STRUCTURE AND FUNCTION IN 

 MAMMALIAN EGGS 



Nucleus 



Oocyte Nucleus 



Primary oocytes exist in large numbers in the ovarian cortex of 

 young animals. They themselves seem incapable of division and 

 their abundance is owing to the earlier multiplication of the oogonia 

 from which they have differentiated. As a feature of differentiation, 

 the oocyte nucleus starts upon the early prophase changes of the 

 first meiotic division, the chromosomes become somewhat con- 

 densed, and the nucleus then passes into the dictyate stage. Those 

 oocytes that are not destined, as many are, for early degeneration 

 remain in this stage until meiosis is suddenly resumed shortly before 

 or soon after ovulation. The precise form assumed by the chromo- 

 somes in the dictyate stage is uncertain though they clearly lose their 

 earlier partially condensed appearance. In oocyte nuclei in fish, 

 amphibians, reptiles and birds, the chromosomes take on the form 

 of fine long threads bearing numerous lateral loops, and are referred 

 to as lampbrush chromosomes. Their special significance is still 

 conjectural — they may play a part in yolk synthesis. Equivalent 

 structures have yet to be demonstrated in mammalian oocytes. 

 Recent observations of Ohno, Kaplan and Kinosita (i960) showed 

 that the two X chromosomes in rat oocytes are isopycnotic, both 

 at the first meiotic prophase and the second meiotic metaphase. 

 This is in contrast to the positively heteropycnotic state of the XY 

 bivalent in spermatocytes, as previously demonstrated by these 

 workers (Ohno, Kaplan and Kinosita, 1957, 1958), and they suggest 

 that the condition in spermatocytes represents an evolved mechan- 

 ism that prevents crossing-over and ensures isolation of the female- 

 determining chromosome from the male-determining chromosome. 

 Crossing-over between the two X chromosomes in oocytes, on the 

 other hand, would not impair the sex-determining mechanism. 



The early oocyte is distinguished from the other cells of the 

 ovarian cortex by its larger size, and correspondingly larger nucleus, 

 and by the presence of yolk materials in the cytoplasm. As seen in 



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