Reproductive Physiology - Through the Life Cycle

The reproductive systems of females and males (Illustration 2.1 on the next page) begin developing in the first months after conception and continue to grow in size and complexity of function through puberty. Females are born with a complement of immature ova and males with sperm- producing capabilities. The capacity for reproduc-tion is established during puberty when hormonal changes cause the maturation of the reproductive system over the course of 3 to 5 years.

Approximately 7 million immature ova, or primor-dial follicles, are formed during early fetal development, but only about one-half million per ovary remain by the onset of puberty. During a woman’s fertile years, some 400–500 ova will mature and be released for possible fertilization. Very few ova remain by menopause.

For men, sperm numbers and viability decrease some-what after approximately 35 years of age, but sperm are still produced from puberty onward.⁸ Because females are born with their lifetime supply of ova, the number with chromosomes damaged by oxidation, radioactive particle exposure, and aging increases with time. Consequently, children born to women older than roughly 35 years of age are more likely to have disorders related to defects in chromosomes than are children born to younger women.¹

Female Reproductive System

During puberty females develop monthly menstrual cycles, the purpose of which is to prepare an ovum for fertilization by sperm and the uterus for implantation of a fertilized egg. Menstrual cycles result from complex interactions among hormones secreted by the hypotha-lamus, the pituitary gland, and the ovary. Knowledge of hormonal changes during the menstrual cycle is expanding,

Table 2.1 2010 nutrition objectives for the nation related to preconception

Increase the proportion of adults who are at a

•

healthy weight from 42 to 60%.

Reduce the proportion of adults who are obese from

•

23 to 15%.

Reduce iron deficiency among females of

•

childbearing age from 4–11% to 1–7%.

Reduce the incidence of spina bifida and other

•

neural-tube defects from 7 to 3 per 10,000 live births and birth defects from 1.7 to 1.2 per 1000 live births.

Increase the proportion of women who receive

•

preconceptional counseling.

Puberty The period in life during which humans become biologically capable of reproduction.

Ova Eggs of the female produced and stored within the ovaries (singular 5 ovum).

Menopause Cessation of the menstrual cycle and reproductive capacity in females.

Menstrual Cycle An approximately 4-week interval in which hormones direct a buildup of blood and nutrient stores within the wall of the uterus and ovum maturation and release. If the ovum is fertilized by a sperm, the stored blood and nutrients are used to support the growth of the fertilized ovum. If fertilization does not occur, they are released from the uterine wall over a period of 3 to 7 days. The period of blood flow is called the menses, or the menstrual period.

Pituitary Gland A pea-sized gland located at the base of the brain. It is connected to the hypothalamus and produces and secretes growth hormone, prolactin, oxytocin, follicle-stimulating hormone, luteinizing hormone, and other hormones in response to signals from the hypothalamus.

Corpus Luteum (corpus 5 body, luteum 5 yellow) A tissue about 12 mm in diameter formed from the follicle that contained the ovum prior to its release. It produces estrogen and progesterone. The

“yellow body” derivation comes from the accumulation of lipid precursors of these hormones in the corpus luteum.

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Chamber in which embryo develops; its narrowed-down portion (the cervix) secretes mucus that helps sperm move into uterus and that bars many bacteria UTERUS

Thick muscle layers of uterus that stretch enormously during pregnancy MYOMETRIUM

Anus

Inner lining of uterus;

site of implantation of blastocyst (early embryonic stage);

becomes thickened, nutrient-packed, highly vascularized tissue during a pregnancy; gives rise to maternal portion of placenta, an organ that metabolically supports embryonic and fetal development ENDOMETRIUM

CLITORIS

Small organ responsive to sexual stimulation LABIUM MINOR One of a pair of inner skin folds of external genitals LABIUM MAJOR One of a pair of outermost, fat-padded skin folds of external genitals OVARY

One of a pair of primary reproductive organs in which oocytes (immature eggs) form and mature;

produces hormones (estrogens and progesterone), which stimulate maturation of oocytes, formation of corpus luteum (a glandular structure), and preparation of the uterine lining for pregnancy

OVIDUCT

One of a pair of ciliated channels through which oocytes are conducted from an ovary to the uterus; usual site of fertilization

Urinary bladder

Urethra Opening

of cervix

Organ of sexual intercourse;

also serves as birth canal VAGINA

SEMINAL VESICLE One of a pair of glands that secrete fructose and prostaglandins, which become part of semen

BULBOURETHRAL GLAND

One of a pair of glands that secrete a lubricating mucus Anus

VAS DEFERENS One of a pair of ducts for rapid transport of sperm

EPIDIDYMIS One of a pair of ducts in which sperm complete maturation; the portion farthest from testis stores mature sperm URETHRA

PROSTATE GLAND Secretion of substances that become part of semen

Dual-purpose duct; serves as channel for ejaculation of sperm during sexual arousal, also for urine excretion at other times

EJACULATORY DUCT One of a pair of sperm-conducting ducts

Urinary bladder

PENIS Organ of sexual intercourse

TESTIS

One of a pair of primary reproductive organs; packed with sperm-producing tubules and cells that secrete testos-terone and other hormones Erectile tissue

Urethra

Illustration 2.1 Mature female and male reproductive systems.

of GnRH, and thus the secretion of FSH and LH. Without sufficient FSH and LH, ova within follicles do not mature and are not released. (This is also how estrogen and pro-gesterone in some birth control pills inhibit ova maturation

and release.) Estrogen and progesterone secreted by the corpus luteum further stimulate the development of the en-dometrium. If the ovum is not fertilized, the production of hormones by the corpus luteum declines, and blood levels

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increase in size and to continue to produce enough estrogen and progesterone to maintain the nutrient and blood vessel supply in the endometrium. The corpus luteum ceases to function within the first few months of pregnancy, when it is no longer needed for hormone production.

Male Reproductive System

Reproductive capacity in males is established by com-plex interactions among the hypothalamus, pituitary gland, and testes. The proc-ess in males is ongoing of progesterone and estrogen fall. This decline removes the

inhibitory effect of these hormones on GnRH release, and GnRH is again able to stimulate release of FSH for the next cycle of follicle development, and of LH for the stimulation of progesterone and estrogen production. Decreased levels of progesterone and estrogen also cause blood vessels in the uterine wall to constrict, allowing the uterine wall to release its outer layer in the menstrual flow. Cramps and other side effects of menstruation can be traced to the pro-duction of prostaglandins by the uterus. These substances cause the uterus to contract and release the blood and nutrients stored in the uterine wall.

If the ovum is fertilized, it will generally implant in the lining of the uterus within 8 to 10 days. Hormones secreted by the dividing, fertilized egg signal the corpus luteum to

Hypothalamus

Anterior pituitary GnRH

Blood levels of FSH (light line) and LH (dark line)

Midcycle peak of LH (triggers ovulation) LH

LH LH

Progesterone, estrogen FSH

FSH

Estrogens

Estrogens Endometrium

of uterus Ovary

5 1

Follicular phase of

menstrual cycle Luteal phase of

menstrual cycle 14

Progesterone, estrogen Blood levels of estrogens

(light line) and

progesterone (dark line)

Menstruation

Days of one menstrual cycle (using 28 days as the average duration)

28 D

Ovulation

Corpus luteum Growth of follicle

Illustration 2.2 Changes in the ovary and uterus, correlated with changing hormone levels during the follicular and luteal phases of the menstrual cycle.

Prostaglandins A group of physiologically active substances derived from the essential fatty acids. They are present in many tissues and perform such functions as the constriction or dilation of blood vessels and stimulation of smooth muscles and the uterus.

Testes Male reproductive glands located in the scrotum. Also called testicles.

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severe stress, infection, tubal damage and other structural problems, and chromosomal abnormalities (Table 2.3).^5,6 Conditions that modify fertility appear to affect hormones that regulate ovulation, the presence or length of the luteal phase, sperm production, or the tubular passageways that ova and sperm must travel for conception to occur. Sexu-ally transmitted infections, for example, can result in pelvic inflammatory disease (PID), which may lead to scarring and blockage of the fallopian tubes.⁹ Endometriosis is also a common cause of reduced fertility. It develops when por-tions of the endometrial wall that build up during men-strual cycles become embedded within other body tissues.

Endocrine abnormalities that modify hormonal regulation of fertility are the leading diagnoses related to infertility.

“Unknown cause” is the second leading diagnosis, how-ever, and is applied to about one-half of all cases of male and female infertility.^10,11

Dalam dokumen Through the Life Cycle (Halaman 81-84)