Electron microscope observations on chick embryo liver. Glycogen, bile canaliculi, inclusion bodies and hematopoiesis

Histological analyses demonstrate the temporary contribution of yolk sac, liver, and bone marrow to hematopoiesis during chicken development

The use of avian animal models has contributed to the understanding of many aspects of the ontogeny of the hematopoietic system in vertebrates. However, specific events that occur in the model itself are still unclear. There is a lack of consensus, among previous studies, about which is the intermediate site responsible for expansion and differentiation of hematopoietic cells, and the liver's contribution to the development of this system. Here we aimed to evaluate the presence of hematopoiesis in the yolk sac and liver in chickens, from the stages of intra-aortic clusters in the aorta-genital ridges-mesonephros (AGM) region until hatching, and how it relates to the establishment of the bone marrow. Gallus gallus domesticus L. embryos and their respective yolk sacs at embryonic day 3 (E3) and up to E21 were collected and processed according to standard histological techniques for paraffin embedding. The slides were stained with hematoxylin-eosin, Lennert's Giemsa, and Sirius Red at pH 10.2, and investigated by light microscopy. This study demonstrated that the yolk sac was a unique hematopoietic site between E4 and E12. Hematopoiesis occurred in the yolk sac and bone marrow between E13 and E20. The liver showed granulocytic differentiation in the connective tissue of portal spaces at E15 and onwards. The yolk sac showed expansion of erythrocytic and granulocytic lineages from E6 to E19, and E7 to E20, respectively. The results suggest that the yolk sac is the major intermediate erythropoietic and granulopoietic site where expansion and differentiation occur during chicken development. The hepatic hematopoiesis is restricted to the portal spaces and represented by the granulocytic lineage.

THE OCCURRENCE AND STRUCTURE OF MICROBODIES : A Comparative Study

Livers from chickens, rats, mongrel dogs, Dalmatian dogs, and man have been examined in the electron microscope in order to compare the microbodies with the known content of uricase. It is concluded that microbodies with inclusion are present in rats, mongrel dogs, and, although the inclusion generally is smaller, in Dalmatian dogs. The inclusion has a characteristic structural appearance. These species (rat, dog) have uricase. Chickens and man lack both the enzyme uricase and the microbody inclusion. This evidence and that from previously published electron micrographs in the literature on microbodies support the notion of a positive correlation between uricase and microbodies with an inclusion. It is recommended that the term "uricosome" be used for such microbodies that have an inclusion of the appearance here described.

Ultrastructure of in situ perfusion-fixed avian liver, with special reference to structure of the sinusoids

Broiler chicken and laying hen livers were fixed using a simple technique of in situ puncture perfusion of cacodylate-buffered fixative, which allowed characterisation of the fine structure of hepatic parenchyma, hepatocytes, bile ductules, and, in particular, the sinusoidal cells including endothelial, Kupffer, and Ito cells. Sinusoidal endothelial cells with their bulging perinuclear cytoplasm, evident in both transmission and scanning electron micrographs, were easily distinguishable from Kupffer cells, which possessed numerous pseudopodia. Bile ductular epithelium and hepatocytes of the laying hens contained large amounts of lipid. The ultrastructural characteristics of intercalated cells (putative extra-sinusoidal macrophages of chicken liver) are described and their possible role as precursors of Kupffer cells is discussed.

Cytochemistry and electron microscopy. The preservation of cellular ultrastructure and enzymatic activity by aldehyde fixation

The aldehydes introduced in this paper and the more appropriate concentrations for their general use as fixatives are: 4 to 6.5 per cent glutaraldehyde, 4 per cent glyoxal, 12.5 per cent hydroxyadipaldehyde, 10 per cent crotonaldehyde, 5 per cent pyruvic aldehyde, 10 per cent acetaldehyde, and 5 per cent methacrolein. These were prepared as cacodylate- or phosphate-buffered solutions (0.1 to 0.2 M, pH 6.5 to 7.6) that, with the exception of glutaraldehyde, contained sucrose (0.22 to 0.55 M). After fixation of from 0.5 hour to 24 hours, the blocks were stored in cold (4°C) buffer (0.1 M) plus sucrose (0.22 M). This material was used for enzyme histochemistry, for electron microscopy (both with and without a second fixation with 1 or 2 per cent osmium tetroxide) after Epon embedding, and for the combination of the two techniques. After fixation in aldehyde, membranous differentiations of the cell were not apparent and the nuclear structure differed from that commonly observed with osmium tetroxide. A postfixation in osmium tetroxide, even after long periods of storage, developed an image that—notable in the case of glutaraldehyde—was largely indistinguishable from that of tissues fixed under optimal conditions with osmium tetroxide alone. Aliesterase, acetylcholinesterase, alkaline phosphatase, acid phosphatase, 5-nucleotidase, adenosine triphosphatase, and DPNH and TPNH diaphorase activities were demonstrable histochemically after most of the fixatives. Cytochrome oxidase, succinic dehydrogenase, and glucose-6-phosphatase were retained after hydroxyaldipaldehyde and, to a lesser extent, after glyoxal fixation. The final product of the activity of several of the above-mentioned enzymes was localized in relation to the fine structure. For this purpose the double fixation procedure was used, selecting in each case the appropriate aldehyde.

Biogenesis and function of lipolysosomes in developing chick hepatocytes

Proliferation of lipolysosomes is one of the characteristic aspects of embryonic chick hepatocytes. Formation of lipolysosomes is observed in the well-developed trans-Golgi network, with the highest frequency occurring from 11 to 14 days of incubation. The lipolysosomes usually contain a small and electron-dense lipid inclusion; however, during development, they gradually enlarge with an accompanying reduction in the electron density of the inclusion. Lipolysosomes isolated from neonatal chick liver homogenates were mainly composed of esterified cholesterol and showed considerably high activity of lysosomal enzymes. Moreover, the lipolysosome fraction is clearly shown to be a function of intralysosomal lipolysis via acid lipase. This accumulation of esterified cholesterol within lipolysosomes might be attributed to an excessive uptake and conversion of plasma lipoproteins to lipolysosomes. This concept is supported by the appearance of an abundance of coated pits and both "early" and "late" endosomes. The major components of plasma lipoprotein are low density lipoprotein (LDL) and high density lipoprotein (HDL), the cholesterol-rich lipoproteins, whose cholesterol content increases during the last week of incubation when the lipolysosomes quickly enlarge. Plasma lipoprotein particles are produced in the yolk sac epithelium from yolk very low density lipoprotein (VLDL) and transferred via the vitelline circulation to the chick liver. After hatching, when the supply of nutrients from the yolk sac is terminated, lipolysosomes immediately decrease in size and number. The cholesterol and fatty acids released are useful as an energy source and lipid metabolism in general, especially after hatching. Food intake induces the use of and accelerates the disappearance of lipolysosomes. Instead of lipolysosomes, lipid droplets appear and increase in number and size with concomitant increases of triglyceride concentrations in the liver homogenates, suggesting that lipogenesis has begun in the chick hepatocyte.

ON THE SITE OF SULFATION IN COLONIC GOBLET CELLS

The location of bound S³⁵ in the goblet cell of the rat colon at time points from 2 to 60 minutes after administration of S³⁵ as sodium sulfate has been observed in vivo and in vitro by radioautographic techniques. Grains were first observed by electron microscopy over the stacked lamellae of the paranuclear part of the Golgi apparatus. The label was subsequently found associated with the supranuclear Golgi lamellae and was then seen associated with the smooth membranes limiting the mucin granules in the goblet. Finally, between ½ and 1 hour, the secreted mucus product in the crypts became radioactive. Neither mitochondria nor the endoplasmic reticulum was labeled. It is concluded that the Golgi apparatus is the organelle in which sulfation occurs.

Formation and accumulation of lipolysosomes in developing chick hepatocytes

Formation and accumulation of lipolysosomes in developing chick hepatocytes were investigated by means of electron microscopy in combination with biochemical analyses of the lipid composition in liver homogenates. The lipolysosomes occurred with highest frequency from days 11 to 14 of incubation. They were usually small and electron-dense, but during development they gradually enlarged with an accompanying reduction in electron density. Coinciding with this enlargement was an accumulation of esterified cholesterol in the liver homogenates. After hatching, an immediate decrease in the size and number of lipolysosomes occurred along with a reduction in the concentration of esterified cholesterol, of which only a very small amount remained by 9 days of age. Instead of cholesterol, triglycerides subsequently increased in concentration and accounted for the major lipid content of the liver homogenates. In keeping with the ultrastructural changes, the total volume of cytoplasmic lipid droplets rapidly increased with increasing age. This transient accumulation of esterified cholesterol within lipolysosomes may be attributed to an excessive uptake and processing of plasma lipoprotein particles, probably derived from the egg yolk. This concept is supported by an abundance of coated pits, endosomes and multivesicular bodies in the embryonic hepatocytes.

Development of the liver in the chicken embryo. II. Erythropoietic and granulopoietic cells

Hepatic hemopoiesis is apparent in the chicken embryo on day 7 of incubation (Hamburger and Hamilton Stage 30), and a peak in hemopoietic activity occurs on day 14 (Stage 40). During this period, the differentiation of hemopoietic cells was examined by light microscopy and by transmission and scanning electron microscopy. Glycol methacrylate sections were used in lieu of smears to study hemopoietic cells, thus minimizing the problems of cell shrinkage and rupture. The sections were superior to smears for close examination of nuclear and cytoplasmic morphologies and for precise localization of hemopoietic cells to intravascular and extravascular sites. The avian liver is involved directly with erythropoiesis and granulopoiesis only. Erythropoietic cells, occurring in intravascular and extravascular locations, appear throughout the time frame examined. Blood islands with granulopoietic cells were not observed until days 8-9 (Stage 35). Granulopoiesis in the liver produces only eosinophilic leukocytes. Individual granulopoietic cells appear first in the connective tissue sheaths of hepatic vessels, and these cells subsequently congregate into blood islands. Endothelial cells of the sinusoidal linings, through asymmetric divisions, frequently release daughter cells into the circulation, and Kupffer cells are actively engaged in phagocytosis of erythrocytes. From a comparative standpoint, the elements deemed critical to hemopoiesis in the mammalian liver--prehepatocyte population, hepatic vasculature, and compartments for stem cell differentiation--may not hold the same importance in the bird, owing to an inordinate reliance on intravascular hemopoiesis in this vertebrate class.

Development of the liver in the chicken embryo. I. Hepatic cords and sinusoids

Hemopoiesis in the liver of the chicken embryo begins on day 7 of incubation (Hamburger and Hamilton Stage 30) and peaks on day 14 (Stage 40). During this time frame, the differentiation of hepatic cells was examined by light microscopy, transmission and scanning electron microscopy, and morphometry. The avian liver is a closely packed mass of dendriform cords and discontinuous sinusoids. Hepatocytes are pyramidal in shape, and they ring the bile canaliculi which run through the centers of the cords. Semithin sections, made possible by infiltration and embedding in glycol methacrylate, were stained with hematoxylin and eosin to assess the general architecture of the organ and the lipid content of the hepatocytes and by the periodic acid-Schiff reaction and hematoxylin to visualize the cytoplasmic stores of glycogen. The number of hepatocytes with demonstrable glycogen fluctuates erratically in early hemopoiesis, and the proportion of glycogen-containing cells progressively increases as hemopoiesis climbs to a peak. Most differentiating hepatocytes are devoid of lipid droplets until Stages 39 and 40. From Stage 30 to 35, hepatocyte volume falls to its lowest value. Subsequently (Stages 36 to 40), cell volume increases and hepatocytes achieve a relatively uniform size. Ultrastructural changes in the differentiating hepatocytes, including alterations to the mitochondria, endoplasmic reticulum, and Golgi apparatus, are documented. These morphological and morphometric findings on the prehepatocyte population and hepatic vasculature cover 2 of the 3 elements deemed critical to hepatic hemopoiesis in many vertebrates.

Ultrastructural and biochemical alterations in the livers from chick embryos maintained in shell-less culture

Ultrastructural and biochemical studies were conducted on the livers from chick embryos maintained in shell-less culture up to stage 39 (Hamburger-Hamilton) and from control embryos developed in ovo up to the same stage. The ultrastructural characteristics of hepatic cells from the cultured embryos were similar to those found in the controls except that they contained many large lipid droplets and were almost devoid of lipoprotein granules normally associated with the Golgi complex and the smooth endoplasmic reticulum. These changes suggest the existence of alterations in the lipid metabolism. The livers from cultured embryos showed also a decreased incorporation of tritiated leucine into proteins, which indicates a reduced rate of protein synthesis. These results are consistent with previous reports showing that cultured embryos possess hypoproteinemia. Lactic dehydrogenase activity was similar and pyruvic kinase higher in the livers from cultured with respect to control embryos. This appears to indicate that both aerobic and anaerobic glycolysis were not depressed and that the changes observed in the rate of protein synthesis should not be attributed to hypoxia. "Fat-storing cells" similar to those described in mammals were found both in control and cultured embryos. They had not been previously described in the livers from chick embryos.

Extravascular cells within the perisinusoidal space of the avian liver

Ultrastructural studies of the perisinusoidal space in the avian liver have demonstrated the presence of 2 extravascular cell types--a fat-storing cell and a free mesenchyme cell or histiocyte. This latter cell type is capable of participating in the formation of a bile canaliculus with the hepatic parenchymal cell. The possibility of the fat-storing cell differentiating from the histiocyte is suggested.

An ultrastructural study of early morphogenetic events during the establishment of fetal hepatic erythropoiesis

Morphogenetic events are described which characterize early stages of the interaction between mesenchyme and expanding epithelial cell cords derived from the hepatic endodermal diverticulum in the C57BL/6J mouse. This interaction culminates in the differentiation of hepatic epithelial and hematopoietic tissues. No basement membrane separates the presumptive hepatic epithelial cells from the adjacent mesenchyme, while intercellular attachments, both adherent junctions and desmosomes, are established transiently between heterologous cell types across this epithelio-mesenchymal interface. Yolk sac-derived erythroblasts found in the primitive liver are distinguished morphologically from endogenous hepatic erythroid cells; they are confined to the vascular compartment and are not, apparently, precursors for hepatic erythropoiesis. The earliest recognizable endogenous hepatic hematopoietic cells appear, extravascularly, among those mesenchymal cells in intimate contact with the endodermal epithelium between the 10(1/4) and 10(1/2) gestational day. Definitive erythropoiesis commences between the 10(1/2) and 11th fetal days. The ultrastructure of these primitive hepatic erythroid cells (proerythroblasts) and their transition to more mature forms (basophilic and polychromatophilic erythroblasts) are described.

Biogenesis of endoplasmic reticulum membranes. I. Structural and chemical differentiation in developing rat hepatocyte

The development of the endoplasmic reticulum of rat hepatocytes was studied during a period of rapid cell differentiation, i.e., from 3 days before to 8 days after birth. Before birth, the ER increases in volume, remaining predominantly rough surfaced; after birth, the increase continues but affects mainly the smooth-surfaced part of the system. These changes are reflected in variations of the RNA/protein and PLP/protein ratios of microsomal fractions: the first decreases, while the second increases, with age. The analysis of microsomal membranes and of microsomal lipids indicates that the PLP/protein ratio, the distribution of phospholipids, and the rate of P(32) incorporation into these phospholipids show little variation over the period examined and are comparable to values found in adult liver. Fatty acid composition of total phosphatides undergoes, however, drastic changes after birth. During the period of rapid ER development in vivo incorporation of leucine-C(14) and glycerol-C(14) into the proteins and lipids of microsomal membranes is higher in the rough-than in the smooth-surfaced microsomes, for the first hours after the injection of the label; later on ( approximately 10 hr) the situation is reversed. These results strongly suggest that new membrane is synthesized in the rough ER and subsequently transferred to the smooth ER.