Polarizing light microscopy of intestine and its relationship to mechanical behaviour

On a phase-field approach to model fracture of small intestine walls

We address anisotropic elasticity and fracture in small intestine walls (SIWs) with both experimental and computational methods. Uniaxial tension experiments are performed on porcine SIW samples with varying alignments and quantify their nonlinear elastic anisotropic behavior. Fracture experiments on notched SIW strips reveal a high sensitivity of the crack propagation direction and the failure stress on the tissue orientation. From a modeling point of view, the observed anisotropic elastic response is studied with a continuum mechanical model stemming from a strain energy density with a neo-Hookean component and an anisotropic component with four families of fibers. Fracture is addressed with the phase-field approach, featuring two-fold anisotropy in the fracture toughness. Elastic and fracture model parameters are calibrated based on the experimental data, using the maximum and minimum limits of the experimental stress-stretch data set. A very good agreement between experimental data and computational results is obtained, the role of anisotropy being effectively captured by the proposed model in both the elastic and the fracture behavior. STATEMENT OF SIGNIFICANCE: This article reports a comprehensive experimental data set on the mechanical failure behavior of small intestinal tissue, and presents the corresponding protocols for preparing and testing the samples. On the one hand, the results of this study contribute to the understanding of small intestine mechanics and thus to understanding of load transfer mechanisms inside the tissue. On the other hand, these results are used as input for a phase-field modelling approach, presented in this article. The presented model can reproduce the mechanical failure behavior of the small intestine in an excellent way and is thus a promising tool for the future mechanical description of diseased small intestinal tissue.

Three-Dimensional Quantification of Collagen Microstructure During Tensile Mechanical Loading of Skin

Skin is a heterogeneous tissue that can undergo substantial structural and functional changes with age, disease, or following injury. Understanding how these changes impact the mechanical properties of skin requires three-dimensional (3D) quantification of the tissue microstructure and its kinematics. The goal of this study was to quantify these structure-function relationships via second harmonic generation (SHG) microscopy of mouse skin under tensile mechanical loading. Tissue deformation at the macro- and micro-scale was quantified, and a substantial decrease in tissue volume and a large Poisson’s ratio was detected with stretch, indicating the skin differs substantially from the hyperelastic material models historically used to explain its behavior. Additionally, the relative amount of measured strain did not significantly change between length scales, suggesting that the collagen fiber network is uniformly distributing applied strains. Analysis of undeformed collagen fiber organization and volume fraction revealed a length scale dependency for both metrics. 3D analysis of SHG volumes also showed that collagen fiber alignment increased in the direction of stretch, but fiber volume fraction did not change. Interestingly, 3D fiber kinematics was found to have a non-affine relationship with tissue deformation, and an affine transformation of the micro-scale fiber network overestimates the amount of fiber realignment. This result, along with the other outcomes, highlights the importance of accurate, scale-matched 3D experimental measurements when developing multi-scale models of skin mechanical function.

Variation of Passive Biomechanical Properties of the Small Intestine along Its Length: Microstructure-Based Characterization

Multiaxial testing of the small intestinal wall is critical for understanding its biomechanical properties and defining material models, but limited data and material models are available. The aim of the present study was to develop a microstructure-based material model for the small intestine and test whether there was a significant variation in the passive biomechanical properties along the length of the organ. Rat tissue was cut into eight segments that underwent inflation/extension testing, and their nonlinearly hyper-elastic and anisotropic response was characterized by a fiber-reinforced model. Extensive parametric analysis showed a non-significant contribution to the model of the isotropic matrix and circumferential-fiber family, leading also to severe over-parameterization. Such issues were not apparent with the reduced neo-Hookean and (axial and diagonal)-fiber family model, that provided equally accurate fitting results. Absence from the model of either the axial or diagonal-fiber families led to ill representations of the force- and pressure-diameter data, respectively. The primary direction of anisotropy, designated by the estimated orientation angle of diagonal-fiber families, was about 35° to the axial direction, corroborating prior microscopic observations of submucosal collagen-fiber orientation. The estimated model parameters varied across and within the duodenum, jejunum, and ileum, corroborating histologically assessed segmental differences in layer thicknesses.

Structure and collagen crimp patterns of functionally distinct equine tendons, revealed by quantitative polarised light microscopy (qPLM)

Structure-function relationships in tendons are directly influenced by the arrangement of collagen fibres. However, the details of such arrangements in functionally distinct tendons remain obscure. This study demonstrates the use of quantitative polarised light microscopy (qPLM) to identify structural differences in two major tendon compartments at the mesoscale: fascicles and interfascicular matrix (IFM). It contrasts functionally distinct positional and energy storing tendons, and considers changes with age. Of particular note, the technique facilitates the analysis of crimp parameters, in which cutting direction artefact can be accounted for and eliminated, enabling the first detailed analysis of crimp parameters across functionally distinct tendons.

IFM shows lower birefringence (0.0013 ± 0.0001 [−]), as compared to fascicles (0.0044 ± 0.0005 [−]), indicating that the volume fraction of fibres must be substantially lower in the IFM. Interestingly, no evidence of distinct fibre directional dispersions between equine energy storing superficial digital flexor tendons (SDFTs) and positional common digital extensor tendons (CDETs) were noted, suggesting either more subtle structural differences between tendon types or changes focused in the non-collagenous components.

By contrast, collagen crimp characteristics are strongly tendon type specific, indicating crimp specialisation is crucial in the respective mechanical function. SDFTs showed much finer crimp (21.1 ± 5.5 µm) than positional CDETs (135.4 ± 20.1 µm). Further, tendon crimp was finer in injured tendon, as compared to its healthy equivalents. Crimp angle differed strongly between tendon types as well, with average of 6.5 ± 1.4° in SDFTs and 13.1 ± 2.0° in CDETs, highlighting a substantially tighter crimp in the SDFT, likely contributing to its effective recoil capacity.

Statement of Significance

This is the first study to quantify birefringence in fascicles and interfascicular matrix of functionally distinct energy storing and positional tendons. It adopts a novel method – quantitative polarised light microscopy (qPLM) to measure collagen crimp angle, avoiding artefacts related to the direction of histological sectioning, and provides the first direct comparison of crimp characteristics of functionally distinct tendons of various ages.

A comparison of matched picrosirius red stained and unstained tendons sections identified non-homogenous staining effects, and leads us to recommend that only unstained sections are analysed in the quantitative manner.

qPLM is successfully used to assess birefringence in soft tissue sections, offering a promising tool for investigating the structural arrangements of fibres in (soft) tissues and other composite materials.

Diabetes-induced mechanophysiological changes in the small intestine and colon

The disorders of gastrointestinal (GI) tract including intestine and colon are common in the patients with diabetes mellitus (DM). DM induced intestinal and colonic structural and biomechanical remodeling in animals and humans. The remodeling is closely related to motor-sensory abnormalities of the intestine and colon which are associated with the symptoms frequently encountered in patients with DM such as diarrhea and constipation. In this review, firstly we review DM-induced histomorphological and biomechanical remodeling of intestine and colon. Secondly we review motor-sensory dysfunction and how they relate to intestinal and colonic abnormalities. Finally the clinical consequences of DM-induced changes in the intestine and colon including diarrhea, constipation, gut microbiota change and colon cancer are discussed. The final goal is to increase the understanding of DM-induced changes in the gut and the subsequent clinical consequences in order to provide the clinicians with a better understanding of the GI disorders in diabetic patients and facilitates treatments tailored to these patients.

Differential diagnosis of periapical cyst using collagen birefringence pattern of the cyst wall

Objectives

Periapical lesions, including periapical cyst (PC), periapical granuloma (PG), and periapical abscess (PA), are frequently affected by chemical/physical damage during root canal treatment or severe bacterial infection, and thus, the differential diagnosis of periapical lesions may be difficult due to the presence of severe inflammatory reaction. The aim of this study was to make differential diagnosis among PC, PG, and PA under polarizing microscope.

Materials and Methods

The collagen birefringence patterns of 319 cases of PC (n = 122), PG (n = 158), and PA (n = 39) obtained using a polarizing microscope were compared. In addition, 6 cases of periodontal fibroma (PF) were used as positive controls.

Results

Collagen birefringence was condensed with a thick, linear band-like pattern in PC, but was short and irregularly scattered in PG, and scarce or absent in PA. PF showed intense collagen birefringence with a short, palisading pattern but no continuous band-like pattern. The linear band-like birefringence in PC was ascribed to pre-existing expansile tensile stress of the cyst wall.

Conclusions

In this study all PCs (n = 122) were distinguishable from PGs and PAs by their characteristic birefringence, despite the absence of lining epithelium (n = 20). Therefore, the authors suggest that the presence of linear band-like collagen birefringence of the cyst wall aids the diagnostic differentiation of PC from PG and PA.

Modeling optical behavior of birefringent biological tissues for evaluation of quantitative polarized light microscopy

Quantitative polarized light microscopy (qPLM) is a popular tool for the investigation of birefringent architectures in biological tissues. Collagen, the most abundant protein in mammals, is such a birefringent material. Interpretation of results of qPLM in terms of collagen network architecture and anisotropy is challenging, because different collagen networks may yield equal qPLM results. We created a model and used the linear optical behavior of collagen to construct a Jones or Mueller matrix for a histological cartilage section in an optical qPLM train. Histological sections of tendon were used to validate the basic assumption of the model. Results show that information on collagen densities is needed for the interpretation of qPLM results in terms of collagen anisotropy. A parameter that is independent of the optical system and that measures collagen fiber anisotropy is introduced, and its physical interpretation is discussed. With our results, we can quantify which part of different qPLM results is due to differences in collagen densities and which part is due to changes in the collagen network. Because collagen fiber orientation and anisotropy are important for tissue function, these results can improve the biological and medical relevance of qPLM results.

Spontaneous and bolus-induced motility in the chronically obstructed guinea-pig small intestine in vitro

Partial obstruction of the small intestine results in dysmotility and morphometric changes proximal to the site of obstruction. However, our understanding of the relation between the morphometric remodeling and change in the motility pattern during chronic obstruction is sparse. The aim of this study was to investigate the effect of partial chronic intestinal obstruction on motility, morphology, and collagen content proximal and distal to the site of obstruction. Twenty guinea-pigs with partial intestinal obstruction and eight sham-operated controls lived for four weeks. Spontaneous and bolus-induced motility was recorded in isolated intestinal segments proximal and distal to the site of obstruction using a perfused low-compliance pressure-measuring system in vitro. After the motility experiments, the specimens were fixed at 2 kPa luminal distension pressure and sampled for histomorphometric determination of luminal radius, layer thickness, and wall thickness. Total wall collagen was also determined. The area under the curve (AUC) of spontaneous contractions and the amplitude, frequency, and AUC for the bolus-induced motility were higher in the proximal segments of the banded animals compared to distal segments and to the intestinal segments in the control animals (P < 0.05). The radius-to-thickness ratio was lowest in the proximal segments of the obstructed animals (P < 0.01). The collagen content was three times higher proximal to the site of obstruction when compared to distal locations and to the controls (P < 0.01). The AUC at 2 ml bolus injections plotted against the radius-to-thickness ratio showed a strong association (r = 0.97 for control, and r = 0.99 for obstruction, P < 0.01). No correlation was found between the collagen content and AUC. In conclusion, partial intestinal obstruction in guinea pigs caused pronounced changes in morphology and motility. An association was found between the radius-to-thickness ratio and bolus-induced motility.

Biomechanical remodeling of the chronically obstructed Guinea pig small intestine

Small intestinal obstruction is a frequently encountered clinical problem. To understand the mechanisms behind obstruction and the clinical consequences, data are needed on the relation between the morphologic and biomechanical remodeling that takes place in the intestinal wall during chronic obstruction. We sought to determine the effect of partial obstruction on mechanical and morphologic properties of the guinea pig small intestine. Partial obstruction was created surgically in 2 groups of animals living for 2 and 4 weeks. Controls were sham operated and lived for 4 weeks. A combined impedance planimetry-high-frequency ultrasound system was designed to measure the luminal cross-sectional area and wall thickness. These measures were used to compute the circumferential stress and strain of the excised intestinal segments. The incremental elastic modulus was obtained by using nonlinear fitting of the stress-strain curve. Histologic analysis and the measurements of total wall collagen were also performed. The luminal cross-sectional area, wall thickness, and elastic modulus in circumferential direction increased in a time-dependent manner proximal to the obstruction site (P < 0.01), whereas no differences in these parameters were found distal to the obstruction site (P > 0.25). The circumferential stress-strain curves of the proximal segments in 2- and 4-week groups shifted to the left, indicating the intestinal wall became stiffer. Histologic examination revealed a massive increase in the thickness of the muscle layer especially the circular smooth muscle layer (P < 0.05). The collagen content proximal to the obstruction site was significantly larger in the partially obstructed animals compared to controls (P < 0.05). No difference was found distal to the obstruction site. Strong correlation was found between the collagen content and the elastic modulus at stress levels of 70 kPa stress (P < 0.01) and 10 kPa (P < 0.05) proximal to the obstruction site suggesting that the alteration of collagen has great impact on the mechanical remodeling. The morphologic and biomechanical remodeling likely influence the function of the intestine affected by partial obstructed intestine.

Quantitative analysis of collagen fiber angle in the submucosa of small intestine

It is of interest to know how distension changes the angle and content of collagen in the submucosa of small intestine. We describe the application of a two-dimensional quantitative analysis technology to determine the angles between collagen fibers in the submucosa using digital image processing. A polarization microscope was used to obtain a series of animal intestinal slice images. The images were studied by analyzing the relationship between the pixel values of each of the polarized angles to obtain the collagen fiber angle. The statistical distribution of the angle as function of the degree of distension can be analyzed.

The effect of epidermal growth factor on the incremental Young's moduli in the rat small intestine

Biomechanical remodelling of the rat small intestine after treatment with epidermal growth factor (EGF) subcutaneously for 2 days (n=6), 4 days (n=6), 7 days (n=6), and 14 days (n=4) was studied. The incremental circumferential, longitudinal and cross moduli close to the in vivo state were computed from bi-axial test data (combined inflation and axial stretching) by a least square method. The moduli in the circumferential direction and the longitudinal direction differed in all groups, i.e. the mechanical properties were anisotropic in both normal and EGF-treated rats. Time-dependent variation existed for the Young's moduli in all directions during EGF treatment (P<0.05). The circumferential modulus decreased during the first 7 days of EGF treatment and it almost remodelled back to that of the control group after 14 days treatment. The incremental modulus in the circumferential direction ranged between 17.4 and 24.2 kPa. The modulus in the longitudinal direction ranged between 22.9 and 32.4 kPa. The longitudinal modulus after 4 days EGF treatment was significantly larger than that of control group (P<0.02). The cross modulus decreased during the first 4 days of EGF treatment thereafter it increased to a maximum at 7 days. The values for the cross moduli were between 4.7 and 6.6 kPa. In conclusion, the mechanical properties in the intestinal wall are anisotropic and remodel during treatment with EGF.

Collagen fiber angle in the submucosa of small intestine and its application in gastroenterology

AIM: To propose a simple and effective method suitable for analyzing the angle and distribution of 2-dimensional collagen fiber in larger sample of small intestine and to investigate the relationship between the angles of collagen fiber and the pressure it undergoes.

METHODS: A kind of 2-dimensional visible quantitative analyzing technique was described. Digital image-processing method was utilized to determine the angle of collagen fiber in parenchyma according to the changes of area analyzed and further to investigate quantitatively the distribution of collagen fiber. A series of intestinal slice’s images preprocessed by polarized light were obtained with electron microscope, and they were processed to unify each pixel. The approximate angles between collagen fibers were obtained via analyzing the images and their corresponding polarized light. The relationship between the angles of collagen fiber and the pressure it undergoes were statistically summarized.

RESULTS: The angle of collagen fiber in intestinal tissue was obtained with the quantitative analyzing method of calculating the ratio of different pixels. For the same slice, with polarized light angle’s variation, the corresponding ratio of different pixels was also changed; for slices under different pressures, the biggest ratio of collagen fiber area was changed either.

CONCLUSION: This study suggests that the application of stress on the intestinal tissue will change the angle and content of collagen fiber. The method of calculating ratios of different pixel values to estimate collagen fiber angle was practical and reliable. The quantitative analysis used in the present study allows a larger area of soft tissue to be analyzed with relatively low cost and simple equipment.

Distribution of collagen fibers in the aggregated lymphoid follicles of swine terminal ileum

The arrangement of the collagen bundles was studied in the Peyer's patches of swine terminal ileum, by means of light microscopy (using silver-impregnation technique and picrosirius F3BA staining) and scanning electron microscopy (after NaOH-maceration). The lymphoid tissue forms a large and continuous patch along the antimesenteric border. The follicles are disposed mainly in the tela submucosa and sometimes they reached in the tunica mucosa surface (follicle/dome structures). Some follicles are located in the lamina propria of the tunica mucosa. Light microscopy showed black and brown-stained fibers, and yellow and red, and green-stained fibers, respectively by silver impregnation technique and picrosirius red staining, in the tela submucosa. In this tela, by scanning electron microscopy, the collagen fibers appeared as thick bundles forming a network of parallel layers. This network was denser in the interfollicular than in the follicular area, and formed a capsule surrounding the lymphoid follicles. Our results pointed out that a clear correspondence exists between the findings of currently used light microscopy techniques and the scanning electron microscopy after alkali-water maceration method. The arrangement of the collagen fibers in the antimesenteric border of the tela submucosa suggested a functional compartmentalization within the aggregated lymphoid follicles. This could facilitate the antigen-to-cell and cell-to-cell interaction during the immune response and thus create a suitable microenvironment for an active cell metabolism. The tunica mucosa showed a porous structure and its frequent gaps were likely the sites through which lymphocytes and other cells could freely migrate thus participating in the immunological activities of these structures.

Biomechanics of the gastrointestinal tract

As the function of the gastrointestinal tract is to a large degree mechanical, it has become increasingly popular to acquire distensibility data in motility research based on various parameters. Hence it is important to know on which geometrical and mechanical assumptions the various parameters are based. Currently, compliance and tone derived from pressure-volume curves are by far the most often used parameters. However, pressure-volume relations obtained in tubular organs must be carefully interpreted as they provide no direct measure of luminal cross-sectional area and other variables useful in plane stress and strain analysis. Thus, erroneous conclusions concerning tissue distensibility may be deduced. Other parameters, such as wall tension, stress and strain, give more useful information about mechanical behaviour. Distensibility data procure significance in fluid mechanics and in the study of tone, peristaltic reflexes, and mechanoreceptor kinematics. Such data are needed for the determination of the interaction between stimulus, electrical responses in neurons and the mechanical behaviour of the gut. Furthermore, from a clinical perspective, investigation of visco-elastic properties is important because GI diseases are associated with growth and remodelling. For example, prestenotic dilatation, increased collagen synthesis, dysmotility and altered distensibility are common features of obstructive diseases. The purpose of this review is to discuss the physiological and clinical importance of acquiring biomechanical data, distensibility parameters and interpretation of these results and their associated errors. We will also discuss some aspects of the relationship between morphology, growth and biomechanics. Finally, we will outline a number of techniques to study the mechanical properties of the GI tract.

Collagen structural organization of healing colonic anastomoses and the effect of growth hormone treatment

PURPOSE

This experimental study was designed to investigate the collagen fibrils of colonic anastomoses in rats and to compare normal healing with rats treated with biosynthetic growth hormone (bGH).

METHODS

The healing zone of left colonic anastomoses was studied at days 2, 4, and 6 after surgery by means of scanning electron microscopy.

RESULTS

After four days of healing a normal anastomosis was filled with loosely packed and unorganized collagen fibrils, which were organized into collagen fibers after six days. Compared with normal anastomoses, rats treated with bGH showed a more organized healing, characterized by a dense structure of a new-formed collagen framework of fibrils and immature collagen fibers after six days.

CONCLUSIONS

Healing colonic anastomoses are characterized by new-formed collagen fibrils at postoperative day 4, and bGH seems to stimulate structural organization of the anastomotic collagen fibrils into fibers.

Demonstration of quantitative fabric analysis of tendon collagen using two-dimensional polarized light microscopy

The extensional behavior of tendon is closely linked to the directional organization of the collagen fibers in the tendon. We sought to demonstrate that quantitative information about this organization is accessible through the application of polarized light microscopy. Ten tendons (2 canine Achilles and 8 rat tail tendons) were sectioned in a longitudinal plane and stained with either hematoxylin and eosin or toluidine blue. One Achilles tendon was fixed without tension on the fibers, thus leaving the fibers in a wavy state, and the other was fixed while strained with longitudinal tension straightening the fibers. One hundred measurements of collagen fiber orientation were made on a 0.7 mm2 area from each tendon. Recorded orientation angles were plotted on circular histograms and analyzed using circular statistics. The strained fibers revealed a sharply peaked distribution, in contrast the unstrained fibers had a bimodal orientation distribution. The tail tendons were fixed at strains from 0-3%. Analysis of the orientation distributions revealed increasing alignment of the collagen as strain increased. These data, while matching the qualitative descriptions of the fibers, also provide quantitative assessment of organization enabling comparison among tissues of varied physical circumstances and function. This method of quantitative orientation measurement has wide application for the many birefringent tissues such as bone, smooth and striated muscle and elastin.

Hierarchical structure of the intervertebral disc

Optical microscope techniques are used to characterize the hierarchical structure of the collagenous components of the human intervertebral disc. In the anterior annulus fibrosus, the thickness of lamellae increases abruptly 2 mm inward from the edge of the disc, dividing the annulus into peripheral and transitional regions. Lamellae in the lateral and posterior aspects of the disc have a broad distribution of lamellar thicknesses throughout the annulus. In alternating lamellae, fibers are inclined with respect to the vertical axis of the spine in a layup structure. From the edge of the disc inward to the nucleus, this interlamellar angle decreases from +62 to +45 degrees. Within lamellae, the collagen fibers exhibit a planar crimped morphology. The plane of the waveform is inclined with respect to the vertical axis by the interlamellar angle. From the edge of the disc inward, the crimp angle increases from 20 to 45 degrees and the crimp period decreases from 26 to 20 um. A hierarchical model of the intervertebral disc has been developed that incorporates these morphological gradients.

The lattice arrangement of the collagen fibres in the submucosa of the rat small intestine: scanning electron microscopy

The three-dimensional architecture of the submucosal collagen fibres of the rat (3 weeks old) small intestine was examined by scanning electron microscopy using a selective microdissection method. The main framework of the submucosa was composed of two arrays of collagen fibre bundles running diagonally around the intestinal wall, one set in a clockwise direction, the other counterclockwise. These fibre bundles were about 5 micron diameter and were oriented at a range of angle +/- 30 degrees -50 degrees to the longitudinal axis of the intestine. With the advantage of the SEM observation it was demonstrated that these fibres in different arrays did not constitute two separate layers but interwove to form a unified lattice sheet. An irregular network of fine collagen fibrils over the main framework was also seen. The significance of their arrangement is discussed with respect to the skeletal function of the submucosa in the intestine.

Ultrastructural properties of collagen fibrils in rat intestine

Collagen fibers (bundles of fibrils) of rat small intestine as observed by Nomarski differential interference contrast microscopy were densely packed in parallel undulating arrays. At the electron microscopical level the diameters of intestinal collagen fibrils increased gradually from 50 nm at three weeks of age to 80 nm at 12 months of age. At each age the fibril diameters were fairly uniform in size. At three weeks or six months of age collagen fibrils from stretched, salted samples were somewhat larger in diameter (12-16%) than those from unstretched, unsalted samples. A biaxial orientation existed between bundles of collagen fibrils at angles that varied from 32 degrees to 81 degrees with a mean of 60 degrees. Thus, intestinal collagen at the fibril level is aligned at varying angles to the longitudinal and circular directions of the intestine.

Organization of collagen fibers in the intestine

The characteristic extinction pattern which is observed when the submucosa is viewed in the optical polarizing microscope has been analyzed in terms of the configuration and orientation of the 4 micron diameter collagen fibers. It is shown that the observed polarization effects are produced by periodic variations in orientation of fully birefringent fibers. The fiber configuration required to produce the observed polarization effects is a tilted wave configuration with a crimp period of approximately 20 micron. In the model, the tilted waveform fibers are crimped in register and form parallel arrays. The arrays are oriented in layers at approximately +30 degrees and -30 degrees to the longitudinal direction and are mirror images of each other. Analysis of the extinction pattern shows that the model satisfactorily accounts for the observed polarization effects at several different angles of the crossed polaroids. The calculated strain necessary to straighten the wavy fibers of the model correlates well with the observed strain to uncrimp the collagen fibers in the intestine. This suggests that the initial response to stress is gradual uncrimping of the collagen fibers, and concurrently, a decrease in the angle between biaxially oriented fibers, rather than extension of the straight fibers.

Scanning electron microscopy of collagen fibers in intestine

Collagen fibers of rat intestine were observed with and without mechanical stress in the scanning electron microscope. Observations were correlated with the previous results obtained by optical polarized microscopy to provide further insight into the organization and morphology of intestinal collagen. Larger fibers, approximately 4 micrometers in diameter, are densely packed in parallel undulating arrays. The initial response to stress is straightening of the original fibers. The extended fibers are biaxially oriented at +30 degrees and -30 degrees to the longitudinal direction. These large fibers appear as assemblies of subfibers. At higher magnifications, the larger fibers appear to be enmeshed in a network of small randomly oriented fibers approximately 0.2 micrometer in diameter. Study of the effect of age on fiber morphology showed that the length of the large fiber undulations increases during maturation but remains constant during aging. The diameters of large and small fibers appear not to change with age, but more of the 4 micrometers fibers are loosely associated into larger fibers which can be observed at both the optical microscope and scanning electron microscope levels. A hierarchical organization of intestinal collagen is proposed.