Chronic placental insufficiency in the fetal guinea pig affects neurochemical and neuroglial development but not neuronal numbers in the brainstem: A new method for combined stereology and immunohistochemistry

Abstract

This study has examined the development of the brainstem in a suboptimal intrauterine environment induced via chronic placental insufficiency in the fetal guinea pig. Placental insufficiency was produced by unilateral ligation of the maternal uterine artery at mid‐gestation (term = 66–68 days) resulting in the production of growth‐retarded fetuses that are chronically hypoxic and malnourished. The structural and neurochemical development of brainstem nuclei either directly or indirectly related to cardiorespiratory control were analysed by using new stereological methods and immunohistochemistry. A technique was devised to enable the procedures to be performed on alternate frozen sections. There were no significant differences between control and growth‐retarded fetuses in the total number of neurons, area of neuronal somata or volume of the hypoglossal nucleus. Quantitative densitometry was used to measure immunohistochemical staining in the brainstem of growth‐retarded fetuses compared to controls and revealed a significant (P < 0.02) decrease in substance P(SP)‐immunoreactivity in the spinal trigeminal nucleus and a significant (P < 0.05) increase in met‐enkephalin‐immunoreactivity in the hypoglossal nucleus. Counts of stained neurons demonstrated a significant increase in the density of SP‐positive neurons in the nucleus tractus solitarius (P < 0.05) and of met‐enkephalin‐positive neurons in the ventral medullary reticular formation (P < 0.05). There was also a proliferation of astrocytes, as determined by immunoreactivity to glial fibrillary acidic protein in the dorsal motor nucleus of the vagus, nucleus tractus solitarius and more generally around blood vessels throughout the brainstem. Thus, these results have shown that although chronic intrauterine deprivation does not alter neuronal numbers, at least in the hypoglossal nucleus, there is a proliferation of astrocytes, and the expression of neurotransmitters/neuromodulators is markedly effected in some of the nuclei involved with cardiorespiratory control. J. Comp. Neurol. 379:99‐112, 1997.