The thyroid gland of vertebrates is considered to be homologous to the endostyle of non-vertebrate chordates (cephalochordates, urochordates), a key character for understanding the origin and evolution of the chordate body plan. In lampreys, the larval endostyle transforms into an adult thyroid gland during metamorphosis, reflecting evolutionary changes that occurred in the vertebrate lineage. Focussing on thyroid-like cells in the endostyle, we here relate morphologically visible steps of lamprey (Lampetra fluviatilis) endostyle differentiation to embryonic stages and determine the onset of thyroid-like function. Analysing lamprey endostyle development using semi-thin histological sections, immunohistochemical detection of thyroid hormone, and the molecular marker thyroid transcription factor1 (Ttf1) refines our current view of the homology between endostyle and thyroid gland. In contrast to earlier literature, we find that a duct always persists to connect the endostyle lumen to the pharynx, a structure that resembles the thyroglossal duct in thyroid development and could further support the homology between endostyle and thyroid. Before the onset of thyroid-like function, Ttf1 expression becomes restricted to the ventral part of the endostyle, on the one hand showing that dorsal thyroid-like cells produce thyroid hormone in the absence of Ttf1, and on the other suggesting that Ttf1 was initially involved in specifying ventral fates in the endostyle.

SummaryThe course of the cortical reaction in the Platynereis dumerilii egg is described from live observation and from sectioned fixed material and is found to differ in several aspects from the course of cortical reactions in better-known systems. Cortical granules are unusually numerous. They are discharged by exocytosis during a period of about 25 min following fertilisation (18°C). Most of the surplus membrane material brought to the egg surface by exocytosis is set free into the perivitelline space. Swelling of egg jelly precursor secreted by cortical granule exocytosis may be causal for the detachment of the vitelline envelope from the egg cell surface which, however, remains attached punctately to the vitelline envelope by about 30000 microvilli. Under the strain of the distending vitelline envelope, the bases of the microvilli move and line up, pulling the cell surface into a network of ridges. The grooves in between the ridges are the sites of exocytoses. Cytochalasin B, generally destabilising actin filaments, induces rupture of the microvilli and exaggerated distension of the vitelline envelope during the cortical reaction. In a final phase of the cortical reaction the vitelline envelope wrinkles and falls back onto the egg cell surface, the microvilli shorten and the egg cell transiently becomes deformed by local contractions. The cortical reaction in the nereid egg is discussed as a process of distortion and reorganisation of the egg cortex and plasmalemma. The abundance of cortical granules accommodating egg jelly precursor in the Platynereis oocyte is attributed to the mode of so-called diffuse oogenesis characteristic of nereids, i.e. of differentiation of oocytes freely suspended in the coelomic fluid. In nereids, egg jelly therefore forms after fertilisation as opposed to ovulation.

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