Lingual papillae of the growing rat as a model of vasculogenesis

Capillary sprouting is an important mechanism that initiates neovascularization. Because observation of capillary sprouting and its morphological staging can be problematic, we sought to establish a simple model of capillary growth.

Rats were obtained at gestational days 15, 16, and 20, at birth, and at postnatal day 10. Scanning electron microscopy (SEM) of vascular casts, freeze-fractured and epithelium-exfoliated specimens, as well as transmission electron microscopy (TEM) of tissue sections were used.

In day 15 fetuses, the filiform papillae and their connective tissue cores had not been formed, but a simple capillary network without regional differences was present. In day 16 fetuses, mesenchymal cells started to form papillary connective tissue cores, and, inside the epithelium, ridges were found. Capillary sprouts arose from the preexisting sinusoidal capillaries by elongation and widening, invaded into connective tissue cores in day 20 fetuses, and gradually bifurcated to form capillary loops in the prospective giant conical papillae of the newborn rat. In postnatal day 10 rats, the capillary network beneath the papillae became bilayered.

Vascular formation in the lingual papillae in growing rats offers an easy model for the observation of capillary sprouting. In this model, the sprouts arise from preexisting sinusoidal capillaries and not from veins, as usually observed in other models. The mechanism of capillary growth is the elongation of (preexisting) sinusoidal capillaries into the developing connective tissue cores and toward the forming epithelial ridges.