A correlative SEM/TEM examination of the endothelium surface in neural capillaries

The modifications of the endothelial surfaces were analyzed in growing neural microvessels by scanning and transmission electron microscopes in the optic tecta of chick embryos and chickens. The endothelial inner aspect appears regular and smooth in the early stages of the vessel growth (7th incubation day). Later (14th incubation day) both the abluminal and luminal surfaces of the endothelium follow a very sinuous course and the luminal ones appear extremely rich in pleomorphic microprojections. When the endothelium differentiation is concluded (5-day-old chicken), the cells are very thin and again exhibit regular and smooth surfaces. These findings reveal a great mobility of the cell membrane of the endothelial cells when they are growing longer and thinner by a moulding process. Moreover, the presence of a number of pinocytotic pits in the embryo vessels would indicate that the neutral vessels, provided with a typically low pinocytotic activity in the adult life, are engaged in this function during development.

Tight endothelial junctions in the developing microvasculature: a thin section and freeze-fracture study in the chick embryo optic tectum

The development of the interendothelial tight junctions was studied in the microvessels of the otpic tectum of chick embryos, at the 14th-16th and 18th-20th incubation day (i.d.), and in post-hatching chickens, using thin sections and freeze-fracture techniques. At the 14th-16th i.d., the junctional plasmamembranes of the endothelial cells are simply apposed or fused for brief tracts showing a pentalaminar or trilaminar configuration. In the replicas the P-faces of the fractured junctional membranes are either lacking in intramembrane particles (IMPs) and characterized by finger-like depressions, or provided with discrete IMPs aligned in rows. At the 18th-20th i.d., the thin peripheral expansions of the endothelial cells are superimposed and welded by continuous pentalaminar junctions. Their fracture P-faces display junctional strands formed by parallel fibrils of fused IMPs, with or without interconnections. In the 10-day-old chickens the junctions consist of highly complex networks of fibrils. The results have made it possible to recognize precise relationships between the features of the developing endothelial junctions in the ultrathin sections and, respectively, in the replicas. Moreover, the observations suggest that tight junction formation occurs progressively in the cerebral microvessels by processes of alignment and fusion of the IMPs, which conclude with the arrangement of fibrils in networks.

Angiogenesis under normal and pathological conditions

Angiogenesis, i.e. the generation of new blood capillaries, occurs in utero (during embryonal and fetal development) and in both physiological and pathological situations during extrauterine life. Several angiogenic factors have now been isolated, including angiogenin, acidic and basic fibroblast growth factors, and alpha and beta transforming growth factors. Their amino acid sequences have been determined and their genes cloned. Other factors await complete characterisation. An account is given of techniques used in the investigation of angiogenesis, both in vivo (transparent chambers; corneal micropockets; implantation on chick chorioallantoic membrane; employment of polymers for the sustained release of angiogenesis factors) and in vitro (cloning and long-term culture of capillary endothelial cells). The angiogenesis induced by solid tumours differs from other forms in that it is not self-limited and continues indefinitely until eradication of the tumour or death of the host. Anti-angiogenic factors have also been identified, particularly a new class of nonglucocorticoid steroids. Their employment in tumour therapy is a possibility, since neoplastic expansion is essentially dependent on angiogenesis.

[Changes in the subendothelial compartment during the maturation process of cerebral microvessels]

Basement lamina and pericytes of growing blood microvessels were analyzed in the chick embryo optic tectum, from the 8th incubation day to hatching. Formation of the basement lamina and morphological changes of the pericytes take place in a short range of time, but late in the embryonic life, when also the blood brain barrier (bbb) devices are developing. The spatial and temporal coincidence between basement lamina formation, endothelium tight junction differentiation, and perivascular arrangement of the astrocytic glia, indicates that these events are correlated and corroborates the hypothesis that the glia needs an extracellular matrix to induce the junctional system maturation in the neural endothelia. Pericytes are irregular in shape during the early neural angiogenesis and smooth and flattened later, as the basement lamina synthesis is taking place; these cells represent a second line of barrier beyond the endothelium when the bbb is immature, owing to their phagocytic and digestive properties.

[Endothelial vesicles as a transport system in the blood-brain barrier. Morphometric study during development]

The vesicles and vacuoles of the endothelia, morphological expression of endocytosis and transendothelial transport, are quite absent in the mature neural endothelia. In order to study the temporal sequence of the vesicle and vacuole modifications during the blood brain barrier (bbb) setting up, the extent of these structures was morphometrically analyzed on electron micrographs of neural microvessels in the optic tectum of 8, 14, and 17 day chick embryos, fixed after an intracardial injection of the permeability marker horseradish peroxidase. During the development, endocytosis and transendothelial transport change, since a statistically significant reduction of both vesicles and vacuoles was recorded at the 17th incubation day. The temporal coincidence between decrease of endocytosis-transport processes and appearance of astrocytic endfeet close to the vessel wall, suggests that the glial cells might control, besides the tight junction formation, the expression of other properties of the bbb-provided endothelia.

Angiogenesis induced by B-cell non-Hodgkin's lymphomas. Lack of correlation with tumor malignancy and immunologic phenotype

Correlations of malignancy grade, immunologic phenotype and angiogenic capacity were studied in B-cell non-Hodgkin's lymphomas grafted onto chorioallantoic membranes of chicken embryos. The angiogenic response elicited by the tumors was significantly greater than the response to normal lymph nodes, but it did not correlate with either the malignancy grade or the immunologic phenotype of the tumors. The elevated angiogenic capacity of neoplastic tissues is confirmed. The results also suggest that the extensive vasoproliferative response required by a rapidly growing tumor is not only controlled by the neoplastic cell population but, probably, by the host-response as well.

Computerized three-dimensional reconstruction of the developing blood-brain barrier

Computer-aided techniques for three-dimensional reconstruction of images were employed in a morphological and morphometrical study of blood-brain barrier (BBB) development in the microvasculature of the chick embryo optic tectum. The method proved very useful for understanding the spatial relationships between BBB vascular and perivascular components, and of their modifications during embryonic life. The results indicate that the perivascular astrocytic endfeet, the main periendothelial components of the BBB, appear on the 14th incubation day and form an almost continuous perivascular envelope at hatching time.

[Ultrastructure of the myocardial junctions of the chick embryo at an early stage in the organogenesis of the heart]

The ultrastructural features of the myocardiocyte junctional systems have been studied in the heart right myocardium of 6 day chick embryos in order to analyze the relationships between the spatial arrangement of the myocardial fibres and the formation of the myocardiocyte junctions. The myocardiocytes are remarkably branched, joined in small fibres to form a loose network with large intercellular spaces and appear well differentiated, rich in organelles, myofibrils and glycogen. Adherent-type junctions and well differentiated desmosomes are detectable at the level of the termino-terminal contacts between myocardiocytes, where the plasma membranes appear more or less complicately interconnected; nexus-type junctions are seen in the latero-lateral contact regions. The results indicate that already at an early stage of the heart organogenesis the myocardium intercellular junctions are similar, on the whole, to those described in the adult, so that they would not seem to be transitory or modifiable structures. The changes in the spatial orientation of the myocardial fibres, which take place after the 6th incubation day, could be allowed by the network-like arrangement of the primitive myocardium and presumably conditioned by the blood flow in the developing heart.

Right ventricular dysplasia: an ultrastructural study

Ultrastructural observations on myocardial biopsies from patients developing clinical and/or echocardiographic and haemodynamic signs of dysplasia of the right ventricle did not reveal morphological defects of the myocardiocyte contraction apparatus, but showed evidence of fibrosis of the myocardial interstitium and modifications of the structures involved in the transmission of the action potential into the cardiac cells as well as of the junctional system between them. Interstitial adipocytes and intracellular lipid droplets were not increased in number.

Ethyl-oxide effects on a newly formed blood-brain barrier

The morphofunctional effects of ethyl-oxide, utilized as anaesthetic, on the optic tectum blood-brain barrier were investigated in hatching chick embryos (21st incubation day) and young chickens in which the barrier to the marker of vascular permeability horseradish peroxidase is complete after the 18th incubation day. Results indicate that ethyl-oxide administration produces horseradish peroxidase extravasation by opening the tight interendothelial junctions. The escape rate of the marker is higher in the tecta of hatching embryos than in those of chickens and in both horseradish peroxidase extravascular diffusion is more frequent at the level of vessel bifurcation than along the branchless vessel tracts.

Morphometric analysis on the effects of hypoxia during the central nervous system vasculogenesis

A morphometric analysis was carried out on the optic tectum microvascolature in chicken embryos developing under normal and hypoxic conditions in order to evaluate the effects of an environmental O2 reduction on growing neural blood vessels. The vessel number, the vessel diameter and the distance intervening between contiguous vessels were measured. In the developing tectum under hypoxia the results evidenced a remarkable, progressive increase in vessel number, higher than in the controls; a transitory increment of the vessel diameter, which in later embryonic development returned similar to that recorded during normal tectum vasculogenesis; a value of the distance between contiguous vessels always much lower than under normal conditions. The possible significances of these morphological evidences were discussed and a capacity on the part of the growing neural vascular bed for adapting itself to a reduced oxygen concentration was noted.

The barrier systems in the choroidal plexuses of the chick embryo studied by means of horseradish peroxidase

The development of the blood-cerebrospinal fluid (CSF) and cerebrospinal fluid-blood barriers, and the differentiation of the choroidal vessel wall, have been ultrastructurally investigated in the choroidal plexuses of the lateral ventricles of 10, 15, 18 and 21 day chicken embryos, fixed in normal conditions and also after intracardial and intraventricular injection of horseradish peroxidase. In all the analyzed developmental stages the choroidal epithelial cells seem able to endocytose and degradate, in their lysosomal apparatus, the tracer molecules reaching their ventricular and basolateral sides. The intravascularly injected horseradish peroxidase can enter the ventricular cavity by interepithelial route only at 10th incubation day, when the tight intercellular junctions are not formed everywhere, while a transepithelial transport is always hindered. The marker injected into the ventricular cavity reaches the subepithelial compartment and the blood stream, at 10th incubation day by interepithelial as well as transepithelial route and, successively, by transepithelial vesicular transport alone. The differentiation processes undergone by the choroidal blood vessels consist in a progressive thinning of the endothelial cells, appearance of pores, already numerous at 15th incubation day, and formation of the endothelial basement lamina, the final event in vessel wall maturation.

Mast cells in the chick embryo choroid plexuses

The existence of mast cells in the choroid plexuses of the chick embryo lateral ventricles has been investigated by a microscopical and ultrastructural study carried out from the 9th incubation day to hatching. These cells have been seen within the perivascular connective of the choroid plexuses in the later stages of their embryonic development and are morphologically similar to those present in the connective stroma of other organs. A role of these cells in the regulation of the choroid vessel permeability as well as of the exchanges between plasma and cerebrospinal fluid (and viceversa) is suggested.

Ultrastructural features of the vasculogenetic processes in the thyroid and suprarenal glands of the chick embryo

This paper reports the results of an ultrastructural investigation on the differentiation of the blood capillary network in the thyroid and suprarenal glands of the chick embryo, from the 6th to the 21st incubation day. Apart from some morphological features of the capillary wall, which change in the same way during gland organogenesis (thickness of the endothelium, quantity and distribution of cytoplasmic organelles, pores), differences have been pointed out concerning the beginning and temporal evolution of the maturative events of the endothelium lining and the pericapillary spaces. Close relationships between morphofunctional maturation of the secretory cells and blood vessel differentiation have been emphasized, and the roles played by intraendothelial vacuoles and endothelial basement lamina during the capillarogenesis are discussed.

Effects of exogenous heparin on the vasculogenesis of the chorioallantoic membrane

On the basis of the well-known stimulating role played by heparin on blood vessel neoformation in the chorioallantoic membrane (CAM), demonstrated by means of slow-release polymers soaked in this substance and placed on CAMs, a new technique of introducing heparin directly into the allantoic sac is proposed. According to observations carried out in ovo and after fixation, morphological modifications are demonstrated in the developing vasculature of the heparin-treated CAMs which, compared with the control CAMs, show dilated and sinuous arterial and venous branches, denser and irregular capillary networks, and a high number of vascular primordia. The results, confirming that heparin is involved in angiogenesis, indicate the suitability of the technique and suggest that the effects of this substance could be enhanced by growth factors released by CAM tissues rapidly growing up from the 5th to the 7th incubation day.

Process of differentiation of cerebellar Purkinje neurons in the chick embryo

The microscopic and ultrastructural differentiation of Purkinje neurons has been studied in 40 chicken embryo cerebella, from the 10th incubation day to hatching, and the transverse diameter of the cell body measured, for each developmental stage, on 30 electron micrographs of sagittally cut Purkinje cells. The developing Purkinje cell bodies, bipolar, at first, given the presence of two processes emerging from the opposite poles of the oval perikaryon, grow progressively in size. After the 12th incubation day, they develop a branched dendritic tree, and, shortly before hatching time, the cells acquire the characteristic flask or pear-shaped configuration. On the 10th incubation day, microtubules are already detectable together with Golgi complexes and a few vesicles of rough endoplasmic reticulum; on the 14th incubation day, RER cisterns are recognizable in the supranuclear cytoplasm, later extending into the whole perikaryon, and attaining their definitive distribution by the 18th incubation day. Pinocytotic and coated vesicles, as well as subsurface cisterns are seen during the whole embryonic life. In the earliest stages of development, three distinct types of junctional contacts between Purkinje cells and surrounding axons are described, and their functional role in relation to synaptogenetic processes is discussed. Beginning with the 16th incubation day, some Purkinje neurons undergo degenerative changes similar to those described in other types of neurons of the central and peripheral nervous system.

Effects of the chronic hypoxia on the developing blood-brain barrier in the chick embryo optic tectum

The blood-brain barrier to horseradish peroxidase has been investigated microscopically and ultrastructurally in the optic tectum of hatching chicken embryos which were kept under hypoxic conditions from the 3rd incubation day. The results indicated that the weakness sites of the blood-brain barrier correspond to the tight interendothelial junctions, whose breakdown allows extravasation of the marker into limited areas of the tectum wall.

[Effects of carbaryl on the morphogenesis of the extremities in chick embryos]

Experimental studies have been carried out to evaluate the effects of carbaryl (l.-naphthyl-methyl-carbamate)- an anticholinesterase pesticide - on the morphogenesis and development of the chick embryo. A solution containing 0.5 mg of carbaryl has been injected in the allantoic sac of the embryos treated (n = 130), at the 5th or 6th incubation day. In comparison with a control group (= 70), some morphological alterations have been found in the embryos treated, at the expense of the skeleton apparatus, particularly in the most distal portions of the lower limbs, as tibial and metatarsal angulations, with their curtailment. Such effects might be related to an interference in the synthesis of NAD (nicotinamide-adenine-dinucleotide), as reported for other organophosphoric pesticides.

Microscopical and ultrastructural investigations on the development of the blood-brain barrier in the chick embryo optic tectum

The formation of a blood-brain barrier to horseradish peroxidase was microscopically and ultrastructurally investigated in the tectum opticum of the chick during development of the intraneural blood vessel network from the 6th incubation day to hatching, and in adult specimens. Extravasation of the circulating marker, apparently unimpeded during early stages of vasculogenesis, starts to diminish from the 14th incubation day (i.d.) and is prevented after the 18th i.d. The tracer seems to get out of the vessel lumina through the sites of reciprocal contact between adjacent endothelial cells, and the differentiation of tight junctions there hinders the passage of peroxidase particles. The formation of numerous endothelial vacuoles during early vasculogenesis and the setting of the blood-brain barrier are discussed in connection with the mechanisms of transendothelial transport, and respectively, the processes of moulding of the growing endothelia.

[Use of horseradish peroxidase as a marker in studies of the formation of the blood-brain barrier (optic tectum of the normal and experimentally-treated chick embryo)]

The development of the blood brain barrier (BBB) and the vessel permeability to horseradish peroxidase (HRP) have been analyzed in the optic tectum of chick embryos developed under normal and hypoxic conditions, normal chickens, and chickens born from fertilized eggs incubated under hypoxia but kept in the open air after hatching. The development of chick embryos under a situation of chronic hypoxia was obtained by covering, a half of the shell of fertilized eggs with a thick layer of melted paraffin to obtain a reduction of the exchanges normally occurring between embryonic blood vessels and open air. In the tecta developed in normal conditions the BBB to HRP begins to form on the 14th i.d. and it is complete on the 17th i.d. The O2 deprivation, producing remarkable alterations of the neural substratum, does not affect the development of the BBB to HRP, since in chicks of 17 i.d., grown up under hypoxic conditions, the tectal microvessels are not permeable to the tracer, being it mainly confined within the vessel lumina. Nevertheless in specimens kept under hypoxia until hatching, areas of perivascular spread of the marker have been observed corresponding to the vessel wall tracts presumably damaged by the experimental conditions along which the BBB to HRP is not complete.