Apical cytoplasm

About: Apical cytoplasm is a research topic. Over the lifetime, 1080 publications have been published within this topic receiving 36131 citations.

Frequency specificity of vibration dependent discharge of nematocysts in sea anemones

Abstract: Hair bundles on tentacles of sea anemones are similar to vertebrate hair bundles in terms of structure and function. Anemone hair bundles are involved in regulating discharge of nematocysts, "stinging capsules," used to capture prey. N-acetylated sugars from the prey includ- ing N-acetylneuraminic acid (NANA) induce hair bundles to elongate while shifting vibration dependent discharge of nematocysts to lower frequencies matching prey movements. In the present study, we find that vibration dependent discharge of nematocysts exhibits sharp frequency dis- crimination to within one Hz. Testing at one-Hz intervals over the range of frequencies spanning 1-75 Hz, we find that seven of these are stimulatory in seawater alone. A total of twenty-six frequencies are stimulatory in the presence of NANA. Stimulatory frequencies in NANA are lower than those in seawater alone. We find that antagonists of ryanodine receptors including ryanodine, procaine and tetracaine shift discharge to lower frequencies. Fluorescently tagged ryanodine labels numerous small loci in the apical cytoplasm of supporting cells. We propose that calcium induced calcium release (CICR) via ryanodine receptors may sharpen frequency specificity and/or cause shortening of hair bundles to shift frequency specificity to higher fre- quencies. J. Exp. Zool. 281:582-593, 1998. © 1998 Wiley-Liss, Inc. Sea anemones are slow moving, predominantly sessile invertebrates that rely on prey locomotion or water currents to bring prey into contact with their tentacles. Anemones with small, fine ten- tacles or long, filamentous tentacles are prima- rily planktivorous (Shick, '91). Prey are captured by nematocysts and other cnidae, complex secre- tory products consisting of capsules containing eversible tubules (Mariscal, '74, '84). In response to appropriate chemical and mechanical stimuli (Pantin, '42), effector cells called cnidocytes trig- ger cnida discharge, rapid eversion of the tubule (Skaer and Picken, '65; Holstein and Tardent, '84). Depending on the type of cnida, the everting tubule may adhere to the surface of the prey, en- tangle its appendages, or penetrate its integu- ment to inject potent toxins (Mariscal, '74, '84). In anemones, most of the published work on the regulation of cnida discharge has concerned microbasic p-mastigophore nematocyts, selected for study because this nematocyst type is abun- dant in the cnidom (complement of cnidae) of the tentacle. Furthermore, microbasic p-mastigophore nematocysts discharged into gelatin coated test probes are relatively easy to count using phase contrast optics (e.g., Watson and Hudson, '94). Mechanical stimuli alone are sufficient to trig- ger discharge. Relevant mechanical stimulation can comprise contact alone or contact with a vi- brating test probe. Contact between a tentacle and nonvibrating test probe elicits discharge of a baseline number of nematocysts. Provided the test probe is vibrating at a preferred frequency (here- after referred to as a "key" frequency), about twice as many cnidocytes discharge nematocysts into the test probe as baseline. The number of nema- tocysts discharged into test probes vibrating at frequencies other than key frequencies is compa- rable to baseline (reviewed in Watson and Mire- Thibodeaux, '94). Thus far, two chemoreceptors are known to affect discharge of nematocysts. These chemore- ceptors bind N-acetylated sugars and amino com- pounds, respectively, compounds derived from prey (Thorington and Hessinger, '88). N-acety- lated sugars are contained in prey mucins and other glycoproteins. Amino compounds are con- tained in hemolymph of prey. Chemical stimuli

Formation of membrane-bounded secretory granules in the midgut epithelium of a termite, Cubitermes severus , and a possible intercellular route of discharge

Abstract: Mature columnar cells of the midgut of Cubitermes contain a prominent secretion product observed at light- and electron-microscopic levels. At the ultrastructural level the product is resolved as an electron dense material contained in vesicles up to 1 μm diameter that accumulate in the apical cytoplasm. The vesicles are composite, apparently formed by coalescence of at least two types of precursor vesicle both of which originate from the Golgi apparatus. Discharge of the product takes place by exocytosis into intercellular space at or in the vicinity of the apical septate junction complex. Augmentation of apical surface area by microvilli is less prominent in Cubitermes than in other termites for which data are available. This and other evidence suggests that absorptive functions are reduced in the midgut of this insect.

Spermiogenesis in Polyplacophora, with special reference to acrosome formation (Mollusca)

Abstract: The present study examines spermiogenesis, and in particular the formation of the acrosome, in ten species of chitons belonging to four families. This study emphasizes the formation of the acrosome but brings to light several other structures that have received little or no mention in previous studies. The process of spermiogenesis is essentially similar in each species, although Chaetopleura exhibits some significant differences. In early spermiogenesis the Golgi body secretes numerous small pro-acrosomal vesicles that gradually migrate into the apical cytoplasm. The chromatin condenses from granules into fibres which become twisted within the nucleus. A small bundle of chromatin fibres projects from the main nuclear mass into the anterior filament; this coincides with the appearance of a developing manchette of microtubules around the nucleus that originates from the two centrioles. Radiating from the distal centriole is the centriolar satellite complex, which is attached to the plasma membrane by the annulus. The distal centriole produces the flagellum posteriorly and it exits eccentrically through a ring of folded membrane that houses the annulus. Extending from the annulus on one side of the flagellum, in all but one species, is a dense fibrous body that has not been previously reported. The proximal centriole lies perpendicular to the end of the distal centriole and is attached to it by fibro-granular material. Pro-acrosomal vesicles migrate anteriorly through the cytoplasm and move into the anterior filament to one side of the expanding nucleus. Eventually these vesicles migrate all the way to the tip of the sperm, where they fuse to form one of two granules in the acrosome. In mature sperm the nucleus is bullet-shaped with a long anterior filament and contains dense chromatin with occasional lacunae. The mitochondria vary in both number and position in the mature sperm of different species. Both centrioles are housed eccentrically in a posterior indentation of the nucleus, where the membranes are modified. The elongate flagellum tapers to a long filamentous end-piece that roughly corresponds to the anterior filament and may be important in sperm locomotion for hydrodynamic reasons. An acrosome is present in all ten species and stained positively for acid phosphatase in three species that were tested.

Effects of progesterone on the oviductal epithelium in estrogen-primed prepubertal beagles: light and electron microscopic observations.

Abstract: A total of 45 prepubertal beagles 6 to 8 weeks of age were used to study the cytological changes that accompany regression of the oviductal epithelium. The oviductal epithelium in untreated pups consisted of undifferentiated low cuboidal cells that measured 10.3 ± 2.0 μm in height. In response to estradiol (E2), low cuboidal cells underwent hypertrophy, hyperplasia, and cytodifferentiation and gave rise to columnar ciliated and secretory cells. After 12 days of E2 treatment the epithelium was fully differentiated and measured 29.4 ± 2.6 μm in height with 56% of the cells possessing cilia. When E2 treatment was continued for an additional 12 days, the epithelium was maintained in a differentiated state. However, if E2 treatment was terminated or progesterone (P) given alone or in conjunction with E2, the oviductal epithelium regressed and after 6 days was composed of low cuboidal cells that ranged in height from 9 to 14 μm with approximately 25% of the cells possessing cilia. A variety of cytological changes characterized the process of regression. The most immediate signs that regression was underway were a reduction in the height of the epithelium and the presence of cells with shrunken, pleomorphic nuclei that lacked prominent nucleoli. Degenerative events included: (1) pinching off and shedding of the apical cytoplasm of cells comprising the epithelium, (2) extrusion of whole cells and/or nuclei, and (3) resorption of cilia and basal bodies. During the first 6 days following E2 withdrawal or P treatment, macrophages and cellular debris were frequently present within the lumen of the oviduct. The process of regression did not proceed synchronously throughout the ampulla of the oviduct, nor did all cells appear to degenerate in the same manner. The cytological changes that accompanied oviductal regression following P treatment were identical to those observed following E2 withdrawal. Results from experiments conducted in the present study show that: (1) E2 induces the oviductal epithelium to differentiate and is required to maintain the epithelium in a differentiated state, (2) E2 withdrawal or P treatment causes the oviductal epithelium to regress, (3) at least three distinct degenerative processes are involved in the transition of columnar ciliated and secretory cells into low cuboidal cells, and (4) regression does not occur synchronously throughout the ampulla region of the oviduct.