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While current consensus suggests the absence of collagen in osteonal cement lines, the extent of cement line mineralization and the nature of the ground substance within the cement line are unclear. Samples of human radius were examined by using scanning electron microscopy, electron microprobe, and histochemical techniques. X-ray intensities were used to compare the amount of calcium, phosphorus, and sulfur in cement lines with amounts in surrounding lamellar bone. The results indicate that cement lines contain significantly less calcium and phosphorus, but significantly more sulfur, than surrounding bone matrix. The Ca/P ratio of cement lines was significantly greater than that of lamellar bone, suggesting that the mineral in cement lines may not be in the form of mature hydroxyapatite. No selective staining of the cement lines could be demonstrated by using periodic acid-Schiff, Sudan black B, or alcian blue critical electrolyte concentration techniques.

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Tissues from adult Sprague-Dawley rats fixed by perfusion with buffered aldehydes for a combined study of the vascular system of the brain are described in light and electron microscopy. In these preparations lack of shrinkage prevents the formation of perineuronal and perivascular spaces. However, connective tissue stains indicate restricted tissue space along the course of small arteries and veins. In fine structure this space is found within the walls of the vessels. It consists of a tubular extension of tissue space bounded inwardly by the endothelial boundary (basement) membrane and outwardly by the neural boundary membrane. Between these boundaries the formed elements of the media and the adventitia are found. The media consists of a thin layer of smooth muscle cells; each cell being enclosed in its own boundary membrane. The adventitia consists of cells and fibrous elements of the connective tissues which are derived, near the surface of the brain, from the intermingling of pial and vascular leptomeninges. This "neural" portion of the tissue space extends from the depths of the capillary bed (where it is obliterated by the fusion of boundary membranes), along the course of the blood vessels, through the subarachnoid space and into the general tissue space of the body.

The subdural region within the cranial meninges is examined in guinea pigs by electron microscopy. The fine structures of the arachnoid membrane and dura are described separately in specimens that have been isolated from each other during removal from the cranial cavity. In addition, the fine structure of the intact dura-arachnoid is described, where the subdural space would be present in an undisrupted state. Lastly, the inner surface of the dura and the outer surface of the arachnoid membrane are examined at the point of separation between the two specimens where the dura is reflected from the arachnoid by experimental dissection. From these observations morphologic criteria are established for identifying the constituents and boundaries of the subdural space and for explaining mechanisms in the histogenetic process of "opening" or enlarging this space. The morphologic identity of the classic subdural space is reinterpreted in light of the findings. The subdural space, traditionally described as a fluid-filled potential cavity existing in an extracellular compartment, is not apparent in the guinea pig. Instead, fragile cells designated as light cells occupy the compartment between the dura and arachnoid, with very little extracellular space available. Experimental opening of the subdural space occurs, significantly, along pathways extending by fracture through the cytoplasm and intercellular separation of these light cells rather than by enlargement of a preexisting mesothelial-lined intercellular space between these cells and the true arachnoid cells. Cytoplasmic fine structure of light cells suggests a close kinship with cells in the meningeal layer of the dura. The functional significance of the light cells and their possible role in subdural hematomas is discussed.

A B S T R A C TMicrofibrils in the notochordal sheath of chick embryos of two to four days incubation age were analyzed electron microscopically. Collagenase, trypsin, hyaluronidase and alpha amylase were administered to both "living" and "fixed" embryos. Living embryos were either explanted on an agar-albumen growth medium or were allowed to survive within the shell during experimental procedures. Fixed embryos were immersed in Karnovsky's aldehyde fixative prior to enzyme digestion. Both living and fixed specimens yielded similar results.Control embryos possessed microfibrils of ca. 200 A, ca. 100 A and less than 100 A in diameter in the notochordal sheath. The smaller fibrils (100 A and less) were present in all regions of the sheath, but were especially concentrated near the notochordal boundary (basement) membrane. The larger fibrils (200 A ) were farther removed from the notochord. Flaky amorphous material was attached to all fibrils and was abundant especially in the inner region of the sheath.Collagenase produced elimination of the 200 A microfibrils, but the smaller ones remained intact. Trypsin caused digestion of all fibrils. Hyaluronidase digested fibrils chiefly 100A and less in diameter. Alpha amylase was similar to hyaluronidase in digesting the small fibrils. This study supports the hypothesis that microfibrils are primarily mucopolysaccharide-protein precursors of larger collagen-rich fibrils.

The anatomy and histology of the adrenal gland in the adult opossum were found to be typical for mammals. The development of the adrenal medulla was also found to follow the typical mammalian pattern. Primitive sympathetic cells were found in both intra- and extra-adrenal locations in the newborn at a time when chromaffin precursor cells were migrating to the adrenal anlage. Pheochromoblasts first appeared within the forming medulla where at a later stage chromaffin cells could be observed forming columns of cells between adjacent sinusoids. Unlike in other mammals, much of this development takes place postnatally when the neonate is in the mother's marsupium. The value of the developing opossum adrenal medulla as an experimental model is stressed, since a significant amount of development takes place in an environment that is accessible to experimental manipulation.

High-voltage electron microscopy was employed to observe developing extracellular connective tissue elements in the cervical perinotochordal and perivertebral regions in the chick embryo from 2 through 15 days' incubation. During days 2 and 3, small (10 nm) and large (18-20 nm) microfibrils surrounded the notochord, becoming evident around fibroblast-like cells in day 4. Amorphous material, globular granules and microfibrillar bundles were present at this time. Microfibrillar length increased as did the total population of microfibrils. At four days microfibrils 3-5 nm in diameter arose in all directions from globular granules. During day 9 and thereafter to day 15, microfibrillar diameters increased. This growth formed unit collagenous fibrils 30 nm in diameter or greater. Axial periodicity became evident at day 14. Small microfibrils appear to be composed largely of glycoproteins and do not contain a significant amount of collagen. The globular granules and associated filaments are probably proteoglycans. The amorphous material is believed to provide molecular collagen to developing fibrils. Large microfibrils and unit collagenous fibrils contain significant amounts of molecular collagen.