Development of thymus, parathyroids, and ultimo‐branchial bodies in NMRI and nude mice

Abstract

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 an 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 as the cervical vesicle, develop normally for the first 11½ days. From this point on, the 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 NMRI embryos, the parathyroids develop at about 11½ 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 at 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 at the precise moment at which the parathyroid primordium appears.