Tissue from the central nervous system of 11 adult SpragueDawley rats fixed by vascular perfusion with buffered aldehydes was used in a study of the fine structure of the pia mater. Surface tissue from the cerebral and cerebellar cortices, ventral pons, olfactory bulb, tuber cinereum, spinal cord, superior colliculus, optic chiasm, and median longitudinal fissure was examined. Variations in pial composition occurred within each of these selected areas although a basic pattern prevailed throughout.
The formation of the chordae tendineae of the left atrioventricular valve in the chick embryo is described using scanning electron microscopy. These supportive structures for the valve cusps develop between days 6 and 13 of incubation. Elevations which represent the primitive papillary muscles form on the ventricular wall. These elevations bifurcate into thin, web-like folds which are attached to the primitive valve cusps. The folds are the primordia of the chordae tendineae. Linear ridges develop on the web between the cusp and papillary muscle. These ridges alternate with depressions. The depressions become perforate to create the individual chorda from the linear ridges. Multiple perforations form initially but they typically consolidate to create one large aperture between two chordae. Some interchordal connections of tissue do persist throughout the period studied. During the period of perforation, prominent rounded cells are typical of the endocardium between the chordae. These cells are similar at the scanning electron microscope level to those present in the formation of the foramina secunda of the atrial septum. Primary, secondary, and tertiary chordae tendineae appear to develop in the same manner. First order chordae (those attached at the free margin of a cusp) are not found in the chick embryo. The majority of the chordae are second order, which insert into the ventricular surface of the cusp a short distance from the free edge. These chordae typically have a horizontal banding or grooving along their length. Third order chordae which extend from the papillary muscle to the ventricular wall are also present. It is suggested that chordal development is a programmed cellular and hemodynamic event.
Pleuroperitoneal canal development and closure were studied with light microscopy and transmission and scanning electron microscopy in 12.75- to 16-day fetuses. The major chronological events described in this paper are 1) the caudal tips of the lung buds projecting to the pleuroperitoneal canal (12.75 through 13.50 days); 2) the caudal tips of the lungs becoming situated medial to the canal areas at 14 days; and 3) both canals becoming crescent shaped with a uniform diameter until closure. Concurrently, the developing diaphragm and associated pleuroperitoneal folds assume more caudal positions. Both canal regions are bordered by the liver, lung, gonadal ridge, and suprarenal glands. In addition, on the left side, the stomach and mesogastrium also border the early canal. The right canal closes before the left (right, 14.75-15 days; left, 15-15.25 days). The results suggest that the pleuroperitoneal folds are pushed together, thereby closing the canals. This may be accomplished by one or a combination of the following: 1) enlargement of the liver pushing the ventral fold dorsad and a molding of the liver to the dorsal body wall caudal to the canal; 2) liver and thorax enlargement which appears to pull the dorsal fold taut against the central fold; and 3) a change in the orientation of the canal near the time of closure. Each canal is fully closed by the mergence of the dorsal and ventral fold mesothelia and mesenchyme. This study provides a basis for relating pleuroperitoneal canal development and closure to the surrounding organs and tissues.
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