An electronmicroscopic study of developing amphibian ectoderm

Summary

• 1. Small pieces of ectoderm were excised from gastrulae, neurulae, and tailbud embryos ofXenopus laevis andTriturus alpestris, preserved inLehmann's fixatives, sectioned at 0.025–0.75μ, and photographed with a Trüb-Täuber electron microscope.

• 2. The following features characterize the early gastrular ectoderm: endoplasmic reticulum coarse, vesicular, and predominantly loose; mitochondria mostly globular and irregular; lipoid droplets and yolk-platelets with investing plasma membranes; pigment granules ofTriturns, but not ofXenopus, composed of subunits; nuclei polymorphic, especially inTriturus, with deep infoldings of nuclear membrane; cells frequently connected only by cytoplasmic bridges which may be anastomosed, cells otherwise separated by spaces or canals.

• 3. Presumptive medullary plate from the very early neurula shows certain differences from the above features: endoplasmic and nuclear reticula considerably more dense; cytoplasmic vesicles, fibers, and granules more delicate; mitochondria smooth and rod-like with increased number of cristae; intercellular spaces less prominent, except between presumptive neural plate and chordamesoderm, the cytoplasmic processes of which may anastomose.

• 4. Presumptive epidermis of the very early neurula shows a wide meshed fibrous reticulum and distally situated mitochondria, foreshadowing the development of highly differentiated outer zones in the epidermal cells of the tailbud embryo.

• 5. Mitochondria of neural cells of the late tailbud embryo are predominantly perinuclear, quite elongate, relatively narrow, and possess many thin longitudinally or diagonally placed cristae.

• 6. Mitochondria of the tailbud epidermis are shorter and thicker than those of the neural tube cells, exhibit pores and swollen tubules, and occur in large numbers below a distal zone of dense cytoplasmic reticulum and secretory vesicles, which seem to form in waves and to discharge periodically their secretion to the surface of the skin.

• 7. The concept of mitochondrial “differentiation” is further developed and the function of epidermal mitochondria and the importance of intercellular contacts, especially in relation to neural induction, are discussed.