EARLY NEUROMERIC DISTRIBUTION OF TYROSINE-HYDROXYLASE-IMMUNOREACTIVE NEURONS IN HUMAN EMBRYOS

A segmental mapping of brain tyrosine-hydroxylase-immunoreactive (TH-I R) neurons in human embryos between 4.5 and 6 weeks of gestation locat es with novel precision the dorsoventral and anteroposterior topograph y of the catecholamine-synthetizing primordia relative to neuromeric u nits. The data support the following conclusions. (1) All transverse s ectors of the brain (prosomeres in the forebrain, midbrain, rhombomere s in the hindbrain, spinal cord) produce TH-IR neuronal populations. ( 2) Each segment shows peculiarities in its contribution to the catecho lamine system, but there are some overall regularities, which reflect that some TH-IR populations develop similarly in different segments. ( 3) Dorsoventral topology of the TH-IR neurons indicates that at least four separate longitudinal zones (in the floor and basal plates and tw ice in the alar plate) found across most segments are capable of produ cing the TH-IR phenotype. (4) Basal plate TH-IR neurons tend to migrat e intrasegmentally to a ventrolateral superficial position, although s ome remain periventricular; those in the brainstem are related to moto neurons of the oculomotor and branchiomotor nuclei. (5) Some alar TH-I R populations migrate superficially within the segmental boundaries. ( 6) Most catecholaminergic anatomical entities are formed as fusions of smaller segmental components, each of which show similar histogenetic patterns. A nomenclature is proposed that partly adheres to previous terminology but introduces the distinction of embryologically differen t cell populations and unifies longitudinally analogous entities. Such a model, as presented in the present study, is convenient for resolvi ng problems of homology of the catecholamine system across the diversi ty of vertebrate forms. (C) 1998 Wiley-Liss, Inc.