The hatching of an embryo from its egg is a crucial point in its development. This is the moment that decides whether the animal will be able to emerge and survive or whether it will remain trapped and die. Cephalopods usually produce enzymes in a system of glands known as the Hoyle organ, which weakens the chorion and enables hatching. In addition to this chemical approach, four cephalopod genera develop a terminal spine to further support the hatching process.
The presence of such a spine has been mentioned for the well-known cephalopod species Euprymna scolopes. However, little is known about either its structure or its cellular details. Our present results indicate that the spine consists purely of musculature, and no intermediate connective tissue was observed. No nerves were detected within the spine, although nerve fibers are present within the surrounding epithelium and dermal muscle layer, indicating that the spine stretching is controlled by this dermal musculature.
Our data show that the spine of Euprymna is similar in its composition to those of the other three cephalopod genera in which a spine has evolved. However, we found morphological differences in the spine tip and in a collagenous layer located between the spine and the normal epithelium. It remains unclear why two completely different but synergetic systems for hatching, the glandular Hoyle organ and a spine, exist in some, but not all, cephalopod genera.