4
and (3) a cell-sparse marginal layer. His further subdivided
the inner layer into four plates: a roof plate, a fl oor plate,
and two lateral plates (alar and basal). The sulcus limitans
forms the dividing line between the two lateral plates. That
terminology is modifi ed as described below.
This Atlas keeps His’ designation of the roof and
fl oor plates because these are different cytologically and
developmentally from the thick lateral plate that contains
the neuroepithelium, a term used by Langman et al. (1966).
The neuroepithelium contains the stem cells that give rise
to neurons and glia. This Atlas modifi es His’ segregation
of the lateral plate by partitioning it into three parts rather
than two, the dorsal neuroepithelium (the source of second-
ary sensory neurons in the dorsal horn), the intermediate
neuroepithelium (the source of neurons in the intermedi-
ate gray), and ventral neuroepithelium (the source of moto-
neurons and interneurons in the ventral horn). The sulcus
limitans marks that part of the spinal canal that will per-
sist as the central canal and is surrounded by the interme-
diate neuroepithelium. His’ terms mantle and marginal
layers are not useful because developing structures should
be named according to what they will become at maturity,
not be given a different set of names during development.
That is a policy followed throughout the Atlas of Prenatal
Rat Brain Development (Altman and Bayer, 1995). In the
spinal cord, the cells that leave the neuroepithelium are
specifi c types of neurons that migrate in a variety of dif-
Introduction
ferent directions, settle in specifi c locations, and begin to
differentiate in the primordial gray matter, not the mantle
layer. Cell groups in the gray matter are named as soon as
they are recognizable; for example, cells in the early ven-
tral horn, intermediate gray, and dorsal horn appear as early
as GW4.75 (Plate 4). The cell-sparse zone beneath the pial
membrane in the developing spinal cord is the primordial
white matter, not the marginal layer. Defi nite axons appear
in the primordial white matter around GW5.25 (Plate 5)
simultaneously in the ventral and dorsal funiculi; axons
appear later in the lateral funiculus.
Two other terms are used throughout this Atlas that
refer to developing features not widely known. Sojourn
zone describes an accumulation of premigratory neurons
in the neuroepithelium; the evidence that these are aggre-
gates of postmitotic neurons rather than neuroepithelial
cells comes from short survival^3 H–thymidine autoradiog-
raphy (see Figure 3-10 in Altman and Bayer, 2001). The
dorsal root boundary cap is an aggregate of specialized
glial cells adjacent to the external wall of the spinal cord
(Altman and Bayer, 1984; see Chapter 4, Section 4.1.3
in Altman and Bayer, 2001). Boundary caps mark spots
on the pia where dorsal root fi bers penetrate the spinal
cord. Indeed, specifi c boundary caps mark nerve entry
or exit points in the developing brain, indicating that they
are important developmental structures (see Figure 62 in
Altman and Bayer, 1981).
