Liquid-tissue behavior and differential cohesiveness during chick limb budding

Emerging chick limb-buds at first grow only in length, not width. The growth parameters of limb mesoderm — cell shapes, distributions, division patterns and cleavage orientations — are incompatible with representations of this tissue as an elongating solid composed of proliferating but immobile cells. We observe that samples of both early limb mesoderm and also surrounding flank mesoderm round up like liquid droplets in organ culture. Therefore, liquid-like tissue rearrangments, including cell shuffling movements and neighbor exchanges, may occur in limb and flank mesoderm during in vivo limb budding. If so, differences in limb-flank surface tension properties would have to be present to keep these two fluid cell populations segregated into distinct tissues and properly positioned underneath limb and flank ectoderm. Previous studies have shown that tissue surface tensions are reflected in the spreading behavior of fused pairs of cell aggregates. To determine whether or not they possess differing surface tension properties, we pair excised pieces of early leg-bud, wing-bud or intervening flank mesoderm with pieces of 5¾-day heart or liver in hanging drop cultures. For more rapid determinations of relative liquid-tissue cohesiveness than can be obtained in conventional, long-term experiments, aggregate pairs are fixed shortly after fusion. Since partial-envelopment configurations depend upon relative aggregate sizes as well as their tissue surface tensions, new procedures are used to deduce relative aggregate cohesiveness from cross-sections of these briefly fused aggregate pairs. The envelopment tendencies of aggregates fixed 6–9 h after fusion are similar to those fixed 15–19 h after fusion: heart tends to surround leg; heart and wing surround each other with similar frequencies, but flank tends to surround heart. Also, liver tends to surround leg and wing, but flank tends to surround liver. When the effects of relative aggregate size are taken into account, these non-random, tissue-specific patterns of aggregate envelopment indicate that the relative cohesiveness of these tissues falls into the sequence: leg > heart ∼ wing > liver > flank. The in vitro behavior of early limb-bud and neighboring flank mesoderm in these studies suggests that they are not simply mechanically identical portions of a single liquid tissue. We have previously proposed that early limb-bud mesoderm may act like a non-dispersing, cohesive liquid droplet which is embedded within a less cohesive fluid layer of flank tissue (and which is molded distally into paddle-shaped conformations by solid-like limb ectoderm and/or subjacent extracellular matrix). This proposal is not only compatible with the growth parameters of limb-bud mesoderm in vivo , but is also consistent with our observation that flank mesoderm surrounds tissues which surround limb mesoderm in these aggregate-fusion-experiments. Our model suggests that differences in the surface tension properties of limb vs. flank mesoderm may combine with differential cell proliferation, and possibly with active limb ectoderm expansion, to generate initial proximodistal limb outgrowth.