R. Ekholm

The ultrastructural organization of the rat exocrine pancreas: I. Acinar cells

Owing to the many previous publications on the ultrastructure of the pancreatic acinar cell only some of the structural details are dealt with here. The microvilli of the apical cell surface display in sections cut lengthwise a longitudinal striation and in cross-sections a pattern of concentrically disposed spots. In the Golgi apparatus the well-known membranes, vacuoles and vesicles are recognized. Some of the vacuoles often contain varying amounts of a substance of moderate to high density. Immediately outside the Golgi zone zymogen granules are regularly encountered which have a polygonal or irregular shape and are more or less filled with a material less dense than that in ordinary zymogen granules. These granules are very similar to the Golgi vacuoles containing dense material, and it is often difficult to decide whether a particular formation should be classified as a Golgi vacuole or an immature zymogen granule. These findings seem to support the hypothesis that zymogen granules are formed from Golgi vacuoles. Occasionally one observes a direct continuity between the content of an apically situated zymogen granule and the content of the acinar lumen, indicating that zymogen granules are capable of opening at the cell surface and emptying their content into the lumen. One also observes continuity between two zymogen granules one of which communicates with the lumen which suggests that the zymogen granules can empty indirectly via the accompanying granule.

Ultrastructure of the human exocrine pancreas

The acini of the human exocrine pancreas are separated from one another by an interacinar space which contains two basement membranes, one belonging to each acinus. These membranes form an unbroken investment around the acinus and are continuous with the membranes of the intercalary ducts. The acinous lumina are bounded by acinous cells only or by acinous and centroacinous cells intermixed. The cytoplasm of the acinous cell is rich in α-cytomembranes which are most numerous basally. The rounded zymogen granules, predominating apically, have a homogeneous and fairly dense content and a distinct surface membrane. Their diameter is about 0.6 μ. The mitochondria, usually rounded with a minor diameter of about 0.5 μ, are bounded by a triple layered membrane and contain similarly layered inner membranes and a fairly dense ground substance. The Golgi apparatus is comprised of membranes, vacuoles and vesicles. The cytoplasm of the centro-acinous cell is characterized by scarcity of structures and low density. The mitochondria, considerably smaller than those of the acinous cell and usually rod-shaped, have a three-ply surface membrane and similar inner membranes. Centro-acinous cells lack a typical system of α-cytomembranes but contain small dense particles often aggregated in groups and occasionally encountered along a short membrane or a rounded profile. Similar profiles lacking dense particles are more numerous. Ductules interpreted as intercalary ducts are composed of a single layer of cells of apparently two types. The ultrastructure of one type resembles that of the centro-acinous cell but the other type differs therefrom inasmuch as its cytoplasm has a denser ground substance and contains more vacuolar and vesicular elements and vacuolized mitochondria.

The ultrastructural organization of the mouse thyroid gland.

The thyroid cells are bounded by a plasma membrane that appears as a single dark line about 80 A thick. In those areas where two cells lie close together these lines are separated by a space of fairly constant width, about 145 A. That part of the plasma membrane which bounds the cell from the colloid forms microvilli, which have a maximum length of about 0.44 μ and a maximum width of about 0.14 μ . The endothelial lining of the capillaries exhibits cytoplasmic discontinuities, the cytoplasm being replaced by a thin (50 A) membranous structure. The width of the discontinuities is about 400 A. The thyroid mitochondria are mostly rod-shaped. They are bordered by a triple-layered membrane consisting of two opaque layers with a less opaque layer interposed. In the interior of the mitochondria there is a large number of similarly triple-layered membranes. The mean thickness of the outer membranes is 165 A and of the inner membranes 180 A. The Golgi zone contains pairs of membranes, vacuolar spaces and small vesicles. In the cytoplasm there is a well developed system of membranes ( α -cytomembranes). These consist of basic membranes, 55 A thick, to one side of which dense particles with a diameter of about 145 A are attached. The membranes bound more or less wide spaces which contain a homogeneous material of low opacity. Two types of big granules are observed delimited by a single membrane, about 50 A thick. One type has a homogeneous, rather dense content and a regular rounded shape. The granules of the other kind are filled with an inhomogeneous matter and their outline is somewhat wavy. The nuclear membrane appears as a double membrane consisting of two opaque layers separated by a less opaque interspace. The outer opaque layer seems to belong to the α -cytomembrane system.

On dense bodies and droplets in the follicular cells of the guinea pig thyroid

A large number of dense bodies are found in the follicular cells of nonstimulated as well as TSH-stimulated guinea pig thyroids. Administration of TSH induces the formation of numerous intracellular droplets of various appearances. The dense bodies are often topographically intimately related to the droplets. In tissues incubated for acid phosphatase activity, reaction products are found in the dense bodies and in some droplets. When I125 is administered several hours and TSH 1 or 2 hours before autopsy autoradiographic reaction is observed over the luminal colloid and over most, but not all, of the droplets. When TSH is given 1 or 2 hours and radioiodide 15 minutes before sacrifice, the luminal colloid is labeled but only an occasional intracellular droplet. The observations seem to corroborate the conclusions reached by previous authors that thyroglobulin is resorbed from the luminal colloid in the form of intracellular droplets and that the thyroglobulin in the droplets is hydrolyzed by enzymes derived from the dense bodies inside the cell. The heterogeneity of the intracellular droplets both with respect to morphology and labeling seem to indicate, however, that some of them represent other processes than thyroglobulin resorption and breakup.

The ultrastructure of the parafollicular cells of the thyroid gland in the rat

The parafollicular cells of the rat thyroid gland have been studied in the light and electron microscope on a material fixed by perfusion with glutaraldehyde followed by immersion in osmic acid. By studying alternating light and electron microscopical sections it was established that, with the preparation technique used here, the follicle cells and parafollicular cells can be differentiated with great safety in the light microscope. In the electron microscope the parafollicular cells are characterized by an abundance of dense granules; a very extensive Golgi apparatus; an endoplasmic reticulum consisting of narrow, orderly arranged cisternae; slender mitochondria with obliquely and longitudinally arranged inner membranes; and an evenly outlined cell nucleus. The specific granules seem to develop in the Golgi region, where all transitions from Golgi saccules to granules of mature appearance are seen. In an intermediate stage of the development the granules are furnished with a bristle-coated bounding membrane.

Transport of thyroglobulin and peroxidase in the thyroid follicle cell

The purpose of this study was to explore the nature of the protein(s) in the exocytotic vesicles in the thyroid follicle cells and to ascertain whether or not thyroglobulin and peroxidase are transported by the same vesicles through the apical region of the cells to the follicle lumen. The study was performed on rats pretreated with thyroxine for 2 days in order to inhibit endocytosis. A fraction of exocytotic vesicles was isolated by centrifugation in continuous and discontinuous sucrose density gradients. The protein content of the vesicles were analysed by electrophoresis in continuous polyacrylamide gradient gels. The vesicles contained (uniodinated) thyroglobulin, 12-S protein and thyralbumin. Parallel histochemical studies in the electron microscope. These observations have important bearings on the mechanisms for thyroglobulin iodination, since it has been demonstrated that iodination does not occur in the exocytotic vesicles but in connection with the opening of the vesicles at the apical cell surface.

Hydrogen peroxide degradation and glutathione peroxidase activity in cultures of thyroid cells

The degradation rate of H2O2, added to the incubation medium, and glutathione (GSH) peroxidase activity were measured in cultures of FRTL-5 cells and porcine thyroid cells. The H2O2 degradation rate increased proportionally to the H2O2 concentration and was in FRTL-5 cells, cultured with TSH, approximately 50 nmol/min and mg DNA at 0.01 mM H2O2 and approximately 3 x 10(4) nmol/min and mg DNA at 10 mM H2O2. The GSH peroxidase activity in the same cells was equivalent to an H2O2 degradation of approximately 400 nmol/min and mg DNA. The involvement of enzymes in H2O2 degradation was studied by inhibiting catalase with aminotriazole (ATZ) and reducing GSH peroxidase by omitting glucose in the incubation medium. At 0.1 mM H2O2, ATZ or glucose omission alone did not measurably reduce H2O2 degradation but did so when combined. At 10 mM H2O2 ATZ caused a clear inhibition whereas glucose omission had no additive effect. These observations indicate that GSH peroxidase was involved in H2O2 degradation only at low H2O2 concentrations. The GSH peroxidase activity decreased by reduction of the selenite supply and increased after replenishment. The recovery of the enzyme activity required the presence of TSH in FRTL-5 cells but not in porcine thyrocytes.

Studies on the protein synthesis in the thyroid: III. In Vivo incorporation of leucine-3H into thyroglobulin of microsomal subfractions of the rat thyroid1

Rats were given leucine-3H at different intervals, ranging from 30 minutes to 3 hours, before sacrifice. The thyroids were homogenized, and one mitochondrial fraction and four microsomal subfractions were prepared. The composition of the subfractions was studied electron microscopically and chemically. Distinct patterns of protein labeling appeared only in two microsomal subfractions. One of these, consisting of ribosome-studded vesicles with a rather dense content, shows labeling of 3–8 S and 12 S proteins as well as thyroglobulin already at 30 minutes. At 1 hour there is a very high labeling of 3–8 S and 12 S proteins and a pronounced radioactivity in thyroglobulin. At 3 hours the thyroglobulin labeling is very high in this subfraction and exceeds that of 12 S. The other microsomal subfraction containing labeled material is composed of smooth-surfaced vesicles of various size and density. This fraction shows no labeled components at 30 minutes, but at 1 hour it displays two radioactivity peaks, one corresponding to thyroglobulin and the other, lower, to 12 S protein. At 3 hours labeling is found only in thyroglobulin. The results strongly indicate that the microsomal subfraction consisting of ribosome-studded vesicles is alone responsible for the thyroglobulin synthesizing capacity. The presence of labeled thyroglobulin in the subfraction consisting of smooth-surfaced vesicles is thought to represent in vivo transfer of thyroglobulin from the endoplasmic reticulum to the follicle lumen.

The ultrastructure of the rat exocrine pancreas after brief ethionine exposure

Adult albino rats were given ethionine in a dosage of 0.5 or 1.0 mg per g body weight once daily for 1–5 days. The rats were sacrificed 24 hours after the last ethionine dose. Pancreatic tissue was excised, duly prepared, and examined in the electron microscope. The following were the most important changes seen in the acinar cells of the pancreas after ethionine treatment. The endoplasmic reticulum loses its normally regular organization; the RNP particles diminish in number and, after prolonged treatment, both RNP particles and membranes undergo a pronounced reduction; particularly after prolonged ethionine exposure, the cells exhibit focal regions where the cytoplasmic structure is lost and replaced by different amounts of variously organized, very dense material; the mitochondria at an early stage exhibit a rearrangement of the inner membranes and after prolonged ethionine administration display swelling, reduced matrix density, and breaking up of the inner membranes to marginally situated fragments; although the Golgi apparatus usually retains a fairly normal structure during the first days of ethionine treatment, it undergoes degenerative changes after the longest adopted period of ethionine treatment; dense granules are encountered within the cisternae of the endoplasmic reticulum (intracisternal granules); zymogen granules of aberrant size and shape appear. The observations made are discussed in the light of previous light microscopical, electron microscopical, and biochemical findings.

The effect of TSH on the acid phosphatase and thyroglobulin hydrolyzing activities in the guinea pig thyroid

Abstract Acid phosphatase and thyroglobulin hydrolyzing activities were assayed in the thyroids from three groups of guinea pigs. The treatments of the animals were: thyroxine only for 4 days; thyroxine for 4 days + TSH 1 h before sacrifice; thyroxine for 4 days + TSH 24 h before sacrifice. It was found that 1 h action of TSH did not measurably change the enzyme activities. Twenty-four hours of TSH-stimulation resulted in a statistically significant increase of both enzyme activities when calculated as total glandular activity or on DNA basis. Parallel cytochemical studies in the electron microscope revealed that acid phosphatase activity was localized, in all three groups, to the lysosomes and, in the animals stimulated by TSH for 1 h, also to the colloid droplets. It is concluded that these findings, combined with earlier data demonstrating a rapid action of TSH on the thyroxine release, indicate that the mechanism for the increase of hormone release by TSH is not accelerated biosynthesis of thyroglobulin hydrolyzing enzymes but induction of endocytosis.