Cerebellar gross anatomy of the African grasscutter (Thryonomys swinderianus - Temminck, 1827) during foetal and postnatal development

Neocortex neurogenesis and maturation in the African greater cane rat

BACKGROUND

Neocortex development has been extensively studied in altricial rodents such as mouse and rat. Identification of alternative animal models along the "altricial-precocial" spectrum in order to better model and understand neocortex development is warranted. The Greater cane rat (GCR, Thyronomys swinderianus) is an indigenous precocial African rodent. Although basic aspects of brain development in the GCR have been documented, detailed information on neocortex development including the occurrence and abundance of the distinct types of neural progenitor cells (NPCs) in the GCR are lacking.

METHODS

GCR embryos and fetuses were obtained from timed pregnant dams between gestation days 50-140 and their neocortex was analyzed by immunofluorescence staining using characteristic marker proteins for NPCs, neurons and glia cells. Data were compared with existing data on closely related precocial and altricial species, i.e. guinea pig and dwarf rabbit.

RESULTS

The primary sequence of neuro- and gliogenesis, and neuronal maturation is preserved in the prenatal GCR neocortex. We show that the GCR exhibits a relatively long period of cortical neurogenesis of 70 days. The subventricular zone becomes the major NPC pool during mid-end stages of neurogenesis with Pax6 + NPCs constituting the major basal progenitor subtype in the GCR neocortex. Whereas dendrite formation in the GCR cortical plate appears to initiate immediately after the onset of neurogenesis, major aspects of axon formation and maturation, and astrogenesis do not begin until mid-neurogenesis. Similar to the guinea pig, the GCR neocortex exhibits a high maturation status, containing neurons with well-developed dendrites and myelinated axons and astrocytes at birth, thus providing further evidence for the notion that a great proportion of neocortex growth and maturation in precocial mammals occurs before birth.

CONCLUSIONS

Together, this work has deepened our understanding of neocortex development of the GCR, of the timing and the cellular differences that regulate brain growth and development within the altricial-precocial spectrum and its suitability as a research model for neurodevelopmental studies. The timelines of brain development provided by this study may serve as empirical reference data and foundation in future studies in order to model and better understand neurodevelopment and associated alterations.

Gross Morphology of the Cerebrum and Brainstem of the Adult African Grasscutter (Thryonomys Swinderianus—Temminck, 1827)

In order to meet the increasing protein and income demand in Africa due to the rapid population growth, wildlife, such as the African grasscutter, is currently bred and domesticated as microlivestock. This study is one of the series on the brain morphology of this very large rodent, aimed at providing information that is lacking in the literature. Here, the gross anatomy of the cerebrum and brainstem in nine adult African grasscutters is described. The cerebral cortex was smooth, devoid of gyri and sulci, thus, placing the rodent in the lissencephalic group of mammals. However, blood vessels on the cortex created arterial and venous impressions. The cortex was asymmetrically-tapered oval in shape. The rostral and caudal colliculi were exposed through the cerebral transverse fissure. The rostro-caudal extent of the corpus callosum was from the mid-point of the frontal and parietal lobes, to a point just rostral to the occipital lobe. The rostral colliculi were grossly smaller than the caudal colliculi. The occulomotor and trochlear nerves emerged from the ventral midbrain, rostral to the pons. The pons was exceptionally large; it was pre-trigeminal. On either side of the ventral median fissure of the medulla oblongata were conspicuous pyramids. The trapezoid bodies were also conspicuous. These, and other findings, will be useful in future phylogenetic comparison of rodent brain morphology.

Morphological Characterization of the Developing Greater Cane Rat (Thryonomys swinderianus) Brain

Developmental mode along the altricial-precocial spectrum is well known to be influenced by brain development and maturation. The greater cane rat (GCR) is an indigenous precocial African rodent with uncommon phenotypes and life traits. This study was therefore designed to characterize and describe distinctive external developmental features in the prenatal GCR brain across the entire gestational length using the emergence and differentiation of external features of the brain vesicles. Four gross morphometric brain parameters (weight, length, width, and height) were evaluated and expressed as mean ± SEM. Relationship between all brain morphometrics and gestation length were analyzed using one-way ANOVA and linear regression. Developmental milestones in the prenatal GCR were then compared with closely related precocial mammals. The earliest time point with gross observable features in the prenatal GCR brain was at gestation day (GD) 60. The period with the most remarkable gross developmental features was noted between GD80 and GD100. Some of these gross features include differentiation of the cerebellar plate into vermis and lateral lobes, emergence of the piriform lobes, mammillary bodies, colliculi bodies, cerebral peduncles, and primordial pons. By GD130, most gross topographic neural features were already established. Cerebellar lobation and patterning at GD130 were the last recognizable gross developmental features noticed in the prenatal GCR brain. This coincided with the time of first eye opening in the GCR fetus. The developmental pattern observed in the prenatal GCR brain is similar to those noted in precocial rodent like the guinea pig. However, the onset of these milestones was delayed, and their duration was relatively shorter in the GCR. This study provides a frame of baseline reference of morphological brain features in the GCR embryos and fetuses that will be useful for fetal age estimation, for home grown neurodevelopmental and eco-toxicological studies, as this rodent is being proposed as a research model.