154 BIOLOGY OF THE LABORATORY MOUSE 



The corpora lutea of lactation are formed from follicles which ovulate 

 post partum. They remain small and free from fat. The size of the cells 

 is equal to those of the corpora of ovulation, but the nuclei are very small. 



Atresia. — All of the follicles present in the ovaries do not mature and 

 ovulate. Many of them undergo involution and gradual degeneration. 

 This involution which is called atresia is a normal occurrence in the ovaries. 

 Atresia may take place at any phase of the developing foUicle. In the 

 process of atresia of a primary follicle the ovum shrinks; it becomes wrinkled, 

 the follicular cells become pyknotic and fragment, following which the sur- 

 rounding stroma soon refills the space. In a larger follicle, after the degen- 

 eration of the ovum, the collapsed zona pellucida forms a hyaline clump 

 which may persist for a considerable time. Occasionally the ovum shows 

 pseudomaturation spindles or polar body formation. Atypical cell division 

 of the ovum may lead to the formation of several cells of varying sizes 

 enclosed in the zona pellucida. Such so-called parthenogenetic development 

 is followed by degeneration. In some cases the partial degeneration of the 

 follicular cells preceeds the degeneration of the ovum, and the latter is 

 found "naked" in the middle of the follicle where it soon shows signs of 

 karyorrhexis and cytolysis. Connective tissue cells and capillaries invade 

 the follicle and replace the degenerated cells. The cells of the theca interna 

 hypertrophy and form large polyhedral epithelioid cells, called theca lutein 

 cells, which form the corpus luteum of atresia. Structurally such a corpus 

 is similar to the normal corpus luteum, but usually contains some remains 

 of the degenerated ovum or granulosa cells. These gradually shrink and 

 are replaced by connective tissue. Strands of theca lutein cells may per- 

 sist for a considerable time. 



Occasionally (in virgin females quite often) a peculiar atresia takes place 

 in a ripe follicle which fails to rupture. The granulosa cells do not degen- 

 erate, but hyertrophy and form a corpus luteum at the center of which the 

 ovum is present. Sometimes the antrum of a follicle in the process of this 

 type of atresia contains blood, and later is not filled in entirely by luteal 

 cells but contains a loose connective tissue core. Gradually the ovum 

 degenerates and hyalinization, progressing from the central area toward 

 the periphery, sets in. The hyalinized corpus may persist as a round bod}- 

 for a considerable time, but finally shrinks and is gradually imbedded in 

 the stroma. 



In an Aschheim-Zondek test, after the injection into an immature 

 female mouse of the urine of a pregnant woman, a similar type of atresia 

 takes place. The follicles of the immature mouse ripen, pseudomaturation 



