DEVELOPMENTAL BIOLOGY AND NEUROGENESIS IN THE PRENATAL AFRICAN GREATER CANE RAT (Thryonomys swinderianus, Temminck 1827)

Abstract

The African Greater Cane Rat (AGCR) is a precocial hystricomorph rodent largely restricted to the African ecosystem. The drive for home-grown animal research models within the African context has increased studies on the biology and development of this rodent. However, information on prenatal development of this rodent is currently scarce. This study was designed to describe the sequence of morphogenetic developmental milestones in the developing AGCR embryos/foetuses, and to elucidate the prenatal corticogenesis of the developing AGCR brain. Nineteen timed primi-gravid AGCR does from Gestation Day (GD) 10 to 140 (every 10 days) were used. Sonographic examinations were conducted to determine time-based prenatal parameters. The AGCR were subsequently anaesthetised, dissected and their embryos/foetuses (n = 54) explanted. Embryos/foetuses were then staged based on morphogenetic characteristics of prenatal development in mammals with Carnegie and Štӗrba systems. Brains were harvested for gross morphological description and evaluation of morphometric parameters (height, length and width). Brain samples were processed using immunofluorescence biomarkers to determine the pattern, onset, duration and peak of neurogenesis (Pax6, Tbr1, Tbr2, NF, HuCD, MAP2), gliogenesis (GFAP, Olig2) and myelinogenesis (MBP) in the prenatal AGCR. Quantitative analysis of individual neural progenitor cells and Olig2+ cells, as well as radial thickness of the ventricular zones were evaluated. Data were analysed using one-way ANOVA and linear regression at α0.05. The earliest detectable sonographic feature of pregnancy in the AGCR was the gestational sac at GD20, while at GD 50 the first gross indication of an embryo was seen Staging of the AGCR embryos/foetuses revealed a relatively longer period of embryogenesis in the AGCR compared to humans and other precocial rodents like the guinea pig and agouti. Grossly, gyrencephalization of the neocortex was first noticed by GD90 and continued till GD130. Lobation patterning of the cerebellum was the last distinct gross developmental feature noticed in the prenatal AGCR brain at GD130. Brain height (0.67±0.03 to 1.35±0.07 cm), length (0.88±0.02 to 2.85±0.07 cm) and width (0.56±0.04 to 2.02±0.04 cm) increased significantly from GD60 to GD140. There was a positive correlation between gestational length and each brain parameters measured (height: r = 0.58; length: r = 0.86 and width: r = 0.87). The period of pureneural stem cell proliferation in the developing AGCR was identified at GD50. By GD60, the radial thickness of the ventricular zone had reached its maximum depth, while the peak period of neurogenesis was at GD80. Axonal and dendritic sprouting had begun by GD80 and this progressed until birth. Deep and upper layers of the neocortex were established in an “inside-out” manner by GD120 and GD130, respectively. Active gliogenesis spanned GD110 - 130. Although, oligodendrocyte progenitor cell proliferation had started by GD80, myelinogenesis did not begin earlier than GD120. The sequences of morphogenetic developmental milestones and prenatal corticogenesis in the African Greater Cane Rat were consistent with the precocity of central nervous system development in the guinea pig. The African Greater Cane Rat is therefore suitable as a research model for neurodevelopmental studies.