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A M Burgess

Publications and source records attributed to A M Burgess.

At least 19 recordsLinked to original sources
PubMed2005

Psychophysiological correlates of the inter-individual variability of head movement control in seated humans.

We recently conducted experiments where 24 seated participants were subjected (with eyes closed) to small amplitude, high-jerk impulses of linear acceleration. Responses were distributed as a continuum between two extremes. The "stiff" participants showed little movement of the head relative to the trunk, whereas the "floppy" participants showed a large head rotation in the direction opposite the sled movement. We hypothesized that the stiff behavior resulted from the spontaneous use of an imagined visual frame of reference and undertook this larger-scale study to test that idea. The distribution along the "stiff-floppy" continuum was compared with the scores on psychophysiological tests measuring vividness of imagery, visual field-dependence and motion sickness susceptibility. Multivariate regression analysis revealed that the "stiffness" of individuals was loosely, but significantly related to the vividness of their imagery. However, "stiffness" was not linked to visual field-dependence or motion sickness susceptibility. Even if it explains only 20% of the variance of the data, the increase of "stiffness" with vividness of imagery fits our hypothesis. With eyes closed, stiff people may use imagined external visual cues to stabilize their head and trunk. Floppy people, who are poorer imagers, may rely more on "egocentric", proprioceptive and vestibular inputs.

Acceleration↗
PubMed2005

Patient and normal three-dimensional eye-movement responses to maintained (DC) surface galvanic vestibular stimulation.

HYPOTHESIS: That disease or dysfunction of vestibular end organs in human patients will reduce or eliminate the contribution of the affected end organs to the total eye-movement response to DC surface galvanic vestibular stimulation (GVS). BACKGROUND: It was assumed that DC GVS (at current of 5 mA) stimulates all vestibular end organs, an assumption that is strongly supported by physiological evidence, including the activation of primary vestibular afferent neurons by galvanic stimulation. Previous studies also have described the oculomotor responses to vestibular activation. Stimulation of individual semicircular canals results in eye movements parallel to the plane of the stimulated canal, and stimulation of the utricular macula produces changes in ocular torsional position. It was also assumed that the total three-dimensional eye-movement response to GVS is the sum of the contributions of the oculomotor drive of all the vestibular end organs. If a particular vestibular end organ were to be diseased or dysfunctional, it was reasoned that its contribution to the GVS-induced oculomotor response would be reduced or absent and that patients thus affected would have a systematic difference in their GVS-induced oculomotor response compared with the response of normal healthy individuals. METHODS: Three-dimensional video eye-movement recording was carried out in complete darkness on normal healthy subjects and patients with various types of vestibular dysfunction, as diagnosed by independent vestibular clinical tests. The eye-movement response to long-duration bilateral and unilateral surface GVS was measured. RESULTS: The pattern of horizontal, vertical, and torsional eye velocity and eye position during GVS of patients independently diagnosed with bilateral vestibular dysfunction, unilateral vestibular dysfunction, CHARGE syndrome (semicircular canal hypoplasia), semicircular canal occlusion, or inferior vestibular neuritis differed systematically from the responses of normal healthy subjects in ways that corresponded to the expectations from the conceptual approach of the study. CONCLUSION: The study reports the first data on the differences between the normal response to GVS and those of patients with a number of clinical vestibular conditions including unilateral vestibular loss, canal block, and vestibular neuritis. The GVS-induced eye-movement patterns of patients with vestibular dysfunction are consistent with the reduction or absence of oculomotor contribution from the end organs implicated in their particular disease condition.

Adolescent↗
PubMed2004

Convergence reduces ocular counterroll (OCR) during static roll-tilt.

When humans are roll-tilted around the naso-occipital axis, both eyes roll or tort in the opposite direction to roll-tilt, a phenomenon known as ocular counterroll (OCR). While the magnitude of OCR is primarily determined by vestibular, somatosensory, and proprioceptive input, direction of gaze also plays a major role. The aim of this study was to measure the interaction between some of these factors in the control of OCR. Videooculography was used to measure 3D eye position during maintained whole body (en bloc) static roll-tilt in darkness, while subjects fixated first on a distant (at 130 cm) and then a near (at 30 cm) head-fixed target aligned with the subject's midline. We found that while converging on the near target, human subjects displayed a significant reduction in OCR for both directions of roll-tilt--i.e. the interaction between OCR and vergence was not simple addition or subtraction of torsion induced by vergence with torsion induced by roll-tilt. To remove the possibility that the OCR reduction may be associated with the changed horizontal position of the eye in the orbit during symmetric convergence, we ran an experiment using asymmetric convergence in which the distant and near targets were aligned directly in front of one eye. We found the magnitude of OCR in this asymmetric convergence case was also reduced for near viewing by about the same amount as in the symmetric vergence condition, confirming that the convergence command rather than horizontal position of the eye underlies the OCR reduction, since there was no horizontal movement of the aligned eye in the orbit between fixation on the distant and near targets. Increasing vergence from 130 to 30 cm reduced OCR gain by around 35% on average. That reduction was equal in both eyes and occurred in both the symmetric and asymmetric convergence conditions. These results demonstrate the important role vergence plays in determining ocular counterroll during roll-tilt and may support the contention that vergence acts to reduce the conflict facing a stereopsis-generating mechanism.

Adult↗
PubMed2001

Variability in the control of head movements in seated humans: a link with whiplash injuries?

The aim of this study was to determine how context and on-line sensory information are combined to control posture in seated subjects submitted to high-jerk, passive linear accelerations. Subjects were seated with eyes closed on a servo-controlled linear sled. They were asked to relax and received brief accelerations either sideways or in the fore-aft direction. The stimuli had an abrupt onset, comparable to the jerk experienced during a minor car collision. Rotation and translation of the head and body were measured using an Optotrak system. In some of the subjects, surface electromyographic (EMG) responses of selected neck and/or back muscles were recorded simultaneously. For each subject, responses were highly stereotyped from the first trial, and showed little sign of habituation or sensitisation. Comparable results were obtained with sideways and fore-aft accelerations. During each impulse, the head lagged behind the trunk for several tens of milliseconds. The subjects' head movement responses were distributed as a continuum in between two extreme categories. The 'stiff' subjects showed little rotation or translation of the head relative to the trunk for the whole duration of the impulse. In contrast, the 'floppy' subjects showed a large roll or pitch of the head relative to the trunk in the direction opposite to the sled movement. This response appeared as an exaggerated 'inertial' response to the impulse. Surface EMG recordings showed that most of the stiff subjects were not contracting their superficial neck or back muscles. We think they relied on bilateral contractions of their deep, axial musculature to keep the head-neck ensemble in line with the trunk during the movement. About half of the floppy subjects displayed reflex activation of the neck muscles on the side opposite to the direction of acceleration, which occurred before or during the head movement and tended to exaggerate it. The other floppy subjects seemed to rely on only the passive biomechanical properties of their head-neck ensemble to compensate for the perturbation. In our study, proprioception was the sole source of sensory information as long as the head did not move. We therefore presume that the EMG responses and head movements we observed were mainly triggered by the activation of stretch receptors in the hips, trunk and/or neck. The visualisation of an imaginary reference in space during sideways impulses significantly reduced the head roll exhibited by floppy subjects. This suggests that the adoption by the central nervous system of an extrinsic, 'allocentric' frame of reference instead of an intrinsic, 'egocentric' one may be instrumental for the selection of the stiff strategy. The response of floppy subjects appeared to be maladaptive and likely to increase the risk of whiplash injury during motor vehicle accidents. Evolution of postural control may not have taken into account the implications of passive, high-acceleration perturbations affecting seated subjects.

Acceleration↗
PubMed2000

Semicircular canal occlusion causes permanent VOR changes.

We measured the guinea pig horizontal vestibulo-ocular reflex (hVOR) to high acceleration impulsive head rotations following a unilateral lateral semicircular canal (LSCC) occlusion. We found a significant hVOR deficit for rotations toward the side of the occluded LSCC and this deficit did not show systematic changes over 3 months. We considered the LSCC nerve was still functional as shown by the normal appearance of the crista of the LSCC ampulla and also electrical stimulation of the LSCC. We conclude that the VOR during angular acceleration in response to high acceleration shows no adaptive plasticity following a unilateral LSCC occlusion.

Animals↗
PubMed1999

The planes of the utricular and saccular maculae of the guinea pig.

To establish a link between otolith anatomy and function it is necessary to know the regions of the utricular and saccular maculae, which are stimulated by any arbitrary linear acceleration stimulus. That requires accurate information about the location and orientation of the spatially extended maculae in head-fixed coordinates and referred to head-fixed landmarks (such as Reid's line). New data showing the location of the otolithic maculae in the guinea pig with respect to head-fixed stereotaxic coordinates are presented. Guinea pigs were perfused with Karnovsky's fixative and the maculae were exposed while the head was held in a guinea pig stereotaxic device. An electrolytically sharpened fine wire held in a calibrated micromanipulator was touched to points all over the surface of each macula under visual observation with the aid of a high-power operating microscope. The x, y, z coordinates of these points were plotted using a three-dimensional plotting program. Both maculae have pronounced curvature so that dorsoventral shear forces will stimulate regions of both the utricular and saccular maculae.

Acoustic Maculae↗
PubMed1999

Human ocular counterrolling during roll-tilt and centrifugation.

To test a hypothesis about how otoliths resolve roll-tilts from translations, we measured human ocular torsion position [ocular counterrolling (OCR)] to maintained linear acceleration stimuli. All subjects (n = 8) were tested in two conditions where the same magnitude of shear along an interaural axis was generated in one of two ways: either by roll-tilt on a tilt-chair in a 1-g environment, or by centripetal linear acceleration during constant velocity rotation 1 m from the axis of rotation on a fixed-chair human centrifuge. The interaural shear to the otoliths was the same for these two conditions, but the dorsoventral shear was different and for all eight subjects the OCR on the centrifuge was significantly greater than the torsion on the tilt-chair, although the resultant angle was in fact smaller on the centrifuge than on the tilt-chair. The results confirm that dorsoventral shear is important for determining OCR. The otoliths may resolve potential stimulus ambiguities between tilts and translations by virtue of the different patterns of interaural and dorsoventral shear that these stimuli generate.

Acceleration↗
PubMed1998

High acceleration impulsive rotations reveal severe long-term deficits of the horizontal vestibulo-ocular reflex in the guinea pig.

While there is agreement that unilateral vestibular deafferentation (UVD) invariably produces an immediate severe horizontal vestibulo-ocular reflex (HVOR) deficit, there is disagreement about whether or not this deficit recovers and, if so, whether it recovers fully or only partly. We suspected that this disagreement might mainly be due to experimental factors, such as the species studied, the means chosen to carry out the UVD, or the nature of the test stimulus used. Our aim was to sort out some of these factors. To do this, we studied the HVOR of alert guinea pigs in response to low and high acceleration sinusoidal and high acceleration impulses after UVD by either labyrinthectomy or by vestibular neurectomy. The HVOR in response to high acceleration impulsive yaw rotations was measured before, and at various times after, either unilateral labyrinthectomy or superior vestibular neurectomy. Following UVD, there was a severe impairment of the HVOR for ipsilesional rotations and a slight impairment for contralesional rotations, after either operation. This asymmetrical HVOR deficit in the guinea pig parallels the deficit observed in humans. Between the first measurement, which was made 1 week after UVD, and the last, which was made 3 months after UVD, there was no change in the HVOR. This lack of recovery was the same after labyrinthectomy as after vestibular neurectomy. The HVOR to low and high acceleration sinusoidal yaw rotations were measured after UVD, and the results were compared with those in response to impulsive rotations. For low acceleration sinusoidal rotations (250 degrees/s2), the gain was symmetrical, although reduced bilaterally. As the peak head acceleration increased, the HVOR became increasingly asymmetric. The HVOR asymmetry for sinusoidal rotations was significantly less than for impulsive rotations that had the same high peak head acceleration (2500 degrees/s2). Our results show that the HVOR deficit after UVD is the same in guinea pigs as in humans; that it is the same after vestibular neurectomy as after labyrinthectomy; that it is lasting and severe in response to high acceleration rotations; and, that it is more obvious in response to impulses than to sinusoids.

Afferent Pathways↗
PubMed1998

Off-center yaw rotation: effect of naso-occipital linear acceleration on the nystagmus response of normal human subjects and patients after unilateral vestibular loss.

Dual search coils were used to record horizontal, vertical and torsional eye movement components of one eye during nystagmus caused by off-center yaw rotation (yaw centrifugation). Both normal healthy human subjects (n=7) and patients with only one functioning labyrinth (n=12) were studied in order to clarify how the concomitant linear acceleration affected the nystagmus response. Each subject was seated with head erect on the arm of a fixed-chair human centrifuge, 1 m away from the center of the rotation, and positioned to be facing along a radius; either towards (facing-in) or away from (facing-out) the center of rotation. Both yaw right and yaw left angular accelerations of 10 degrees s(-2) from 0 to 200 degrees/s were studied. During rotation a centripetal linear acceleration (increasing from 0 to 1.24xg units) was directed along the subject's naso-occipital axis resulting in a shift of the resultant angle of the gravitoinertial acceleration (GIA) of 51 degrees in the subject's pitch plane and an increase in the total GIA magnitude from 1.0 to 1.59xg. In normal subjects during the angular acceleration off-center there were, in addition to the horizontal eye velocity components, torsional and vertical eye velocities present. The magnitude of these additional components, although small, was larger than observed during similar experiments with on-center angular acceleration (Haslwanter et al. 1996), and the change in these components is attributed to the additional effect of the linear acceleration stimulation. In the pitch plane the average size of the shift of the axis of eye velocity (AEV) during the acceleration was about 8 degrees for a 51 degrees shift of the GIA (around 16% of the GIA shift) so that the AEV-GIA alignment was inadequate. There was a very marked difference in the size of the AEV shift depending on whether the person was facing-in [AEV shift forward (i.e. non-compensatory) of about 4 degrees] or facing-out [AEV shift forward (i.e. compensatory) of around 12 degrees]. The linear acceleration decreased the time constant of decay of the horizontal component of the post-rotatory nystagmus: from an average of 24.8 degrees/s facing-in to an average of 11.3 degrees/s facing-out. The linear acceleration dumps torsional eye velocity in an manner analogous to, but independent of, the dumping of horizontal eye velocity. Patients with UVD had dramatically reduced torsional eye velocities for both facing-in and facing-out headings, and there was little if any shift of the AEV in UVD patients. The relatively small effects of linear acceleration on human canal-induced nystagmus found here confirms other recent studies in humans (Fetter et al. 1996) in contrast to evidence from monkeys and emphasizes the large and important differences between humans and monkeys in otolith-canal interaction. Our results confirm the vestibular control of the axis of eye velocity of humans is essentially head-referenced whereas in monkeys that control is essentially space-referenced.

Acceleration↗
PubMed1998

Prognostic factors for survival of patients treated systemically for disseminated melanoma.

PURPOSE: The current American Joint Commission on Cancer (AJCC) staging system distinguishes between soft tissue and visceral metastases in advanced (stage IV) melanoma. We sought to verify these staging criteria and to identify prognostic variables that could be used to evaluate the impact of systemic therapy on long-term survival during the prior decade. PATIENTS AND METHODS: We conducted a retrospective study of patients with advanced cutaneous melanoma enrolled in clinical trials between 1979 and 1989 at The University of Texas M.D. Anderson Cancer Center. Pretreatment age, sex, number of organs with metastases, serum levels of lactate dehydrogenase (LDH) and albumin, and period of enrollment were analyzed using a Cox proportional hazards model of survival. RESULTS: In univariate and multivariate analyses that involved 318 stage IV patients, normal serum levels of LDH and albumin, soft tissue and/or single visceral organ metastases (especially lung), female sex, and enrollment late in the decade were independent positive predictors for survival. In multivariate analyses, the current AJCC criteria did not significantly predict outcome. Systemic treatment response did not bias these results, and only 4% of patients had a complete response. Patients who lived more than 2 years (11%) had a mix of favorable prognostic characteristics and a high frequency of systemic or surgically induced complete response. CONCLUSION: This study supports the use of stratification parameters that reflect the favorable prognostic impact of soft tissue or single visceral organ metastases and normal serum levels of LDH and albumin at time of enrollment in advanced melanoma trials. Improved survival over the prior decade probably reflects advances in diagnostic and palliative interventions.

Adult↗
PubMed1996

Inflammation of the lungs, umbilical cord and placenta associated with meconium passage in utero. Review of 123 autopsied cases.

It is uncertain how often the passage of meconium in utero is a response to some event causing fetal distress as opposed to being simply the physiologic functioning of a maturing intestinal tract. The extent to which meconium may produce injury or inflammation in pulmonary and placental tissues after intrauterine exposure is also unclear. This study was a retrospective review of 123 cases, 79 stillborn and 44 liveborn less than one month of age, autopsied at The Johns Hopkins Hospital, and showing histologic evidence of intrauterine meconium exposure by aspirated meconium or meconium macrophages in placental tissues. Of 55 cases with pulmonary inflammation, 13 (24%) had fetal pneumonia, 5 (9%) had postnatal bronchopneumonia, and 37 (67%) had inflammation secondary to meconium aspiration. There was inflammation of the umbilical cord in 31 (41%) of the 75 cases with available slides, 11 (15%) had funisitis associated with chorioamnionitis and 18 (58%) were secondary to meconium exposure. There were 19 cases with focal injury of cord vessels from meconium, two of which had cord ulceration. Inflammation of the membranes and chorionic plate was present in 24 (33%) of the 72 cases where it could be assessed, and was due to chorioamnionitis in 11 (46%) and to meconium in 13 (54%). In general, meconium-related inflammations was much less severe in the membranes than in the cord. There were 67 (54%) cases with definite or probable evidence of fetal distress. In 38 (48%) stillborns no cause of fetal death in utero was identified and in 18 (41%) liveborns there was no known prenatal problem. The results support the concept that meconium passage in utero may occur either as a response to fetal distress or as a physiologic process. Inflammation in the lung and placental tissues, and vascular injury in the umbilical cord may arise secondary to in utero exposure to meconium.

Female↗
PubMed1995

Return to well-being and function after infrainguinal revascularization.

PURPOSE: The purpose of this study was to assess functional status, well-being, and symptom relief of patients after infrainguinal revascularization for severe peripheral vascular disease. METHODS: Two questionnaires were used to assess symptoms, functional status, and well-being before operation and 6 months after operation. Sociodemographics, comorbidities, indications for surgery, graft location, and morbidity, mortality, patency, and limb salvage rates were obtained via vascular registry. RESULTS: Of 318 patients who underwent revascularization over a 1-year period, 276 patients were asked to complete the questionnaires. Of the 156 patients who completed both questionnaires, mean age was 66 years, 67% were men, 84% had diabetes mellitus, and 83% had various heart-related conditions. Mean length of stay was 15.3 days. Distal graft sites were popliteal (29%), tibial/peroneal (40%), and pedal/plantar (31%). The operative morbidity rate was 21%, the cumulative primary graft patency rate was 93%, the cumulative secondary graft patency rate was 95%, and the limb salvage rate was 97% at 6 months. At follow-up, improved functioning of instrumental activities of daily living, mental well-being, and vitality were reported. Symptoms of calf cramping and toe or foot pain when walking and at rest were also improved. Sores or ulcers improved, but leg swelling did not. The only independent predictor of improved function and well-being was the patients' perception of their status at baseline: those patients who functioned better before operation reported improved function and well-being at 6 months. Only 45% of patients reported feeling "back to normal" at 6 months. CONCLUSION: Reported health status at baseline was a predictor of improved function, mental well-being, and resolution of symptoms after infrainguinal revascularization. Expected return to "normal" may take longer than 6 months.

Activities of Daily Living↗
PubMed1995

Dorsalis pedis arterial bypass: durable limb salvage for foot ischemia in patients with diabetes mellitus.

PURPOSE: Although the technical feasibility of pedal artery bypass for limb salvage is now well established, questions remain about its most appropriate use and its long-term durability. METHODS: We reviewed our experience over an 8-year period in 367 consecutive patients undergoing 384 vein bypass grafts to the dorsalis pedis for limb salvage. RESULTS: Ninety-five percent of the patients had diabetes mellitus. Infection complicated ischemia at initial presentation in 55.2% of patients. The preoperative arteriogram demonstrated a patent dorsalis pedis in 362 extremities (92.8%). Four hundred two patients underwent exploration for bypass, including 29 patients without demonstrated arteries on the arteriogram but audible pedal Doppler signals. Successful bypasses were carried out in 357 of 362 cases, where preoperative arteriography demonstrated a patent dorsalis pedis artery (98.6%), 16 of 28 cases explored on the basis of a Doppler signal alone (57%), and 11 of 12 patients where angiographic status was unknown. All procedures were performed with vein: in situ 38.5%, reversed 29%, nonreversed 18%, arm vein 7%, and composite vein 8%. Inflow was taken from the common femoral artery in 34%, superficial femoral or popliteal arteries in 60%, a previously placed graft in 5%, and a tibial artery in 1%. There were seven perioperative deaths (1.8%) and 21 myocardial infarctions (5.4%). Twenty-nine grafts failed within 30 days (7.5%), but 19 were successfully revised. Eight of the 10 failed grafts resulted in major amputation (80%). Over the remaining study period, there were 39 additional graft failures, of which 17 were successfully revised, and 17 additional major amputations. Actuarial primary and secondary patency and limb salvage rates were 68%, 82%, and 87%, respectively, at 5 years' followup. The actuarial patients survival rate was 57% at 5 years. Patency rates were similar for in situ and translocated saphenous vein grafts. CONCLUSIONS: Dorsalis pedis arterial bypass is an effective limb salvage procedure with long-term durability comparable to distal vein grafts placed into more proximal arteries.

Arteries↗
PubMed1995

A quantitative investigation of gonadal feminization by diethylstilboestrol of genetically male embryos of the quail Coturnix coturnix japonica.

The effect of diethylstilboestrol on gonad development in quail embryos has been quantitatively analysed. Quail embryos at 4 days of incubation were treated with diethylstilboestrol (DES), using the egg dipping method. At 10 days of incubation, embryos were removed and killed by decapitation. Tissues were prepared for chromosome analysis, and the parts of the abdomen containing the gonads were prepared for serial sectioning and quantitative assessment. Left gonads of DES-treated male embryos resembled ovaries histologically, while their right gonads were markedly reduced in size. Right gonads of DES-treated female embryos were also further reduced by treatment with DES. There was no statistically significant effect by DES treatment on the size of left gonads, although the ratio of left compared with right gonadal volumes was highly significant. Since, in birds, the left embryonic gonad has ambisexual potential, while the potential of the right gonad is exclusively masculine, these results exemplify the adverse effect exerted by oestrogen on male sexual development in vertebrates.

Animals↗
PubMed1993

Improved quality of diabetic foot care, 1984 vs 1990. Reduced length of stay and costs, insufficient reimbursement.

Ischemic foot ulceration in the diabetic patient is a source of great physical and emotional strain for the patient and represents a significant financial burden for the health care system responsible for the cost of such care. Limb salvage remains the primary therapeutic goal; yet, fiscal constraints imposed by diagnosis related group-based reimbursement systems require maximal cost efficiency in the care process. Between 1984 and 1990, the changes in our team management approach to this problem, emphasizing aggressive surgical revascularization of threatened limbs, have improved the quality of care and dramatically reduced the major and minor amputation rate. In the process, we have reduced the length of hospital stay and the overall cost of care. Despite this improvement in outcome and efficiency, Medicare reimbursement remains insufficient, with an average loss of $7480 per admission.

Aged↗
PubMed1993

Gonadal sex differentiation in embryos and neonates of the marsupial, Monodelphis domestica: arrest of testis development in postterm embryos.

Growth and histological differentiation were studied in 8 litters of embryos and 4 litters of neonate grey short-tailed opossums, Monodelphis domestica. The embryonic litters included 2 that had passed their expected birth date, and whose weights exceeded the usual birthweights; we refer to these litters as 'postmature'. There was an abrupt increase in the growth rate of XY gonads after birth, but this was not seen in XX gonads. Although there was evidence of testicular differentiation in XY gonads on the day before the expected birth, testicular differentiation was found to be blocked in postmature litters. The growth of XX gonads in postmature embryos was not affected. In view of evidence that exogenous oestrogens feminise the gonads of genetic males in some species of marsupials including Monodelphis domestica, the question arises whether oestrogen is responsible for the failure of testes to continue their development in utero. We suggest that the ability of functional testes to develop in the presence of oestrogen may be a fundamental requirement distinguishing eutherian mammals from other vertebrates, including marsupials.

Animals↗
PubMed1990

Gonadal sex differentiation in the neonatal marsupial, Monodelphis domestica.

A quantitative and histological study of the gonads of newborn grey short-tailed opossums, Monodelphis domestica, is described. The pups were karyotyped, and comparisons were made within litters segregating for XX and XY sex chromosomes. A total of four litters including 25 pups were available. On the day of birth, developing testes were significantly larger than the ovaries of litter mates, and testes could be histologically distinguished by the formation of sex cords and a tunica albuginea. The data suggest that in this marsupial species gonadal differentiation may be initiated in utero.

Animals↗