Structure Of Meshwork Research Articles

Cryo-Electrospinning Generates Highly Porous Fiber Scaffolds Which Improves Trabecular Meshwork Cell Infiltration.

Human trabecular meshwork is a sieve-like tissue with large pores, which plays a vital role in aqueous humor outflow. Dysfunction of this tissue can occur, which leads to glaucoma and permanent vision loss. Replacement of trabecular meshwork with a tissue-engineered device is the ultimate objective. This study aimed to create a biomimetic structure of trabecular meshwork using electrospinning. Conventional electrospinning was compared to cryogenic electrospinning, the latter being an adaptation of conventional electrospinning whereby dry ice is incorporated in the fiber collector system. The dry ice causes ice crystals to form in-between the fibers, increasing the inter-fiber spacing, which is retained following sublimation. Structural characterization demonstrated cryo-scaffolds to have closer recapitulation of the trabecular meshwork, in terms of pore size, porosity, and thickness. The attachment of a healthy, human trabecular meshwork cell line (NTM5) to the scaffold was not influenced by the fabrication method. The main objective was to assess cell infiltration. Cryo-scaffolds supported cell penetration deep within their structure after seven days, whereas cells remained on the outer surface for conventional scaffolds. This study demonstrates the suitability of cryogenic electrospinning for the close recapitulation of trabecular meshwork and its potential as a 3D in vitro model and, in time, a tissue-engineered device.

Melt Electrowriting of Graded Porous Scaffolds to Mimic the Matrix Structure of the Human Trabecular Meshwork.

The permeability of the human trabecular meshwork (HTM) regulates eye pressure via a porosity gradient across its thickness modulated by stacked layers of matrix fibrils and cells. Changes in HTM porosity are associated with increases in intraocular pressure and the progress of diseases such as glaucoma. Engineered HTMs could help to understand the structure–function relation in natural tissues and lead to new regenerative solutions. Here, melt electrowriting (MEW) is explored as a biofabrication technique to produce fibrillar, porous scaffolds that mimic the multilayer, gradient structure of native HTM. Poly(caprolactone) constructs with a height of 125–500 μm and fiber diameters of 10–12 μm are printed. Scaffolds with a tensile modulus between 5.6 and 13 MPa and a static compression modulus in the range of 6–360 kPa are obtained by varying the scaffold design, that is, the density and orientation of the fibers and number of stacked layers. Primary HTM cells attach to the scaffolds, proliferate, and form a confluent layer within 8–14 days, depending on the scaffold design. High cell viability and cell morphology close to that in the native tissue are observed. The present work demonstrates the utility of MEW for reconstructing complex morphological features of natural tissues.

Microstructure and mechanics of the bovine trachea: Layer specific investigations through SHG imaging and biaxial testing

The trachea is a complex tissue made up of hyaline cartilage, fibrous tissue, and muscle fibers. Currently, the knowledge of microscopic structural organization of these components and their role in determining the tissue’s mechanical response is very limited. The purpose of this study is to provide data on the microstructure of the tracheal components and its influence on tissue’s mechanical response. Five bovine tracheae were used in this study. Adventitia, cartilage, mucosa/submucosa, and trachealis muscle layers were methodically cut out from the whole tissue. Second-harmonic generation(SHG) via multi-photon microscopy (MPM) enabled imaging of collagen fibers and muscle fibers. Simultaneously, a planar biaxial test rig was used to record the mechanical behavior of each layer. In total 60 samples were tested and analyzed. Fiber architecture in the adventitia and mucosa/submucosa layer showed high degree of anisotropy with the mean fiber angle varying from sample to sample. The trachealis muscle displayed neat layers of fibers organized in the longitudinal direction. The cartilage also displayed a structure of thick mesh-work of collagen type II organized predominantly towards the circumferential direction. Further, mechanical testing demonstrated the anisotropic nature of the tissue components. The cartilage was identified as the stiffest component for strain level < 20% and hence the primary load bearing component. The other three layers displayed a non-linear mechanical response which could be explained by the structure and organization of their fibers. This study is useful in enhancing the utilization of structurally motivated material models for predicting tracheal overall mechanical response.

Effect of intracameral human cord blood-derived stem cells on lasered rabbit trabecular meshwork.

This study aimed to investigate the effect of intracameral human cord blood stem cells on lasered rabbit trabecular meshwork. Immediately following diode laser application to the trabecular meshwork, human cord blood stem cells were injected intracamerally, in one eye of 12 albino rabbits. The other eye of ten rabbits was lasered controls and two eyes were normal controls. Rabbits were killed after 4, 8 and 12weeks. Lasered control rabbit eyes showed significant disruption of trabecular architecture, loss and pleomorphism of trabecular endothelial cells and progressive narrowing of trabecular spaces till 12weeks. In contrast, lasered eyes, concurrently injected with human cord blood stem cells, showed relatively preserved endothelial cellularity and structure of the trabecular meshwork, at all time points. Human CD34- and CD44-positive cells were identified in 7/8 eyes treated with stem cells, at 4 and 8weeks, and 2 of 3 at 12weeks. Many PKH26-labeled human cord blood cells were visible throughout the trabecular area at 4weeks. They gradually decreased in number by 8weeks, and at 12weeks, they appeared to be oriented along trabecular beams. There was a relative preservation of cellularity and architecture of the trabecular meshwork in eyes injected with human cord blood stem cells, as compared to lasered control eyes up to 12weeks, without significant inflammation. This suggests a probable role for such stem cells in eyes with glaucoma, having trabecular dysfunction.

Expression of eNOS and CAV gene in trabecular meshwork cells and effects on glaucoma pathogenesis

Objective: Glaucoma is the second popular blinding disease worldwide, and is mainly caused by abnormal function or structure of trabecular meshwork and consequent aqueous humor effluent dysfunction, ocular hypertension, leading to vision deficit. eNOS and CAV1 might be correlated with glaucoma onset. This study thus investigated their expression in trabecular meshwork cells and effects on glaucoma pathogenesis. Patients and Methods: Trabecular meshwork tissues were collected during surgery from POAG patients, and eNOS/CAV1 gene expressions were analyzed by qRT-PCR and Western-Blot. eNOS and CAV1 genes were over-expressed in cultured HTMC cells, whose survival rate was analyzed by MTT assay, whilst cell free radicals clearance rate was analyzed by chemiluminescence method. Results: HE staining revealed significant pigment deposition in trabecular meshwork of POAG patients compared to normal group, accompanied with stenosis of mesh. Expression levels of eNOS and CAV1 were significantly higher in POAG group (p<0.05 compared to healthy control group). In vitro overexpression of eNOS and CAV1 significantly suppressed cell survival rate and free radical clearance rate (p<0.05). Conclusion: Trabecular meshwork cells in glaucoma patients showed elevated eNOS and CAV1 expression. Up-regulation of eNOS expression may suppress cell survival rate at cellular level, impaired ability to clear free radicals, eventually leading to cell or tissue damage.

Morphology and porosity of the spines of the sea urchin Heterocentrotus mamillatus and their implications on the mechanical performance

Spines of the slate pencil sea urchin Heterocentrotus mamillatus Linnaeus, 1758, are in focus of biomimetic research as they feature a “graceful” failure behaviour under uniaxial compression dissipating energy and resisting high loads even after high strain. This study elucidates and quantifies the organization of calcitic trabeculae and pores in large primary spines of the slate pencil urchin H. mamillatus by image analysis from scanning electron microscopy, X-ray micro-computed tomography (µCT) and gravimetry. This study delivers a detailed distribution of porosities within the whole spine and shows that parts of the spines have a much higher porosity then hitherto thought. The central part (medulla) of the high-magnesium calcitic stereom of H. mamillatus spines has a porosity range of 75% to nearly 90%. From this innermost structure, more than 200 radially aligned, but often sinuous trabeculae extend to the spine rim. The structure of this complicated meshwork (radiating layer) is best seen in basal cross sections and was confirmed by µCT scans. The radiating layer has a porosity range from 40–70% and is irregularly separated by the dense growth layers (15–35% porosity). Growth layers were classified in proximal and distal growth layers with numbers ranging within a single animal between 3–14 and 2–7, respectively. These growth layers are characteristic for H. mamillatus spines and play a major role in their remarkable mechanical properties. The porosity of the spine increases from base to tip. Biological and mechanical implications of the variations are discussed.

The structure of the trabecular meshwork, its connections to the ciliary muscle, and the effect of pilocarpine on outflow facility in mice.

To determine the connections between the ciliary muscle (CM), trabecular meshwork (TM), and Schlemm's canal (SC) and their innervations that allows CM contraction (by pilocarpine) to influence conventional outflow in mice. Sequential sections and whole mounts of murine corneoscleral angles were stained for elastin, α-smooth muscle actin (αSMA), vesicular acetylcholine transporter (VAChT), neuronal nitric oxide synthase (nNOS), vasoactive intestinal peptide (VIP), and tyrosine hydroxylase (TH). Elastic (EL) fibers between the CM, TM, and SC were examined in ultrathin, sequential sections from different planes. The effect of pilocarpine (100 μM) on conventional outflow facility was measured by perfusion of enucleated mouse eyes. The mouse TM contains a three-dimensional (3D) net of EL fibers connecting the inner wall of SC to the cornea anteriorly, the ciliary body (CB) internally and the choroid and CM posteriorly. The CM bifurcates near the posterior TM, extending outer tendons to the juxtacanalicular tissue and inner wall of SC and internal connections to the lamellated TM and CB. Ciliary muscle and lamellated TM cells stain with αSMA and are innervated by VAChT-containing nerve fibers, without TH, VIP, or nNOS. Pilocarpine doubled outflow facility. Mouse eyes resemble primate eyes not only by their well developed SC and TM, but also by their 3D EL net tethering together the TM and SC inner wall and by the tendinous insertion of the CM into this net. The increase in outflow facility following cholinergic stimulation in mice, as in primates, supports using mice for studies of aqueous humor dynamics and glaucoma.

In Situ Autofluorescence Visualization of Human Trabecular Meshwork Structure

To characterize the three-dimensional (3-D) structure of the human trabecular meshwork (TM) by two-photon excited (TPEF) autofluorescence (AF) and optical sectioning without conventional histologic embedding and sectioning. Viable human ex vivo explants of the anterior chamber angle containing the aqueous humor drainage tissue in situ were imaged by TPEF to localize AF and Hoechst 33342 nuclear fluorescence. An autofluorescent marker in Schlemm's Canal (SC) aided SC situ visualization. En face and orthogonal views of the TM were generated. In the innermost uveal TM, AF signals outlined an intricate 3-D network of fine branching beams with large openings between the beams. In the adjacent corneoscleral TM, beams were thicker and coalesced as plate-like structures with pore-like openings. Linear and coiled AF fibers were visible on the background AF of beams. Deeper, in the external TM, this organization changed to fine fiber arrays orientated in the tissue's longitudinal axis, reminiscent of the cribriform plexus of the juxtacanalicular TM (JCT). In the outermost JCT, AF of fine fibers was sparse, then undetectable as optical sections approached the inner wall of SC. Cell nuclei were closely associated with the TM structural extracellular matrix. We have used TPEF and optical sectioning to exploit AF as a useful method to visualize the structure of the human conventional aqueous drainage pathway in situ. Ancillary nuclear staining allowed cell association with the AF structures to be seen. This approach revealed a unique 3-D perspective of the TM that is consistent with known TM structural characteristics.

Combustion Synthesis and Characterization of Bulk Nanocrystalline ${\bf Fe}_{\bf 88} {\bf Si}_{\bf 12}$ Alloy

Bulk nanocrystalline Fe88Si12 alloy is fabricated by a combustion synthesis processing that is convenient, low in cost, and capable of being scaled up. The Fe88Si12 alloy consists of two phases: ?-Fe(Si) and Fe3Si. It is a composite structure of meshwork dispersed in a matrix. The meshwork is a few micrometers in length and 0.5-1 ?m in diameter, and the grain size of the matrix is in the range of 5-15 nm. The nanocrystalline Fe88Si12 alloy is yielded at 1760 MPa with a large ductility of 14.6% in compressive test. Moreover, the coercive force of the product is approximately 3.6 A/m and saturation magnetization is about 196 emu/g. The results suggest that the product exhibits good mechanical behavior and soft magnet properties.

Complex organizational defects of fibroblast architecture in the mouse spleen with Nkx2.3 homeodomain deficiency

The capacity of secondary lymphoid organs to provide suitable tissue environment for mounting immune responses is dependent on their compartmentalized stromal constituents, including distinct fibroblasts. In addition to various members of the tumor necrosis factor/lymphotoxin beta family as important morphogenic regulators of peripheral lymphoid tissue development, the formation of stromal elements of spleen is also influenced by the Nkx2.3 homeodomain transcription factor in a tissue-specific fashion. Here we extend our previous work on the role of Nkx2.3-mediated regulation in the development of spleen architecture by analyzing the structure of reticular fibroblastic meshwork of spleen in inbred Nkx2.3-deficient mice. Using immunohistochemistry and dual-label immunofluorescence we found both distributional abnormalities, manifested as poor reticular compartmentalization of T-zone and circumferential reticulum, and developmental blockade, resulting in the absence of a complementary fibroblast subpopulation of white pulp. The disregulated distribution of fibroblasts was accompanied with an increased binding of immunohistochemically detectable complement factor C4 by T-cell zone-associated reticular fibroblasts, distinct from follicular dendritic cells with inherently high-level expression of bound C4. These data indicate that the impact of Nkx2.3 gene deficiency on fibroblast ontogeny within the spleen extends beyond its distributional effects, and that the formation of various white pulp fibroblast subsets is differentially affected by the presence of Nkx2.3 activity, possibly also influencing their role in various immune functions linked with complement activation and deposition.

Construction of an S-layer protein exhibiting modified self-assembling properties and enhanced metal binding capacities

The functional S-layer protein gene slfB of the uranium mining waste pile isolate Bacillus sphaericus JG-A12 was cloned as a polymerase chain reaction product into the expression vector pET Lic/Ek 30 and heterologously expressed in Escherichia coli Bl21(DE3). The addition of His tags to the N and C termini enabled the purification of the recombinant protein by Ni-chelating chromatography. The Ni binding capacity of the His-tagged recombinant S-layer protein was compared with that of the wild-type S layer. The inductively coupled plasma mass spectrometry analyses demonstrate a significantly enhanced Ni binding capability of the recombinant protein. In addition, the self-assembling properties of the purified modified S-layer proteins were studied by light microscopy and scanning electron microscopy. Whereas the wild-type S-layer proteins re-assembled into regular cylindric structures, the recombinant S-layer proteins reassembled into regular sheets that formed globular agglomerating structures. The nanoporous structure of the protein meshwork, together with its enhanced Ni binding capacity, makes the recombinant S-layer attractive as a novel self-assembling biological template for the fabrication of metal nanoclusters and construction of nanomaterials that are of technical interest.

Augendrucksenkung nach selektiver Lasertrabekuloplastik (SLT) mit zwei unterschiedlichen Lasersystemen und nach Argonlasertrabekuloplastik (ALT) - Eine kontrollierte prospektive klinische Studie an 284 Augen

Selective laser trabeculoplasty (SLT) is a new method to reduce intraocular pressure in eyes with primary open angle glaucoma. The laser parameters of a Q-switched, frequency-doubled Nd:YAG-laser are set to selectively target pigmented trabecular meshwork cells without visible damage to the adjacent non-pigmented tissue. SLT acts non-thermally, the intracellular microdisruptions triggered by the laser are confined to the targeted cells, the laser pulses are so short that heat caused within the targeted cells does not have time to spread to the surrounding tissue. A controlled prospective randomised clinical study was conducted to compare the long-term results, safety and efficacy after SLT with two different laser systems and after ALT in the treatment of ocular hypertension and medically uncontrolled open angle glaucoma. About two years ago the authors performed a selective laser trabeculoplasty in 119 eyes using the SLT laser unit Otello (Glautec AG, ARC, EC), here named as SLT-I, and in 124 eyes using the SLT laser unit Selecta II (Lumenis, Palo Alto, CA), here named as SLT-II. In 41 eyes the authors performed argon laser trabeculoplasties using the argon laser Argus (Aesculap Meditec, EC). Two months after treatment mean IOP reduction from baseline was 1.9 mmHg or, respectively, 8.8 % after SLT with the SLT-System I, 2.0 mmHg or, respectively, 9.5 % after SLT with SLT-System II, and 2.2 mmHg or, respectively, 9.9 % after ALT with the argon laser. Twelve months after LTP mean pressure reductions were 1.7 mmHg (7.9 %) after SLT-I, 1.8 mmHg (8.5 %) after SLT-II, and 2.1 mmHg (9.4 %) after ALT. The response curve of the eyes with SLT-I greatly resembled that of the eyes with SLT-II and those eyes with ALT. Pressure reduction was highest after ALT, followed by SLT-II, in SLT-I reduction was the least, but the differences were not significant. Our findings did not correspond with those of other authors reporting an average IOP reduction of 25 % after SLT and ALT. SLT has shown reasonable efficacy in lowering IOP in eyes with ocular hypertension and primary open angle glaucoma, SLT achieves about the same level of IOP reduction compared with ALT. As a result of the preservation of the structure of the trabecular meshwork and low rate of complications, SLT is a safe alternative to ALT. The exact biological effect induced with SLT is still not yet understood. For the early glaucoma stages SLT provides an alternative to drug treatment, for the advanced glaucomas SLT is an additional option for further pressure reduction. More long-term follow-up studies are needed to determine whether the IOP lowering effect is sustained over time, and to assess the efficacy of repeated SLT.

Kofaktorielles Glaukom - Ursachen

The various types of glaucoma may be classified more accurately by studying the structure and physiology of the trabecular meshwork. Seen in this light, the development of glaucoma is linked to "extrinsic" and "intrinsic" factors. The intrinsic factors represent the predisposition to glaucoma; the secondary factors intervene in a secondary phase, creating a resistance to aqueous humor outflow. In so-called primary glaucoma the predisposition plays a decisive role, while the extrinsic factors are not so important. In cofactorial glaucoma the predisposition is only of slight significance, while the extrinsic factors are extremely important. In true secondary glaucoma predisposition plays a minimal role, while the extrinsic factors are all the more important. In correlated glaucomas the predisposition may be extremely relevant, but the extrinsic factors which represent the key causes are still unknown. Several types of glaucoma are described as examples.

The fine structure of the trabecular meshwork at graded levels of intraocular pressure: (2) Pressures outside the physiological range (0 and 50 mmHg)

The effects of 60 min exposure to high (50 mmHg) and low (0 mmHg) intraocular pressure in the outflow apparatus of the rhesus monkey have been assessed by transmission electron microscopy. At high pressure there is marked distension of the endothelial and outer corneoscleral parts of the meshwork with occlusion of the anterior part of Schlemm's canal, but in this region cellular integrity is maintained. In the posterior part of the meshwork, intercellular connections are disrupted and degenerative changes are observed in the trabeculae. At 0 mmHg intraocular pressure, the outer half of the meshwork is bowed inwards and compressed, and degeneration of the lining cells and trabecular cores is progressively advanced toward the inner meshwork. The graded preservation of the meshwork is attributed to diffusion of oxygen from the packed erythrocytes in the dilated canal of Schlemm.

The fine structure of the trabecular meshwork at graded levels of intraocular pressure: (1) Pressure effects within the near-physiological range (8–30 mmHg)

The outflow apparatus from rhesus monkeys which had been subjected to graded levels of intraocular pressure (8–30 mmHg) was studied by transmission electron microscopy. At the higher pressures marked distension of the outer corneoscleral and the endothelial layers of the meshwork occurred, but cell-to-cell contact across the intertrabecular spaces and in the endothelial meshwork was maintained by extended endothelial cell processes. The elasticity of the native cells was attributed to the presence of abundant intracytoplasmic microfilaments. “Button” junctions were observed at the sites of process apposition and these were considered to be an important factor in the cellular adhesion. The endothelium lining the trabecular aspect of Schlemm's canal remained in close contact with the underlying meshwork throughout the pressure range and cellular rather than extracellular contacts were thought to be the major anchoring mechanism. With increase in intraocular pressure, there was an apparent decrease in extracellular material in the endothelial meshwork. A statistical analysis demonstrated a significant increase in the counts of lysosomes, lysosome complexes, multivesicular bodies and lipid vesicles in the meshwork cells at 30 mmHg when compared to the counts at 15 mmHg (the control pressure).

The structure of the trabecular meshwork of the primate eye: A light and electron microscopic study with peroxidase

The trabecular meshwork of the anterior chamber angle is composed of two regions. Adjacent to Schlemm's canal is the meshwork region which is formed by cells with interdigitating processes and large intercellular spaces containing collagen. Between this meshwork region and the anterior chamber is the trabecular region in which the cells closely envelop a collagenous core. The endothelium of Schlemm's canal is a complete, nonfenestrated membrane. Protrusions of meshwork tissue into Schlemm's canal form villi that are invested by the intact endothelium of Schlemm's canal. There are no canals, large pores, or other open channels of communication between the anterior chamber and Schlemm's canal. Peroxidase injected into the anterior chamber is trapped appreciably in the spaces of the trabecular meshwork. The connective tissue cells and infiltrating leukocytes become phagocytic, and within 4 days have disposed of the injected protein. No evidence was found for the direct passage of peroxidase into Schlemm's canal through endothelial cell vacuoles. The results of this study suggest that the anterior chamber and the spaces of the chamberangle connective tissue are structurally delimited from Schlemm's canal and that the trabecular meshwork may function as a biological filter.

Structure of the trabecular meshwork and the canal of Schlemm.

Structure of the trabecular meshwork and the canal of Schlemm.

Schlemm's canal and the trabecular meshwork. An electron microscopic study of the normal structure in man and monkey (Cercopithecus ethiops)

Schlemm's canal and the trabecular meshwork of eight human and eleven monkey eyes have been analyzed by means of electron microscopy. 1. The size, shape and delineation of the lumen of Schlemm's canal is given. 2. The wall of Schlemm's canal in a strict sense is a single endothelial layer. Through some of the cells there is a small channel, which connects the intertrabecular spaces with Schlemm's canal. The details of the endothelium is discussed. 3. The wall in a broader sense is a 4–8 μ thick layer of endothelial cells, collagen material, homogeneous material and “empty” spaces. 4. The first trabecular sheet has a very intimate relationship to the wall. 5. The topography of the trabecular sheets is discussed. Data about the size of the sheets are given. The increasing thickness with age is especially stressed. The composition of the individual sheet has been evaluated and change in composition with age is noted. 6. The topography, size and interconnections of the intertrabecular spaces are discussed. There exist open communications through the trabecular meshwork which connect the anterior chamber with Schlemm's canal. 7. Nerves are present in different parts of the trabecular meshwork, even in immediate vicinity of the inner wall of Schlemm's canal. There are suggestions that some of the nerves are adrenergic. 8. Besides red and white blood cells there are found plasma cells and mast cells in the trabecular meshwork. 9. The difference in appearance of the trabecular meshwork between human and monkey eyes is noted. 10. To a limited extent the structure of the trabecular meshwork is discussed with regard to the function.

Experimental studies on the trabecular meshwork in primates.

There is a considerable amount of information about the normal structure of the trabecular meshwork.* However, little is known about its pathological reactions.2,12,39,40This paper is an attempt to study the morphological behavior of chamber angle tissue under 3 different conditions: (1) after perfusion of the anterior chamber with various solutions; (2) after autologous implantation into the anterior chamber, and (3) after experimentally elevated intraocular pressure. Material and Methods Because of great differences in the structure of the trabecular meshwork between primates and other mammals described previously,37,41human and monkey eyes only were used in this study. A total of 28 eye-bank eyes and 43 freshly enucleated monkey eyes (Macaca mulatta and Cercopithecus aethiops) were studied. 1. Perfusion Experiments. —Eighteen monkey and 28 autopsy eyes were perfused with a polyethylene tube fixed to a hypodermic needle. It was thrust through the anterior chamber so that a double perforation

The fine structure of the DNP component of the nucleus. An electron microscopic study utilizing autoradiography to localize DNA synthesis.

In the present investigation, the sites of deoxyribonucleic acid (DNA) synthesis and the fate of labeled deoxyribonucleoprotein (DNP) were studied in autoradiographs of ultrathin sections viewed with the electron microscope. Tritiated thymidine was employed as a label for DNA in the nuclei of proliferating cells of regenerating salamander limbs. In the autoradiographic method reported here, dilute NaOH was used to remove the gelatin of the emulsion after exposure and development. The exposed silver grains are not displaced by this treatment and the resolution of fine structure in the underlying section is greatly improved. Our observations suggest that the DNP component is a meshwork of interconnected filaments 50 to 75 A in diameter, which may be cross-linked to form what Frey-Wyssling would term a "reticular gel." The filamentous DNP meshwork is dispersed throughout the interphase nucleus during DNA synthesis, whereas in chromosomes, which are relatively inert metabolically, the meshwork is denser and is aggregated into compact masses. Dense chromatin centers in interphase nuclei are similar in fine structure to chromosomes and are also inert with respect to DNA synthesis. In the Discussion, the structure of the filamentous meshwork in chromatin is compared with that in chromosomes, and speculations are made as to the functional significance of the variations in DNP fine structure observed.