Cell division and differentiation in asexual and sexual development

Abstract

In ascidians the development of eggs and of buds are in sharp contrast. In bud development cell sizes are minimal throughout. Gross differentiation of form becomes apparent from the beginning, histological differentiation only when cell division is ending. The position of a cell relative to the whole determines its nature. Multiplication of cells continues until sufficient have been formed for the expression of all specific and other characters. In sexual development the egg is a large cell which divides until the minimal cell sizes characteristic of the species are obtained. The course of cleavage is a curve suggesting the attainment of a state of equlibrium. Commencing before fertilization and continuing during cleavage is a precocious differentiation of certain parts that inhibits further cell division and results in the formation of special larval structures that function when a mere fraction of the whole developmental period has elapsed. This differentiation may be suppressed, or may be retarded, without affecting the development of the rest of the egg. In the remaining parts cell division continues until minimal cell sizes are reached and only then does histological differentiation become apparent, as in asexual development. The number of cells thus formed is very small compared with that necessary for the expression of the full character of the species, and the newly functional postlarval organism is necessarily peculiar in structure.

Regulation and evolution of muscle development in tunicates

“…Coloniality evolved in the tunicates more than once [371] though perhaps repeatedly built upon an ancestral ability to regenerate body parts [42]. Colonial ascidians are thus a polyphyletic group of benthic tunicates united by their ability to reproduce through two complementary routes: sexually via gametes and embryogenesis, and asexually through blastogenesis [23]. Both processes converge on a similar end product, the zooid, or individual within a colony.…”

Early Chordate Evolution Part 2. Amphioxus and Ascidians To Settle or Not to Settle

“…An ascidian egg, such as those of Ciona, Styela and Ascidia, produces about 3400 cells altogether. Amphioxus eggs produce about 9000 cells, the final sizes being somewhat smaller at the end of the primary developmental process [1]. The comparatively smaller eggs of polychaetes and polyclads form less than one thousand.…”

DNA SYNTHESIS, MITOSIS, AND DIFFERENTIATION IN PANCREATIC ACINAR CELLS IN VITRO

“…Many discussions are available on the exclusivity of differentiation and division in embryonic cells (2,6,12,35,36), but it is only recently that critical evidence has been forthcoming on the precise stages in the cytodifferentiative sequence beyond which DNA is no longer replicated (21,27,38). Particularly pertinent is the work of Stockdale and Holtzer (32) who used tritiated thymidine to demonstrate that differentiating chick muscle cells cease incorporating the DNA-precursor before myoblasts fuse to form multinucleate myotubes or synthesize myosin.…”