The fine structural development of rat fetal liver sinusoids from 10 to 22 days gestation was studied. Colloidal carbon (Pelikan ink) was injected into 14-22 day gestation fetuses via the umbilical vein to assess the continuity of the sinusoidal lining and the phagocytic ability of the developing lining cells. Endothelial cells, devoid of an underlying basal lamina, form the bulk of the vascular lining at all gestational ages. These cells possess typical intercellular junctions and fenestrae with diaphragms before 17 days gestation. Transendothelial open fenestrations, typical of the adult liver, appear around 17 days gestation, increasing in number for the remainder of gestation. Although fenestrae possessing diaphragms are permeable to carbon before 16 days gestation, open fenestrations, first seen at 17 days gestation, allowed large amounts of carbon to reach the extravascular space. Endocytosis of carbon by endothelial cells was accomplished exclusively by large bristle-coated vesicles. Endothelial cells were also seen to be involved in transmural diapedesis of newly formed erythrocytes and megakaryocyte processes from the extravascular space by forming a temporary migration pore allowing these cells and processes to enter the circulation. At the end of gestation, blood-forming activity had nearly ceased, and only the space of Dissé separated the lining cells from the parenchymal cells. Kupffer cells were easily identified as early as 13 days gestation by their content of phagosomes and engulfed erythrocytes. The Kupffer cells are much more avid in the phagocytosis of carbon than are endothelial cells. Toward the end of gestation, some Kupffer cells develop a homogeneous "sticky coat" to carbon.

The buoyant density ofEscherichia coli was shown to be related to the osmolarity of the growth medium. This was true whether the osmolarity was adjusted with either NaCl or sucrose. When cells were grown at one osmolarity and shocked to another osmolarity, their buoyant density adjusted to nearly suit the new osmolarity. When cells were subjected to hyperosmotic shock, they became denser than expected. When cells were subjected to hypoosmotic shock they occasionally undershot the new projected density, but the undershoot was not as dramatic as the overshoot seen with hyperosmotic shocks. Shrinkage and swelling of the cells in response to osmotic shocks could account for the change in their buoyant density. The changes in cell size after osmotic shocks were measured by two independent methods. The first method measured cell size with a Coulter Counter, and the second method measured cell size by stereologic analysis of Nomarski light micrographs. Both methods gave qualitatively similar results and showed the cells to be flexible. The maximum swelling recorded was 23% of the original cell volume, while the maximum shrinkage observed was 33%.

This study is concerned with the passage of carbon particles through the sinusoidal lining cells of bone marrow and embryonic liver of the rat. A carbon suspension (Pelikan C11/1431A, Gunther Wagner, Hanover) diluted 1 : 1 with double strength Tyrode solution, was administered through the aorta for the bone marrow studies and through the umbilical vein for observations on the embryonic liver. The carbon particles have a diameter ranging from 220 to 380 A with a mean diameter of 280 A. Within three minutes after the injection, the particulate was present in the extravascular spaces. Neither the sinusoidal walls of the bone marrow nor of the embryonic rat liver prior to 16 days gestation have preformed apertures. In both cases, the carbon particles enter the extravascular space through fenestrae with diaphragms. No carbon particles occur in the junctional spaces between the lining cells. The temporary pores caused by diapedetic blood cells maintain a tight seal and no particulate was observed leaving the vascular space at these sites. At 17 days of gestation, open gaps develop in the endothelial lining of the embryonic rat liver and particulate material leaves the vascular lumen through these openings. The presence of bristle-coated vesicles containing particulate material at the abluminal side of the lining cells is interpreted as a retrograde uptake by these phagocytic cells rather than as evidence for vesicular transmural transport.

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The ultrastructural morphology of the cellular and extracellular components of the developing myocardial capillary wall--from the 16-day-gestation fetus of the rat to the 21-day neonate--was examined. A morphometric analysis of plasmalemmal vesicles and of coated vesicles and pits of capillary endothelial cells was performed during the same developmental period. As the lateral extensions of the capillary endothelial cells change from irregular to regular in their thickness during development, there is an increase in the number of plasmalemmal vesicles and a progression from clusters of plasmalemmal vesicles to a uniform distribution in the endothelial cell. The ratio of vesicles which are open to the luminal front, which are "free" in the cytoplasm, or which are open to the abluminal front of the endothelial cell was consistent throughout development. The numerical density of plasmalemmal vesicles demonstrates a gradual and significant increase. In contrast, the numbers of coated vesicles and pits are variable within a very narrow range, and no pattern of increase or decrease is discernible during development. Similarly, there is no change in interendothelial cell junctions, which consist of occluding and primitive adhesive junctional types, during development. The lamina densa of the basal lamina gradually develops from discontinuous, patchy densities along the abluminal surface of the endothelial cells to a continuous and distinct layer by 21 days gestation. The presence of the proteoglycan species in the developing basal lamina was assessed with the cationic dye ruthenium red (RR), and the appearance of RR-marked proteoglycans was found to parallel the appearance of lamina densa material. found to parallel the appearance of lamina densa material. The RR sites appear discontinuously in patches; and later, the RR sites appear in a continuous and regular planar lattice in the lamina rara interna and externa at 21 days gestation. A complete array of RR-stainable anionic sites outside a continuous lamina densa near birth indicates that the basal laminae of developing capillaries in the heart are morphologically, and in part biochemically, mature by the end of the first neonatal week. Our results show that the endothelial cells and the subtending basal lamina of myocardial capillaries gradually mature morphologically during the final days of gestation and the first neonatal week.(ABSTRACT TRUNCATED AT 400 WORDS)