An electron microscopic study of the intestinal villus. I. The fasting animal

Brush border development in the intestinal absorptive cells of Xenopus during metamorphosis

The differentiation of the brush border which makes up the apical free surface of intestinal absorptive cells has been studied by electron microscopy. Specimens of Xenopus small intestine were fixed at various stages during metamorphosis, the time when a new intestinal epithelium forms. The interpretation of details described herein emphasizes the role of "surface-forming" vesicles. These vesicles are thought to provide membrane both for the initial expansion of the apical surface and for the later elongation of the microvilli. The latter are believed to be "molded" around filamentous cores that appear early in differentiation. The cores are attached to the apical membrane and extend vertically into the supranuclear cytoplasm. This interpretation rests chiefly on (a) the resemblance, both in morphology and in staining properties with colloidal thorium, between the membrane that limits the vesicles and that which limits the microvilli and (b) the distribution and time of appearance of the vesicles with respect to development of the microvilli. According to this view, the specific properties of surface membrane reside in preformed units that arise within the supranuclear cytoplasm. This morphogenetic process probably involves participation of the Golgi region as the site where the complex macromolecular architecture of the cell surface is assembled.

A study of the small intestinal mucosa using the scanning electron microscope

In this paper we describe the features of small intestinal structure in normal control subjects using the scanning electron microscope.

Morphological and biochemical characterization of goldfish erythrophores and their pterinosomes

The fine structure of integumental erythrophores and the intracellular location of pteridine and carotenoid pigments in adult goldfish, Carassius auratus, were studied by means of cytochemistry, paper and thin-layer chromatography, ionophoresis, density-gradient centrifugal fractionation, and electron microscopy. The ultrastructure of erythrophores is characterized by large numbers of somewhat ellipsoidal pigment granules and a well-developed system of tubules which resembles endoplasmic reticulum. The combined morphological and biochemical approaches show that pteridine pigments of erythrophores are located characteristically in pigment granules and are the primary yellow pigments of these organelles. Accordingly, this organelle is considered to be the "pterinosome" which was originally found in swordtail erythrophores. Major pteridines obtainable from goldfish pterinosomes are sepiapterin, 7-hydroxybiopterin, isoxanthopterin, and 6-carboxyisoxanthopterin. Density-gradient fractions indicate that carotenoids are mostly associated with the endoplasmic reticulum. Both tyrosinase and possibly a tyrosinase inhibitor containing sulfhydryl groups are present in the pterinosome. The possible existence of a tyrosinase inhibitor is suggested by the marked increase of tyrosinase activity upon the addition of iodoacetamide or p-chloromercuribenzoic acid. In the light of their fine structure, pigmentary composition, and enzymatic properties, the erythrophores and pterinosomes are discussed with respect to their probable functions and their relationship to melanophores.

Rat intestinal microvillus membranes. Purification and biochemical characterization

1. A technique is described for the removal of subcellular contaminants from intact rat intestinal brush borders, and for the subsequent separation of a microvillus membrane fraction from a fibrillar residue. 2. Increments in invertase activity, microscopic homogeneity and low nucleic acid content indicate that the microvillus plasma membrane has been extensively purified. Multiple membrane preparations have been shown to be highly reproducible with respect to their invertase specific activity, cholesterol content and phospholipid content. Alkaline phosphatase, leucine aminopeptidase, Mg(2+)- and Ca(2+)-dependent adenosine triphosphatase and seven separate disaccharidases were shown to be predominantly confined to the membrane fraction. 3. The fibrillar fraction has been shown to contain approximately 30% of the total protein of purified brush borders, plus most of the residual nucleic acid contaminant. No evidence was found for the localization of any specific enzyme in this fraction.

A comparative study of the ultrastructure of microvilli in the epithelium of small and large intestine of mice

A comparative analysis of the fine structure of the microvilli on jejunal and colonic epithelial cells of the mouse intestine has been made. The microvilli in these two locations demonstrate a remarkably similar fine structure with respect to the thickness of the plasma membrane, the extent of the filament-free zone, and the characteristics of the microfilaments situated within the microvillous core. Some of the core microfilaments appear to continue across the plasma membrane limiting the tip of the microvillus. The main difference between the microvilli of small intestine and colon is in the extent and organization of the surface coat. In the small intestine, in addition to the commonly observed thin surface "fuzz," occasional areas of the jejunal villus show a more conspicuous surface coat covering the tips of the microvilli. Evidence has been put forward which indicates that the surface coat is an integral part of the epithelial cells. In contrast to the jejunal epithelium, the colonic epithelium is endowed with a thicker surface coat. Variations in the organization of the surface coat at different levels of the colonic crypts have also been noted. The functional significance of these variations in the surface coat is discussed.

Intestinal triglyceride absorption in the rat. An electron microscopical study

This report provides information on the morphology of fat absorption in rat intestinal epithelial cells. Three types of experiments were performed: (a) intubation of corn oil into fasted rats, (b) injection of physiological fatty-chyme prepared from fat-fed donor rats into ligated segments of jejunum of fasted animals, and (c) administration of electron-opaque particles in corn oil and markers given concurrently with the fat. These results support the hypothesis that fat is absorbed by selective diffusion of monoglycerides and fatty acids from micelles rather than by pinocytosis of unhydrolized triglycerides. Evidence is presented that the pits between the microvilli, previously believed to function in the transport of fat, are not involved in this process. Instead they appear to contribute their contents to lysosomes in the apical cytoplasm. Arguments are offered that the monoglycerides and fatty acids diffuse from the micelle while the latter is associated with the microvillous membrane of the absorptive cell. These micellar components penetrate the plasma membrane and diffuse into the cytoplasmic matrix where they encounter the SER. Triglyceride synthesis occurs in the SER and results in the deposition of fat droplets within its lumina. The synthesis of triglycerides and their sequestration into the SER establishes an inward diffusion gradient of monoglycerides and fatty acids.

Cell junctions in ommatidia of Limulus

The intercellular relationships in the ommatidia of the lateral eye of Limulus have been investigated. The distal process of the eccentric cell gives origin to microvilli which interdigitate with the microvilli of the retinular cells. Therefore, both types of visual cells contribute to form the rhabdom and may have an analogous photoreceptor function. Quintuple-layered junctions are found within the rhabdom at the lines of demarcation between adjoining microvilli, whether the microvilli originate from a single retinular cell, from two adjacent retinular cells, or from a retinular cell and the eccentric cell. Furthermore, quintuple-layered junctions between the eccentric cell and the tips of the microvilli of the retinular cells occur at the boundary between the distal process and the rhabdom. These findings are interpreted to indicate that the rhabdom provides an extensive electrotonic junction relating retinular cells to one another and to the eccentric cell. Quintuple-layered junctions between glial and visual cells, as well as other structural features of the ommatidial cells, are also described.

Cellular mechanism of intestinal permeability alterations produced by chelation depletion

The absorption of phenolsulfonphthalein (phenol red) was used as a measure in vivo of intestinal permeability in anesthetized rats. A chelating agent, sodium ethylenediaminetetraacetate (NaEDTA), placed in the lumen evoked a fivefold increase in membrane permeability; at the same time the mucosal content of magnesium and calcium decreased significantly. Making either magnesium or calcium available to the luminal surface of the membrane in isotonic solution restored normal permeability and brought the cation contents above the original levels. Electron micrographs of tissues treated in vivo with NaEDTA revealed (a) rounded swellings on the microvilli in the area of the junctional complexes between adjacent epithelial cells, (b) widening of intercellular channels particularly in the region of the intermediate junctions (zonulae adhaerentes), and (c) loss of architectural detail in the region of the desmosomes (maculae adhaerentes) with separation of their dense borders. All of these alterations in fine structure could be reversed by in vivo cation replacements which reinstated normal permeability. The implications of these findings on mechanisms of fluid transport across epithelial membranes are discussed, and a working hypothesis for the role of divalent cations in membrane permeability regulation is presented.

The fine structure of the absorptive epithelial cells of the developing small intestine of the rat

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