Helvik J V, Oppen-Berntsen D O, Walther B T
Department of Biochemistry, University of Bergen, Norway.
Int J Dev Biol. 1991 Mar;35(1):9-16.
In general fish larvae emerge from the protective egg after secreting a hatching enzyme (HE) from diffusely located hatching gland cells (HGCs). This proteolytic enzyme is distributed over the entire inner part of the eggshell (zona radiata). In a marine flatfish halibut, (Hippoglossus hippoglossus), we have found a more specialized hatching process. A strategic location of the HGCs in a narrow belt on the anterior part of the yolk sac leads to restricted degradation of the eggshell resulting in cleavage of the eggshell into two distinct rigid parts. This hatching process--termed "rim-hatching"--results in an empty eggshell with a lid approximately 1/4 the size of the bottom shell. During the hatching process the yolk sac is reshaped. The posterior part of the yolk sac contracts and the yolk mass is squeezed forward before hatching. This mechanism ensures close contact between the HGCs and the eggshell during the release of the hatching enzyme, which is a prerequisite for restricted degradation of the eggshell.
一般来说，鱼类幼虫在从分散分布的孵化腺细胞（HGCs）分泌出孵化酶（HE）后，会从保护性的卵中孵化出来。这种蛋白水解酶分布在整个卵壳（放射带）的内部。在一种海洋比目鱼——大比目鱼（Hippoglossus hippoglossus）中，我们发现了一种更为特殊的孵化过程。HGCs在卵黄囊前部的一条狭窄带中的战略位置，导致卵壳的降解受到限制，从而使卵壳分裂成两个不同的刚性部分。这种孵化过程——称为“边缘孵化”——会产生一个空的卵壳，其盖子的大小约为底部壳的1/4。在孵化过程中，卵黄囊会重新塑形。卵黄囊的后部收缩，卵黄物质在孵化前被向前挤压。这种机制确保了在孵化酶释放过程中HGCs与卵壳之间的紧密接触，这是卵壳受限降解的先决条件。
