| Definition: |
Hematopoietic organ of the larva, located along the dorsal vessel, behind the brain. It produces the plasmatocytes, crystal cells and lamellocytes which are released at the end of the larval instar and onset of metamorphosis. It is composed of 3 pairs of bilateral lobes: an anterior primary lobe, a posterior secondary lobe and the most posterior lobe (or multiple lobes), the tertiary lobe. Each lobe is separated by pericardial cells. In the late embryo, the lymph gland consists of a single pair of lobes (primary lobes) containing around 20 cells each that flank the dorsal vessel. By the second larval instar, the lymph gland is composed of two or three new pairs of posterior lobes, and the primary lobes have increased in size to contain around 200 cells. During the first and early second larval instars, the lymph gland comprises only the hematopoietic progenitor population. The lymph gland increases in size ten-fold by the late third instar, and at this stage the progenitor cells (prohemocytes) become restricted to the medial region (medullary zone) of the primary lobe and become quiescent. The differentiated hemocytes are found in the periphery (cortical zone) of the primary lobe and these cells proliferate extensively. The posterior signalling center is a group of about 30 cells at the posterior tip of the primary lobe that secretes several signalling molecules and functions as a stem-cell niche. The secondary and tertiary lobes contain prohemocytes, and rarely a differentiated cell. Lymph glands are eliminated at metamorphosis. |