Nuclear morphologies: their diversity and functional relevance

Abstract

Studies of chromosome and genome biology often focus on condensed chromatin in the form of chromosomes and neglect the non-dividing cells. Even when interphase nuclei are considered, they are often then treated as interchangeable round objects. However, different cell types can have very different nuclear shapes, and these shapes have impacts on cellular function; indeed, many pathologies are linked with alterations to nuclear shape. In this review, we describe some of the nuclear morphologies beyond the spherical and ovoid. Many of the leukocytes of the immune system have lobed nuclei, which aid their flexibility and migration; smooth muscle cells have a spindle shaped nucleus, which must deform during muscle contractions; spermatozoa have highly condensed nuclei which adopt varied shapes, potentially associated with swimming efficiency. Nuclei are not passive passengers within the cell. There are clear effects of nuclear shape on the transcriptional activity of the cell. Recent work has shown that regulation of gene expression can be influenced by nuclear morphology, and that cells can drastically remodel their chromatin during differentiation. The link between the nucleoskeleton and the cytoskeleton at the nuclear envelope provides a mechanism for transmission of mechanical forces into the nucleus, directly affecting chromatin compaction and organisation.

Keywords: Chromatin; Differentiation; Eukaryote; Gene expression; Shape.

Fig. 1

Examples of some of the human cell types mentioned in the main text. Nuclei are drawn in blue against the cytoplasm in pink. a Spherical and ovoid nuclei. b The lobed granulocyte lineage. c The lobed monocyte, and some of its differentiated macrophage stages. d Other shapes, including the polyploid megakaryocyte, fusiform fibrocyte and smooth muscle nuclei, and the condensed nucleus of a sperm

Fig. 2

A selection of sperm head morphologies from across metazoa; acrosomal regions are shaded in grey and nucleus cross-sections denoted by a dashed outline. a The typical ovate or paddle head shape seen in many mammals. b Examples of giant acrosomes (including sagittal cross-sections) and falciform hooks seen in rodents. c Atypical mammalian head shapes. d Examples of morphologies from outside mammalia, including the anomalous sperm head of the Eurasian bullfinch (Pyrrulah pyrrulah), the rounded acrosome-less sperm head of the sea bream (Sparus aurata) and the spiralling acrosome sperm head of the ‘living fossil’, Tubiluchus troglodytes—the nucleus of which also forms a remarkable double spiral in the anterior portion of the sperm head, seen here in cross-section

Fig. 3

Layers of structure impacting nuclear shape, and their functional relevance. The levels of structure within a cell are schematically shown. Different ranges of structures have different effects upon the function of the cell, and are involved in different functional roles. The shape of the nucleus is determined by the cytoskeleton, the nuclear lamina, chromatin distribution and chromatin compaction. The nucleus can be repositioned and reoriented within the cell via actin- and microtubule-based transport, while mechanical stresses on the cell transmitted to the nucleus via the cytoskeleton can affect gene expression. Invaginations of the cytoplasm into the nucleus can provide additional transport for signalling molecules and RNAs