Development of intraparotid lymph nodes with special reference to entrapment of the parotid acini in human fetuses

Lymph nodes within the parotid gland (PG) are known as 1) an origin of Warthin's tumors and 2) a metastatic focus of nearby cancers. This study aimed to investigate factors that provide the specific features.

Using histological sections from 28 human fetuses at approximately 15-31 weeks, we examined developing acini, ducts and lymph vessels (LVs) in the PG and compared the results with the other exocrine glands.

In the PG at 15-16 weeks, developing acini were embedded in a meshwork of LVs that corresponds to a major lymphatic route from the posterolateral head to the jugular lymph sac. Moreover, lymphocyte aggregations (=initial lymph node) protruded into LVs and some aggregations surrounded acini and ducts. In contrast, without entering the exocrine pancreas, the LVs and aggregations extended along and beneath the peritoneum. The submandibular and lacrimal glands contained a few LVs and, the sublingual gland and pancreas did not. Interstitial tissues between acini were loose in the PG and lacrimal gland, whereas they were relatively tight in the submandibular and sublingual glands and tightest between clusters of the pancreatic acini and endocrine tissue. The entrapped parotid acini appeared to maintain the basal lamina but, after 26 weeks, lymphocytes contacted acinar cells. The encapsulation occurred much later than nodal development in all those exocrine glands.

The PG at 3-4 months of gestation was characterized by active lymph node development accelerated by the highest density of intraglandular LVs. These LVs might drain the physiological nuchal translucency in fetuses.