ESTROGEN AND PROGESTERONE 



557 



(1937), Latz and Reiner (1942), Haman 

 (1942), Knaus (1950), Mazer and Israel 

 (1951), and Crossen (1953). 



The earlier concepts regarding the men- 

 strual cycle were based primarily on the 

 changes occurring in the human endome- 

 trium and for convenience of description the 

 cycle was divided into four stages or pe- 

 riods. The first of these was the period of 

 active menstruation, and the length of the 

 cycle was dated from its onset. Most au- 

 thors agreed that menses began by leaking 

 of blood from superficial vessels to form 

 lakes under the surface epithelium and that 

 there was some sloughing of tissue after the 

 beginning of bleeding. There was consider- 

 able disagreement as to the amount of de- 

 struction and loss of tissue; estimates of 

 various authors ranged from very little to 

 almost complete denudation of the surface. 

 Bartelmez (1933) emphasized both the wide 

 individual variability of the amount of tis- 

 sue lost and differences in the stage of de- 

 velopment of the endometria at the time of 

 menstruation. 



The second period immediately following 

 menstruation began with regeneration of the 

 surface epithelium, which started sometimes 

 before menstrual bleeding had ceased and 

 was completed in a very short time. This 

 lieriod included the 5 to 7 days after cessa- 

 tion of menses, during which the endome- 

 trium grew in thickness. Frequent mitoses 

 were recognized, especially in the glands 

 which lengthened but remained straight and 

 tubular. 



The third ("interval") period, lasting 6 

 to 10 days, was characterized by a some- 

 what thickened endometrium, still with 

 straight glands and showing little evidence 

 of secretory activity. At first this was con- 

 sidered a quiescent period as indicated by 

 the term "interval." However, as will be 

 shown later, such a characterization was not 

 justified from the physiologic viewpoint. 



The fourth period, called the premen- 

 strual period, included the 10 days or 2 

 weeks before menstruation. During this 

 phase the glands continued to increase in 

 size and became distended, coiled, or even 

 sacculated. The glandular cells increased 

 in height, and there was evidence of glyco- 

 gen mobilization and secretion. Next, the 

 epithelium became "frayed out" along the 



outer borders, then decreased in height, in- 

 dicating secretory depletion. Decidual cells 

 appeared in the stroma at this time. The 

 endometrium was much thickened and ex- 

 tremely hyperemic. At the height of this 

 period the endometrium was approximately 

 5 mm. in thickness, as compared with V2 

 mm. toward the end of menses. The term 

 premenstrual was usually applied to this 

 phase but today the term progestational 

 would seem preferable. 



During and after these descriptions of the 

 changes in the human endometrium, many 

 attempts were made to locate the time of 

 ovulation in the menstrual cycle. When it 

 was found, as will be discussed later, that 

 ovulation occurred approximately midway 

 between two menses, and was preceded by 

 follicular growth and followed by develop- 

 ment of a corpus luteum, it became custo- 

 mary to refer to the two halves of the men- 

 strual cycle as the follicular phase and the 

 luteal phase. One advantage of this descrip- 

 tive terminology was the emphasis it placed 

 on the homology of the two phases of the 

 menstrual cycle in primates with the fol- 

 licular and luteal phases of the estrous cy- 

 cles of lower mammals. 



A theory to explain menstruation, widely 

 adopted in 1920, was formulated from this 

 morphologic evidence. The essentials were 

 that menstruation occurs because the lining 

 of the uterus, prepared for implantation of 

 the ovum, degenerates if fertilization of the 

 egg does not occur. This required that ovu- 

 lation and corpus luteum formation precede 

 the i^remenstrual changes in the endome- 

 trium. Subsequent research disclosed that 

 menstrual cycles frequently occur in which 

 ovulation does not take place and bleeding 

 results from the breakdown of an "interval" 

 rather than a progestational endometrium. 



The discovery of anovulatory cycles not 

 only brought about a revision of ideas re- 

 garding an explanation for menstruation 

 but also raised questions as to what consti- 

 tuted a normal menstrual cycle. The length 

 of the cycle and the amount and duration of 

 bleeding are approximately the same re- 

 gardless of whether or not ovulation has 

 taken place. The gross features of menstru- 

 ation under these two conditions are indis- 

 tinguishable one from the other. However, 

 the biologic purpose of the menstrual cycle 



