Vascular endothelial growth factor signaling affects both angiogenesis and osteogenesis during the development of scleral ossicles.

Intramembranous ossification is a complex multi-step process which relies on extensive interactions among bone cells and surrounding tissues. The embryonic vasculature is essential in regulating endochondral ossification; however, its role during intramembranous ossification remains poorly understood, and in vivo studies are lacking. Previous research from our lab on the development of the intramembranous scleral ossicles has demonstrated an intriguing pattern of vascular development in which the areas of future osteogenesis remain avascular until after bone induction has occurred. Such avascular zones are located directly beneath each of the conjunctival papillae, epithelial structures which provide osteogenic signals to the underlying mesenchyme. Here we provide a high-resolution map of the developing vasculature from the time of ossicle induction to mineralization using a novel technique. We show that vegfa is expressed by the papillae and nearby mesenchymal tissue throughout HH 34-37, when vascular growth is taking place, and is down-regulated thereafter. Localized inhibition of Vegf results in expansion of the avascular zone surrounding the implanted papilla and mispatterning of the scleral ossicles. These results demonstrate that Vegf signaling could provide important insights into the complex relationship between bone and vasculature during intramembranous bone development.

Coarctation of the stapedial artery: an unusual adaptive response to competing functional demands in the middle ear of some eutherians.

In primitive eutherians, the stapedial artery is the primary supplier of blood to the nonneural tissues of the head. Beyond a certain body size, the stapedial artery can no longer function as the sole supplier to its original territory because the diameter of its stem is limited by the size of the intercrural foramen of the stapes, which exhibits strong negative allometry. Some eutherians have extended the upper limit that the diameter of the stapedial stem can attain by developing a coarctation (narrowing) at the transcrual portion of the vessel. In the Norway rat (Rattus norvegicus) and the golden hamster (Mesocricetus auratus) the coarctation develops in postnatal life and is evidently caused by a retardation in growth that keeps the diameter of the vessel at infantile dimensions. In the rat, additional reduction in the external diameter is produced by a thinning of the tunica media of the arterial wall. A comfortable gap between the wall of the artery and the sides of the intercrural foramen is maintained that most likely facilitates the attenuation of potentially disruptive low-frequency vibrations produced by the arterial pressure pulse. The only negative side effect of a coarctation in rat-sized animals is that resistance to flow is increased and volume flow rate is concomitantly diminished. The coarctation does not create flow disturbances downstream of the constriction. One possible additional benefit of the coarctation is a flattening out of the arterial pressure pulse. It is speculated that the capacity to develop a coarctation once a certain body size is reached is an ancient trait that dates at least as far back as the Early Cretaceous.

Persistence of the stapedial artery: a histopathologic study.

Persistence of the stapedial artery is a rare event. Fewer than 30 cases have been reported since the discovery of this artery in 1836. We carried out a histopathologic study on three temporal bones from two patients who had this anomaly, and were able to trace the full course of the artery. In two specimens a large stapedial artery persisted and substituted for the middle meningeal artery. In the third, a small, persistent stapedial artery ended in the arterial plexus surrounding the facial nerve. Persistence is discussed in terms of embryogenesis, developmental theories, histologic findings, and clinical significance. The material suggests that the stapedial artery can persist to varying degrees.

Morphometric investigation on osteocytes in human auditory ossicles.

An osteocyte lacunae differential count (1-lacunae with live osteocytes, 2-lacunae with degenerating osteocytes, 3-empty lacunae) was carried out on ear ossicles and clavicles from cadavers as well as on stapes removed by stapedotomy. The distance of the three types of lacunae from the vascular source was also determined by a computer-assisted light microscope. Results showed that the delayed fixation of bone from cadavers does not significantly interfere with osteocyte preservation, at least with the scope of this investigation. The results of osteocyte differential count show that the number of empty lacunae and lacunae with degenerating osteocytes: (a) is significantly higher in ear ossicles than in clavicles, (b) increases with age, (c) is higher in stapes than in incuses and mallei, (d) increases with the distance from the vascular sources in both ear ossicles and clavicles. Additionally it appeared that the process of osteocyte degeneration in ear ossicles is very rapid and widespread, over 40% of the cells being dead within the 2nd year of age. In the light of the recent literature and personal findings, which ascribe to osteocytes the function of mechanical detectors, and considering that bone remodeling occasionally occurs in ear ossicles, it is postulated that osteocyte death in these bones could be a programmed phenomenon (apoptosis?), due to which they lose the ability to react to strains and stresses and achieve the structural stability they need to perform their peculiar stereotyped function.

Preformed autologous ossicles. Experimental studies.

On the basis of earlier experimental investigations concerning autogenous bone transplants a project is going on with the view to have a patient to create his own bone graft suitable for being used in reconstructive surgery of the middle ear. This theory was tested in 6 rabbits and 6 dogs where a titanium form was installed in the proximal tibial metaphysis. The healing time was 6-8 months. After extraction histologic examination of the facsimile showed that it consisted of an outer form-giving thin layer ocal bone and a system of spongious bone surrounded by marrow with haemopoetic cells. Microradiologic analyses revealed a well mineralized trabecular bone tissue. By means of microangiography a microvascular architecture with a mature appearance could be demonstrated.

The stapedial artery in the rat. A microscopical study under normal conditions and in otitis media with effusion.

Arteria stapedialis in the rat was studied in healthy rats and in experimental otitis media with effusion (OME). The mean diameters of the vessel was calculated to approximately 460 microns (healthy) and 430 microns (otitic) (no significant difference). The tunica intima consisted of an endothelium and a well-developed elastica interna. The tunica media was composed of 2-3 layers of smooth muscle cells. The adventitia was thicker than the media and consisted of bundles of collagen fibers. The artery in the middle ear cavity was quite naked covered only with the middle ear mucosa. The stapedial artery did not appear to be involved in the production of effusion in experimentally induced OME.

Persistent stapedial artery associated with otosclerosis.

A 43-year-old female had bilateral otosclerosis with a persistent stapedial artery in the left ear which measured approximately 0.4 mm in diameter, lying on the floor of the tympanic cavity, running between the crura of the stapes and entering the facial canal. Stapedectomy was performed following section of this vessel and controlling the hemorrhage. Satisfactory hearing gain was obtained in both ears after stapedectomy.

The facial nerve coursing across the promontory with a persistent stapedial artery.

A case of congenital bilateral conductive deafness is described. Some years ago absence of the oval window was found in the left ear. Exploratory tympanotomy of the right ear revealed a large persistent stapedial artery while the facial nerve took its course from the geniculate ganglion straight down across the promontory anteriorly of the stapedial artery.

The vascular supply of the lenticular and long processes of the incus.

Previous reports of the blood supply to the lenticular and long processes of the incus have failed to consider the potential contribution of the periosteal vascular plexus. This scanning electron microscopic study of vascular foramina on the cortical bone surface demonstrates that there is no statistical difference between the numbers of vascular foramina on the lenticular and long processes, as opposed to the body and short process. These results suggest that the blood supply to this portion of the ossicular chain may not be as tenuous as has been thought.