Advances in the specific fluorescent labeling of chromatin in fixed and living 
human cells in combination with three-dimensional (3D) and 4D (space plus time) 
fluorescence microscopy and image analysis have opened the way for detailed 
studies of the dynamic, higher-order architecture of chromatin in the human cell 
nucleus and its potential role in gene regulation. Several features of this 
architecture are now well established: 1. Chromosomes occupy distinct 
territories in the cell nucleus with preferred nuclear locations, although there 
is no evidence of a rigid suprachromosomal order. 2. Chromosome territories 
(CTs) in turn contain distinct chromosome arm domains and smaller chromatin foci 
or domains with diameters of some 300 to 800 nm and a DNA content in the order 
of 1 Mbp. 3. Gene-dense, early-replicating and gene-poor, 
middle-to-late-replicating chromatin domains exhibit different higher-order 
nuclear patterns that persist through all stages of interphase. In mitotic 
chromosomes early replicating chromatin domains give rise to Giemsa light bands, 
whereas middle-to-late-replicating domains form Giemsa dark bands and C-bands. 
In an attempt to integrate these experimental data into a unified view of the 
functional nuclear architecture, we present a model of a modular and dynamic 
chromosome territory (CT) organization. We propose that basically three nuclear 
compartments exist, an "open" higher-order chromatin compartment with chromatin 
domains containing active genes, a "closed" chromatin compartment comprising 
inactive genes, and an interchromatin domain (ICD) compartment (Cremer et al., 
1993; Zirbel et al., 1993) that contains macromolecular complexes for 
transcription, splicing, DNA replication, and repair. Genes in "open," but not 
in "closed" higher-order chromatin compartments have access to transcription and 
splicing complexes located in the ICD compartment. Chromatin domains that build 
the "open" chromatin compartment are organized in a way that allows the direct 
contact of genes and nascent RNA to transcription and splicing complexes, 
respectively, preformed in the ICD compartment. In contrast, chromatin domains 
that belong to the "closed" compartment are topologically arranged and compacted 
in a way that precludes the accessibility of genes to transcription complexes. 
We argue that the content of the ICD compartment is highly enriched in DNA 
depleted biochemical matrix preparations. The ICD compartment may be considered 
as the structural and functional equivalent of the in vivo nuclear matrix. A 
matrix in this functional sense is compatible with but does not necessitate the 
concept of a 3D nuclear skeleton existing of long, extensively arborized 
filaments. In the absence of unequivocal evidence for such a structural matrix 
in the nucleus of living cells we keep an agnostic attitude about its existence 
and possible properties in maintaining the higher-order nuclear architecture. 
Quantitative modeling of the 3D and 4D human genome architecture in situ shows 
that such an assumption is not necessary to explain presently known aspects of 
the higher-order nuclear architecture. We expect that the interplay of 
quantitative modeling and experimental tests will result in a better 
understanding of the compartmentalized nuclear architecture and its functional 
consequences.