Compartmentalization of Human Thymic Medulla: Facts and Hypotheses

*Ildiko Bodi, Krisztina H.-Minko, Zsolt Prodan and Imre Olah*

## **Abstract**

The thymus function was discovered in the middle of the last century. The role of the thymus in the adaptive immune system facilitated its histological and experimental studies. Before the role of the thymus was discovered, the thymus was called as a gland on the basis of lobulation; even some histological textbook listed it up among the endocrine glands. In addition to the cortex and medulla, the immunohistochemical studies revealed a further compartmentalization in the thymic medulla, which is related to the epithelium-free areas (EFA) and keratin-positive network (KPN). The two medullary compartments have different cellularity that determines their role. This chapter would concentrate on the medullary compartmentalization and their cellularity. Furthermore, this chapter discusses the relationship of thymic septae with the perivascular space, the vascular embedding and thymic dendritic cells.

**Keywords:** medulla, epithelium-free area, keratin-positive network, dendritic cells

### **1. Introduction**

In the histological textbooks, the main morphological landmarks of the thymus are (1) the lobulation—like the glands; (2) the darkly and lightly stained cortex and medulla, respectively; and (3) the Hassall's bodies. The relatively simple structure of the thymus did not help to "decode" the enigmatic thymic function until the middle of the last century. The three morphological landmarks may be supplemented by the keratin-negative area (KNA) [1, 2] or "epithelium-free area" (EFA) [3–6]. EFA can be found in both the cortex and medulla [6–8]. In the cortex, the EFA is a nest of double-positive (CD4+, CD8+) T lymphocytes [4] and various macrophages [7]. Others suggest that the cortical EFA is a pathological phenomenon [3, 6].

If the KNA/EFA is a permanent compartment of the human thymic medulla, then it should be added to the general histological features of the thymus of the warm-blooded vertebrates.
