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PubMed1976

Microtubules and control of insect egg shape.

This study evidence for tension transmission by microtubules and desmosomes in the follicular epithelium during anisometric growth of certain insect eggs. Most insect oocytes, and the follicles which surround them, grow anisometrically as they assume shapes which approximate to those of long prolate spheroids. Surface growth is most rapid in directions which parallel the polar axis of an oocyte and slowest in circumferential directions at right angles to this axis. The longitudinal axes of microtubule bundles in follicle cells of the gall midge Heteropeza and the cockroach Periplaneta are oriented circumferentially with respect to the surfaces of developing eggs and at right angles to the polar axes of eggs. At cell boundaries, the tubules appear to be attached to spot desmosomes. It is suggested that microtubules and desmosomes form a mechanical continuum throughout a follicular epithelium which transmits tensile forces around the circumference of a growing egg. Follicular resistance to circumferential expansion may be largely responsible for defining the elongate form of insect eggs.

Animals↗
PubMed1997

Control of insect ecdysis by a positive-feedback endocrine system: roles of eclosion hormone and ecdysis triggering hormone.

A successful ecdysis in insects requires the precise coordination of behaviour with the developmental changes that occur late in a moult. This coordination involves two sets of endocrine cells: the peripherally located Inka cells, which release ecdysis triggering hormone (ETH), and the centrally located neurosecretory neurones, the VM neurones, which release eclosion hormone (EH). These two sets of endocrine cells mutually excite one another: EH acts on the Inka cells to cause the release of ETH. ETH, in turn, acts on the VM neurones to cause the release of EH. This positive-feedback relationship allows the Inka cells and the VM neurones to be the peripheral and central halves, respectively, of a decision-making circuit. Once conditions for both halves have been satisfied, their reciprocal excitation results in a massive EH/ETH surge in the blood as well as a release of EH within the central nervous system. This phasic signal then causes the tonic activation of a distributed network of peptidergic neurones that contain crustacean cardioactive peptide. The relationship of the latter cells to the subsequent maintenance of the ecdysis motor programme is discussed.

Animals↗