Exercise and the brain: a mechanical model for pulsation on flow of cerebrospinal fluid

Exchange of molecules between cerebrospinal fluid (CSF) and brain cells contributes to brain function and protection from dementia, but the route by which CSF is brought close enough to the neural tissue to be exchanged by extracellular diffusion is not clear. Exogenous molecules injected into CSF are carried along channels outside arteries and reach the basement lamina that surrounds the dense capillary network. Transport of solutes by diffusion along the basement lamina, a gel of macromolcules about 100 nm thick, would be too slow; bulk flow in a static geometry would require unphysiologically high pressures. However, it is known that the pulsation of blood aids transport of CSF, and we hypothesized that this is because the pulsation intermittently squeezes the pericapillary lamina. In a primitive mimicry, we have tested whether intermittent squeezing increases flow through an agar gel. In all but one of 216 tests, pulsation caused a reversible increase, sometimes by a factor of 100 or more. The enhancement was greatest for frequencies 5-11 Hz and, over the tested range of pressure heads (20 - 50 cmH2O), was greatest for the lowest pressure. The results suggest one reason why exercise slows the aging of the brain.

An ultrastructural study of the early cercarial development in Prosorhynchoides borealis (Digenea: Bucephalidae) with special reference to formation of the primitive epithelium

Primitive epithelium and outer tegumental layer formation during early cercarial development was studied in Prosorhynchoides borealis using electron microscopy. It demonstrated that germinal cells freely floating in the sporocyst body cavity divide to give rise to naked cell aggregates. These early embryos are highly irregular in outline and are composed of blastomeres differing in size and structure. In embryos consisting of about 12–14 cells a few (possibly only two) superficial macromeres become concave and produce thin extensions which envelop the embryonic mass before fusing to form a syncytial primitive epithelium. This primitive epithelium forms syncytial connections with underlying embryonic cells. Primordial tegumental cells become apparent in late germinal balls below the primitive epithelium. These cells expand and fuse to give rise to an embryonic nucleated tegument. The embryonic tegument is connected to peripheral embryonic cells by thin cytoplasmic bridges until the basement lamina is formed. Subsequently, the primitive epithelium is shed by the embryos and the nuclei in the embryonic tegument undergo pyknotic degeneration. These results are analysed and compared with data from studies on other trematode species and it is concluded that the primitive epithelium is derived from the embryo in at least the majority of digeneans.

Coated Vesicle-Mediated Transport and Deposition of Vitellogenic Ferritin in the Rapid Growth Phase of Snail Oocytes

The main yolk component in oocytes of the pulmonate freshwater snails Planorbarius corneus L. and Lymnaea stagnalis L. consists of the iron storage protein ferritin and iron-free apoferritin. Both compounds are deposited in the yolk in the form of large paracrystalloids, tubular structures and randomly dispersed particles. In addition, the plasm contains lysosome-like inclusions with depositions of haemosiderin. Haemosiderin is interpreted as the product of proteolytic degradation of ferritin. During the rapid growth phase of the oocytes vitellogenic ferritin is transported across the basement lamina and taken up by adsorptive endocytosis via coated pits and vesicles. Formation of yolk bodies occurs by fusion of ferritin-containing vacuoles and empty vesicles that are probably derived from the Golgi apparatus. Uptake of ferritin is restricted to the basal region of the oocyte. No involvement of the follicle cells in synthesis and deposition of ferritin could be detected. Secretory cells of the midgut gland are the most likely site of synthesis of vitellogenic ferritin. Under conditions of iron overload large masses of ferritin are encountered in the basement lamina of the oocytes. However, no significant increase in the uptake of ferritin could be observed. With the use of a tannic acid-glutaraldehyde fixation procedure a hitherto unobserved filamentous or rod-like material was detected inside the lamina and in coated pits. This material is probably also taken up by the oocytes and integrated into yolk platelets. Though ferritin is a rather unusual vitellogenic protein, the mode of its uptake and deposition in the oocyte plasm is highly reminiscent of that of typical hormone-induced vitellogenins in other animal groups.

An ultrastructural method for the use of polyvinylpyrrolidone and dextrans as electron opaque tracers.

A simple method has been developed whereby polymers of polyvinylpyrrolidone and dextran can be visualized at the ultrastructural level by postfixing tissue with osmium tetroxide partially reduced by potassium ferrocyanide. Further enhancement of polymer electron opacity is obtained by counterstaining thin sections with bismuth subnitrate chelated by alkaline tartrate. These tracers remain adequately dispersed in serum and, depending on molecular size and concentration, are easily detected at the ultrastructural level within vascular lumena, basement lamina, apical vacuoles of proximal and distal tubules and endothelial vesicles. This technique offers the potential of studying nephron and cell permeability with families of tracers, the members of which are similar in chemical affinities, charge density and molecular shape.

The development of the tail and the excretory system in the cercaria of Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae from Littorina Littorea (L.))

The tail of the cercaria of Cryptocotyle lingua develops within the redia (intraredial stage) the process being completed in the molluscan haemocoel (intramolluscan stage). During growth epidermal cell bodies are carried into the proximal region of the tail while the single type of epidermal (secretory) cell body, containing two types of secretions develops later in the same region. The vesicular secretion bodies disperse throughout the outer cytoplasmic epidermis while the clear secretion bodies collect between the inner plasma membrane of the outer cytoplasmic epidermis and the basement lamina, near the tail root, forming the proximal caudal dilation. The epidermal and epidermal (secretory) cell bodies soon degenerate. The longitudinal caudal muscles, at first similar to and continuous with those of the body, gradually develop into the separate system of striated caudal muscles characteristic of the free swimming cercaria. The nuclei of most of the muscle cells degenerate before emer­gence of the cercaria. A secondary excretory pore is formed at the posterior end of the body, in the intraredial cercaria, the caudal excretory vessel and primary pores subsequently degenerating. The tail is shed when the cercaria contacts the fish second intermediate host. The mechanical break at the body–tail junction is caused by the movement of the tail against the resistance of the fixed body. The resulting ‘wound area’ on the body, at first enclosed in the caudal pocket, is soon covered by a layer of outer cytoplasmic epidermis.

PERMEABILITY OF THE OVARIAN FOLLICLE OF AEDES AEGYPTI MOSQUITOES

The passage of tracers of various molecular weights into resting and vitellogenic ovarian follicles of Aedes aegypti mosquitoes was studied ultrastructurally. The outermost layer of the follicular sheath (the basement lamina) is a coarse mechanical filter. It is freely permeable to particles with molecular weights ranging from 12,000 to 500,000 (i.e. cytochrome c, peroxidase, hemoglobin, catalase, ferritin, immunoglobulin (IgG)-peroxidase, iron dextran and Thorotrast) that have dimensions less than 110 A. Molecules as large as carbon (300–500 A) are totally excluded. Whereas proteins and polysaccharide tracers permeate the basement lamina with apparent ease, certain inert particles (e.g. Thorotrast, Fellows-Testager Div., Fellows Mfg. Co., Inc., Detroit, Mich.) penetrate more slowly. With respect to the tracers tested, resting follicles are as permeable as vitellogenic follicles. The follicle epithelium of resting or vitellogenic follicles is penetrated by narrow intercellular channels. Our observations suggest that these spaces are lined with mucopolysaccharide material. After permeating the basement lamina, exogenous tracers fill these channels, while the bulk of material accumulates in the perioocytic space. Within 3 hr after imbibing blood, the pinocytotic mechanism of the oocyte is greatly augmented. Pinocytosis is not selective with regard to material in the perioocytic space, since double tracer studies show that exogenous compounds are not separated, but are incorporated into the same pinocytotic vesicle. During later stages of vitellogenesis, 36–48 hr after the blood-meal, the pinocytotic mechanism of the oocyte is diminished. Simultaneously, the intercellular channels become occluded by desmosomes, and the vitelline membrane plaques separate the oocyte and follicle epithelium.