The Menstrual Cycle PDF

I3 Chapter The Menstrual Cycle HIGHLIGHTS The term menstrual cycle is applied to cyclical changes that occur in the endometrium every month. The most obvious feature is a monthly flow of blood (menstruation). T h e m e n s t r u a l...

I3 Chapter The Menstrual Cycle HIGHLIGHTS The term menstrual cycle is applied to cyclical changes that occur in the endometrium every month. The most obvious feature is a monthly flow of blood (menstruation). T h e m e n s t r u a l cycle is divided into the following phases: postmenstrual, proliferative, secretory, menstrual (Fig. 3.3). The menstrual cycle is also divided into the follicular phase (in which changes are produced mainly by oestrogens) and the luteal phase (in which effects of progesterone predominate). Both phases are of roughly equal duration. The main changes in the endometrium are: (a) increase in thickness, (b) growth of uterine glands, (c) changes in epithelial cells lining the glands and (d) increase in thickness and fluid c o n t e n t of t h e e n d o m e t r i a l s t r o m a The period of a woman's life in which she can (Figs 3.4, 3.5). bear children is called the reproductive period. Just before onset of menstruation the blood T h e m o s t o b v i o u s feature of this period is a supply to superficial parts of the endometrium monthly flow of blood from the uterus that is is cut off (Fig. 3.6). This part is shed off and referred to as menstruation (or menses). The onset rhere is bleeding. of menstruation (menarche) takes place at about The menstrual cycle is influenced by oestrogens, 12 years of age. Menstruation ceases to occur at by progesterone, by the follicle stimulating about 45 years of age, and this is referred to as h o r m o n e ( F S H ) , a n d by the l u t e i n i z i n g menopause. hormone (LH). T h e m o n t h l y m e n s t r u a t i o n is the external manifestation of a series of cyclic changes taking place in the uterus. These changes constitute the menstrual cycle. Simultaneously, cyclic changes also take place in the ovaries, and these constitute the ovarian cycle. The most important event in the ovarian cycle is ovulation (Chapter 2).  Human Embryology bleeding bleeding \ Days r One Menstrual Cycle - Fig. 3.1 Diagram illustrating the definition of a menstrual cycle. To understand the menstrual cycle it is (b) The stroma fills the interval between surface necessary to know the structure of the uterus. The epithelium and myometrium. It contains wall of the uterus is made up of three layers. numerous simple tubular glands (uterine glands). 1. The outermost layer or perimetrium is made (c) The arteries that supply the endometrium up of peritoneum. tend to run vertically towards the surface. 2. The main thickness of the wall is made up Some of these run spirally and supply the of smooth muscle. This is the myometrium. whole thickness of the endometrium, while 3. The innermost layer (corresponding to others that remain straight are confined to mucous membrane) is called the endo- metrium. It is this layer which undergoes the basal part. changes during the menstrual cycle. The constituents of the endometrium are as follows PHASES OF THE MENSTRUAL CYCLE (tig- 3.2). The menstrual cycle is usually divided into the (a) The surface of the endometrium is covered following phases, on the basis of changes raking by a lining epithelium. place in the uterine endometrium (Fig. 3.3): Lining epithelium Straight artery Myometrium Arterial plexus I Fig. 3.2 Components of the uterine endometrium. These undergo changes during each menstrual cycle.  The Menstrual Cycle 5 - Cycle beams ? ''^fl ;V8 s\V here. 28^W O if) < Q_ 26 1 ?° / %U11 < LU Corpus luteum ^U Secretory I12 Proliferative n I begins to 24 \ U 13 degenerate. Jr14 m 23 N ^ ^ ^ 1 5 Ovulation 21^^C= — ^ 1 6 20 17 Fig. 3.3 Phases of the menstrual cycle. 1. Postmenstrual into the phases mentioned above is, however, 2. Proliferative arbitrary. T h e changes are really continuous, and 3. Secretory or premenstrual may be summarized as follows: 4. Menstrual 1. The endometrium progressively increases in The changes during rhe postmenstrual phase thickness (Fig. 3.4}. In the postmenstrual and during most of the proliferative phase take p h a s e it is 0.5 to 1 m m t h i c k ; in t h e place under the action of oestrogens produced by the developing follicles in the ovary. Hence this period is referred to as the follicular phase of the menstrual cycle. Secretory The follicular phase constitutes the first- half of the menstrual cycle. Following ovulation, the corpus luteum is formed and starts secreting progesterone. During the second-half of the menstrual cycle, this h o r m o n e (along with oestrogens) m p r o d u c e s s t r i k i n g c h a n g e s in t h e '- e n d o m e t r i u m. As these changes take place under the influence of the corpus luteum, this half of the menstrua! cycle Postmenstrual i 7 IT is called the luteal phase. Just before the CM o n s e t of t h e n e x t b l e e d i n g , t h e r e is 1 lowering of levels of both progesterone and oestrogens, and it is believed that this 'withdrawal' leads to the onset of menstrual bleeding. Fig. 3.4 Uterine glands at various phases of the The division of the menstrual cycle menstrual cycle.  Human Embryology proliferative phase it is 2 to 3 mm thick; surrounding the bodies of the uterine glands a n d in the secretory phase its thickness becomes spongy. The deepest part of the reaches 5 to 7 mm. stroma also remains compact. The stroma The uterine glands grow in length. At first can, therefore, be divided into the following they are straight (Fig. 3.4A), but gradually three layers (Fig. 3.6A). become convoluted (Fig. 3.4B). Because of (a) Stratum compactum these convolutions, the glands acquire a (b) Stratum spongiosum ' s a w - t o o t h e d 1 a p p e a r a n c e when seen in (c) Stratum basale l o n g i t u d i n a l s e c t i o n. T h e g l a n d s also increase in diameter (Fig. 3.4C). The most During the secretory phase, these layers basal p a r t s of uterine g l a n d s , however, become better defined. The endometrium remain tubular and do not undergo these becomes soft and oedematous, because of changes. the fluid secreted by the uterine glands. D u r i n g the p o s t m e n s t r u a l phase the 5. The arteries of the endometrium are small epithelium lining the glands is cuboidal to begin with. They grow in length during (Fig. 3.5A). During the proliferative stage the proliferative phase. During the secretory it becomes columnar (Fig. 3.5B). Glycogen phase, the arteries supplying the superficial accumulates in the basal portion of the two-thirds of the endometrium become very epithelial cell, pushing the nucleus nearer tortuous, and are called spiral arteries. The the lumen (Fig. 3.5C). During the secretory a r t e r i e s t o t h e b a s a l t h i r d of t h e phase the apical part of the cell is shed off endometrium (which does not participate in as part of the secretion. The cell again the changes associated with the menstrual becomes cubical, but the edge towards the cycle) remain straight and short. lumen becomes irregular (Fig. 3.5D). During the postmenstrual phase, the cells Towards the end of the secretory phase the of the stroma are uniformly distributed and endometrium is thick, soft, and richly supplied are compactly arranged (Fig. 3.2). As the with blood. The secretory activity of the uterine endometrium increases in thickness (during glands not only makes the endometrium soft, but the proliferative phase), the superficial part also provides nutrition to the e m b r y o. These of the stroma remains compact, but the part changes are, therefore, an obvious preparation for % B ^ Postmenstrual Secretory Fig. 3.5 Changes in the epithelium of uterine glands during a menstrual cycle.  The Menstrual Cycle Lining epithelium. These layers are shed off. Lining epithelium is reformed by proliferation of cells lining the basal parts of the uterine glands. in Secretory Endometrium Endometrium endometrium at end of during the post- menstruation menstrual phase Fig. 3.6 Shedding off, and regeneration, of uterine endometrium, during a menstrual cycle SC = stratum c o m p a c t u m ; SS = stratum spongiosum; SB = stratum basale. providing a suitable environment for the fertilized The mechanism for onset of menstrual bleeding ovum, when it reaches the uterus. In the absence is as follows. A few hours before the onset of of pregnancy, however, these measures are menstrual bleeding the spiral arteries get abortive: the superficial parts of the thickened constricted so that blood supply to superficial endometrium (stratum compactum and stratum parts of the endometrium is cut off. This spongiosum) are shed off (Fig. 3.6B), and this is ischaemia leads to degeneration of the accompanied by menstrual bleeding. endometrium and also damages the walls of Menstrual bleeding causes the endometrium to the blood vessels themselves. Subsequently be shed off bit by bit, and the blood along with when the arteries relax and blood again flows shreds of endometrium flows out through the into the endometrium, it leaks out through the vagina. At the end of menstruation, the damaged blood vessels. This leaking blood is endometrium that remains is only 0.5 mm thick. responsible for gradual shedding of It consists of the stratum basale along with the endometrium. basal portions of the uterine glands (Fig. 3.6B). The epithelium of these glands rapidly proliferates and reforms the epithelial lining (Fig. 3.6C). Time of Ovulation in Relation to Menstruation The endometrial changes associated with the menstrual cycle are confined to the body of the In a 28-day menstrual cycle, ovulation takes place uterus. The cervical mucosa is not affected. at about the middle of the cycle (Fig. 3.3). The  Human Embryology Fig. 3.7 Graph showing the morning temperature of a woman, on various days of the menstrual cycle. There is a fall in temperature at about the time of ovulation, followed by a rise. period between ovulation and the next menstrual occur only if intercourse takes place during a bleeding is constant at about 14 days, but the time period between four days before ovulation to of ovulation does not have a constant relationship two days after ovulation. The remaining days with the preceding menstruation. This is so because have been regarded as safe period as far as the length of the menstrual cycle in women may prevention of pregnancy is concerned. This vary from one m o n t h to another. Hence, it is forms the basis of the so-called rhythm-method difficult to predict the date of the next ovulation of family planning. from the date of menstruation unless the woman has very regular menstrual periods. B. Where pregnancy is desired There are many methods of finding out the Knowledge regarding the time of ovulation is exact time of ovulation, but the one commonly also of importance in cases of sterility (difficulty used is the temperature method. In this technique, in having children), where the couple can be the woman's body temperature is recorded every advised to have intercourse during the days most morning. When these temperatures are plotted on favourable for conception. a g r a p h , we get a curve like t h a t s h o w n in Fig. 3.7. The temperature is low during actual Correlation between Ovarian and menstruation. Subsequently it rises. At about the Uterine Cycles middle of the cycle, there is a sudden fall in The ovarian and uterine cycles run parallel to temperature followed by a rise. This rise is believed each other. Both are of 28 days duration. The to indicate that ovulation has occurred. uterine cycle is dependent on the ovarian cycle. The uterine endometrium shows cyclic changes, CLINICAL CORRELATION which are dependent on the hormones secreted by developing ovarian follicles a n d c o r p u s Importance of Determining the Time of luteum of the ovary (Fig. 3.8). Ovulation and 'Safe Period' A. Where pregnancy is not desired HORMONAL CONTROL OF OVARIAN AND UTERINE CYCLES After ovulation, the ovum is viable {i.e..it can be fertilized) for n o t m o r e t h a n t w o d a y s. These cycles are under the control of various hormones, Spermatozoa introduced into the vagina die which can be briefly summarized here (Fig. 3.9). within four days. Therefore, fertilization can The hypothalamus acts as a major centre for  The Menstrual Cycle OVARIAN CYCLE Developing Mature Corpus Degenerating follicles follicles luteum corpus luteum FOLLICULAR PHASE LUTEAL PHASE Menstrual Postmenstrual & Secretory phase phase proliferative phase Fig. 3.8 Diagram showing correlation between ovarian and uterine cycles. t h e c o n t r o l of r e p r o d u c t i o n. It secretes the pituitary (LH surge) about 24 to 36 hours before Gorutdotropin-releasing hormones (GnRH), which ovulation (Fig. 3.10). in turn control the secretion of gonadotrophs The LH surge leads to o v u l a t i o n ; and the hormones by t h e a n t e r i o r p i t u i t a r y g l a n d Graafian follicle is transformed into the corpus (adenohypophysis). luteum. There are two g o n a d o t r o p h s hormones. They The LH stimulates the secretion of progesterone are the follicle stimulating hormone (FSH) and by the corpus luteum. Though the secretion of the luteinizing hormone (LH). progesterone predominates, some oestrogen is also In the first-half of the menstrual cycle the C n R H produced. The combined action of oestrogen and acts on the anterior pituitary to release FSH. The progesterone stimulates the endometrial glands to FSH acts on the ovary and stimulates the formation secrete glycogen rich mucoid material (Fig. 3.9). and maturation of ovarian follicles (Fig. 3.9). If fertilization does not occur the granulosa cells T h e m a t u r i n g o v a r i a n follicles n o w s t a r t produce the protein inhibin, which acts on the secreting oestrogens. The repair and proliferation anterior pituitary and inhibits the secretion of of endometrium takes place under the influence of gonadotrophins. This leads to regression of the oestrogens. The endometrial stroma progressively corpus luteum. thickens, the glands in it elongate and the spiral Due to the regression of the corpus luteum there a r t e r i e s begin t o g r o w t o w a r d s t h e surface is a fall in the b l o o d level of o e s t r o g e n a n d epithelium. progesterone. The withdrawal of these hormones The level of oestrogen rises to a peak about causes the endometrium to regress and triggers two days before ovulation. This leads to sudden the onset of menstruation. increase in the level of L H secreted by the anterior If fertilization occurs the corpus luteum does  Human Embryology HYPOTHALAMUS produces releasing factors f ADENOHYPOPHYSfS produces FSH LH 1 Formation & maturation Ovulation Formation of of ovarian follicles corpus luteum OVARY 0es,r °9en Progesterone Produced produced Repair & Proliferative phase Secretory phase ENDOMETRIUM Fig. 3.9 H o r m o n a l control of ovarian and uterine cycles. not regress. It continues to secrete progesterone form of synthetic compounds). Better results are and oestrogen. The secretory phase of endometrium obtained when a small amount of oestrogen is continues and menstruation does not occur. also given. In the most common variety of pill CLINICAL CORRELATION (distributed by government agencies in India) Use of Hormones for Contraception the progestin is norethisterone acetate (1 mg); and the oestrogen is in the form of oestradiol Ovulation in a woman (and by corollary, (50 ug). The pills are distributed in packets, pregnancy) can be prevented by administering each packet containing 28 pills out of which contraceptive pills. The most important 21 pills contain these hormones, and 7 pills do ingredients of such pills are progestins (in the not (for use in the last 7 days). The use of pills  The Menstrual Cycle 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Concentration maximum two days before ovulation PROGESTERONE Level increases to maximum in the secretory phase under influence ot LH. Fig. 3.10 Concentration of the hormones FSH, L H , estrogen and progesterone during a normal menstrual cycle. O v u l a t i o n occurs because of a LH surge just before ovulation. is started 5 days after onset of menstruation. because the progesterone in the pill prevents They are taken continuously without any break the secretion of FSH and LH by the pituitary. as long as contraception is desired. N o r m a l This interferes with the maturation of follicles menstruation occurs during the 7 days in which and ovulation. pills without hormones are being taken. If the Stoppage of pills reduces levels of these pills are taken regularly there is a regular hormones in blood. It is this withdrawal that menstrual cycle of 28 days duration. leads to menstrual bleeding. Contraceptive pills Presence of progesterone in the preovulatory have almost 100 per cent success in suppressing phase prevents occurrence of ovulation. This is maturation of follicles and ovulation.