Book - Aids to Embryology (1948) 3
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter III Changes In The Female Genital Tract
Before describing the mechanism of implantation of the embryo and the formation of the foetal membranes, reference must be made to the cyclic changes which take place in the ovary and uterus during the reproductive period of life in the human female.
Cyclic Changes in the Ovary
The growth of the ovarian follicle was described earlier in this book (p. 6), and when a mature follicle ruptures at ovulation the resultant cavity is converted into a structure known as the corpus luteum. In the absence of fertilization this degenerates after a life of about two weeks and the cycle recommences with the growth of more follicles. The whole series of events is repeated approximately every twentyeight days.
At the beginning of the ovarian cycle a number of follicles commence growth. Normally only one of them becomes mature in the human. The others sooner or later degenerate, a phenomenon known as follicular atresia, and are ultimately replaced by fibrous tissue. In the follicle which will eventually rupture at ovulation, growth is very marked just before this event. Certain cells of the follicle (probably the theca interna) produce a hormone known as oestrin, which influences the remainder of the female genital tract ( vide infra). The whole process of follicular ripening in the ovary is controlled by a hormone produced by the anterior lobe of the hypophysis and called the follicle stimulating hormone (F.S.H.).
The Corpus Luteum
After ovulation a structure is formed called, from its characteristic yellow colour, the corpus luteum. The wall of the ruptured follicle consists of stratum granulosum cells. This wall crumples up around some central blood clot and the granulosa cells become greatly enlarged, accumulating granules of yellowish pigment in their cytoplasm. These cells are now termed luteal cells and the whole structure becomes vascularized by ingrowth of capillaries from the theca. The luteal cells produce a hormone called progesterone which has an influence upon the uterine endometrium, and this will be discussed later, while the formation of the corpus luteum itself is controlled by the anterior hypophysis by means of the luteinizing hormone (L.H.).
The fate of the corpus luteum depends on whether fertilization of the ovum occurs or not. In the latter case the structure, a corpus luteum of menstruation, grows for some days until it may be one-third the size of the ovary. Before the next menstruation occurs however, degenerative changes set in and eventually it is represented by a fibrous scar in the ovary, a corpus albicans. If the ovum be fertilized the corpus luteum persists as a corpus luteum of pregnancy until the fourth month of development since its presence is necessary for maintaining pregnancy during this time. Then degenerative changes commence and it slowly regresses.
Cyclic Changes in the Uterus
During the whole of a woman's sexual life certain changes occur roughly every twenty-eight days in the mucosa of the uterus, in preparation for the possible implantation of a fertilized ovum. There is discharge of blood from the uterus for several days during these changes and , the first day of such discharge is reckoned as the first day of the menstrual cycle. Although commonly of twenty-eight days duration, the cycle may be longer or shorter in different individuals.
The endometrial changes may be described in four phases : (a) menstrual ; ( b ) post-menstrual ; (c) interval ; (d) pre-menstrual ; these however pass gradually from one to another and it is not easy to say when one ends and another commences.
The menstrual phase lasts three to five days. Before it commences the endometrium is thick (5 mm. or more) and the superficial part is congested. Leakage of blood from the capillaries into the subepithelial tissues results in the molecular disintegration of the superficial parts (stratum compactum and stratum spongiosum) which, with the blood cells, pass to the exterior as the menstrual flow. As a consequence the thickness of the endometrium is greatly reduced (0*5 mm.). The amount of blood and debris lost averages some 50 to 60 cc., but may be considerably more.
In the post-menstrual or early proliferative phase, repair of the denuded surface takes place by reepithelialization from the terminal parts of the glands remaining in the stratum basale of the endometrium. This process occurs rapidly (about four days or so) and passes insensibly into the next phase, that of the interval.
The interval phase (twelve to thirteen days) corresponds to late proliferation and early secretion and shows progressive increase in the thickness of the mucosa ; the uterine glands increase in length and become somewhat coiled and at the end of this period the endometrium may be divided into three strata. Superficially there is a compact layer; next, a zone where the stroma cells are more loosely arranged (the stratum spongiosum), and next to the uterine muscle is a stratum basale, in which the terminal parts of the uterine glands are embedded. This endometrium is 3 to 4 mm. in thickness.
The last phase of the menstrual cycle is the premenstrual one. Its duration is from six to eight days.
Here the endometrium increases greatly in thickness and the stroma becomes oedematous. The uterine glands become markedly dilated and convoluted presenting in microscopic sections a characteristic appearance called â€œ saw-tooth Their secretion contains glycogen and mucin. The veins and capillaries become dilated while the small arteries are described as arranged in spiral fashion in the tissue.
These changes in the endometrium are shown diagrammatically in Fig. 7.
Relation of Uterine Changes to Ovarian Cycle
Broadly speaking, the first half of the menstrual cycle is associated with the presence of a ripening follicle in the ovary. During the second half a corpus luteum is found. The events taking place in the endometrium are controlled by hormones secreted by these two structures, and so we may distinguish a first or follicular and a second or progestional phase. Follicle growth and corpus luteum formation are in turn controlled by hormones elaborated by the anterior lobe of the hypophysis so the sequence of events may be considered as follows :
The anterior lobe of the hypophysis produces a hormone â€” follicle stimulating hormone (F.S.H.) â€” which controls development of the ovarian follicle. This developing follicle produces a second hormone, an oestrogen â€” which controls the proliferation phase of the endometrium. This corresponds to the postmenstrual and early interval stages. About the fourteenth day of a normal cycle the anterior lobe of the hypophysis commences secretion of another substance called the luteinizing hormone (L.H.). This is principally concerned with the formation and maintainance of the corpus luteum, but the early small amounts of it, secreted by the hypophysis, cause rupture of the follicle and thus the ovum is released from the ovary. There does not seem to be any evidence for a specific factor causing ovulation. A corpus luteum is formed at the site of the ruptured follicle by the stratum granulosum cells of the follicle wall, and this formation, and the persistence of it, is due to the luteinizing hormone. The cells of the corpus luteum secrete progesterone which is responsible for the secretory phase of the endometrium, that is the late interval and pre-menstrual stages. The changes induced in the endometrium are such as to prepare it for the reception of a fertilized ovum, and if fertilization does not occur the corpus luteum degenerates and the influence of progesterone upon the uterine mucosa is withdrawn. The latter breaks down as the menstrual discharge and the whole cycle begins once more.
Fig. 7. - Diagram to show Inter-relations between the Hypophysis, Ovary and Endometrium during the Menstrual Cycle.
Time of Ovulation. â€” A great deal of work has been carried out in recent years to determine the relation of ovulation to the menstrual cycle. Experiments on the macaque monkey, which resembles the human in its menstrual cycle, point to the mid-interval period as being the most usual time for rupture of the ovarian follicle. Various observations suggest that this is essentially correct for the human female. Probably the most important of these has been the recovery of living unfertilized ova from the uterine tube by Allen and his co-workers (1930), who determined in this way that ovulation took place about the fourteenth day of the cycle. Microscopic examination of specimens of endometrium removed by biopsy demonstrate early secretory changes under progesterone influence and thus it is possible to determine if ovulation has occurred. It is not possible to state exactly when.
A widely held theory (Knaus-Ogino) states that ovulation occurs during a short period of time which bears a constant relation to the succeeding menstruation. According to this view, ovulation occurs fourteen days before the first day of the next menstrual flow. The theory, assuming that the ovum and the spermatozoon have a comparatively short life (see below), claims to afford a means of determining that period in the menstrual cycle when the human female is practically sterile. This period would be, then, the six or seven days preceding menstruation. Reports from different observers vary as to the practical value of the Knaus-Ogino theory.
Viability of Ovum and Spermatozoon
It was formerly thought that the unfertilized human ovum could remain alive for some two weeks after ovulation. This view is now very seriously questioned, since it is known that the life span of the unfertilized ovum of lower mammals is a short one, ranging from five to six hours in the rabbit (Hammond, 1934) R> lsss than thirty hours in the ferret (Hammond and Walton, 1934)- It seems commonly agreed at this day that the unfertilized human ovum cannot much exceed these times in its viability which may therefore be put at twenty-four hours or somewhat more.
Spermatozoa deposited in the female genital tract remain motile for varying periods of time depending on their environment. Thus, judging by their motility, they survive only a few hours in the vagina owing to the acid reaction of the vaginal secretions. In the cervix uteri survival is longer since they are in an alkaline medium. Motility of a spermatozoon does not, however, constitute ability to fertilize an ovum, and it seems unlikely on the fragmentary evidence to hand that the human spermatozoon retains its fertilizing power for more than forty-eight hours within the female genital tract.
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Cite this page: Hill, M.A. (2021, April 20) Embryology Book - Aids to Embryology (1948) 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_3
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