这个词是什么意思？
A single, unpaired primary lymphoid organ situated in the MEDIASTINUM, extending superiorly into the neck to the lower edge of the THYROID GLAND and inferiorly to the fourth costal cartilage. It is necessary for normal development of immunologic function early in life. By puberty, it begins to involute and much of the tissue is replaced by fat.
中文名称索引；定义为 MeSH 英文原文，尚未逐条译审。MESH:D013950 · MeSH 2026。
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