Cellular organization and fine structure of type II microtrich sensilla in gammaridean amphipods (Crustacea)

The cellular organization of type II microtrich sensilla was studied in male Anonyx lilljeborgi Boeck, 1871 (Lysianassoidea) by light and transmission electron microscopy. The sensillum consists of two bipolar sensory neurons and three concentric sheath cells. The sensory cell bodies are subepidermal. In each sensillum both dendrites are enclosed by the thecogen cell process. The inner dendritic segments are filled with mitochondria and lucent vesicles and expand in the epidermis into a spindle-shaped swelling. One of the neurons gives rise to two cilia and the second to a single cilium. These three outer dendritic segments lie in the receptorlymph cavity. The dendritic sheath, secreted by the thecogen cell process, completely ensheaths the outer dendritic segments. The trichogen (middle) cell and the tormogen (outer) cell incompletely enclose the thecogen cell, but their processes form autojunctions around the dendritic sheath in the apical epidermis. In premolt, the trichogen cell processes project into the exuvial space. The cytoplasm of the tormogen cell and the bordering epidermal cells contains coarse osmiophilic inclusions. All the cells of the sensillum are joined by desmosomes. The sensilla structurally resemble chemosensory (gustatory) insect sensilla.

Cis-interactions between Delta and Notch modulate neurogenic signalling in Drosophila

We find that ectopic expression of Delta or Serrate in neurons within developing bristle organs is capable of non-autonomously inducing the transformation of the pre-trichogen cell into a tormogen cell in a wide variety of developmental contexts. The frequencies at which Delta can induce these transformations are dependent on the level of ectopic Delta expression and the levels of endogenous Notch signalling pathway components. The pre-trichogen cell becomes more responsive to Delta- or Serrate-mediated transformation when the level of endogenous Delta is reduced and less responsive when the dosage of endogenous Delta is increased, supporting the hypothesis that Delta interferes autonomously with the ability of a cell to receive either signal. We also find that a dominant-negative form of Notch, ECN, is capable of autonomously interfering with the ability of a cell to generate the Delta signal. When the region of Notch that mediates trans-interactions between Delta and the Notch extracellular domain is removed from ECN, the ability of Delta to signal is restored. Our findings imply that cell-autonomous interactions between Delta and Notch can affect the ability of a cell to generate and to transduce a Delta-mediated signal. Finally, we present evidence that the Fringe protein can interfere with Delta- and Serrate-mediated signalling within developing bristle organs, in contrast to previous reports of the converse effects of Fringe on Delta signalling in the developing wing.

Development of adult sensilla on the wing and notum of Drosophila melanogaster

We have investigated the temporal pattern of appearance, cell lineage, and cytodifferentiation of selected sensory organs (sensilla) of adult Drosophila. This analysis was facilitated by the discovery that the monoclonal antibody 22C10 labels not only the neuron of the developing sensillum organ, but the accessory cells as well. The precursors of the macrochaetes and the recurved (chemosensory) bristles of the wing margin divide around and shortly after puparium formation, while those of the microchaetes and the stout and slender (mechanosensory) bristles of the wing margin divide between 9 h and 18 h after puparium formation (apf). The onset of sensillum differentiation follows the terminal precursor division within a few hours. Four of the cells in an individual microchaete organ are clonally related: A single first-order precursor cell divides to produce two second-order precursors; one of these divides into the neuron and thecogen cell, the other into the trichogen cell and tormogen cell. Along the anterior wing margin, two rounds of division generate the cells of the mechanosensory sensilla; here, no strict clonal relationship seems to exist between the cells of an individual sensillum. At the time of sensillum precursor division, many other, non-sensillum-producing cells within the notum and wing proliferate as well. This mitotic activity follows a spatially non-random pattern.

In vitro analyses of cuticular differentiation at the chromosomal level in Lucilia sericata

The large bristles of flies sclerotize and melanize during the pupal phase well before the rest of the cuticle. Each bristle is the product of two cells, the trichogen cell and the tormogen cell. Both their nuclei exhibit analyzable polytene chromosomes. The pupal metathoracic integument of Lucilia sericata has been excised and cultivated in Schneider's Drosophila medium for extended periods of time. Among the differentiations taking place in vitro, the most obvious is the stiffening and coloring of the bristles. The in vitro differentiation very much resembles the in vivo differentiation in its timing and appearance. Explants excised early in pupal development cannot differentiate in vitro, while those excised and cultured at later stages can. Actinomycin D was administered to the culture medium and the incorporation of [3H]uridine was analyzed after long incubations to determine if this process was a reaction of dead cuticle or controlled by living cells. The analysis of chromosome structure and puffing patterns in the five polytene chromosomes of the trichogen cell after in vitro cultivation shows that transcriptional activity is not dependent on the existence of normal chromosome conformation. The degree of pycnotic reaction and puff induction of the nuclei, however, varied under different culture conditions. The capacity of this system for differentiation can be used to study in vitro chromosomal activity during the course of development.Key words: in vitro, polytene chromosomes, sclerotization, cuticle.

Morphology of the elongate sensillum placodeum on the antennae of Aphidius smithi (Hymenoptera: Aphidiidae)

Elongate placoid sensilla were present on all 20 flagellar segments in five male Aphidius smithi and all but the basal 1 of 17 segments in five females. They are oriented longitudinally, approximately equidistant around the circumference of a segment. Scanning electron microscopy of the internal cuticular surface disclosed lamellae which divide the sensillum into lateral and median channels. Many transverse ridges are present in the ceiling of the median channel. Unlike other species, there is no cuticular floor of the sensillum. Numerous nerve cell bodies in the subcuticular tissue give rise to dendritic processes, a ciliary region, and a region of dendritic branches which lie parallel to each other within the 1-μm-wide median channel underneath the 0.1- to 0.25-μm-thick dome. Each dendritic branch is composed of a neurotubule surrounded by a plasma membrane. The ascending dendrites are flanked by the microvilli of the trichogen cell. A tormogen cell encloses the trichogen cell and extends into the lateral channels of the sensillum. The relationship of minute pores in the dome with the underlying dendritic branches is unclear. The hypothesis is advanced that the sensilla may be involved in host finding through perception of infrared radiation.

Morphology of a unique sensillum placodeum on the antennae of Coeloides brunneri (Hymenoptera: Braconidae)

A previously undescribed and unique placoid sensillum was discovered on the antennae of Coeloides brunneri. This elongate, dome-shaped plate organ was examined by light microscopy, and transmission and stereoscan electron microscopy. It differs from other plate organs in that it has two cuticular lamellae suspended internally from the dome of the sensillum. These lamellae separate the internal structure of the plate organ into three channels running the full length of the sensillum. Dendritic branches which enter the plate organ through an aperture in the center of the sensillum's cuticular floor fill the upper two-thirds of the median channel. A tormogen cell fills the two lateral channels and the basal third of the median channel. The linear structure and placement of these plate organs suggest that they are highly directional wave guides capable of perceiving infrared radiation in a manner that would enable C. brunneri to locate its host with a very high degree of accuracy.

The Function and Fine Structure of the Cephalic Airflow Receptor in Schistocerca Gregaria

Electron micrographs of parts of the sense organ showed that the dendritic axis consisted of a large and a small envelope containing microtubules as their main inclusion. The envelopes are supported by a thick-walled tube believed to be part of the Ist-tier sheath cells. The small envelope is segregated from the large envelope near its apex by a fold of the tube wall. The packing of the neurotubular array within the small envelope is both more dense and more regular than within the large envelope. The tube is separated by an extracellular space from the trichogen-tormogen cell. Sections through the apex of the dendrite reveal a homogeneous cap unlikely to be part of a structure continued into the upper region of the hair shaft. No ciliary structures were visible within the dendrite, whose microtubules pass into the neuron cell body proximally. Sections through the neuron cell body reveal branched mitochondria, and numerous microtubules. Rates of discharge in sensory axons from these hair organs produced by deflexion of the hair shaft were found to be within the range 300-100 impulses/sec. There is an initial phase of rapid adaptation which gives place to a steady rate. It is suggested that the fine structure of the receptor may indicate mechano-electrical transduction at a more proximal level than is believed to be the case in some other types of receptor. The diaphragms that support the hair shaft laterally can be seen to be composed of fine cuticular strands.