Biomedical subjects

I S Alvarez

Publications and source records attributed to I S Alvarez.

24 records · Page 2Linked to original sources

PubMed1988

Cell death in the ventral region of the neural retina during the early development of the chick embryo eye.

The present study deals with morphologic and quantitative changes that take place in the area of cell death in the ventral part of the presumptive retinal wall of the chick embryo. These changes were followed from the optic vesicle stage until the first optic fiber fascicles leave the neural retina. Our results show that both the volume occupied by the area of cell death and the density of its pyknotic fragments undergo considerable variation during the period between Hamburger and Hamilton's (1951) stages 12 to 20. In the optic vesicle stages, cell death in the ventral wall of the vesicle was observed in 50 to 75% of the embryos studied. During stages 14 and 15, this zone was seen in more than 90%. By the time invagination of the optic cup was complete, the ventral retinal zone of cell death had disappeared entirely in a large proportion of embryos; in all others, it shrank significantly both in volume and density of pyknotic fragments. In stage 19, when the first optic fiber fascicles begin to emerge from the retina, a dramatic increase occurs in the number of pyknotic fragments in the posterior pole of the retina. The appearance of dying cells, in a region shortly to be traversed by developing ganglion cell axons, supports the hypothesis that cell death processes are apparently somehow related to the creation of a suitable environment for the emergence of fibers toward the optic stalk. Densities of mitotic and interphasic cells as well as the mitotic index were determined in both the retinal zone of cell death and in areas devoid of dead cells. In all developmental stages analyzed, the mitotic index was notably lower in the former than in non-necrotic zones, suggesting that cell proliferation is partially inhibited in retinal areas of cell death.

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PubMed1987

Glioblast migration in the optic stalk of the chick embryo.

Light and electron microscopy was used to study the chick embryo optic stalk from Hamburger and Hamilton stages 21 to 29. Observations of glioblast morphology in different developmental stages suggest that these cells undergo radial migration toward peripheral regions of the stalk. Immediately previous to and during migration, morphological changes were noted in the glioblasts, including the appearance of lateral prolongations which contribute to the subdivision of optic fiber fascicles and the radial elongation of their nucleus, which gives the impression of squeezing itself into the peripheral glioblastic prolongation. These phenomena occur in a retino-diencephalic direction, commencing in the distal optic stalk during stage 23 and continuing in subsequent stages. The significance of glioblastic migration is discussed in relation to possible mechanisms through which optic fiber fascicles, initially located on the surface of the stalk, come to lie in deeper areas of the stalk wall.

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PubMed1987

Extra-axonal environment and fibre directionality in the early development of the chick embryo optic chiasm: a light and scanning electron microscopic study.

The events that occur during the early development of the optic chiasm of the chick embryo have been studied by light and scanning electron microscopy. In developmental stages previous to the arrival of the first optic fibres in the floor of the diencephalon, as well as during the arrival of the leading fibres, extracellular spaces can be seen in the diencephalon ventral wall. These spaces are defined by external cell prolongations which end in a foot-shaped formation. During stages 25 and 26 a prechiasmic degenerative centre appears in the area immediately rostral to the early chiasm, leading to a notable degree of disorganization in the diencephalon wall. This centre appears to be related to the reorganization of the system of external cell processes and extracellular spaces which become progressively more irregularly distributed, coinciding with the arrival of the first optic fibre fascicles to the midline of the floor of the diencephalon. The optic fibre fascicles change their latero-medial directionality in the medial-most regions of the ventral diencephalon, where their course becomes rostrocaudal. This reorientation of the optic fibres seems to be mediated by primitive glial cells which first appear in the ventrorostral region of the early chiasm (previously occupied by the system of external cell processes and extracellular spaces) in stage 26, increasing in number from this stage on. The morphology of the primitive glial cells is laminar in nature and the cells are seen to be densely packed together with no large extracellular spaces between them.

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PubMed1986

Differential staining of dead and dying embryonic cells with a simple new technique.

A new staining technique with toluidine blue and safranin is described which yields satisfactory results in histological sections of chick embryo tissue fixed in a mixture of glutaraldehyde and formaldehyde and embedded in Spurr's resin. In zones of embryonic cell death, this technique allows the blue-staining of dying cell pyknotic nuclei to be clearly distinguished from red-staining healthy cells. The possible factors underlying this differential staining of pyknotic nuclei are discussed.

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PubMed1985

Proliferation of glial precursors during the early development of the chick optic nerve.

The mitotic patterns and cytoarchitecture of the optic stalk were studied in the chick embryo during the period of formation of the optic cup until the elimination of the stalk lumen. Cell proliferation in the superficial regions of the stalk ventral wall is described. Superficial cell proliferation, whose beginning coincides with penetration of the earliest optic fibers, gives rise to an early glioblast plate located internally with respect to the marginal lamina of ganglion cell axon fascicles. The early glioblasts are transformed into marginal glioblasts, which undergo radial mitosis. Radially oriented division seems to favour glioblast penetration towards more internal zones of the stalk. Thus the marginal glioblasts are transformed into inner glioblasts, which continue to proliferate. Cell death in the ventral wall of the optic stalk is in close topographical relation with extracellular spaces which loosen up the consistency of the stalk tissue, favouring invasion of the ventral stalk by optic fibers and the addition of new glioblasts by proliferation of preexisting cells.

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PubMedDate not supplied

Vertical regulation of En-2 expression and eye development by FGFs and BMPs.

The formation of the midbrain region depends mainly on the activity of a signalling center located in the isthmus region, on the border between the prospective mesencephalon and metencephalon. FGF-8 has been proposed as a signalling molecule responsible for this specification because of its expression pattern and its ability to elicit duplication of the midbrain region when expressed ectopically in the neuroepithelium. Here we present evidence that members of the FGF family of growth factors when released in the cephalic mesenchyme are able to extend the expression of the mesencephalic marker En-2 to both the anterior and the posterior regions of its original landmark. This alteration in the expression pattern of En-2 is not accompanied by a significant alteration in the later development of the midbrain-cerebellar anlage, although the eye development is severely altered. Members of the bone morphogenetic protein family ectopically released from the mesenchyme down-regulate the expression of En-2 and also have an effect on the development of the eye. These results demonstrate that growth factor molecules produced in the mesenchyme (vertical signalling) participate in the correct establishment of the antero-posterior patterning of the cephalic nervous system during development.

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