Development of anencephaly and its variants

Abstract

Extreme variants of anencephaly in two human embryos of the same stage, namely 22 (54 days), shed new light on problems such as craniocerebral interrelationships and the timing of developmental events. Embryo X had a chondrocranium that possessed features typical of a holoacranial anencephalic skull and an extremely well-preserved brain, in which some of the neural tracts were comparable to those in a normal control. On the other hand, embryo Y of the same stage had a completely degenerated brain, although the chondrocranium was more nearly normal and represented the precursor of a meroacranial skull. A comparison of the two cases seems to indicate a certain independence between skull and brain. Moreover, it appears possible that the disturbances are related primarily to the skeletal, and only secondarily to the nervous, component. Comparisons with experimental data allow the conclusion that the maldevelopment involves mostly paraxial mesenchyme and little or no disturbance of neural crest. The timing of the mesenchymal defect is probably as early as stages 8 and 9 (18-20 days). This is also the time at which mesenchymal defects can result in failure of the neural tube to close.

Neuro-Osteology

“…(Kbaer et al, 1994b). This provides some support to the conclusion of Muller and O'Rahilly (1991) that the maldevelopment in anencephaly involves mainly the para-axial mesenchyme and little or no disturbance of the neural crest. It is remarkable that the multilocular ethmoid cartilage malformation was observed behind an apparently normally developed surface ectoderm of the face (Kjaer et al, 1994b).…”

“…10, II). Various types of malformation, formerly described by Muller and O'Rahilly (1991), were seen, such as cleft vertebral bodies, butterfly-shaped bodies, or complete median clefting of vertebral bodies. In the clivus region of the cranial base, a narrowed, completely cleft or short, blunted basilar part of the occipital bone was seen.…”

Primary Craniovertebral Anomalies and the Hindbrain Herniation Syndrome (Chiari I): Data Base Analysis

“…This involves both the osseous as well as the neural structures. It is more than likely that an insult to both may occur between the 4th and 7th week of intrauterine life resulting in the combina tion of anomalies [4,[30][31][32][33][34]. A careful look at the embryology and development of the craniocervical region makes it obvious that anomalies in this region would con sist of failures of segmentation, failures at fusion of differ ent components of each bone, or hypoplasia and ankylo sis.…”

Development and differentiation of pituitary cells

“…The lack of close relationships between the adenohypophysis and hypothalamus disturbed the further development of the pituitary gland as observed in human embryos with mediobasal prosencephalic defects (Mü ller and O'Rahilly, 1989). However, the disturbance of pituitary development in human anencephalic fetuses (Bégeot et al, 1977;Dubois and Dubois, 1974;Hagen and McNeilly, 1975) could be the result of a secondary phenomenon since the degenerative process inducing anencephaly occurs later than the appearance of the pituitary gland (Mü ller and O'Rahilly, 1991). It is worth noting that the future adenohypophysis is located close to nasal as well as hypothalamic presumptive territories (Fig.…”

“…However, pituitary response to adenohypophyseal regulatory factors varies according to the individual cases (Grasso et al, 1980;Hayek et al, 1973;Kaplan et al, 1976), thus obscuring any possible interpretation. Moreover, uncertainty about the developmental pattern of anencephaly prevents us from using this cerebral malformation as a model to study interrelationships between the floor of the brain and the adenohypophyseal gland at the early stages of development (Mü ller and O'Rahilly, 1991;Warkany, 1971). Attempts to mimic the conditions of human anencephaly have been made using encephalectomy in rat fetuses (Jost et al, 1970).…”