The dendritic organization of the human spinal cord: the dorsal horn.

The human spinal cord was studied with the Golgi method from 26 weeks gestational age onto adult life. Impregnated neurons were analyzed morphometrically by an adaptation of Sholl's concentric circle method in order to specify dendritic geometry, ramification richness and branching pattern. Neurons were classified according to Rexed's laminar scheme, identified on adjacent Nisslstained sections. The following features were found to be characteristic of the dorsal horn laminae. Lamina I is formed of a uniform population of large, poorly ramified neurons. Their main dendritic domaine is disk-shaped in the tangential plane with a mean diameter of about 800 micrometers. Orthogonal, spiny dendrites penetrating into lamina II are numerous even in adult material. Four cell types, all lying in the sagittal plane, are found in lamina II. (1) "Islet cells' (about 30% of impregnated neurons) have a rostrocaudal, cylindrical dendritic domaine with a long axis of +/- 600 micrometers, a few scattered spines and a richly branched axonal network, confined to the dendritic territory. They are more numerous in the central part of lamina II. A few islet cells have two axons. (2) "Filamentous cells' (about 20%), so called because of their multiple filiform, spiny dendrites, are vertico-sagittally oriented. Their soma is located in inner or outer lamina II, their dendritic tree, trapezoïd of about 280 micrometers in height, being dorsal or ventral. The axon emits some collaterals in the vicinity of the dendritic tree, then it penetrates into lamina I or Lissauer's tract. (3) "Curly cells' (about 10%) have a complex, twisted, spine-rich dendritic tree. The dendritic domaine can be schematized by a sagittal disk with a diameter of about 200 micrometers, the cell body being eccentric. Curly cells are mainly found in outer lamina II. Their axon penetrates into lamina I or Lissauer's tract. (4) "Stellate cells' (about 40%) are multipolar neurons preferentially found in inner lamina II. They have straight, spine-poor dendrites, which cover a large (longest diameter ca. 500 micrometers), elliptical territory extending into laminae I and III. Their axon gives longitudinal collaterals to lamina II before penetrating deeply into laminae III and IV. Lamina III contains a mixed population of "antenna-like neurons' with a vertical, cone-shaped dendritic domaine and "radiate cells' characterized by a small, spherical territory. All lamina IV neurons are medium or large sized "antenna-like neurons' whose dorsally oriented, cone-shaped dendritic domaine may have a height of 1000 micrometers. It can be concluded that the dorsal horn of the human spinal cord has several distinct dendroarchitectonic features, different from those reported in animals. The possible functional implication of some dendritic features is examined and a laminar dendroarchitectonic scheme of the human cord is proposed as a morphological tool for future neuroanatomical and neuropathological studies.