Biomedical subjects
Publications and source records attributed to Y Michaeli.
A detailed anatomic and histologic description of the cheek teeth of the rabbit, both deciduous and permanent, is presented, together with the chronology of their pre- and postnatal development. The deciduous teeth are of limited growth, while the permanent ones grow continuously. Certain aspects of cementogenesis in these teeth are discussed.
A method is described, which is applicable to cell renewal systems with an anatomical structure in which all cell locations may be uniquely mapped. Its use is demonstrated on the rat incisor inner enamel epithelium, which forms a one cell thick column in the sagittally sectioned tooth. Cells born in the apical part of the column migrate toward the distal end of the tooth, where they mature. As the cells migrate along the column, they traverse the various cell cycle phases. The present study has been designed to estimate the probability of a cell being in a given phase; all cells touching the basement membrane were numbered, and the number of cells separating any two cells was taken as a measure of distance. Since generally all cells move in one direction (lateral cell migration may occur), it is possible to solve the problem with the aid of functions describing the renewal counting stochastic process in which cell distance serves as an independent variable. The method predicts labelled cell and mitotic rates which agree with those estimated in the usual way. It was then utilized to estimate the fraction of cells in G2.
An attempt is presented to extract cell kinetic information from histomorphological features. It is applicable to rapidly proliferating tissues like the intestinal epithelium. Each replicating tissue has an origin where cells are formed and a periphery toward which cells migrate. The migration path along which they move is denominated as tissue radius on which all cell positions are mapped. Cell migration on the radius is associated with cell proliferation at tissue origin. Each mitosis there is associated with the displacement of all cells distal to it by one cell position. The more mitoses positioned between a cell and tissue origin, the greater its migration velocity. It is possible therefore to derive the cell migration velocity v(x) from the cumulative mitotic distribution on the radius, N(x). v(x) = N(x)/tm (tm = mitotic time). In this form v(x) represents also cell production at any point on the radius and may serve for the computation of other cell kinetic parameters like generation time. These arguments are illustrated on the rat incisor tooth inner enamel epithelium which has been studied in the normal and rapidly erupting tooth.
Thirty male albino rats weighing 200 g. each were separated into two groups. 15 rats received 0.2% 3-amino propionitrile fumarate (BAPN) in drinking water. 15 rats served as controls. Within three weeks eruption rate diminished by 50%. On the 22nd experimental day each rat received one dose of 1 microCi/g. body weight tritiated thymidine (1.9 Ci/mM). 5 rats of each group were then killed after 1 hour; 1 week and 2 weeks. The PDL cell migration rate was evaluated autoradiographically. It diminishes proportionally with the eruption rate. In the BAPN treated animals, the mean total periodontal ligament (PDL) cell count is 55.5% higher than that of the control.
74 rat incisor odontogenic organs were autotransplanted intradermally into the auricle. The animals were killed in groups of four up to eight months after grafting. Out of 61 grafts examined from the third week onward, 46% exhibited a typical odontogenic organ or a well differentiated tooth with pulp, vessels and nerve fibers. In 33% only scattered epithelial islands were formed. 21% of the grafts ended as osteodentin. Following the initial trauma leading to aseptic necrosis, the most primitive odontogenic organ cells survive to generate a new tooth. The grafted odontogenic organ resumes its existence as a complex structure which mobilizes its own vascular and nervous supply from the host dermis as evident from nerve fibers which penetrated the tooth pulp. Since all original nerve fibers of the graft had to be disconnected from their cell bodies, they obviously degenerated. Thus any nerve fiber observed in the regenerating tooth pulp had to be of dermal origin.
The inner enamel epithelium (IEE) covers the labial tooth aspect as a one cell layer which, when cut sagittally, appears as a longitudinal cell column extending from the tooth origin toward the periphery. Following sudden tooth shortening, the IEE responds by an increased cell production which later declines below normal values. The perturbation affects all cell kinetic parameters; the progenitor compartment, which initially increases, diminishes in size toward end of the experiment. The cell cycle transition times, which initially decline, rise toward the end of the experiment. The mean normal daily cell production rate of 70 cell % (i.e. 70 cells are produced by 100 progenitors) increases to 111 cell % and then declines to a low of 51 cell %. The IEE response typifies the behavior of other cell renewal systems such as intestinal epithelium and epidermis.
Torsion dystonia in 13-year-old girl progressed to severe opisthotonus and dystonic posturing of the limbs, accompanied by severe psychic changes, cachexia and calcification around joints. Drug therapy was ineffective. Ventrolateral thalamotomy and chlorpromazine brought about a delayed but remarkable amelioration of all symptoms and signs, still present 4 years later.
The odontogenic organ of 10 male albino rats weighing 200 g has been exposed by a longitudinal incision through the masseter muscle. Its site could be identified as a bluish elevation of the mandible. Into this region through the bone, a 2 X 2 mm. tinfoil has been inserted, deep into the odontogenic organ. The tinfoil direction was generally parallel or slightly oblique to the tooth longitudinal axis. The animals were killed after 1 month. Their lower incisors in the bony sockets were fixed and decalcified, cut sagittally and processed histologically. In 3 sockets two separate teeth could be identified, a labial and a lingual tooth. Generally only the labial tooth was covered by enamel, in one case however both teeth exhibited a regular enamel layer. Both teeth were separated by a periodontal ligament and exhibited separate tooth cavities. This experiment demonstrates the capability of the odontogenic organ to reproduce a whole new complex organ, the tooth. A phenomenon hitherto observed only in Urodela in which a severed limb regenerates from primitive pluripotential stem cells known as blastema. Contrary to the newt however, the incisor blastema reproduces continuously throughout the life of the individual and responds to the tinfoil stimulus by a production of an extra tooth.
In order to examine the aging of dentin and cementum, seventy-two impacted human canines, nonfunctional and not affected by external factors, such as attrition, abrasion, caries, etc., were surgically extracted. Ground sections were prepared and the areas of sclerotic dentin and cementum were measured with the aid of a planimeter and expressed as a percentage of the total root area. It was found that there is a significant increase in sclerotic dentin and apposition of cementum with an increase in age.
Under conditions of high impediment, the rate of eruption of the rat incisor decreases considerably as does the rate of cell production of the inner enamel epithelium (IEE). The latter is acomplished by a decrease in the size of the proliferative compartment and an increase in the generation time. A linear relation exists between IEE cell production and tooth eruption, and cell production can be estimated from eruption measurements.
A histologic and anatomic investigation of the symphyseal region in rabbits did not reveal a bony fusion between the two halves of the mandible; these two bones are united in the anterior part by a synchrondrosis, and a definite histologic suture with interdigitating bony rugosities and interposed connective tissue, in the posterior part. Functionally, the two parts are immobile and thus constitute one bone.
Crowning of all four rat incisors slows down their eruption rate, leading to a diminution of cell production by the inner enamel epithelium (IEE). The production rate by 100 progenitor cells is dominated as production rate %. The normal IEE in rats weighing 200 g produces daily 67 cells %. In the crowned incisor this rate drops to 15 cells % while during accelerated eruption rate and IEE cell production rate remains unchanged and an eruption of 6-4 mum reflects a production of 1 cell % by the IEE. A method which relates labelled cell displacement along the IEE with its cell production rate is described and its applicability to other cell renewal systems discussed.
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