During their studies of paired meiotic chromosomes, Moses (1956) and Fawcett (1956) independently observed ribbon-like structures in the electron microscope. These were called synaptonemal complex (SC). The complexes were considered to represent the state of chromosome pairing.
The complex consists of two dense parallel lines called lateral elements (LE) and a less dense filament in the centre called the central element (CE). Across the space from the CE run transverse filaments towards each lateral element. This tripartite structure has chromatin material of paired homologous chromosomes lying on either side of the complex, next to each lateral element. Thus each LE with its associated chromatin comprises one homologue. However, sister chromatids belonging to each homologue are not distinguishable in the complex. The dimensions of the complex are known; the LE is about 40 nm thick, and the CE 60-80 nm thick.
Usually SCs are attached at both ends to the nuclear envelope. Digestion studies with trypsin and some other proteolytic enzymes have established that both LE and CE are made up of protein; they are resistant to DNAse digestion.
There may be one or more dense bodies in the central element called nodes, first observed in Neurospora and yeast, later in other fungi. Nodes are a common feature in SCs of Drosophila oocytes where they are about 100 nm in diameter (Carpenter, 1975). They are also called recombination nodules although their involvement in genetic exchange is not clear.
In spermatocytes of the Chinese hamster, Dresser and Moses (1980) found that at early pachytene the LEs are single elements, at mid pachytene they start appearing double; by late pachytene all the LEs are double along their lengths.
Besides the typical tripartite form, SCs with different configurations have been observed. In some insects the complex consists of multiple elements. In the triploid anthers of lily, each of the 3 homologues has its own dense axis at leptotene. Each axis then joins first with one and then with another of the remaining two homologues to form a double synaptonemal complex (Moens,
1969). The haploid spermatids of Gryllus contain multiple core complexes resembling stacked SCs somewhat separated from the chromosomes (Sotelo and Trujillo, 1960).
The synaptonemal complex first becomes visible at leptotene when unpaired chromosomes develop single dense axes, each ofwhich will become a lateral element of a future SC. At zygotene
the dense axes form parallel pairs similar to SC although CE is not yet clearly visible. As pairing advances, the complete tripartite structure of SC is seen. At pachytene full length SCs are visible, often twisted around their long axes. At diplotene SCs disintegrate and the paired homologues separate. However, the dense single axes remain visible resembling the leptotene configurations.
The relationship between SCs and crossing over is not fully known.