The pollen tube: a soft shell with a hard core.

Plant cell expansion is controlled by a fine-tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in-depth knowledge of cell wall mechanics. Pollen tubes are tip-growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope (CFM) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method (FEM), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MPa and 20-90 MPa, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM, in combination with an FEM-based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue.

Cellular force microscopy for in vivo measurements of plant tissue mechanics.

Although growth and morphogenesis are controlled by genetics, physical shape change in plant tissue results from a balance between cell wall loosening and intracellular pressure. Despite recent work demonstrating a role for mechanical signals in morphogenesis, precise measurement of mechanical properties at the individual cell level remains a technical challenge. To address this challenge, we have developed cellular force microscopy (CFM), which combines the versatility of classical microindentation techniques with the high automation and resolution approaching that of atomic force microscopy. CFM's large range of forces provides the possibility to map the apparent stiffness of both plasmolyzed and turgid tissue as well as to perform micropuncture of cells using very high stresses. CFM experiments reveal that, within a tissue, local stiffness measurements can vary with the level of turgor pressure in an unexpected way. Altogether, our results highlight the importance of detailed physically based simulations for the interpretation of microindentation results. CFM's ability to be used both to assess and manipulate tissue mechanics makes it a method of choice to unravel the feedbacks between mechanics, genetics, and morphogenesis.

Pulsatile tinnitus: a review of the literature and an unusual case of iatrogenic pneumocephalus causing pulsatile tinnitus.

BACKGROUND: Pulsatile tinnitus is frequently attributed to identifiable and treatable causes, in contrast to the more common subjective non-pulsatile tinnitus. It usually originates from vascular structures as a result of either increased blood flow or lumen stenosis; atherosclerotic carotid or subclavian artery disease; arterial, venous, or arteriovenous malformations, fistulas, or dissection; and paragangliomas. Other causes have also been reported, with often unclear pathophysiology. OBJECTIVE: The aim of this paper is to present a case of pulsatile tinnitus secondary to iatrogenic pneumocephalus and to review the literature on pulsatile tinnitus. SUBJECT: A 48-year-old white woman had a roaring, very disturbing, pulsatile tinnitus after the removal of a cerebellar lobe meningioma. When the patient experienced the symptom of tinnitus, a pulsatile movement of the tympanic membrane could be clearly seen, and this was synchronous with the patient's heartbeat. Computed tomography revealed an epidural pneumocephalus in the left posterior fossa communicating freely with the air cell system of the left mastoid cavity without any sign of residual tumor. A simple mastoidectomy was performed. The whole air cell system was removed and the mastoid cavity was filled with abdominal fat. After the operation, the pulsatile tinnitus ceased completely and the pneumocephalus disappeared gradually. The patient is free of symptoms 11 months after surgery. CONCLUSION: Otologists, neurosurgeons, and skull base surgeons should be aware of this surgical complication and be careful to identify any accidental opening to the air cell system of the temporal bone and meticulously close it when it happens. The review of the literature leads to the conclusion that pulsatile tinnitus should be thoroughly investigated, as it may be related to diseases that may have serious complications.