[Histogenesis of the immune system of the "nude" mouse. Postnatal development of the thymus: a light microscopical...

Mice homozygous for the mutation "nude" display a dysgenetic thymus (Pantelouris, 1968). The anterior mediastinum of these animals contains a rudimentary epithelial organ (pantelouris and Hair, 1970), which has been assumed to represent not a maldeveloped thymus, but rather a remnant of the parathyroid gland or of the ultimobranchial body (Wortis et al., 1971). We have investigated the development of the thymus in homo- and heterozygous nude by both light and electron microscopy. This paper describes the light microscopical findings in embryos and fetuses from the 11th up to the 18th day post conceptionem (pc).

Homo- and heterozygous nude (nu/nu + nu/+) mice with the genetic background BALB/c were kept under specific pathogen-free (spf) conditions. Virgin nu/+ females were mated with nu/nu males. In addition, homozygous females were mated with homozygous males in order to investigate 11-day-old homozygous embryos. The embryos and fetuses were fixed in Bouin's solution and embedded in paraffin wax. Serial frontal sections (5mu) were stained with hematoxylin and eosin.

Up to the 12th day pc, the thymus anlage of the n/nu embryo is indistinguishable from that of the nu/+ animal. The paired anlage develops from the epithelium of the ventral and dorsal diverticles of the 3rd pharyngeal pouch, and from the ectodermal vesicula cervicalis. In the nu/+ embryo, the thymic epithelium begins to proliferate between the 12th day and 13th day pc. In the central part of the anlage, the epithelial cells dissociate and begin to form a reticulum. At the same time, blood vessels from the surrounding mesenchyme invade the anlage. From the 14th day onwards, lymphoblasts are recognizable..