Chapter 8. Reproductive Cycles: The Ovarian Cycle and Ovulation - Review of Medical Embryology Book

Introduction: the recurring periods of sexual excitement in adult females, other than primates, are called estrus or "heat." In mammals other than man we speak of the estrous cycle (series of physiologic uterine, ovarian, and other changes that occur which consist of proestrus, estrus, postestrus, and anestrus or diestrus). In humans there is the menstrual cycle (period in which the ovum matures, is ovulated, and enters the uterine lumen via the uterine tubes)

MENSTRUATION begins at puberty, about 12 to 15 years of age, and continues throughout the reproductive years in the human female
REPRODUCTIVE OR SEXUAL CYCLES occur monthly and involve activities of the hypothalamus, hypophysis, ovaries, uterus, uterine tubes, vagina, and mammary glands
CYCLES prepare the reproductive system for pregnancy. A hormone-releasing factor synthesized in the hypothalamus and carried via the hypophyseal portal system of vessels to the anterior lobe of the hypophysis, causes the cyclic release of the gonadotropic hormones, follicle-stimulating hormone (FSH), and luteinizing hormone (LH)

THE GONADOTROPINS (FSH and LH) produce cyclic changes in the ovaries such as development of follicles, ovulation, and corpus luteum formation - ovarian cycle

FSH promotes growth of several ovarian follicles, but usually only one forms a mature follicle which finally expels its oocyte

Growth and differentiation of the primary oocyte
Proliferation of the follicular cells
Formation of the zona pellucida
Development of the theca folliculi (connective tissue capsule) from ovarian stroma

THE FOLLICULAR CELLS actively divide, producing a stratified layer around the ovum

The follicle becomes oval and the oocyte eccentric in position because the follicular cell proliferation is greater on one side
Fluid-filled spaces then appear around the cells, coalesce, and form a single large cavity, the follicular antrum, and the ovarian follicle is now called a secondary or vesicular follicle
The primary oocyte is pushed to one side of the follicle where it is surrounded by a follicular cell mound, the cumulus oophorus which projects into the antrum

EARLY FOLLICLE DEVELOPMENT is induced by FS Final stages of maturation require LH as well

Growing follicles produce estrogen (female sex hormone) that regulates development and functions of reproductive organs

Under influence of FSH and LH, around midcycle or 14 days ? 1 day, the follicle grows rapidly producing a bulge or cystic swelling on the ovarian surface, and a small oval avascular spot, the stigma, is seen on the swelling
Before ovulation, the oocyte and some cells of the cumulus oophorus detach from the inside of the distended follicle
At ovulation, there is a "surge" of LH release, the stigma balloons out, forming a surface vesicle, then it ruptures, expelling the oocyte with follicular fluid
The oocyte is covered by the zona pellucida and one or more layers of follicular cells which radially arrange themselves as the corona radiata
Signs of ovulation include mittelschmerz or intermenstrual pain and basal body temperature rise (slightly). Although the time; between ovulation and succeeding menstrual bleeding is constant, the time between ovulation and the preceding menstruation is highly variable and depends on how long the follicle needs to matur One cycle of maturation may need more time than another