Comparisons of capillary maturation in control and hypothyroid rat spinal cord: an ultrastructural study

Quantitative and qualitative features of capillary maturation were examined in the ventral horn of the lumbar spinal cord of control and neonatally induced hypothyroid rats from birth to 6 weeks, using light (LM) and electron microscopy (EM). Quantification of the capillary densities by LM in the control animals and their hypothyroid litter mates have shown three- and twofold increases, respectively, from birth to Postnatal (Pn) Day 21. The following features were observed in the control animals at the EM level: (a) The newborn animals showed varying degrees of capillary maturation; (b) The majority of capillaries possessed mature characteristics by Pn Day 21; (c) By Pn Day 42, mature characteristics were found in nearly all capillary profiles. The hypothyroid animals demonstrated: (1) reactive perivascular cells and astrocytes; (2) delayed appearance of glycogen in the early Pn period and its persistence in extensive amounts during the latter part (3-6 weeks) of development; (3) cytoplasmic extensions of endothelial cells and perivascular cells, and (4) the presence of mitotic endothelial cells and perivascular cells even during the latter period of development. The observations suggest that the peak period of vasculogenesis in the lumbar spinal cord of the normal rat occurs during the second and third Pn weeks. The results from the hypothyroid rats point toward a delay in development and maturation of capillaries resulting in a hypoplastic vascular bed of the ventral horn. The reactive cells and the accumulation of glycogen particles could be morphological expressions of biochemical changes in hypothyroidism during the critical period of CNS development.