Anatomy and Embryology

Histologic and microradiographic analyses of 47 human mandibles from 42 fetuses and 5 neonates have been carried out to study the development of the mandibular body. This not only entails woven bone formation along Meckel's eartilage but also endochondral ossification in the condylar cartilage, the pillars of which are included in the posterior part of the mandibular body. Progressively, woven bone is replaced by lamellar bone, and typical Haversian system are already present at the fifth month of fetal life. This early occurrence of bone remodelling is related to the fact that the mandible is subjected to intense activity from sucking and swallowing.

It has become customary to distinguish between two stages in descent of the testis: first transabdominal migration of fetal testes from the posterior body wall to the inguinal region, and second, true descent into the developing cremaster sacs. The present study of rats examines the validity of the concept of transabdominal testis migration by histological analysis, between days 15 and 22 of fetal life, of the position of testes and ovaries relative to the site where, postnatally, the male cremaster sacs are to develop. The analysis revealed that rat testes do not migrate from the posterior body wall to the inguinal region during the last days of fetal life. It appeared that, during that period of fetal life, ovaries ascend in a cranio-lateral direction, together with the caudo-lateral poles of the kidneys, and are closely connected to them via the ovarian cranial suspensory ligaments. A similar ascent of testes seemed to be prevented by failure of cranial suspensory ligament growth. This failure may have occurred through the exposure of male fetuses to androgen, since cranial ligaments developed in male fetuses exposed to anti-androgen. The above results allow for the following conclusions. There is no evidence for active testis migration from the posterior abdomen towards the inguinal region during the latter part of rat fetal life. There is clear evidence of cranial migration of the the ovaries during the latter part of rat fetal life together with the ascending kidneys. Androgens are thought to be responsible for the failure of cranial testis ligament growth and, therefore, for failure of cranial testis migration in ovary-like fashion. For the proper understanding of the normal and disturbed process of testis descent, it seems preferable to eliminate the concept ‘first phase of testis descent’ from the description of this process, at least in the rat. Further work should reveal the interspecific generality of these conclusions.

We have examined the distribution of immunoreactivity for GAP-43 in the developing and adult brain of a diprotodontid metatherian, the tammar wallaby (Macropus eugenii). The distribution of GAP-43 immunoreactivity in the neonatal wallaby brain was strikingly heterogeneous, in contrast to that reported for the newborn polyprotodontid opossum. Immunoreactivity for GAP-43 in the developing wallaby brain showed a caudal-to-rostral spatiotemporal gradient, with the brainstem well in advance of the telencephalon throughout the first 100 days of postnatal life. In many regions examined, GAP-43 immunoreactivity passed through the following phases: 1. intense immunoreactivity in developing fiber tracts and occasional somata; 2. diffuse homogeneous immunoreactivity; 3. selective loss of immunoreactivity in particular nuclei or cortical regions. In the isocortex, selective loss of GAP-43 immunoreactivity in the somatosensory and visual cortex (at postnatal day 115) coincided with the maturation of the laminar distribution of terminal thalamocortical axonal fields. Within adult cortical regions, GAP-43 immunoreactivity was highest in layer I of all regions, lower layers (V and VI) of primary somatosensory and visual cortices, layers II/III of motor and cingulate cortex, and layer IV of entorhinal cortex. Our findings suggest that, while patterning of GAP-43 immunoreactivity in the mature brain is similar across meta- and eutheria, there may be early developmental differences in the distribution of GAP-43 immunoreactivity between poly- and diprotodontid metatheria.

Das Neuralrohr von 39 Std alten Hühnerembryonen zeigt einen kompakten Verband von großen Zellen. Ihre Zellmembranen verlaufen bevorzugt radiär zur Längsachse des Neuralrohrs. Dünne faserartige Zellfortsätze wurden hier bisher nicht gefunden. Das Neuralrohr von 8 Tage alten Hühnerembryonen gliedert sich in graue und weiße Substanz. Die weiße Substanz läßt regelmäßig eine äußere und innere Zone unterscheiden, die sowohl nach Osmiumfixierung als auch nach Bouin-Fixierung darstellbar ist. Beide Gewebszonen haben je nach der angewandten Fixierung und je nach der benutzten Optik ein verschiedenes Erscheinungsbild. Nach Bouin-Allen-Fixierung erscheint die äußere Zone schon bei schwachen Vergrößerung typisch zerklüftet, dagegen nach Osmiumfixierung fast homogen. Erst bei stärkerer elektronenoptischer Vergrößerung erweist sich diese Zone als dichtestes Aggregat feinster Zellfortsätze, die vorzugsweise parallel zur Längsachse des embryonalen Rückenmarks angeordnet sind. Sie haben eine auffallend gleichmäßige Dicke von ungefähr 0,2 μ. Die tiefe Schicht der weißen Substanz zeigt sehr verschieden große bis zu 2 μ dicke Zellfortsätze.