As cells grow, the size and number of their internal organelles increase in order to keep up with increased metabolic requirements. Abnormal size of organelles is a hallmark of cancer and an important aspect of diagnosis in cytopathology. Most organelles vary in either size or number, or both, as a function of cell size, but the mechanisms that create this variation remain unclear. In some cases, organelle size appears to scale with cell size through processes of relative growth, but in others the size may be set by either active measurement systems or genetic programs that instruct organelle biosynthetic activities to create organelles of a size appropriate to a given cell type.

In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.

We often think about regeneration in terms of replacing missing structures, such as organs or tissues, with new structures generated via cell proliferation and differentiation. But at a smaller scale, single cells, themselves, are capable of regenerating when part of the cell has been removed. A classic model organism that facilitates the study of cellular regeneration in the giant ciliate Stentor coeruleus. These cells, which can grow to more than a millimeter in size, have the ability to survive after extensive wounding of their surface, and are able to regenerate missing structures. Even a small piece of a cell can regenerate a whole cell with normal geometry, in a matter of hours. Such regeneration requires cells to be able to trigger organelle biogenesis in response to loss of structures. But subcellular regeneration also relies on intracellular mechanisms to create and maintain global patterning within the cell. These mechanisms are not understood, but at a conceptual level they involve processes that resemble those seen in animal development and regeneration. Here we discuss single-celled regeneration in Stentor from the viewpoint of standard regeneration paradigms in animals. For example, there is evidence that regeneration of the oral apparatus in Stentor follows a sender-receiver model similar to crustacean eyestalk regeneration. By drawing these analogies, we find that many of the concepts already known from the study of animal-scale regeneration and development can be applied to the study of regeneration at the cellular level, such as the concepts of determination, induction, mosaic vs. regulative development, and epimorphosis vs. morphallaxis. We propose that the similarities may go beyond analogy, and that some aspects of animal development and regeneration may have evolved by exploiting pre-existing subcellular developmental strategies from unicellular ancestors.

All of the same conceptual questions about size in organisms apply equally at the level of single cells. What determines the size, not only of the whole cell, but of all of its parts? What ensures that subcellular components are properly proportioned relative to the whole cell? How does alteration in organelle size affect biochemical function? Answering such fundamental questions requires us to understand how the size of individual organelles and other cellular structures is determined. Knowledge of organelle biogenesis and dynamics has advanced rapidly in recent years. Does this knowledge give us enough information to formulate reasonable models for organelle size control, or are we still missing something?

Quantitative analysis of the giant ciliate Stentor roeselii shows that a single cell can make decisions, based on the ability to switch between several different behaviors in a non-random order.

Cells are not just amorphous bags of enzymes, but precise and complex machines. With any machine, it is important that the parts be of the right size, yet our understanding of the mechanisms that control size of cellular structures remains at a rudimentary level in most cases. One problem with studying size control is that many cellular organelles have complex 3D structures that make their size hard to measure. Here we focus on linear structures within cells, for which the problem of size control reduces to the problem of length control. We compare and contrast potential mechanisms for length control to understand how cells solve simple geometry problems.

Abstract Cells are highly complex and orderly machines, with defined shapes and a startling variety of internal organizations. Complex geometry is a feature of both free-living unicellular organisms and cells inside multicellular animals. Where does the geometry of a cell come from? Many of the same questions that arise in developmental biology can also be asked of cells, but in most cases we do not know the answers. How much of cellular organization is dictated by global cell polarity cues as opposed to local interactions between cellular components? Does cellular structure persist across cell generations? What is the relationship between cell geometry and tissue organization? What ensures that intracellular structures are scaled to the overall size of the cell? Cell biology is only now beginning to come to grips with these questions.

The mechanisms that determine the shape and organization of cells remain largely unknown. Green algae such as Chlamydomonas provide excellent model systems for studying cell geometry owing to their highly reproducible cell organization. Structural and genetic studies suggest that asymmetry of the centriole (basal body) plays a critical determining role in organizing the internal organization of algal cells, through the attachment of microtubule rootlets and other large fiber systems to specific sets of microtubule triplets on the centriole. Thus to understand cell organization, it will be critical to understand how the different triplets of the centriole come to have distinct molecular identities.

The relation of organelle size to cellular function is a basic question in cell biology about which almost nothing is known. Reporting in this issue of Developmental Cell, Ferraro et al. (2014) show that the size and topology of the Golgi apparatus determines the size and functionality of a medically important secretory granule.