[Histogenesis of the immune system of the "nude" mouse. II. Postnatal development of the thymus: a light microscopical study (author's transl)].

UNLABELLED: The thymus of the homozygous nude mouse embryo is laid down normally. From the 13th day post conceptionem (pc) onwards, however, a maldevelopment takes place (Groscurth und Kistler, 1974). As a result, the anterior mediastinum of the newborn nu/nu mouse contains a rudimentary epithelial organ which resembles parts of an embryonic exocrine gland. In this paper, the light microscopical findings of the postnatal development of the thymus in homo- and heterozygous nude mice are reported.

MATERIAL AND METHODS: Homo- and heterozygous nude (nu/nu and nu/+) mice with the genetic background BALB/c were kept under specific pathogen-free (spf) conditions. Three nu/nu and three nu/+ female mice were sacrificed at the following developmental stages: newborn animal, 1st, 2nd, 3rd, 6th, 9th as well as 12th week postpartum (pp). The organs were fixed in Bouin's solution and embedded in paraffin wax. Serial sections (5 mu) were stained with hematoxylin and eosin.

RESULTS AND DISCUSSION: The bilobed thymus rudiment of the newborn nu/nu mouse consists of branching ducts. From the 2nd week post partum (pp) onwards, their epithelial cells proliferate. At the same time, some of these cells begin to differentiate into serous and mucous cells. In some parts of the rudiment, they also form thyroid-like follicles. The exocrine cells secrete their products into the lumina of the ducts. As there is no connection between the thymus rudiment and either the epidermis or the digestive tract, an accumulation of secretory products takes place, resulting in a dilatation of the whole duct system. Therefore, the thymus rudiment of the 12-week-old nu/nu mouse consists of a number of communicating cysts which are lined by a flattened epithelium. Cysts lined by an epithelium can also be observed within the thymus of the nu/+ mouse. Two types must be distinguished..