Zur Fr�hentwicklung der herzeigenen Strombahn. Elektronemikroskopische Untersuchung an der Ratte

Development of the coronary arteries and cardiac veins in the dogfish (Scyliorhinus canicula)

The development of the coronary system in the Dogfish was studied using light microscopy. The sample examined consisted of 44 embryos and four newborns. The chronology of events occurring during the process was referred to the total length (TL) of the specimens. The nourishment of the developing myocardium first takes place by means of intertrabecular sinusoids. This system is later switched to a circulation through coronary vessels. The cardiac veins develop earlier than the coronary arteries. The earliest evidence of development of heart vessels in the present sample was the appearance of a diverticulum from the sinus venosus in three embryos of 31 mm TL. This diverticulum outlined the future coronary sinus. Both the atrioventricular and conoventricular venous rings were completely developed in an embryo of 36 mm TL. Coronary artery vessels appeared for the first time in embryos of 40 mm TL. In these specimens, two arteries arose from the midventral hypobranchial artery and divide to give rise to the four coronary artery conal trunks. In a 51-mm TL embryo it was already possible to follow the course of the coronary arteries, from the hypobranchial artery to the conoventricular groove. All main coronary vessels were fully developed in embryos of more than 58 mm TL. However, the arteries supplying the atrium were firstly recorded in a newborn of 77 mm TL. Birth usually occurs when the shark reaches about 72-76 mm TL.

The fine structure of the developing human choriocapillaris during the first trimester

In human embryos with a gestational age ranging from 6.5 to 13 weeks the development of the choriocapillaris was studied by electron microscopy. A continuous maturation process was observed over the whole period, which was most pronounced between weeks 6.5 and 9.7. During the earlier weeks, the endothelium appeared to be rather thick and contained many cytoplasmic vesicles; it was markedly flattened and less vesiculated than at week 9.7. This process was accompanied by an enlargement in the vascular lumina. Fenestrations directed towards the primitive Bruch's layer were found to be already present at week 7.2 but increased in number from week 9.7 to week 13. In the early weeks, basement membrane structures were present in small, discontinuous tufts of cloudy material adjacent to the endothelium. These structures became continuous at week 9.7 and increased in thickness towards week 13. In the earlier weeks, punctate junctions were seen. Beginning with week 9.7, adultlike tight junctions between the endothelial cells became observable. Pericytes were already present at week 6.5. It is suggested that both pericytes and endothelial cells are derived from the differentiating mesenchymal cells of the surrounding ocular stroma.

Functional maturation of the capillary wall in the fetal and neonatal rat heart: permeability characteristics of developing myocardial capillaries

The development of the functional components of the myocardial capillary wall was characterized by time-course studies of transendothelial transport of intravascularly injected probes of graded size from 16 days of gestation in the fetal rat to seven days postpartum. Despite the morphological changes occurring in the developing endothelial cells, the interaction of the probes was similar throughout the developmental period studied. The carbon particles were retained within the capillary lumina without any association with interendothelial junctions or with plasmalemmal vesicles. Carbon also was associated with coated vesicles. In contrast to carbon, ferritin was localized sequentially, over 60 sec of circulation, in plasmalemmal vesicles on the lumenal surface, in the cytoplasm, and on the ablumenal surface of the endothelial cells as well as in the interstitial space. Ferritin was located also in coated pits and vesicles and, after 90 sec of circulation, in multivesicular bodies. Within 30 sec of circulation, reaction product of myoglobin was located in plasmalemmal vesicles, coated vesicles, and transendothelial cell channels. Also within 30 sec, myoglobin partially filled the interendothelial space from the capillary lumina to the level of the tight junction. At all developmental ages studied, the interendothelial cell junctions appeared structurally tight and were impermeable to all of the probes. Once ferritin or myoglobin had reached the ablumenal space, the basal lamina did not appear to restrain the passage of the probes. Plasmalemmal vesicles are the capillary structures which transendothelially transport ferritin and myoglobin in developing myocardial capillaries.

Maturation of myocardial capillaries in the fetal and neonatal rat: an ultrastructural study with a morphometric analysis of the vesicle populations

The ultrastructural morphology of the cellular and extracellular components of the developing myocardial capillary wall--from the 16-day-gestation fetus of the rat to the 21-day neonate--was examined. A morphometric analysis of plasmalemmal vesicles and of coated vesicles and pits of capillary endothelial cells was performed during the same developmental period. As the lateral extensions of the capillary endothelial cells change from irregular to regular in their thickness during development, there is an increase in the number of plasmalemmal vesicles and a progression from clusters of plasmalemmal vesicles to a uniform distribution in the endothelial cell. The ratio of vesicles which are open to the luminal front, which are "free" in the cytoplasm, or which are open to the abluminal front of the endothelial cell was consistent throughout development. The numerical density of plasmalemmal vesicles demonstrates a gradual and significant increase. In contrast, the numbers of coated vesicles and pits are variable within a very narrow range, and no pattern of increase or decrease is discernible during development. Similarly, there is no change in interendothelial cell junctions, which consist of occluding and primitive adhesive junctional types, during development. The lamina densa of the basal lamina gradually develops from discontinuous, patchy densities along the abluminal surface of the endothelial cells to a continuous and distinct layer by 21 days gestation. The presence of the proteoglycan species in the developing basal lamina was assessed with the cationic dye ruthenium red (RR), and the appearance of RR-marked proteoglycans was found to parallel the appearance of lamina densa material. found to parallel the appearance of lamina densa material. The RR sites appear discontinuously in patches; and later, the RR sites appear in a continuous and regular planar lattice in the lamina rara interna and externa at 21 days gestation. A complete array of RR-stainable anionic sites outside a continuous lamina densa near birth indicates that the basal laminae of developing capillaries in the heart are morphologically, and in part biochemically, mature by the end of the first neonatal week. Our results show that the endothelial cells and the subtending basal lamina of myocardial capillaries gradually mature morphologically during the final days of gestation and the first neonatal week.(ABSTRACT TRUNCATED AT 400 WORDS)

TEM-studies on development and canalization of the dorsal aorta in the chick embryo

Transmission electron microscopic investigations were carried out on the early development of the dorsal aorta in chick embryos during the period ranging from the 6-somite stage to the 16-somite stage. Special attention was paid to the formation of the vascular lumen. The initial angioblasts tended to aggregate into a cluster with a narrow intercellular space. Only a few punctate thickenings of the plasma membrane were found at junctional sites of the angioblasts. The vascular cavity appeared to be produced by expansion of the intercellular space and possibly also by enclosure of the extracellular space with attenuated vasoformative cells. The wall of the early vessels so formed was frequently observed to be open to the extravascular space through clefts originating probably from incomplete closure of the early endothelium. No specific ultrastructural features characterizing the angioblasts or early endothelial cells were recognized.

Relations between development of the capillary wall and myoarchitecture of the rat heart

The type of the blood supply to the myocardium appears to be closely related to its structural arrangement. The heart of adult poikilotherm animals is either entirely spongious, supplied from the ventricular cavity or its spongious musculature is covered by an outer compact layer with vascular supply. The size of the compact layer increases with increasing heart weight. The changes in the heart size during the ontogenetic development of honoiotherms are accompanied by the gradual transformation of the vascularless spongious musculature into a compact myocardium supplied thrugh coronary vessels. Up to the development of coronary arteries (in the rat up to the 17th day of embryonic life - ed) the myocardium is entirely spongious and supplied from mentricular cavity. Two types of primitive vascular bed are characteristic for this period: a) intertrabecular spaces, which penetrate deep into the ventricular wall as direct continuation of the endocardium, and b) intramyocardial clefts without endothelial lining. During further development of the terminal mascular bed, the outgrowth of endothelial cells into the myocardial clefts is important. The first capillaries with closed endothelial wall can be observed on the 18th ed. At this time various developmental stages o the terminal blood bed can be observed simultaneously. Within the following period (20-21 ed) the thick capillary walls become narrow and pericytes occur. The process of differentiation spreads in both ventricles and in septum from the base to the cardiac apex and is practically finished by the 14th day of postnatal life. The longitudinal orientations of myofibres starts between the 20th and 22nd ed. The final arrangement of muscle cells and capillaries into three layers (outer and inner longitudinal, central circular) is terminated during the second postnatal week.