Two‐step consumption of yolk granules during the development of quail embryos

Abstract

The mechanism of yolk consumption was studied morphologically and biochemically in Japanese quail Coturnix japonica . The amount of yolk granules in the yolk (or 'yolk cell') decreased in two steps during embryonic development. In the first step, during days 0-4 of incubation, the yolk-granule weight decreased at a rate of 13 mg/day. This decrease was due to segregation by endodermal cells that were newly formed in the developing yolk sac. In the second step after day 6, the decrease was drastic at a rate of 29.8 mg/day during days 6-12 and very slow thereafter. The decrease at the second step was due to the enzymatic digestion of yolk granules by cathepsin D that coexisted in yolk spheres. This digesting reaction was triggered by the solubilization of the granules with high concentrations of salts that were supplied after disruption of the limiting membrane of yolk spheres. The 'yolk cell' seemed to die around day 5 of incubation. Thus the digestion products might be taken up together with yolk lipids by endocytosis into the endodermal cells and transported to blood vessels.

Protease activity in the egg yolk during the development of Meleagris gallopavo (Galliformes: Phasianidae) embryos

“…During this period, the limiting membranes of the yolk spheres were lost (Yoshizaki et al 2004). As a consequence, the yolk granules and the aspartic protease cathepsin D, initially found inside the spheres, became mixed with the surrounding medium, which was high in solute concentration (Ito et al 2003;Yoshizaki et al 2004). Under these conditions, cathepsin D started the digestion of yolk granules at a high rate; however, the rate of decrease in yolk-granule weight became slow after day 12 (Yoshizaki et al 2004).…”

“…During this period, the limiting membranes of the yolk spheres were lost (Yoshizaki et al 2004). As a consequence, the yolk granules and the aspartic protease cathepsin D, initially found inside the spheres, became mixed with the surrounding medium, which was high in solute concentration (Ito et al 2003;Yoshizaki et al 2004).…”

“…In the Japanese quail Coturnix japonica, it was shown that a drastic reduction in the weight of yolk granules occurred between incubation days 6 and 12 (Yoshizaki et al 2004). During this period, the limiting membranes of the yolk spheres were lost (Yoshizaki et al 2004).…”

“…In contrast, in quail, the activation of acidic proteases preceded the major transfer of egg white into the yolk, which occurred after day 12 (Yoshizaki et al 2002(Yoshizaki et al , 2004Ito et al 2003). The activated proteases appeared to digest the egg white and yolk proteins before their transfer into the growing embryo (Sugimoto & Yamada 1986;Yoshizaki et al 2002Yoshizaki et al , 2004.…”

“…The egg yolk consists of nearly 50% water, 33% fat, 15% protein, less than 1% carbohydrate, and the rest of various minerals and vitamins (Shenstone 1968;Vieira 2007). Egg white is ultimately transferred into the yolk before being consumed by the developing embryo (Saito et al 1965;Saito & Martin 1966;Baintner & Fehér 1974;Sugimoto et al 1978Sugimoto et al , 1984Sugimoto et al , 1989Sugimoto et al , 1999Burley & Vadehra 1989;Yoshizaki et al 2002Yoshizaki et al , 2004Ito et al 2003;Shbailat & Safi 2015). In chicken, the major transfer of egg white into the yolk through the amniotic sac and intestinal lumen of the embryo occurred after day 12 (McIndoe 1960;Saito et al 1965;Saito & Martin 1966;Carinci & Manzoli-Guidotti 1968;Baintner & Fehér 1974;Burley & Vadehra 1989;Sugimoto et al 1989).…”