DEVELOPMENT OF INNERVATION IN TETRAPOD LIMBS
91
of cells in the anterior half of the ventral horn declines, whereas posteriorly there has been an increase. In the anterior region the loss is largely
confined to unpigmented cells, but in the caudal half the ventral horn has
gained a new terminal zone of unpigmented, and, hence, of newly differentiated cells. We may thus conclude that in the growth and development of the ventral horn, the cranial pole is in advance of the caudal. The
question may now be asked whether this fact bears any relationship to
the growth and innervation of the limb. Nerve fibers have been present
within the limb from the earliest stages of development of the ventral
horn. It is then much more difficult to count individual nerve fibers under
the microscope than at later stages. Both in Xenopus and in Eleutherodactylus, the total number of fibers in all of the lumbar ventral roots
has been counted over the whole period of the cellular changes within
the ventral horn which have been described. At the early stage when the
number of ventral horn cells is at its maximum, the proportion of cells
that send an axon into a ventral root is about 1:4 in Xenopus and 1:5 in
Eleutherodactylus.
In both, the number of ventral root fibers rises as the
number of ventral horn cells falls, so that totals in each category reach
similar levels at the end of development (Fig. 3). In Xenopus, the increase
in ventral root fibers is largely concentrated within the period of rapid
change within ventral horn and limb. Pioneer fibers in the early limb
nerves thus become greatly outnumbered by new arrivals. The problem,
therefore, arises whether ventral root axons which are continuously entering the limb, are distributed there in any order to the developing muscles
of each segment.
Although this question cannot at present be answered directly, we have
evidence that the craniocaudal sequence of differentiation within the
ventral horn is matched by a proximodistal order of development within
the limb. The growth of the early limb bud in this manner is well established—by Saunders (1948) in the chick and by Tschumi (1957) in
Xenopus.
Observations at later stages on the progress of innervation,
which will be described, show that this order is then still maintained.
This evidence comes from study of the changes during development in
the reaction of lumbar ventral horn cells to leg amputation. This subject
has been examined in detail by Prestige (1967). In the early neuroblasts horn, the operation has little immediate effect. When cell turnover
has begun, however, amputation at once halts the recruitment of fresh
cells from the mantle layer of the cord. Furthermore, to the cells which
are then undergoing degeneration in the normal process of histogenesis
are then added many others.
91
of cells in the anterior half of the ventral horn declines, whereas posteriorly there has been an increase. In the anterior region the loss is largely
confined to unpigmented cells, but in the caudal half the ventral horn has
gained a new terminal zone of unpigmented, and, hence, of newly differentiated cells. We may thus conclude that in the growth and development of the ventral horn, the cranial pole is in advance of the caudal. The
question may now be asked whether this fact bears any relationship to
the growth and innervation of the limb. Nerve fibers have been present
within the limb from the earliest stages of development of the ventral
horn. It is then much more difficult to count individual nerve fibers under
the microscope than at later stages. Both in Xenopus and in Eleutherodactylus, the total number of fibers in all of the lumbar ventral roots
has been counted over the whole period of the cellular changes within
the ventral horn which have been described. At the early stage when the
number of ventral horn cells is at its maximum, the proportion of cells
that send an axon into a ventral root is about 1:4 in Xenopus and 1:5 in
Eleutherodactylus.
In both, the number of ventral root fibers rises as the
number of ventral horn cells falls, so that totals in each category reach
similar levels at the end of development (Fig. 3). In Xenopus, the increase
in ventral root fibers is largely concentrated within the period of rapid
change within ventral horn and limb. Pioneer fibers in the early limb
nerves thus become greatly outnumbered by new arrivals. The problem,
therefore, arises whether ventral root axons which are continuously entering the limb, are distributed there in any order to the developing muscles
of each segment.
Although this question cannot at present be answered directly, we have
evidence that the craniocaudal sequence of differentiation within the
ventral horn is matched by a proximodistal order of development within
the limb. The growth of the early limb bud in this manner is well established—by Saunders (1948) in the chick and by Tschumi (1957) in
Xenopus.
Observations at later stages on the progress of innervation,
which will be described, show that this order is then still maintained.
This evidence comes from study of the changes during development in
the reaction of lumbar ventral horn cells to leg amputation. This subject
has been examined in detail by Prestige (1967). In the early neuroblasts horn, the operation has little immediate effect. When cell turnover
has begun, however, amputation at once halts the recruitment of fresh
cells from the mantle layer of the cord. Furthermore, to the cells which
are then undergoing degeneration in the normal process of histogenesis
are then added many others.