Postnatal development of the ovarian bursa of the golden hamster (Mesocricetus auratus): Its complete closure and morphogenesis of lymphatic stomata

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Abstract
The golden hamster ovarian bursa was studied by light and electron microscopy to clarify the process of its complete closure and the development of lymphatics that leads to morphogenesis of stomata. The results were as follows. (1) The bursa completely closed at 9 days of age primarily due to development of the mesotubarium superius. (2) With the closure, the ovary and bursa became closely apposed, and most of the original bursal cavity disappeared. (3) Between 9 and 12 days of age U‐shaped folds of the bursal mesothelium began to invade the connective tissue of the bursa. (4) Widening of the internal angle of the U‐shaped folds contributed to reappearance of the bursal cavity, and thus separation of the bursa from the ovary. It also contributed to future geometrical proximity of lymphatics to the cavity of the bursa. (5) The separation of the bursa from the ovary began as early as 12 days of age in the cephalic half of the bursa. It occurred remarkably late in the caudal half. Juxtaposition of the window portion of the bursa to the ovary remained in some adult animals. (6) Development of lymphatics in the cephalic half of the bursa was divided into two stages, before and after days 21–24 of life. In the first stage, lymphatics grew in the submesothelial connective tissue, and the framework of lymphatics was formed. In the second stage, lymphatics extended small branches to form the submesothelial plexus or lymphatic lacuna. (7) Intercellular junctions between contiguous lymphatic endothelial cells were mostly tight and desmosomelike. Open junctions were, if they occurred at all, rare. (8) A smooth‐surfaced area lined with the lymphatic endothelium was found in the bursa on day 27 of life, before the initiation of ovulation. Valvelike stomal orifices were absent before the initiation of ovulation and extremely rare even after the first ovulation. They were commonly present in the bursae after the fourth ovulation, however.The process of complete closure of the ovarian bursa is very complex and may be related to the later development of the bursal mesothelium and lymphatics. Some liplike stomal orifices are of purely developmental origin. However, all valvelike stomal orifices are assumed to be formed as a result of damage to the bursal mesothelium, as well as to the submesothelial connective tissue and lymphatics, by repetition of ovulation. It is possible that liplike stomal orifices may be formed in the process of repairing the damage.