On isopycnotic behavior of the XX-bivalent in oocytes of Rattus norvegicus

Discrepancies between genotype and phenotype in hematology: an important frontier

An African American male infant with sickle cell disease has a devastating stroke; an African American soldier is surprised when he is informed that he has sickle cell disease. They are both homozygous for the same mutation. An Ashkenazi Jewish woman with Gaucher disease has a huge spleen and severe thrombocytopenia; her older brother, homozygous for the same 1226G glucocerebrosidase mutation, is found on routine examination to have a barely palpable spleen tip. The fact that clinical manifestations of genetic diseases can vary widely among patients has been recognized for many decades. In the past, however, it could often be attributed to the pleomorphic nature of mutations of the same gene: the patient with severe disease, it was averred, must have a different mutation than the one with mild disease. Even before a precise definition of mutations could be achieved at the DNA level, such an explanation did not serve to clarify the differences that existed between siblings with the same autosomal recessive disease. Such siblings must surely be carrying the same 2 disease-producing alleles. With the advent of sequence analysis of genes, the great extent of phenotype variation in patients with the same genotype has come to be more fully appreciated, but understanding of why it occurs continues to be meager. It is the purpose of this review to explore some of the variations in phenotype seen by hematologists in patients with identical mutations, to indicate where some progress has been made, and to suggest how understanding in this important area may be expanded.

A quantitative and cytological study of germ cells in human ovaries

Foetal, neonatal and post-natal human ovaries (2 months p . c . to 7 years p . p .) were studied with the object of correlating cytological changes in the germ cells with fluctuations in their numbers. The chromosomal configurations of oogonia and oocytes were examined in both histological and squash preparations. The numbers of normal and degenerating germ cells at different developmental stages up to the time of birth were estimated volumetrically, while those in post-natal ovaries were assessed by means of cell counts. Mitotic divisions in oogonia were observed from the end of the 2nd up to the 7th month p.c. , a period during which some oocytes had already entered the leptotene and zygotene stages of meiotic prophase. Oocytes at the pachytene stage were recorded from 4 months p.c . until the time of birth, and those at diplotene from the 4th month throughout the remainder of the period studied. The so-called dictyate stage was not observed in human oocytes. Three ‘waves’ of degeneration were observed, affecting (i) oogonia undergoing mitosis (‘atretic divisions’), (ii) oocytes principally at the pachytene stage (‘ Z ’ cells) and (iii) oocytes at the diplotene stage. The total population of germ cells appears to rise steadily from some 600 000 at 2 months p . c ., reaching a peak of 6 800 000 at 5 months. By the time of birth, the number has declined to 2 000 000 of which some 50 % are atretic. Of the 1 000 000 normal oocytes in the new-born infant, only some 300 000 survive to the age of 7 years.

The ultrastructure of oogonia and oocytes in the foetal and neonatal rat

Ovaries from eighty foetal and neonatal rats (aged 16·0 dayspost coitumto 4 dayspost partum) were examined under the electron microscope. All the normal developmental stages (oogonia and oocytes in the leptotene, zygotene, pachytene, diplotene and dictyate phases of meiotic prophase) were identified. Patterns of degeneration (‘atretic divisions’, ‘Z’ cells and atresia at the diplotene phase), whose histological appearance and incidence have been recorded by Beaumont & Mandl (1962), were also recognized. The nuclei of oocytes at the leptotene phase contain single electron dense threads which become aligned in parallel pairs at the following phase (zygotene). A third finer fibril half-way between them appears at pachytene (tripartite ribbon). The longitudinal segments of threads, visible in ultra-thin sections, become shorter, presumably due to coiling. Nuclei at the diplotene phase contain single threads essentially similar to those seen at leptotene. As the oocyte enters the dictyate or resting phase, electron dense threads disappear from the nucleus. These observations suggest that the threads represent chromosomal ‘cores’. Nucleolus-like components persist throughout meiotic prophase, and at the diplotene phase regain the more complex form typical of oogonia. The cytoplasmic organelles become more numerous and complex as the oocyte approaches the dictyate phase. ‘Atretic divisions’ in oogonia are characterized by the persistence of long segments of nuclear membrane and the appearance of vesicles enveloped by a double membrane resembling the nuclear envelope. The dense masses of ‘chromatin’ (mitotic chromosomes) are more rounded than in normal cells at metaphase, and tend to coalesce. Spindle fibres have not been observed. Cytoplasmic organelles tend to increase in number and complexity. ‘Z’ cells (cells degenerating largely at the pachytene phase) show heavy ‘chromatin’ condensation around the tripartite ribbons. The major cytoplasmic changes consist in swelling of the endoplasmic reticulum, vacuolation of mitochondria and increase in incidence of multilamellar bodies. Atretic oocytes at the diplotene phase differ markedly from ‘Z’ cells in that ‘chromatin’ condensation around electron dense threads (single) is markedly less prominent. Cytoplasmic changes are similar to those of ‘Z’ cells, but also include a rise in the incidence of multivesicular and other complex bodies. All three types of degenerating cells are removed from the ovary by the phagocytic activity of neighbouring somatic cells.

True hermaphroditism: a clinical description and a proposed function for the long arm of the Y chromosome

True hermaphroditism is a very rare form of ambiguous genitalia characterized by the presence of both ovarian and testicular tissue in the same individual. Sixty percent of these patients have a 46,XX karyotype; however, most have H-Y antigen detectable. Since H-Y antigen is thought to be the gene product of the short arm of the Y, then the short arm must be present somewhere in the karyotype. Products of the short arm loci are thought to initiate testicular differentiation and male determination. The long arm of the Y has not been detected in true hermaphrodites. Testicular histology in true hermaphrodites is characterized by spermatogenic arrest, causing us to speculate that the long arm of the Y might be essential for germ cell maturation beyond the spermatogonia state.