Ultrastructural study on cell differentiation of the rabbit carotid body.

The development of the carotid body in both fetuses from 13 to 30 days of gestation and newborn rabbits was studied by light and electron microscopy. The carotid body anlage first appeared as a cellular aggregation close to the ventral wall of the third branchial artery on the 14th day of gestation. Type-I cells were recognized as such by the presence of dense-cored vesicles in the cytoplasm on the 16th day of gestation, whereas cells destined to develop into Type-II cells became clearly distinguishable by their typical relationship to the Type-I cells on the 20th day of gestation. Afferent and efferent synapses as well as reciprocal-like synapses between Type-I cells and nerve endings were also observed in perinatal fetuses. Although cell differentiation was almost finished by birth, Type-I cells and their innervation did not seem to be fully matured. Thus, it was concluded that ultrastructurally, the rabbit carotid body during the fetal and perinatal periods was relatively inactive.