Fig. 1.

The connection of individual cells to multicellular networks is an essential prerequisite for the development of multifunctional tissues in animals, plants and fungi alike. A Giemsa–Wright combination staining of avian red blood cells (courtesy of Jennifer Owen, Michigan State University, MI, USA). B Pollen tube photomicrographs of different angiosperm species (reproduced with permission from [33]). Scale bars, 20 µm. C Unicellular, dormant fungal spores (conidia, dC) - here shown from Neurospora crassa - are the most common starting point of a fungal colony. Isotropic expansion (iC) precedes germination (gC). Conidal germlings differentiate germ tubes (GT) which extend and explore the substrate, as well as conidial anastomosis tubes (CATs) which establish cell–cell fusion connections (*). GT extension and CAT-mediated cell fusion establish an interconnected germling network in which original cells lose their individuality. Hence, the network is regarded as having a supracellular state. Adapted with permission from Ref. [121]. Scale bar, 10 μm. D Simple squamous epithelium from a frog (courtesy of Fayette Reynolds, Berkshire Community College, MI, USA). Scale bar, 200 µm. E Surface view of lower epidermis of leaf of Kalanchoe (courtesy of John Adds, Science and Plants for Schools; https://www.saps.org.uk/). F With increasing length, GTs - here shown from Neurospora crassa - mature into vegetative hyphae (VH), which branch and differentiate fusion hyphae (FH) to establish vegetative hyphal fusion (VHF) connections (*) that expand the interconnected mycelium network further. Adapted with permission from Ref. [160]. Scale bar, 100 μm. G Simple columnar epithelium of a mammalian gut (courtesy of Fayette Reynolds, Berkshire Community College, MI, USA). Scale bar, 500 µm. H Staining of a shoot apex of Coleus blumei (alias Solenostemon scutellarioides) (courtesy of John Hardy, University of Wisconsin—Stevens Point, WI, USA). The red-stained dome-shaped tip of the cone in the center of the micrograph is the apical meristem. Scale bar, 500 µm. I The interconnected vegetative mycelium forms the basis for further differentiation. Starvation conditions, for instance, trigger sexual development of female fruiting bodies (perithecia) - here shown from Sordaria macrospora - in which highly durable sexual spores (ascospores, AS) develop that can outlast unfavorable conditions. Perithecia comprise different cell and tissue types that form under the participation of cell fusion processes and thus represent some of the most complex multicellular structures that fungi are able to create. (a, asci; cp, centrum pseudoparenchyma cells; pa, paraphyses; pe, periphyses). Modified with permission from Ref. [122]. Scale bar, 25 µm