The chromatoid body was discovered by von Brunn (1876) in the cytoplasm of the young spermatid in the white rat. It was first described in a marsupial by KorfT (1902), in a vertebrate other than mammals by the Schreiners (1905, 1908), and in an invertebrate by Bösenberg (1905).
The word chromatoide was first used in connexion with spermatogenesis by Benda (1891), who called this cytoplasmic inclusion der chromatoide Nebenkörper. The German authors generally call it der chromatoide Körper, the French authors corps chromatoïde. Wilson (1913) referred to it as the chromatoid body and it is generally given this name in papers written in English, though the expression ‘chromatic body’ is sometimes used. It is suggested that the ‘residual body’ described by Gresson and Zlotnik (1945) is identical with the chromatoid body of other authors.
In most species the chromatoid body is spherical or ovoid but in some it assumes other forms as well and in a few it is never spherical or ovoid.
The chromatoid body is usually single in each cell, but sometimes there are 2 or 3 and in a few there are many.
In living cell the chromatoid body generally gives a low phase-change, and is invisible or almost invisible when studied by direct microscopy. In the Mammalia, however, it gives a higher phase-change.
The chromatoid body is highly resistant to acetic acid.
It is deeply stained by basic dyes and basic dye-lakes. It is also stained intensely by acid dyes.
The chromatoid body cannot in most cases be blackened by silver or long osmication techniques.
The histochemical reactions show that the chromatoid body consists mainly of RNA and basic proteins rich in arginine. There is little or no tyrosine. Lipid, carbohydrates, DNA, alkaline phosphatase, and calcium are not shown by histochemical techniques.
As a rule the chromatoid body is homogeneous but in some cases it has a cortex and a medulla. In many cases it is surrounded by a clear, vacuole-like space. Under the electron microscope it has been seen as an opaque irregular body, as an irregular mass of closely aggregated, dense, osmiophil granules, or as a faintly electron-opaque body.
The chromatoid body has so far been recorded in certain species of mammals, a bird, reptiles, cyclostomes, Crustacea, insects, and arachnids. In most cases it appears for the first time during the growth of the primary spermatocyte. Its presence in the spermatid has been recorded in practically all cases. With a few exceptions it has not been found to take any obvious part in the final make-up of the spermatozoon.
The chromatoid body in most cases seems to disappear at the metaphases of meiosis and to be later reconstructed in the daughter cells.
The chromatoid body probably originates from the ground cytoplasm.
On the basis of histochemical studies it is tentatively suggested that the function of the chromatoid body may be to provide basic proteins for the final maturation of the chromatin in the nucleus of late spermatids.
Certain authors have considered that a cytoplasmic inclusion occurring in the young (and in some cases mature) spermatozooids of certain liverworts, mosses, and a gymnosperm is to be regarded as the homologue of the chromatoid body. Reasons are given for denying this supposed homology.