FINE STRUCTURE AND MORPHOGENIC MOVEMENTS IN THE GASTRULA OF THE TREEFROG, HYLA REGILLA

Abstract

The blastoporal groove of the early gastrula of the treefrog, Hyla regilla, was examined with the electron microscope. The innermost extension of the groove is lined with invaginating flask-and wedge-shaped cells of entoderm and mesoderm. The distal surfaces of these cells bear microvilli which are underlain with an electron-opaque layer composed of fine granular material and fibrils. The dense layer and masses of vesicles proximal to it fill the necks of the cells. In flask cells bordering the forming archenteron the vesicles are replaced by large vacuoles surrounded by layers of membranes. The cells lining the groove are tightly joined at their distal ends in the region of the dense layer. Proximally, the cell bodies are separated by wide intercellular spaces. The cell body, which is migrating toward the interior of the gastrula, contains the nucleus plus other organelles and inclusions common to amphibian gastrular cells. A dense layer of granular material, vesicles, and membranes lies beneath the surface of the cell body and extends into pseudopodium-like processes and surface undulations which cross the intercellular spaces. A special mesodermal cell observed in the dorsal lining of the groove is smaller and denser than the surrounding presumptive chordamesodermal cells. A long finger of cytoplasm, filled with a dense layer, vesicles and membranes, extends from its distal surface along the edge of the groove, ending in a tight interlocking with another mesodermal cell. Some correlations between fine structure and the mechanics of gastrulation are discussed, and a theory of invagination is proposed, based on contraction and expansion of the dense layer and the tight junctions at distal cell surfaces.

The Mechanics of Morphogenesis in Multicellular Embryos

“…The apex of a bottle cell may contract when an increase in intracellular calcium stimulates the mechanical activity of apical myosin and microfilaments. Microfilaments have been observed in the apices of bottle cells (Baker, 1965;Perry and Waddington, 1966). The calcium ionophore A23187 accelerates the spreading of the blastopore.…”

Shape: Its development and regulation capacity during embryogenesis

“…(a) Microtubules which show a specific distribution and orientation have been considered as a cytoskeleton (Fawcett and Witebsky, 1964;Porter, 1966). Developmentally, microtubules have been implicated in individual cell shape changes in several morphogenetic processes, such as lens placode formation (Byers and Porter, 1964), sea urchin primary mesenchyme formation Tilney and Gibbins, 1969), neurulation (Schroeder, 1970a;Burnside, 1971;Karfunkel, 1971), axon elongation of spinal ganglionic neurons (Yamada et al, 1970;, bottle cell formation during amphibian gastrulation (Baker, 1965;Perry and Waddington, 1966;Nakatsuji, 1975), etc.…”

“…(b) Microfilaments, of presumed contractile nature, appear to be essential for amphibian gastrulation (Baker, 1965;Perry, 1975), amphibian neurulation (Baker and Schroeder, 1967;Schroeder, 1970a;Burnside, 1971;Karfunkel, 1971), mouse lens invagination (Wrenn and Wessels, 1969), morphogenesis of salivary epithelium (Spooner and Wessells, 1970b;, and axon elongation in embryonic nerve cells (Yamada et al, 1970;. It seems noteworthy that microtubules and micro filaments are involved not only in the maintenance or alteration of cellular shape, but also in cellular migration (Bhisey and Freed, 1975), which represents a main feature in several morphogenetic processes, particularly during gastrulation.…”

“…(b) At the supracellular level: the demonstration of intracellular contractile systems in developing rudiments (Baker andSchroeder, 1967 Wessells et al, 1971) supports the assumption that mechanical stress in cell layers may play a principal role in morphogenetic processes (Beloussov and Dorfman, 1974). In a recent study on mechanical stress during amphibian embryonic development from the late blastula until the early tail bud stage, it was shown that periodically drastic transformations of the stress pattern occur, interspersed with periods in which the pattern of stress remains topologically constant (Beloussov et al, 1975).…”

Introductory class exercises in experimental embryology usingXenopus laevis

“…Changes in shape of invaginating mesoderm cells of the blastopore lip were claimed to be the result of differential water uptake by these cells, and the reversal of the direction of cell movement seen in exogastrulation was the result of mesoderm cells moving towards a hypertonic medium. 225 More recent work, however, suggests that the changes in cell shape are brought about by the action of contractile microfibrils within the mesoderm cells (Balinsky, 1961, Baker, 1965, and it still remains to be shown that the blastocoel is normally hypertonic to the capsular fluid. An added complication is that the invaginating cells of Xenopus gastrulae do not show the characteristic shapes described by Holtfreter (1943) in urodeles and Baker (1965) in Hyla regi//a (see Deuchar, 1975), so perhaps neither proposed mechanism operates in this case.…”