Female Physiology Before Pregnancy and Female Hormones

Female reproductive functions can be divided into two major phases: (1) preparation of the female body for conception and pregnancy, and (2) the period of pregnancy itself. Tliis chapter is concerned with preparation of the female body for pregnancy, and Chapter 82 presents the physiology of pregnancy and childbirth.

Physiologic Anatomy of the Female Sexual Organs

Figures 81-1 and 81-2 show the principal organs of the human female reproductive tract, the most important of which are the ovaries, fallopian tubes, uterus, and vagina. Reproduction begins with the development of ova in the ovaries. In the middle of each monthly sexual cycle, a single ovum is expelled from an ovarian follicle into the abdominal cavity near the open fimbriated ends of the two fallopian tubes. This ovum then passes through one of the fallopian tubes into the uterus; if it has been fertilized by a sperm, it implants in the uterus, where it develops into a fetus, a placenta, and fetal membranes—and eventually into a baby.

During fetal life, the outer surface of the ovary is covered by a germinal epithelium, which embryologically is derived from the epithelium of the germinal ridges. As the female fetus develops,primordial ova differentiate from this germinal epithelium and migrate into the substance of the ovarian cortex. Each ovum then collects around it a layer of spindle cells from the ovarian stroma (the supporting tissue of the ovary) and causes them to take on epithelioid characteristics; they are then called granulosa cells. The ovum surrounded by a single layer of granulosa cells is called a primordial follicle. The ovum itself at this stage is still immature, requiring two more cell divisions before it can be fertilized by a sperm. At this time, the ovum is called a primary oocyte.

During all the reproductive years of adult life, between about 13 and 46 years of age, 400 to 500 of the primordial follicles develop enough to expel their ova—one each month; the remainder degenerate (become atretic). At the end of reproductive capability (at menopause), only a few primordial follicles remain in the ovaries, and even these degenerate soon thereafter.

Female Hormonal System

The female hormonal system, like that of the male, consists of three hierarchies of hormones, as follows:

1. A hypothalamic releasing hormone, gonadotropin-releasing hormone (GnRH)

2. The anterior pituitary sex hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are secreted in response to the release of GnRH from the hypothalamus

3. The ovarian hormones, estrogen and progesterone, which are secreted by the ovaries in response to the two female sex hormones from the anterior pituitary gland

These various hormones are not secreted in constant amounts throughout the female monthly sexual cycle; they are secreted at drastically differing rates during different parts of the cycle. Figure 81-3 shows the approximate changing concentrations of the anterior pituitary gonadotropic hormones FSH and LH (bottom two curves) and of the ovarian hormones estradiol (estrogen) and progesterone (top two curves).

The amount of GnRH released from the hypothalamus increases and decreases much less drastically

Female reproductive organs.

during the monthly sexual cycle. It is secreted in short pulses averaging once every 90 minutes, as occurs in the male.

Monthly Ovarian Cycle; Function of the Gonadotropic Hormones

The normal reproductive years of the female are characterized by monthly rhythmical changes in the rates of secretion of the female hormones and corresponding physical changes in the ovaries and other sexual organs. This rhythmical pattern is called the female monthly sexual cycle (or, less accurately, the menstrual cycle). The duration of the cycle averages 28 days. It may be as short as 20 days or as long as 45 days in some women, although abnormal cycle length is frequently associated with decreased fertility.

There are two significant results of the female sexual cycle. First, only a single ovum is normally released from the ovaries each month, so that normally only a single fetus will begin to grow at a time. Second, the uterine endometrium is prepared in advance for implantation of the fertilized ovum at the required time of the month.

Gonadotropic Hormones and Their Effects on the Ovaries

The ovarian changes that occur during the sexual cycle depend completely on the gonadotropic hormones FSH and LH, secreted by the anterior pituitary gland. In the absence of these hormones, the ovaries remain inactive, which is the case throughout childhood, when almost no pituitary gonadotropic hormones are secreted. At age 9 to 12 years, the pituitary begins to secrete progressively more FSH and LH, which leads to onset of normal monthly sexual cycles beginning between the ages of 11 and 15 years. This period of change is called puberty, and the time of the first menstrual cycle is called menarche. Both FSH and LH are small glycoproteins having molecular weights of about 30,000.

Uterine tube Ovary

Uterine tube Ovary

Uterine tube (sectioned) Uterine cavity Uterine tube
Figure 81-1

Ovarian ligament Isthmus of uterine tube

Ovarian stroma z

Endometrium

Uterine cavity

Myometrium

Uterosacral ligament

Cervical canal -

Cervix

Ampulla of uterine tube

Ovarian ligament Isthmus of uterine tube

Ovarian stroma z

Ampulla of uterine tube

Endometrium

Uterine cavity

Myometrium

Uterosacral ligament

Cervical canal -

Cervix

Palmate Folds Uterus

Mucosal folds of uterine tube imbriae

Ovarian vessels

Vaginal rugae

Broad ligament of uterus

Mucosal folds of uterine tube imbriae

Corpus albicans Ovarian follicles Corpus luteum

Ovarian vessels

Vaginal rugae

Broad ligament of uterus

Figure 81-2

Internal structures of the uterus, ovary, and a uterine tube. (Redrawn from Guyton AC: Physiology of the Human Body, 6th ed. Philadelphia: Saunders College Publishing, 1984.)

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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