In the NMRI mouse embryo, the thymus develops from the third endodermal pouch and the third ectodermal cleft. The cervical vesicle, formed not by the closure of the sinus cervicalis but by a n invagination of the dorsal segment of the ectoderm between the third and fourth branchial clefts, contributes to the formation of the thymus. The intense proliferation of the ectoderm of the third cleft on the eleventh day covers the endodermal part.The thymus is thus composed of a central endodermal region and a peripheral ectodermal region. The normal adult thymus, then, has a mixed origin, the cortical cells being ectodermal, and the medullary cells endodermal in origin.The fourth endodermal pouch gives rise to the ultimo-branchial body, which becomes entirely incorporated within the thyroid on the fourteenth day. There is no formation of a thymus IV nor of a second pair of parathyroids from the fourth endodermal pouch.In the Nude mouse embryo, the third branchial pouch and cleft, as well a s the cervical vesicle, develop normally for the first 11% days. From this point 011, lhe ectoderm of the third cleft ceases to develop further. The endoderm is, now, no longer covered by ectoderm and, deprived of its normal inducing agent, ceases to develop further. Thymic dysgenesis is thus ectodermal in origin. The fourth endodermal pouch develops normally. No development of an accessory "thymus-like" structure from the fourth pouch was observed. The dysgenetic thymus originates entirely from the third branchial pouches and clefts.In both Nude and NMRZ embryos, the parathyroids develop a t about 11% days from a very limited area in the dorsal region of the cranial wall of the third endodermal pouch between the pharyngo-branchial and ecto-branchial ducts. Morphometric analysis shows that the volume of the parathyroids is the same in both strains of mice a t each stage of development; nor does their microscopic appearance differ. Thus, mutation in the Nude mouse does not affect the development of the parathyroids from the third pouch, even though the first anomalies in the development of the thymus are observed a t the precise moment a t which the parathyroid primordium appears.In the Nude mouse mutant, described by Flanagan ('661, the thymus is lacking (Pantelouris, '68). A model of considerable interest for the study of immune responses in the absence of T lymphocytes (Pantelouris, '71; Wortis, '71, '74; Croy and Osoba '73), the Nude mouse accepts grafts from a wide variety of origins (Manning et al., '73; Leclerc et al., '75), and is widely used in the study of human tissue transplants, both normal '76; Bastert et al., '77), and malignant (Houchens and Ovejera, '78).In fact, the adult Nude mouse possesses a rudimentary epithelial thymus, composed of ciliated cysts and clusters of glands (Cordier, '74, '75; Groscurth et al., '751, but completely lacking in lymphocytes (Pantelouris and Hair, '70; Wortis et al., '71). Although a certain reduction in the number of colony-forming cells (CFU) in bone marrow of Nude mice has ...

In this study, the effects of fixation procedures, embedding medium and section thickness on stereological measurements of normal thyroid were analysed. The following conclusions were drawn: A) the use of a single section for the analysis of a lobe is sufficient if this section is located in the central part of the lobe. B) fixation and embedding with glutaraldehyde-Epon leads to a larger shrinkage than Bouin-paraplast, but the difference between the two procedures is not significant. C) osmium post-fixation reduces the shrinkage induced by glutaraldehyde and lowers the axial deformation produced by sectioning. D) Bouin's fixative and paraplast embedding induce considerable shrinkage of the interstitial tissue. The shrinkage obtained with glutaraldehyde-Epon is less. However, it is still not known whether this difference is due to the fixative, or to the embedding procedure or to both. E) only in glutaraldehyde and osmium-fixed material, embedded in Epon, can follicles and colloids be assumed to be spherical in shape without significant errors.

High concentrations of zinc have been found by a histochemical method at the sites of calcification. Zinc has been detected in the developing osteons (Haversian systems) of compact bone exactly at the border line between calcified and uncalcified tissue (Fig. 1), in the cartilaginous partitions of hypertrophic cells (Fig. 3), and in the endochondral bone which has just been deposited in the metaphysis (Fig. 4).

No abstract

Thyroid hyperplasia was induced in C3H mice by a low iodine diet feeding supplemented with propylthiouracil. The morphological modifications associated to the development of hyperplasia were analyzed at light microscopical level and the cellular proliferation was studied by autoradiography after a pulse labelling with [3H]thymidine. The initial modification during the course of hyperplasia is the development of the vascularization. It includes the dilatation of the capillaries, which occurs before any extended modification of the follicular cells and any change of the thyroid weight, and the proliferation of endothelial cells which starts earlier than that of follicular cells.Whenever produced, hyperplastic goitres are characterized by follicular cell hypertrophy and hyperplasia, but also by changes in interfollicular tissues as connective tissue and blood vessels