Ultrastructural studies on postnatal differentiation of neurons in the substantia gelatinosa of rat cervical spinal cord.

Neuroblasts of the substantia gelatinosa at birth were small with large oval nuclei and scanty cytoplasm. The cytoplasm possessed ribosomes and mitochondria. Granular endoplasmic reticulum and Golgi complexes were generally absent or rudimentary. Electron dense bodies were seldom observed. By the end of the first week, the nuclei of several cells demonstrated early nuclear invaginations; cytoplasm exhibited growth cones, a well developed granular endoplasmic reticulum and Golgi complexes. At several points the channels of endoplasmic reticulum became continuous with the perinuclear space. By the end of the second week, differentiation of the neuroblasts was more advanced. More nuclei showed invagination of their contour. The cytoplasm revealed well dev-loped granular endoplasmic reticulum and multiple Golgi complexes. Numerous vesicles and dense bodies were found adjacent to the Golgi complexes. Arrays of agranular endoplasmic reticulum also appeared late in the second week. By the third week, features of neuronal differentiation, such as nuclear invagination, granular endoplasmic reticulum agranular membrane configurations, multiple Golgi complexes and dense bodies in the cytoplasm became well established.     

Ultrastructural studies on postnatal differentiation of neurons in the substantia gelatinosa of rat cervical spinal cord

Neuroblasts of the substantia gelatinosa at birth were small with large oval nuclei and scanty cytoplasm. The cytoplasm possessed ribosomes and mitochondria. Granular endoplasmic reticulum and Golgi complexes were generally absent or rudimentary. Electron dense bodies were seldom observed. By the end of the first week, the nuclei of several cells demonstrated early nuclear invaginations; cytoplasm exhibited growth cones, a well developed granular endoplasmic reticulum and Golgi complexes. At several points the channels of endoplasmic reticulum became continuous with the perinuclear space. By the end of the second week, differentiation of the neuroblasts was more advanced. More nuclei showed invagination of their contour. The cytoplasm revealed well developed granular endoplasmic reticulum and multiple Golgi complexes. Numerous vesicles and dense bodies were found adjacent to the Golgi complexes. Arrays of agranular endoplasmic reticulum also appeared late in the second week. By the third week, features of neuronal differentiation, such as nuclear invaginations, granular endoplasmic reticulum, agranular membrane configurations, multiple Golgi complexes and dense bodies in the cytoplasm became well established.     

Structure of taste buds in foliate papillae of the rhesus monkey,Macaca mulatta

Taste buds in foliate papillae of the rhesus monkey were examined by electron microscopy. Three distinct cell types were identified. Type I cells were narrow elongated cells containing an oval nucleus, bundles of intermediate filaments, several Golgi bodies, and characteristic apical membrane-bounded dense granules. These cells exhibited morphological variations: some had a moderately dense cytoplasm, perinuclear free ribosomes, and flattened sacs of rough endoplasmic reticulum; others had a more lucent cytoplasm, dilated irregular rough endoplasmic reticulum, lysosome-like dense bodies, and lipid droplets. Type II cells typically contained a spherical, pale nucleus, a prominent nucleolus, supranuclear and infranuclear Golgi bodies, mitochondria with tubular cristae, and one or two centrioles. This cell type, too, showed some variation in the relative amounts of ribosomes and smooth endoplasmic reticulum, which varied inversely with each other. Type III cells were characterized by a clear apical cytoplasm essentially devoid of ribosomes and containing microtubules. In a few type III cells, the peri- and infranuclear regions contained many ribosomes and some rough endoplasmic reticulum. In most Type III cells, there were large numbers of dense and clear vesicles in the peri- and infranuclear regions; some of the vesicles were grouped in synapse-like arrangements with adjacent nerves. The morphological variations exhibited by all three cell types could be accounted for by age differences in each of the cells. This would be consistent with the notion that cell renewal occurs in each of the three cell populations.     

Growth cones in differentiated neuroblastoma: A time-lapse cinematographic and electron microscopic study

Summary Growth cones of differentiating neuroblastoma cells in monolayer culture were studied by time-lapse cinematography and electron microscopy. Morphological differentiation, and thus growth cone formation, was induced by the glucocorticoid dexamethasone. Growth cones lengthened gradually at an average rate of 30 m/h, advancing in stages that involved alternating extensions and retractions of the filopodia and lamellar sheets. During neurite growth the cell body usually remained stationary. The ultrastructure of growth cones was typified by several filopodia, each containing a bundle of microfilaments, agranular endoplasmic reticulum, aggregates of large agranular vesicles lying adjacent to filopodia (previously termed vesicle-filled mounds), many dense-cored vesicles, 100–140 nm in diameter, microtubules, bizarre and distorted mitochondria, and scattered free ribosomes. Comparing the findings with previous ultrastructural accounts of growth cones of cultured ganglion cells, similarities outnumbered differences. The organization of the microfilament bundles and the abundance of free ribosomes were remarkable in the neuroblastoma cell as was the profusion of dense-cored vesicles which were most numerous in the proximal portion of the growth cone.     

The cytoskeletons of isolated, neuronal growth cones

We have examined by electron microscopy the cytoskeletons of growth cones isolated from neonatal rat forebrain by the method of Gordon-Weeks and Lockerbie [Gordon-Weeks and Lockerbie (1984) Neuroscience 13, 119-136]. When fixed in suspension with conventional fixatives, isolated growth cones contain a central region filled with a branching system of smooth endoplasmic reticulum and a cortical region immediately beneath the plasma membrane that is relatively free of organelles and is composed of an amorphous granular cytoplasm. The filopodia of isolated growth cones are also devoid of organelles and contain a cytoplasm that is similar in appearance to that in the cortical region. No microtubules or neurofilaments have been found in these growth cones. When isolated growth cones were prepared for electron microscopy by a method which preserves actin filaments [Boyles, Anderson and Hutcherson (1985) J. Histochem. Cytochem. 33, 1116-1128], microfilaments were found throughout the cortical cytoplasm. In the filopodia, the microfilaments were bundled together and oriented longitudinally. Filopodial microfilament bundles often extended into the body of the growth cone and could traverse it completely. Inclusion of Triton X-100 (1% v/v) in the fixative solubilized the membranes and soluble cytoplasmic proteins of growth cones, allowing an unobscured view of the microfilament cytoskeleton including the core bundle of microfilaments in filopodia. Suspended within the cytoskeleton were the coats of coated vesicles. These were particularly numerous at the broad bases of filopodia. Microfilaments bound heavy meromyosin and were cytochalasin B (2.0 X 10(-7) M) sensitive. Individual microfilaments branched and within filopodia they were extensively cross-linked by thin (7 nm) filaments. Microtubules and neurofilaments were not seen in these cytoskeletons despite the fact that the fixative contained a Ca2+ chelator. When growth cones were preincubated in taxol (14 microM) their cytoskeletons were found to contain microtubules. These were located mainly in the centre of the growth cone, were absent from the filopodia and were contiguous with microfilaments. We conclude that the cytoskeletons of isolated neuronal growth cones from neurones of the central nervous system are mainly composed of actin microfilaments. Although microtubules are not normally present, there is a pool of soluble tubulin which will form microtubules in the presence of taxol. This may imply that those microtubule-associated proteins that promote tubulin polymerization are absent in the growth cone or are below the concentration threshold for polymerization.     

Fine structure of the adrenocortical homologue in the north American eel and modifications following seawater adaptation

The ultrastructure of the adrenocortical homologue (AH) of the north American eel (Anguilla rostrata) was studied from freshwater and long-term (1.5 years) seawater (SW) adpated animals. The AH tissue situated in the wall of cardinal veins is surrounded by a thin layer of collagenous capsule; in the region away from the vein wall, parenchymal cells are separated by interstitial lacunae containing collagen bundles, capillaries, chromaffin cells and nerve fibers often applied closely to the surface of AH cells. The free surface of the cell near the vein wall, capillaries or interstitial space extends numerous slender microplicae. The cytoplasm is characterized by the presence of mitochondria with tubular cristae, a network of smooth endoplasmic reticulum, a few cisternae of rough surfaced endoplasmic reticulum, well developed Golgi apparatus, coated vesicles, centrioles, cilium, filaments, microtubules, dense bodies of variable nature and a scarcity of liposomes. The cell nuclei possess invaginated cytoplasmic pseudo-inclusions. Electron histochemical reaction for free cholesterol revealed the occurrence of needle-shaped crystals mainly associated with surface microplicae and smooth endoplasmic reticulum, which seems to be the major organelle for storage or synthesis of this steroid precursor. SW animals indicated ultrastructural signs of stimulated steroid synthesis and secretion, i.e., high degree of pseudo-follicle formation, increased electron density of mitochondria, greater abundance of lysosomal dense bodies, hyperactivity of Golgi apparatus and dilation of smooth endoplasmic reticulum tubules. Some SW fishes showed extensive deposition of osmiophilic inclusions in the mitochondria and stacks of elongated cup-shaped mitochondria. Chronic seawater acclimation enhances AH activity as judged by ultrastructural criteria with ultimate mitochondrial degeneration resulting possibly from prolonged cortisol hypersecretion; the latter may be linked with physiologic re-adjustment of ionic transfer mechanism in hyperosmotic medium.     

Ultrastructure of macrogametogenesis of Eimeria mivati

Summary The development of the macrogamete of Eimeria mivati Edgar and Seibold 1964 was studied with the electron microscope. Development of the young gamont was characterized by a loss of organelles such as the apical complex, subpellicular microtubules, rhoptries and micronemes, followed by an increase in micropores, mitochondria, rough endoplasmic reticulum (rER), and Golgi complexes. Nuclear detachment bodies and canaliculi were present in maturing macrogamonts. Amylopectin was first observed as small electron-dense rod-like bodies that eventually became large electron-transparent bodies. Type II wall-forming bodies developed in the cisternae of the rER. Type I wall-forming bodies appeared shortly thereafter in close association with numerous Golgi complexes. Many small vesicles located between the cisternae of the rER and the Golgi complexes formed what appeared to be a secretory pathway whereby protein formed in the cisternae and, modified by the Golgi complex, may produce the type I wall body material. The outer wall of the oocyst developed between two distal membranes on the surface of the macrogamete. Although the actual mechanism of deposition of the wall material was not seen, it was probably by some secretory process. Wall-forming bodies did not fuse.Abbreviations Ap Amylopectin - CB Crescent body - G Golgi complex - Imc Inner membrane complex - Ivt Intravacuolar tubules - L Lipid body - Mn Microneme - Mp Micropore - N Nucleus - NC Nuclear canaliculi - ND Nuclear detachment body - Nu Nucleolus - Om Outer membrane - rER Rough endoplasmic reticulum - Spm Subpellicular microtubules - Sm Secretory membrane - sv Secretory vesicles - tv Transfer vesicles - V Vesicles - WF I Type I wall body - WF II Type II wall body - Wm Wall material     

Fine structure at the cut ends following section of the sensory component of the vagus nerve of mice

The fine structures at the cut ends following section of the sensory component of the midcervical vagus nerve of mice were examined. No changes were found in the fine structure of the neurons in the nodose ganglion or in that of the axons in the midcervical vagus nerve 14 d after supranodose vagotomy. After supranodose vagotomy followed 14 d later by midcervical vagotomy, the dilated axons present in the vicinity of the cut contained clear vesicles with a mean diameter of 69 nm and dense-cored vesicles with a mean diameter of 78 nm, vesiculotubular membranous structures, smooth endoplasmic reticulum (SER), mitochondria, multivesicular bodies, dense bodies, neurofilaments, and microtubules. The similarity between sensory axons interrupted by cutting and the presynaptic terminals of sensory neurons is discussed.     

Immunocytochemical and electron microscopic study of serotonin neuronal organization in the dorsal raphe nucleus of the monkey

Serotonin neurons in the dorsal raphe nucleus were identified using an antibody to a serotonin-bovine serum albumin conjugate and the peroxidase anti-peroxidase method. Nerve cell bodies showing serotonin-like immunoreactivity ranged in size from 15 to 22 micron in diameter; their dendrites were also immunoreactive. Immunostaining was present in the cytoplasmic matrix, outer membranes of mitochondria, rough endoplasmic reticulum, multivesicular bodies and dense-cored vesicles. Heavily immunoreactive axonal varicosities contained small round vesicles (18-35 nm) and larger dense-cored vesicles (50-90 nm). Both unmyelinated (0.2-0.5 micron) and myelinated (0.8-1.1 micron) serotonin-like immunoreactive axons were found, often interspersed within bundles of similar caliber unlabeled axons. Serotonin-like immunoreactive somata and dendrites were postsynaptic to numerous unlabeled terminals that contained either (a) clear round vesicles (18-25 nm) with many small dense-cored vesicles (30-50 nm), (b) clear round vesicles (18-25 nm) with large dense-cored vesicles (90-110 nm) or (c) clear round vesicles (18-25 nm) with or without flat vesicles. In addition pairs of unlabeled terminals formed crest synapses onto serotonin-like immunoreactive dendritic spines. This variety of unlabeled terminals making contact with serotonin-like immunoreactive elements suggests that several neuronal systems with possibly different transmitters may regulate serotonin raphe neurons. We occasionally observed serotonin-like immunoreactive dendrites and terminals in apposition to other serotonin-like immunoreactive dendrites with membrane specializations at the site of contact. This might represent a possible site for the self inhibition of serotoninergic neurons reported in physiological studies of the serotonin system in the dorsal raphe nucleus.     

The zinc-iodide-osmium reactive sites in the sensory epithelia of the frog labyrinth

Summary In the sensory epithelia of the vestibular system of the frog,Rana temporaria, the zinc-iodide-osmium stain reacts in the sensory cells with mitochondria, multivesicular bodies, lysosomes, coated vesicles, the smooth endoplasmic reticulum, the vesicles surrounding the synaptic bars and large numbers of morphologically similar vesicles scattered throughout the cytoplasm. In supporting cells positive reactions are given by mitochondria, multivesicular bodies, a few vesicles and areas of cytoplasm. Reactive sites in nerve fibres are the synaptic vesicles, mitochondria and the smooth endoplasmic reticulum. Certain regions of the Schwann-cell cytoplasm and myelin sheath of myelinated nerve fibres are also reactive.Energy dispersive X-ray microanalysis (EMMA-4) shows that the zinc-iodide-osmium stain is composed mainly of osmium with possibly a trace amount of zinc. Iodine is not shown to be present.     

Intravascular macrophages in normal calf lung. An electron microscopic study

Intravascular macrophages were found commonly in sections of calf lung capillaries. These cells were large with many pseudopodia of various sizes. The cell membrane was covered with an electron-opaque coat which remains between adjacent pseudopodial walls and results in structures suggesting “micropinocytosis vermiformis.” The cytoplasm of the macrophage was electron-lucent and contained endoplasmic reticulum, numerous mitochondria, free ribosomes, Golgi zones, dense bodies and vesicles of many sizes. These macrophages showed phagocytic activity. During erythrophagocytosis, a dense layer initially appeared at the red blood cell periphery. This layer moved toward the center together with the erythrocyte membrane as phagocytosis progressed. Simultaneously, the electron opacity of the red blood cell decreased until the cell was completely dispersed within the macrophage. This erythrophagocytosis occurred relatively seldom. The possibility that intravascular macrophages are the pre-cursors of alveolar macrophages is discussed.     

The structure of Schwann cells in unmyelinated fibres. A qualitative and quantitative electron microscope study

The structure, size and distribution of many cytoplasmic components of Schwann cells associated with unmyelinated axons in lizard thoracic spinal roots were analysed under the electron microscope. The percentages of Schwann cell cytoplasmic area occupied by the following cytoplasmic components were determined: mitochondria, Golgi apparatus, granular endoplasmic reticulum, multivesicular bodies, smooth endoplasmic reticulum, lipofuscin granules, peroxisome-like bodies, autophagic vacuoles, dense bodies and lipid droplets. A linear correlation was found between the sectional areas of the mitochondria and granular endoplasmic reticulum of the Schwann cell and both length of Schwann cell plasma membrane profile and size of the related axoplasm. The structure of Schwann cells associated with unmyelinated axons and that of Schwann cells associated with myelinated axons were compared in the same species and in the same region of the peripheral nervous system using the same fixative and the same preparation technique. Some differences were detected in the organization of the granular endoplasmic reticulum, in the presence of cilia and in the percentages of cytoplasm occupied by various components. The hypothesis that Schwann cell mitochondria and granular endoplasmic reticulum are involved in the production and storage of proteins for the plasma membrane of this cell as well as the hypothesis that these organelles are involved in the production and storage of protein metabolites which are subsequently transferred to the related axons seem applicable not only to Schwann cells associated with myelinated axons (Pannese et al., in press), but also to those associated with unmyelinated ones.     

Ultrastructural changes in the parathyroids of Mongolian gerbils induced experimentally in vitro. Effects of variations in Ca2+ and Mg2+ concentrations

Isolated parathyroid glands from normal adult Mongolian gerbils were incubated for 15 minutes to 3 1/2 hours either at high or low concentrations of Ca2+ and Mg2+ after which they were studied ultrastructurally, using the pyroantimonate technique and x-ray analysis for identification and son concentrations were mainly composed of suppressed chief cells with moderate or high cytoplasmic density, sparsely developed endoplasmic reticulum, often large Golgi complex, occassional cytoplasmic accumulations of secretory granules, lipoid bodies, glycogen-like particles, and numerous often large mitochondria. Ca2+-containing precipitates were found mainly in mitochondria. Autophagic vacuoles contained Ca2+-loaded degenerating mitochondria. Glands exposed to low concentrations of Ca2+ and Mg2+ were mainly composed of stimulated and active chief cells; characteristic features were a moderate or low cytoplasmic density, prominent endoplasmic reticulum, small Golgi complex, medium-sized, or small mitochondria, and smooth electron lucent vacuoles with or without an association of mitochondria. Ca2+-containing precipitates were found mainly in smooth vacuoles and cytosol, but also in mitochondria and routh vacuoles. Myelin-like figures and crystalloid bodies occurred in some mitochondria, and normal or degenerating mitochondria withoug Ca2+-loading were seen in autophagic vacuoles. In addition, some stimulated chief cells exhibited double membrane-limited sequestered areas of cytoplasm with a rich content of free ribosomes and glycogen-like particles. The chief cell mitochondria seem to possess capacity for rapid accumulation of Ca2+, associated with an increase in volume at functional suppression. At stimulation of parathyroid function the endoplasmic reticulum is prominent in the active cells, and there seems to be a decrease in the volume and Ca2+-content of the mitochondria occasionally associated with degenerative changes, and a decrease also in the number of free ribosomes and glycogen-like particles in the stimulated cells.     

Morphological and morphometric studies in pancreatic acinar cells of the rat at successive post-mortem intervals. The early appearance of a previously undescribed Golgi complex associated tubular vesicular structure

Structural alterations in rat pancreatic acinar cells were studied in thin section at 0.5, 1, 4, 8, 12, 24 and 48 h post-mortem (PM). Morphometric analyses were performed both by light and electron microscopy, at 0.5 and 1 h PM. The parameters evaluated were: a) nuclear, cytoplasmic and cellular volumes; b) volume density and absolute volume of the rough endoplasmic reticulum (RER), mitochondria, zymogen granules (ZG), Golgi complex and its subcompartments [cisternae, condensing vacuole (CV) and 56-nm diameter vesicles], dense bodies (lysosome-like structures, electron-dense vacuoles and unidentifiable granules) and cytoplasmic matrix; c) surface density, surface/volume ratio and total surface area of the RER, mitochondria, ZG, Golgi cisternae, 56-nm diameter vesicles lying at the rough ER-Golgi interface, CV, and apical and basolateral membranes. Between 0.5 and 48 h, the mitochondria were dilated, junctional complexes were preserved and autophagic vacuoles were rare or absent. Flocculent densities were present in the mitochondria and chromatin condensation was observed at 4 h PM. In thin sections from samples obtained between 0.5 and 12 h, we consistently observed a membrane bounded structure formed by tubules and vesicles, designated as a tubular vesicular structure (TVS). These TVS's were observed at positions corresponding to the 4th Golgi cisterna. Fibrillar aggregates and a reduction in the number of 56-nm vesicles on the cis side of the Golgi were seen. Morphometry revealed a 60-70% reduction in the numerical density of the 56-nm vesicles between zero (control) and 0.5 h PM. These analyses also showed a 70% increase in the total volume and 57% increase in the total membrane surface of the Golgi cisternae in the PM period. The current results suggest that during the early PM (0.5 h) there is transport between Golgi compartments, and the 56-nm diameter vesicles fuse with the cisternae.     

Electron microscopic observations of the mesencephalic nucleus of the fifth nerve in the Selachian brain

Summary The mesencephalic nucleus of the trigeminal nerve (mes V) in the brain of the skate (Raja oscellata) was studied by electron microscopy. Mes V neurons are large (40–80 m diameter) and are located in the periventricular grey matter. Their perikaryal cytoplasm is rich in Golgi apparatus, small mitochondria, rough endoplasmic reticulum, polysomes and bundles of neurofilaments. A striking feature is the presence of masses of glycogen granules, at times surrounded by membrane wrappings and lysosomal bodies.Two types of conventional synaptic contacts were made onto mes V perikarya and dendrites. One had round, agranular vesicles and usually also contained dense-cored vesicles, the other had flattened, pleomorphic, agranular vesicles and usually lacked dense-cored vesicles. Additional membrane complexes consisting of a region of gap junction flanked by sites of desmosomal attachment were observed to link neighbouring mes V neurons. Somato-somatic, dendro-somatic, axo-somatic, and dendro-dendritic junctions were noted. Except for the somato-somatic union, one or more chemical synapses were located close to the sites of gap junctions.     

Continuity of intracellular channels with extracellular space in adipose tissue and liver: Demonstrated with tannic acid and lanthanum

Tannic acid was used to demonstrate continuity of intracellular channels with extracellular space in white adipose tissue of adult rats, brown adipose tissue of suckling rats, and liver of diabetic rats. Electron-opaque material resulting from treatment of glutaraldehyde-fixed tissue with tannic acid was found in extracellular space, invaginations of cell surfaces, vesicles, and intracellular channels. Electron-opaque material was present in channels that surrounded lipid droplets in both white and brown adipocytes and in hepatocytes. The small distance between the lumen of marked channels and lipid droplets in adipocytes indicates that a monolayered structure, perhaps a leaflet of membrane lining the channel, separates the lipid droplet from the lumen of the channel, suggesting that the lipid droplet may be located between leaflets of the membrane lining the channel. Similar findings were obtained in brown adipose tissue using lanthanum instead of tannic acid to mark intracellular channels continuous with extracellular space. Since endoplasmic reticulum is the primary site of triacylglycerol synthesis in adipocytes, marked channels near lipid droplets may be elements of endoplasmic reticulum. Some of the channels marked with tannic acid in hepatocytes contained lipoprotein particles, whereas others were located, in relation to mitochondria and lipid droplets, in the same sites as endoplasmic reticulum in untreated tissue. This indicates that some of the channels marked with tannic acid in hepatocytes are endoplasmic reticulum. Presence of electron-opaque material in intracellular channels and vesicles, but not in cytoplasm, of treated tissue indicates the channels and vesicles were open to extracellular space during treatment with tannic acid or lanthanum and, furthermore, that their membranes were continuous with plasma membrane.     

The fine structure of Brunn's nests in human bladder urothelium

The aetiology, morphology and clinical significance of Brunn's nests in human bladder urothelium are poorly understood. In the present study, 9% of a population of 100 consenting patients undergoing diagnostic or review cystoscopies were histologically found to have Brunn's nests, although their presence was not detected endoscopically. Of the nine cases, four were diagnosed with cystitis cystica, two with bladder papillomata, one with transitional cell carcinoma and cystitis cystica, and two were considered to be macroscopically normal. Mucosal biopsies were removed from areas unaffected by macroscopic abnormalities. Brunn's nests consisted of rounded collections of cells, whose nuclei contained prominent nucleoli and occasional nuclear bodies. The cytoplasm frequently contained clusters of mitochondria, together with Golgi membranes and cisternae of granular endoplasmic reticulum while small membrane bound vesicles containing electron dense material were present in some cells. Presumptive lysosomes were rarely observed. Thus Brunn's nest cells generally resembled normal urothelial cells in both their nuclear and cytoplasmic fine structure. Brunn's nests were surrounded by a basal lamina and loose connective tissue containing fenestrated capillaries.     

The distribution and movement of organelles in maturing growth cones: correlated video-enhanced and electron microscopic studies

Summary The morphology of growth cones from identified neurons ofAplysia californica was analysed both with video-enhanced contrast differential-interference contrast (VEC-DIC) microscopy, and through serial electron microscopic reconstructions of the same growth cones. The largest structures seen in the living growth cones, the large irregular refractile bodies (LIRBs), were shown in electron micrographs to be unique structures, composed predominantly of dense-core vesicles but including mitochondria and smooth membrane profiles. The LIRBs were stratified in the growth cones, occurring predominantly in sections distant from the substrate and relatively devoid of microtubules. VEC-DIC observations showed that LIRBs formed in the peripheral regions of the organelle-rich central growth cone, and grew in size through fusion with other LIRBs, accumulating into a large central mass in more proximal regions. The distribution of microtubules and LIRBs and the movements of LIRB suggest that there is an overall circulatory pattern in the growth cones, with the delivery of new vesicles occurring at distal areas close to the substrate, and the accumulation and retrograde processing of organelles occurring in more proximal areas away from adhesive contacts.     

Androgen producing adrenocortical carcinoma

Summary Two cases of androgen secreting adrenocortical carcinoma have been described by light and electron microscopy. The histological and ultrastructural features of the tumour cells were similar to those of compact cells of zona reticularis and to those described in virilizing adenomas. They possess numerous mitochondria with lamellar and tubular cristae, abundant smooth endoplasmic reticulum, lipofuscin bodies and scanty lipid. Irregularly shaped, crenated mitochondria, with outpouchings of the outer limiting membrane have also been observed. The clusters of neoplastic cells were surrounded by basement membrane which demonstrated a focal discontinuity, probably reflecting malignancy of the tumours. Hyperplasia of smooth endoplasmic reticulum and the presence of outpouchings of the mitochondrial outer limiting membrane might be the morphological manifestation of endocrine activity of the tumours.     

Postnatal development of mitral cell perikaryon in the olfactory bulb of the rat. A light and ultrastructural study

The differentiation of the mitral cell perikaryon in postnatal rat olfactory bulb was studied with the light and electron microscope. At birth the mitral cell was distinguishable and occupied a definitive position in the mitral cell layer. The cell contained a large oval nucleus surrounded by a thin rim of cytoplasm. Ribosomes, free and clustered, were scattered in the cell cytoplasm. Rough endoplasmic reticulum was relatively scarce. The Golgi complexes were made up of stacks of smooth-surfaced cisternae and associated vesicles. In certain cases the Golgi complexes projected into cellular processes. Mitochondria were present in all regions of the cytoplasm and contained well developed cristae. At the end of the first week, the mitral cell had developed significantly in size, and the cytoplasm contained well-developed rough endoplasmic reticulum. The Golgi complexes were made up of several stacks of smooth-surfaced cisternae with the association of vesicles and electron dense bodies. The apical dendrites o mitral cells at this periiod had increased significantly in length. Subsequently, during the second and third week, the rough endoplasmic reticulum and Golgi complexes became well developed. Associated with the Golgi complexes were electron dense lysosomal bodies. At three weeks and in older cells it was observed that dense lysosomal bodies. At three weeks and in older cells it was observed that dense lipofuschin granules increased significantly. It is suggested that the mitral cell matures and differentiates earlier than cells in the cerebral cortex.