The roots of certain cranial and spinal nerves are particularly
suitable for studying the growth, myellnation and maturation of
motoneuron fibres. Furthermore, their rootlets can be readily traced on
serial sections in both PtJS and CHS. Accordingly, they were used for a
series of morphometric studies analysing the developmental changes
invloved in the segregation, growth, myellnation and maturation of
central and peripheral segments of large and small motoneuron fibres
(1-9). These included analysis of age changes in the internodal
morphology of the entire myelin sheath of both the Schwann cell and the
oligodendrocyte. This permitted comparison of the myelinogenic
responses to stimuli emerging the same axon bundles, both within and
between the two classes of ensheathing cell (5,6,8,9). Extensive
statistical analysis was performed on ttie myelin sheath thickness - axon
calibre relationship for the developing internodeB of large and small
calibre motoneuron fibres (1,8,9). To test if this relationship was
constant among all motoneuron fibre bundles, its setting was compared
between nerves of similar developmental origin, peripheral distribution
and function (10). The extent to which it varied between different
levels along one and the same fibre bundle was also studied using
dorsolateral vagal rootlets (14).
A series of morphometric studies was performed on the developing
CNS-PNS transitional zones of a number of rat cranial and spinal nerves
to analyse the complex interaction, migration, and sometimes
intermingling, of central and peripheral nervous tissues during this
process (16-25). These contribute to understanding the wide variety of
forms of the mature transitional zones (15,16,20-30), whether these lie
at, peripheral to, or central to the plane of the CNS surface to which
the nerve roots or rootlets which contain them are attached. CNS-PHS
transitional node development was examined to assess the manner in which
oligodendrocytes and Schwann cells modify their developmental behaviour
in response to their unique relationship to one another in this location
(20). These studies also examined the behaviour of myelinated and
unmyelinated fibres as these pass between the CNS and PNS, whether they
cross the interface between them once only (16-21, 24, 29), or alternate
a number of times between the two tissue compartments (28). The
morphology, Including myelin-axon relationships, of isolated islands of
CNS tissue in the PNS (31) and of PNS tissue in the CNS (14, 28) was
also examined quantitatively. All of these studies on the transitional
zone, as well as one experimental study in particular (34) also dealt
with its functional, especially mechanical properties.
Extensive morphometric investigations were carried out to determine
the maturationa1 changes in axons, myelin sheaths and the relationships
between these, in rat mixed peripheral nerves (a) under control
conditions (35) and (b) in animals in which diabetes mellitus had been
experimentally induced (36).
Following an earlier investigation (11) a detailed study was
made of the age-related decrease in the degree of branching of rat
cervical ventral root axons from birtti to adulthood, to determine the
relative contributions of axon loss and Schwann cell proliferation to
axon segregation (12). In another numerical study (13), the size of the
autonomic component of the oculomotor nerve was estimated.
Other developmental studies concerned macrophages related to developing
ventral roots (32) and axon-glial relationships in the developing
vomeronasal nerve (33).
