reduced number of ganglionic neurons differs in the neonate and adult rat. In the developing animal, the degenerative response to injury is much more severe than in the adult, but the reorganizational response is also greater.

Asunto(s)

Asunto(s)

RESUMEN

Synapse formation during postnatal development of the superior cervical sympathetic ganglion was studied in rats after neonatal ganglionic denervation, when reinnervation was either permitted or prevented. In both groups of operated animals, 90% of the synapses were lost by the fourth postoperative day. In the rats in which reinnervation was permitted, restoration of synapse numbers began by one month after surgery and reached 50% of control by two months. There was no further synapse restoration after this time. In the animals in which reinnervation was prevented, synapse numbers increased, but at all times were approximately 10% of controls. Thus the intrinsic ganglionic synapses underwent their normal postnatal developmental increase in number, but did not sprout to any significant degree in response to the massive deafferentation caused by removal of the preganglionic input. Vacant postsynaptic membrane thickenings (those not apposed by presynaptic terminals) appeared in both experimental groups. There was no significant loss of these vacant thickenings in either group over the course of 3 months. Therefore, the vacated postsynaptic sites do not appear to be recontacted during reinnervation, while the reinnervating axons appear to cause the formation of new postsynaptic sites on the ganglion cells.

Asunto(s)

Asunto(s)

Asunto(s)

RESUMEN

The dorsal motor nucleus of the vagus nerve (DMX) of adult cats and young kittens was studied by quantitative light microscopic methods. In normal animals, the DMX was found to contain no distinct subgroupings of neurons, based on somatic volume or Nissl pattern. Retrograde perikaryal responses to axotomy of neurons in the DMX were found to be of a more subtle nature than those seen in other types of neurons. Quantitative methodology applied to the axotomy than could be obtained by routine microscopic observations. Changes which occurred included a slight chromatolytic reaction, and a decrease in the volume of the nucleus followed by an increase in somatic volume. These morphological alterations were affected by the factors of age of the animal, time after axotomy, and length of the intact proximal axon stump. More pronounced perikaryal changes occurred when the vagus nerve was recut at a more proximal level five days after the first vagotomy. Interpretation of the data yielded the conclusion that most if not all neurons of the ipsilateral DMX contribute axons to the cervical vagus nerve. In addition, at least 10% of the neurons on the side contralateral to vagotomy showed signs of retrograde reaction. It was therefore concluded that there exists in the vagus nerve a population of axons with cell bodies located in the contralateral DMX.

Asunto(s)