J. Francis Hartmann

Electron microscopy of neurohypophysis in normal, adrenaline-treated and pilocarpine-treated rabbits

Summary1.The fine structure of the neurohypophysis in normal, adrenaline-treated and pilocarpine-treated rabbits has been studied, using the electron microscope. The normal structure of the nerve terminal and pericapillary region is quite similar to that of the rat as described by Palay (1957) and Hartmann (1958).2.Pituicytes are significantly different, however, and are classified into three types. The first one (Type A) is identical with the astrocyte of Farquhar and Hartmann (1957) and of Schultz, Maynard and Pease (1958). This type cell has a round or oval nucleus of low electron density and abundant watery cytoplasm with few formed elements.3.The second type (Type B) is considered to be an atypical astrocyte. This type cell has a nucleus closely resembling that of Type A, but with abundant cytoplasm containing relatively numerous formed elements. The inverse relationship between relative numbers of Type A and Type B cells in a given gland suggests the possibility of one type being readily transformed into the other under normal conditions. The last type (Type C) has a dark, irregular nucleus and dense cytoplasm with many cell organelles. This cell is structurally identical with the microglia of Farquhar and Hartmann, and of Schultz, Maynard and Pease. A typical oligodendrocyte is not found. No secretory granules are present in pituicytes. The pituicytes are in contact with nerve fibers with a double membrane intervening, and sometimes enclose nerve fibers in invaginations of their cytoplasm. The cell bodies of the pituicytes are located in the parenchymatous region far from blood capillaries, and their processes, along with the nerve terminals, reach to the outer basement membrane of the pericapillary space.4.During 1–2 hours after injection of adrenaline there was found to be a statistically significant increase in number and size of secretory granules, as well as structural changes of mitochondria in the nerve terminals, enlargement of the pericapillary space and endothelial alterations. These phenomena are considered to reflect a blocking of the release of secretory material after injection of adrenaline.5.After injection of pilocarpine, a statistically significant decrease in number and size of neurosecretory granules, plus the appearance of many optically empty granules, structural changes of mitochondria and attenuation of the pericapillary space were observed. These phenomena are considered to be related to an acceleration of the release of secretory granules after injection of pilocarpine.

The relationship between dense bodies and mitochondria in motor neurones.

SummaryAn electron microscopic study was made of the hypoglossal nuclei in a series of rabbits which had been subjected to unilateral hypoglossal neurotomy. Particular attention was given to the dense bodies which were present in the cytoplasm of the motor neurones on both sides. The simplest forms of dense body showed a granular structure without a limiting membrane. Others showed a limiting membrane (with either one or two dense layers) and varying degrees of internal organization in the form of double membranes. Some of the more highly-organized dense bodies possessed electron-transparent areas and were very similar in appearance to mitochondria. The appearances seemed consistent with the possibility that different forms of dense body represented developmental stages leading to or from mitochondria.Counts made on micrographs confirmed that mitochondrial numbers were increased 10–11 days after axonal section and showed in addition that there was an associated increase in the number of dense bodies. These findings are interpreted as evidence in favour of the developmental relationship of dense bodies to mitochondria.

LOCALIZATION OF MELANIN SYNTHESIS WITHIN THE PIGMENT CELL: DETERMINATION BY A COMBINATION OF ELECTRON MICROSCOPIC AUTORADIOGRAPHY AND TOPOGRAPHIC PLANIMETRY.

SummaryLocalization of melanin synthesis within the pigment cells of the Cloudman S-91 mouse melanoma was determined by means of a combination of high resolution autoradiography and topographic planimetry. Initial melanin biosynthesis occurred predominantly in the endoplasmic reticulum and associated ribonucleoprotein particles of the melanocytes. By measuring a number of cell organelles and employing the index of “relative specific localization” it could be shown that the nucleus and mitochondria are of little or no importance in the process of melanogenesis.

Ultrastructural Substrates of Experimental Cerebral Edema

The developing clinical interest in cerebral edema and its neuropathological definition has been paralled by an increasing number of attempts to produce an adequate experimental model for the study of this process. The entity has been vigorously studied and there is a voluminous literature on the subject filled with conflicting reports and divergent opinions with regard to both reliability of methods and morphological changes produced.

Electron microscopy of myelin sheath in sections of spinal cord

Abstract 1. 1. The structure of the myelin sheath in the central nervous system has been studied in sections of the spinal cord of the rat. 2. 2. The sheath has been found to be composed of concentrically arranged placodes of varying electron density, denser types averaging 120 A in thickness and less dense types averaging 61 A. The width of both kinds of plaques was found to vary unsystematically between 540 A and 1150 A, and their length was found to range between 900 A and 1700 A. 3. 3. The placodes in the myelin sheath are imbricated in both the concentric and longitudinal directions so as to mask any possible interposition of complete concentric cylinders between them. 4. 4. Open spaces in the myelin sheath comparable to those described by other authors for peripheral nerve were seen only occasionally, and are here interpreted as artefacts of embedding rather than of fixation.