Thymus. Light micrograph of a section through par of a thymic lobule, showing a densely staining cortex with many developing lymphocytes (white blood cells) and a central pale medulla, with fewer lymphocytes but many epithelial-type cells, dendritic cells, macrophages and blood vessels. The thymus is essential for normal immune function by production of T-lymphocytes (T-cells) and their release into the circulation. Magnification: x36 when printed 10 centimetres wide.