The early stages of vascularizations of the spinal cord of the mouse were studied by graphic reconstruction techniques and electron microscopy. Vascular sprouts arise from the perineural vascular plexus (PNVP) to invade the cord of 10-day embryos. These enter the cord most frequently via the lateral surface between the dorsal root and the ventral root; less frequently, they enter via the ventral and/or dorsal surfaces and anastomose with sprouts that have entered via the lateral surface. During the development of intramedullary blood vessels there are essential changes both in the basal laminae covering the neural parenchyma of the cord and in the relationship between the neural tissue and vascular walls. The basal laminae of the developing spinal cord were classified into three categories. The first is the perineural, external, or primary neural, basal lamina (PNBL), which is the earliest of the three in formation and covers the entire external surface of the cord. The second one is the internal, or secondary neural, basal lamina (INBL), which invests the internal surface of the neural tissue facing the walls of invading blood vessels. The third type is the perivascular basal lamina (PVBL), which surrounds the vascular wall. Blood vessels enter the spinal cord by penetrating the PNBL. Since the PVBL and INBL are absent or incomplete in early stages of vascularization, the neural tissue is in direct contact with intramedullary blood vessels. However, following their development, boundary membranes are formed, separating the neural tissue from neighboring vessels, a situation characteristic of capillaries in the mature CNS. Perivascular spaces are seen along the course of developing vessels and secondarily become continuous with the extramedullary connective tissue space. They are neither artifact nor intramedullary extensions of extramedullary connective tissue space along invading sprouts. The boundary membranes are formed by connection of membrane plaques or by fusion of the INBL and PVBL.
Serial transverse and horizontal sections of the tail of the 26-day larval lamprey, Lampetra japonica, were observed by light and electron microscopy. The axial structures in the tail of the larval lamprey seem to differentiate from the prospective materials derived individually from the tail bud. The latter consists of two closely adjoined cell populations (C1 and C2). C1 is a small cell cluster located posterior to the other group (C2) and consists of loosely arranged polymorphous cells. The cell cluster extends cranially as a cell sheet on the ventral surface of C2; somites differentiate from this cell sheet. C2 is composed of cells elongated mediolaterally and stacked horizontally to form a compact cell mass which is covered on each lateral surface by a basal lamina. The upper one-third of C2 seems to differentiate into the neural tube, anteceding other axial structures. The middle one-third of C2 seems to develop into the notochord, and the lower one-third into the subchord and the undefined cell cord. The central canal develops in the upper one-third of C2 through the following events: 1) appearance of cilia and a small cavity between adjoining cells; 2) appearance of microvilli in the cavity, in addition to cilia; and 3) development of junctional complexes along the luminal borders of cells surrounding the cavity. Together with these events, cells surrounding the cavity increase in number, acquiring apicobasal polarity and radial arrangement. The cavity itself enlarges by incorporation of periciliary clefts and fusion of cavities with each other to establish the central canal. Near the caudal end of the neural tube, the central canal is directly confluent with the connective-tissue space through an opening in the dorsal wall of the neural tube.
Development of the mouse spinal nerves was studied. On E11 (11th day of gestation), the primitive spinal nerve fascicle extended ventrally in the anterior half of the sclerotome. Spinal nerves in the forelimb region united with each other to form the primitive brachial plexus. Their terminal segment was covered by a peculiar cell mass. On E12, five primary branches developed along the primitive spinal nerve trunk. The ramus dorsalis was originally a cutaneous nerve, supplying two series of branches to the skin of the back. The medial series was derived from the dorsal ramus of C2-C8, and the lateral series from C8 and the more caudal dorsal rami. Nerves of the former series took the presegmental course through the intermyotomic space, while those of the latter the postsegmental course. The ramus cutaneous lateralis was a nerve that took the presegmental course to become cutaneous. The ramus intercostalis externus was a muscle branch whose distribution was restricted within the segment. The ramus anterior was a muscle branch from the end of the primitive spinal nerve trunk. The ramus visceralis connected a thoracic nerve with the para-aortic sympathetic cell cord. On E13-16 the ramus anterior secondarily gave off a cutaneous branch (ramus cutaneous anterior). The ramus intercostalis externus extended ventrally deep to the intercostalis externus muscle, crossing just caudal to the ramus cutaneous lateralis that secondarily gave off branches to the obliquus externus abdominis muscle.
Intersections between the coronary veins (CV) and arteries (CA) of 103 adult human hearts were mapped on the heart surface. Then the correlations of these intersection patterns to their localization were studied. Eight spots were selected where one of four major CV (anterior cardiac vein, middle cardiac vein, left posterior ventricular vein, and great cardiac vein) intersected with one of CA and their branches (right coronary artery, posterior interventricular branch, left posterior ventricular branch, circumflex branch, diagonal branch, and anterior interventricular branch). The great cardiac vein (GCV) ran beneath the anterior interventricular branch in 56 specimens out of 103, beneath the diagonal branch in 75 specimens out of 103, and beneath the circumflex branch in 36 specimens out of 103, while the other CV mostly ran over CA. The present observations suggest that the CV on the right side may be formed prior to CA, while the CV on the left side may be formed simultaneously with CA.
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