Interstitial compliance and transcapillary fluid balance in renal hypertensive rats

Local interstitial fluid volume (IFV) and pressure (IFP) were used to estimate interstitial compliance (= delta IFV/delta IFP) in skin and skeletal muscle of normotensive (NT) and renal hypertensive rats (HT). The IFV was measured as the extravascular 51Cr-EDTA space, and IFP with micropipettes (tip diameter 2-4 microns) connected to a servocontrolled counter-pressure system. After control measurements, overhydration was induced by infusion of saline, 10% of body weight i.v. with or without venous stasis. Alternatively, dehydration was induced by peritoneal dialysis with 20% glucose or by furosemide infusion with or without 24 h fluid deprivation. Control ECV averaged 24.94 and 24.73 ml per 100 g in NT and HT, respectively (P greater than 0.05). Control PV averaged 2.81 and 3.28 ml per 100 g in NT and HT, respectively (P = 0.061), and control IFP was more positive in HT than in NT: 0.4 mmHg in skin (P less than 0.05) and 0.2 mmHg in skeletal muscle (P greater than 0.05). Dehydration changed PV significantly more in HT than in NT (P less than 0.05). The interstitial volume-pressure curve was linear in dehydration and the initial part of overhydration but gradually levelled off, and the maximal rise in IFP was 1-1.5 mmHg in skin and muscle. Interstitial compliance was calculated from the dehydration part of the volume-pressure curve and was in NT 14% per mmHg both in skin and skeletal muscle. In HT, compliance during dehydration was 10.2 and 20.7% reduction in IFV per mmHg fall in IFP in skin and muscle, respectively, not significantly different from corresponding values in NT (P greater than 0.05). We conclude that HT had unaltered ECV and a tendency to higher PV, and that interstitial compliance was not significantly different in normotensive and hypertensive rats.     

Muscle fiber conduction velocity during isometric contraction and the recovery period

Changes of muscle fiber conduction velocity (MFCV) detected by surface array electrodes during an isometric contraction and the recovery period were evaluated. The location on skin measured for action potentials of muscle fiber in m. biceps brachii was a distance of 5 mm and 30 mm from the end-plate to the distal tendon. The MFCV was evaluated by averaging raw EMG waves. The MFCVs at both locations declined gradually during the loads of sustained isometric contractions of 30, 50, and 70% of the maximum voluntary contraction (MVC). The degree of the decrease of the MFCV was extremely intense during a sustained contraction of 70% MVC. The values of the MFCV at the location of 5 mm from the end-plate in the period close to the exhaustion state showed a significant decrease compared with the values in the initial period during the contractions for the above three kinds of loads, while the decrease of the MFCV at the location of 30 mm from the end-plate was not found to be so significant during the contractions. In the recovery period, the contraction of 5% MVC was maintained, and the resultant MFCVs restored gradually to the value of the initial period. At the location of 5 mm from the end-plate, the MFCVs at 15 minutes after the end of the load were significantly higher than those just after the end of the load. At the location of 30 mm from the end-plate, the increases of the MFCVs during the recovery period did not show significant changes. Changes of the MFCV during the isometric contraction and the recovery period depend greatly on the location of the electrodes measured for the action potentials of the muscle fibers.     

Optimal choice of vacuum-membrane skeletal muscle extension

Peculiarities of skin extension under vacuum applicators (nozzles) of various design was studied in 20 volunteers aged 18-25 years. The data obtained made it possible to chose optimal parameters of elastic membranes of the vacuum cups that produce stretch sufficient for vacuum-membrane skeletal muscle extension with minimal traumatic damage to the skin. Local negative pressure varied from 10 to 70 kPa. It was found that for 4.5-cm applicator with one 1.4-2.0-cm central orifice the optimal thickness of the membrane is 0.3 cm.     

A new MR-compatible loading device to study in vivo muscle damage development in rats due to compressive loading

To study the aetiology of pressure ulcers an MR-compatible loading device was developed. Magnetic resonance imaging provides the possibility of non-invasive evaluation of muscle tissue after compressive loading. Pressure was applied to the tibialis anterior region of rats by means of an indenter. The developed MR-compatible loading device allowed high quality consecutive MR measurements for up to 6h. Tissue was evaluated both during and after loading. Two loading protocols were used; a large indentation of 4.5mm (mean pressure 150 kPa) was applied for 2h and a small indentation of 2.9 mm (mean pressure 50 kPa) was applied for 4h. T2-weighted MR images after the large indentation showed an immediate increase in signal intensity, associated with damage, following load removal. After 20 h the signal intensity remained higher in the affected regions. Afterwards the tissue was perfusion fixated for histological examination. Histological evaluation revealed an inflammatory response and severe muscle necrosis. No signal increase was observed after small indentation. With this new set-up, the different factors that may play a role in the onset of muscle damage can be studied, what we believe will lead to a better understanding of the contributing factors to pressure ulcer development.     

A novel MRI compatible soft tissue indentor and fibre Bragg grating force sensor

MRI is an ideal method for non-invasive soft tissue mechanical properties investigation. This requires mechanical excitation of the body's tissues and measurement of the corresponding boundary conditions such as soft tissue deformation inside the MRI environment. However, this is technically difficult since load application and measurement of boundary conditions requires MRI compatible actuators and sensors. This paper describes a novel MRI compatible computer controlled soft tissue indentor and optical Fibre Bragg Grating (FBG) force sensor. The high acquisition rate (100 Hz) force sensor was calibrated for forces up to 15 N and demonstrated a maximum error of 0.043 N. Performance and MRI compatibility of the devices was verified using indentation tests on a silicone gel phantom and the upper arm of a volunteer. The computer controlled indentor provided a highly repeatable tissue deformation. Since the indentor and force sensor are composed of non-ferromagnetic materials, they are MRI compatible and no artefacts or temporal SNR reductions were observed. In a phantom study the mean and standard deviation of the temporal SNR levels without the indentor present were 500.18 and 207.08, respectively. With the indentor present the mean and standard deviation were 501.95 and 200.45, respectively. This computer controlled MRI compatible soft tissue indentation system with an integrated force sensor has a broad range of applications and will be used in the future for the non-invasive analysis of the mechanical properties of skeletal muscle tissue.     

Temporal Effects of Mechanical Loading on Deformation-Induced Damage in Skeletal Muscle Tissue

Mechanical loading of soft tissues covering bony prominences can cause skeletal muscle damage, ultimately resulting in a severe pressure ulcer termed deep tissue injury. Recently, by means of an experimental-numerical approach, it was shown that local tissue deformations cause tissue damage once a deformation threshold is exceeded. In the present study, the effects of load exposure time and intermittent load relief on the development of deformation-induced muscle damage were investigated. The data showed that a 2 h loading period caused more damage than 10 min loading. Intermittent load reliefs of 2 min during a 2 h loading period had minimal effect on the evolution of skeletal muscle damage. In addition, a local deformation threshold for damage was found, which was similar for each of the loading regimes applied in this study. For short loading periods, these results imply that local tissue deformations determine whether muscle damage will develop and the exposure time influences the amount of tissue damage. Temporary load reliefs were inefficient in reducing deformation-induced damage, but may still influence the development of ischemia-induced damage during longer loading periods.     

Influence of inspiratory threshold load on cardiovascular responses to controlled breathing at 0.1 Hz

Slow, deep breathing is being used as a self‐management intervention for various health conditions including pain and hypertension. Stimulation of the arterial baroreceptors and increased vagal modulation are among the proposed mechanisms for the therapeutic effects of slow, deep breathing. We investigated whether adding inspiratory threshold load can enhance the cardiovascular responses to controlled breathing at the frequency of 0.1 Hz, a common form of slow, deep breathing. Healthy volunteers (N = 29) performed controlled breathing at 0.1 Hz (6 breaths/minute) without load and with inspiratory threshold loads of 5 cmH₂O and 10 cmH₂O. Respiratory airflow, heart rate, and blood pressure were continuously recorded. The amplitude of the systolic blood pressure variation during respiratory cycles increased with increasing loads. Respiratory sinus arrhythmia was higher during controlled breathing at 0.1 Hz with the load of 10 cmH₂O compared to without load. Baroreflex sensitivity was not affected by loads. The effect of loads on respiratory sinus arrhythmia was mediated by increasing the amplitude of systolic blood pressure variation during respiratory cycles. These results suggest that applying small inspiratory threshold loads during controlled breathing at 0.1 Hz increases cardiac vagal modulation by this breathing exercise. This effect seems to be mediated by stronger stimulation of the arterial baroreceptors because of larger systolic blood pressure swings along the respiratory cycle. The potential benefit of long‐term practice of controlled breathing at 0.1 Hz with inspiratory threshold loads on baroreflex function and cardiac vagal control needs to be investigated, particularly in pain and hypertension patients.     

Oxygen uptake for negative work, stretching contractions by in situ dog skeletal muscle.

Oxygen uptake for negative work, stretching twitch contractions by in situ gastrocnemius-plantaris muscle was calculated from measurements of venous outflow and arterial and venous blood oxygen contents. Contractions were produced by valving air at high pressure into the pneumatic lever 10-50 ms before stimulation of the muscle. The loads produced were up to about 2.5 times isometric. Muscle length was always below optimal isometric length. Oxygen uptake for shortening contractions increased with increasing load up to isometric load. Oxygen uptake for stretching contractions decreased with increasing loads above isometric load. Velocity of shortening decreased with increasing loads up to isometric load whereas velocity of stretching increased with increasing loads above isometric. In shortening contractions external work done by the muscle was greatest at intermediate loads, but in stretching contractions the work done on the muscle increased with increasing loads. In stretching contractions the ratio of the energy equivalent of the work absorbed by the muscle reached 8.0 times the energy equivalent of the oxygen uptake. Since this ratio cannot exceed 1.0 for an engine, muscles must act as brakes during stretching contractions.     

Microhardness of superficial and deep sound human dentin

Our purpose in this study was to determine the microhardness of superficial and deep dentin by means of two indentation methods (Knoop and Vickers) under two different applied loads. Twelve dentin discs approximately 2-mm thick were obtained from both superficial and deep dentin by transversally sectioning the crowns of sound, extracted human third molars with a diamond blade under water irrigation. Dentin surfaces were sequentially polished, and indentations (n = 20 per surface) were performed with either Vickers indentor at loads of 300 and 500 g, respectively, or Knoop indentor at loads of 50 and 100 g, respectively. Average Vickers hardness number (VHN) and Knoop hardness number (KHN) were calculated and treated with two-way analysis of variance (ANOVA) and Student's t test. Microhardness of dentin was not influenced by the different loads applied for both indentation methods. Knoop hardness was significantly higher for superficial than for deep dentin (p < 0.05). Conversely, Vickers hardness was not significantly different for both substrates (p > 0.05). Differences in dentin hardness as a function of depth exist, but they might not be relevant, and no alteration of the distribution of stresses along the adhesive interface is expected.     

Magnetic resonance elastography of skeletal muscle deep tissue injury

The current state‐of‐the‐art diagnosis method for deep tissue injury in muscle, a subcategory of pressure ulcers, is palpation. It is recognized that deep tissue injury is frequently preceded by altered biomechanical properties. A quantitative understanding of the changes in biomechanical properties preceding and during deep tissue injury development is therefore highly desired. In this paper we quantified the spatial–temporal changes in mechanical properties upon damage development and recovery in a rat model of deep tissue injury. Deep tissue injury was induced in nine rats by two hours of sustained deformation of the tibialis anterior muscle. Magnetic resonance elastography (MRE), T₂‐weighted, and T₂‐mapping measurements were performed before, directly after indentation, and at several timepoints during a 14‐day follow‐up. The results revealed a local hotspot of elevated shear modulus (from 3.30 ± 0.14 kPa before to 4.22 ± 0.90 kPa after) near the center of deformation at Day 0, whereas the T₂ was elevated in a larger area. During recovery there was a clear difference in the time course of the shear modulus and T₂. Whereas T₂ showed a gradual normalization towards baseline, the shear modulus dropped below baseline from Day 3 up to Day 10 (from 3.29 ± 0.07 kPa before to 2.68 ± 0.23 kPa at Day 10, P < 0.001), followed by a normalization at Day 14. In conclusion, we found an initial increase in shear modulus directly after two hours of damage‐inducing deformation, which was followed by decreased shear modulus from Day 3 up to Day 10, and subsequent normalization. The lower shear modulus originates from the moderate to severe degeneration of the muscle. MRE stiffness values were affected in a smaller area as compared with T₂. Since T₂ elevation is related to edema, distributing along the muscle fibers proximally and distally from the injury, we suggest that MRE is more specific than T₂ for localization of the actual damaged area.     

Selective recording of electroneurograms from the sciatic nerve of a dog with multi-electrode spiral cuffs

Electroneurograms (ENGs) from superficial regions of the sciatic nerve of a Beagle dog were recorded selectively with a chronically implanted 33-electrode spiral cuff (cuff). By delivering stimulating pulses to groups of three electrodes (GTEs) within the cuff we could define the relative positions of the particular superficial regions that selectively innervated the tibialis anterior (TA) and gastrocnemius muscles (GM). GTEs with and without contractions of the TA and GM muscles were selected and connected to a 4-channel ENG system designed to amplify ENGs by 100,000 times and to pass frequencies between 500 Hz and 10 kHz. In our study, 12 experiments were conducted on three Beagle dogs with a cuff implanted for up to 2 years. We present the results obtained in four experiments conducted on one animal. With the implanted leg mounted in a special electronic brace we applied extending forces to the ankle, rotating it by up to 37 degrees according to the neutral position, eliciting torque to stretch the TA muscle. Only the ENG from a GTE eliciting maximum contraction of the TA muscle showed activities corresponding to the trajectory of the mechanical load of the muscle. Next, we dissected the calcanean tendon (CT) of the implanted leg and applied repetitive pull forces to the CT. Only the ENG from the GTE eliciting maximum contraction of the GM muscle was activated in correspondence to the trajectory of the mechanical load applied on the CT. The results suggest that the cuff, implanted chronically on the sciatic nerve, is useful to record ENGs of the afferent fibers from TA and GM muscles selectively and that the technique could be extended for human use in the field of rehabilitation for paralysis.     

Experimental and computational analysis of soft tissue stiffness in forearm using a manual indentation device

A hand held stiffness meter can be used to measure indentation stiffness of human soft tissues, sensitively altered, e.g., by pathological tissue swelling. Under indentation load, the relative contribution of each soft tissue component (i.e., skin, adipose tissue and muscle) to the biomechanical response is not known. In the present study, we evaluated the biomechanical role of different soft tissues in relaxed, physically stressed and oedemic human forearm. Soft tissue stiffness of the forearms of nine healthy human subjects was measured under four different test protocols: (1) forearm at rest, (2) forearm under isometric flexor loading, (3) forearm under isometric extensor loading, and (4) forearm under venous occlusion. In (2) and (3) the loading forces were monitored using a dynamometer, and in (4) the soft tissue swelling was induced by venous occlusion using a pressure cuff. At the site of indentation, thickness of different tissue layers (skin, adipose tissue and muscle) was measured using B-mode ultrasound imaging. Layered, hyperelastic finite element (FE) model of the indentation measurement was created and the model response was matched with that of the stiffness meter to determine the elastic modulus for each tissue in the model. Optimized values of the elastic modulus for skin and adipose tissue at rest were 210 kPa and 1.9 kPa, respectively. Further, significance of the variations in stiffness of different tissues on the indentation response was tested. Experimentally, indentation stiffness of the forearm increased during isometric extensor and flexor loads as well as under venous occlusion by 53, 91 and 15%, respectively. The FE model could reproduce the experimental responses primarily by the increased modulus of skin; 112% (446 kPa), 210% (651 kPa) and 21% (254 kPa) under flexor and extensor loading as well as during venous occlusion, respectively. The indentation response was 9–16 times more sensitive to changes in the mechanical properties of skin than those of adipose tissue and muscle. In conclusion, the present stiffness meter may be used to quantify in vivo mechanical properties of soft tissues in the forearm, sensitively modulated by soft tissue swelling and muscle loading.     

Non contact method for in vivo assessment of skin mechanical properties for assessing effect of ageing

The assessment of human tissue properties by objective and quantitative devices is very important to improve the understanding of its mechanical behaviour. The aim of this paper is to present a non contact method to measure the mechanical properties of human skin in vivo. A complete non contact device using an air flow system has been developed. Validation and assessment of the method have been performed on inert visco-elastic material. An in vivo study on the forearm of two groups of healthy women aged of 23.2±1.6 and 60.4±2.4 has been performed. Main parameters assessed are presented and a first interpretation to evaluate the reduced Young's modulus is proposed. Significant differences between the main parameters of the curve are shown with ageing. As tests were performed with different loads, the influence of the stress is also observed. We found a reduced Young's modulus with an air flow force of 10 mN of 14.38±3.61 kPa for the youngest group and 6.20±1.45 kPa for the oldest group. These values agree with other studies using classical or dynamic indentation. Non contact test using the developed device gives convincing results.     

An experimental method for the application of lateral muscle loading and its effect on femoral strain distributions

Experimental models that have been used to evaluate hip loading and the effect of hip implants on bone often use only a head load and abductor load. Anatomic considerations and in vivo measurements have lead several investigators to suggest that these models are inaccurate because they do not incorporate the loads imposed by additional muscles. The aim of this study was to evaluate the strains in the proximal and mid diaphysis of the femur for five hip loading models, one with a head load and abductor load only and four which incorporated lateral muscle loads as well. Head load to body weight load ratios were used to evaluate the physiologic accuracy of these models and strains were compared to determine the extent of strain changes as a function of model complexity. All models which incorporated additional lateral muscle loads more accurately simulated head load to body-weight load ratios than the simple abductor-only model. The model which incorporated a coupled vastus lateralis and iliotibial band load in addition to the abductor load provided the simplest configuration with a reasonable body-weight to head-load ratio.     

C57BL/6小鼠胫骨前肌和趾长伸肌肌梭解剖学分析

目的 定位C57BL/6小鼠胫骨前肌(TA)和趾长伸肌(EDL)肌梭分布,分析肌梭在骨骼肌中的固定方式,并统计肌梭各区域长度和赤道横截面积(CAS)等参数分析肌梭形态学共性,为肌梭的形态和功能研究提供解剖学基础.方法 5只正常成年C57BL/6小鼠取TA和EDL,利用改良的骨骼肌冷冻技术得到无冰晶样本,样本连续冷冻切片...     

大鼠胫骨前肌微循环的两种压力损伤效应研究

目的通过两种压力模式作用于大鼠胫骨前肌引起肌组织缺血和血管损伤程度的比较,探讨波浪床预防深部组织损伤的机制。方法 10～12周龄SD大鼠20只,随机分为两组。每只大鼠取单侧后肢胫骨前肌中段部位0.2cm2,实验组给予渐变间歇性压力（8～21.3kPa）,对照组给予持续性压力（13.3kPa）。实验组和对照组未施压一侧后肢作为健康对照。施压2h后间歇30分钟为1个周期,重复3个周期。施压3个周期后手术切开暴露胫骨前肌,采用微循环显微镜观察施压点边缘血管平均管径及平均流速,CD34免疫组化测量两组受压胫骨前肌组织微血管密度。结果施压后大体观察两组皮肤均无开放性溃疡形成,对照组胫骨前肌可肉眼见明显压痕并伴水肿、淤血,实验组损伤较轻微。微循环观测对照组肌肉平均血管管径,平均血流速度均较健侧下降,两组间差异有统计学意义（P〈0.05）。CD34免疫组化测量微血管密度：实验组微血管密度比对照组高,比较两组差异有统计学意义（P〈0.05）。结论渐变的间歇性压力可以减轻压力介导的组织缺血和血管损伤,预防深部组织损伤的形成。     

Biomechanics of the knee joint in deep flexion: a prelude to a total knee replacement that allows for maximum flexion.

A two-dimensional anatomically based mathematical model of the human knee joint was developed to understand its biomechanics in deep flexion. The model was used to determine the internal knee loads as it simulates isometric quadriceps and hamstring co-contractions at different flexion angles during deep squat. It was found that in order to achieve deep flexion, large muscle forces are required, resulting in large tibio-femoral contact forces. In deep flexion, the femoral contact point was located on the most proximal point of the posterior condyle, location which was not affected by the level of quad activation. Conversely, the location of the tibial contact point was highly affected by the level of quad activation. Both anterior and posterior fiber bundles of the posterior cruciate ligament were found to carry high loads when the knee is maximally flexed. These results point to the important role of the posterior cruciate ligament in this position, and suggest the necessity of retaining this ligament during total knee replacement (TKR) procedures that allows for maximum flexion angles. Furthermore, the present data provide an explanation why most TKR's do not allow deep flexion: while contact occurs on the most proximal points of the posterior condyles in normal knees, this portion of the condyles is not presently resurfaced when performing a TKR.     

Changes in reciprocal inhibition across the ankle joint with changes in external load and pedaling rate during bicycling

The purpose of this study was to investigate the role of reciprocal inhibition in the regulation of antagonistic ankle muscles during bicycling. A total of 20 subjects participated in the study. Reciprocal inhibition was induced by stimulation of the peroneal nerve (PN) at 1.2 times threshold for the M-response in the tibialis anterior muscle (TA) and recorded as a depression of the rectified soleus (SOL) EMG. Recordings were made during tonic plantar flexion and during bicycling on an ergometer bicycle. During tonic contraction, the amount of inhibition in the SOL EMG was linearly correlated to the amount of background EMG. This linear relation was used to calculate the expected amount of reciprocal inhibition at corresponding EMG levels during bicycling. During the early phase of down-stroke of bicycling at 60 revolutions per minute (RPM) and an external load of 1.0 kg, the amount of recorded reciprocal inhibition was significantly smaller than that calculated from the linear relation during tonic contraction. In nine subjects, the SOL H-reflex was used to evaluate the amount of inhibition. At a short conditioning test interval (2-3 ms), the PN stimulation depressed the SOL H-reflex when the subjects were at rest. This short latency inhibition was absent during downstroke, but appeared during upstroke just prior to and during TA activation. A positive linear relation was found between the level of SOL background EMG in early downstroke and the external load (0.5-2.5 kg) as well as the rate of pedaling (30-90 RPM at 1.0 kg external load). The amount of inhibition in the SOL EMG when expressed as a percentage of the background EMG activity decreased significantly with increasing load. During increased pedaling rate, a similar decrease was seen, but it did not reach a statistically significant level. The data illustrate that reciprocal inhibition of the soleus muscle is modulated during bicycling being small in downstroke when the SOL muscle is active and large in upstroke where the muscle is inactive and its antagonist becomes active. The depression of the inhibition in relation to increased load and pedaling rate likely reflects the need of reducing inhibition of the SOL motoneurons to ensure a sufficient activation of the muscle.     

Spinal muscles can create compressive follower loads in the lumbar spine in a neutral standing posture

The ligamentous spinal column buckles under compressive loads of even less than 100N. Experimental results showed that under the follower load constraint, the ligamentous lumbar spine can sustain large compressive loads without buckling, while at the same time maintaining its flexibility reasonably well. The purpose of this study was to investigate the feasibility of follower loads produced by spinal muscles in the lumbar spine in a quiet standing posture. A three-dimensional static model of the lumbar spine incorporating 232 back muscles was developed and utilized to perform the optimization analysis in order to find the muscle forces, and compressive follower loads (CFLs) along optimum follower load paths (FLPs). The effect of increasing external loads on CFLs was also investigated. An optimum solution was found which is feasible for muscle forces producing minimum CFLs along the FLP located 11 mm posterior to the curve connecting the geometrical centers of the vertebral bodies. Activation of 30 muscles was found to create CFLs with zero joint moments in all intervertebral joints. CFLs increased with increasing external loads including FLP deviations from the optimum location. Our results demonstrate that spinal muscles can create CFLs in the lumbar spine in a neutral standing posture in vivo to sustain stability. Therefore, its application in experimental and numerical studies concerning loading conditions seems to be suitable for the attainment of realistic results.     

Protective effect of tenuazonic acid against dimethyl benz(a)antracene-induced skin carcinogenesis in mice

Anticarcinogenic potential of tenuazonic acid (TA), a mycotoxin isolated from a fungus, Alternaria alternata, on skin tumorigenesis induced by 7,12-dimethyl benz(a) antracene (DMBA) was investigated. Female Swiss albino mice were exposed topically to 100 nmole of DMBA twice weekly for 20 weeks. Another group of animals was treated with 250 microg TA in acetone daily for a period of 1 week, followed by the same dose of TA prior to every application of DMBA. At the end of 14 weeks, all the animals in the group that was exposed to DMBA alone developed tumors, while 40% of the animals in TA-treated group were found to be tumor free. After 20 weeks, there was no further increase in the number of tumor-bearing animals. Results indicated that prior application of TA significantly delayed the onset of tumorigenesis and also reduced the cumulative number of tumors per tumor-bearing animals. The present studies reveal the antitumor and protective potential of TA against polycyclic aromatic hydrocarbon induced skin carcinogenesis.