Chapter 20

Reproduction

Reproduction

Reproductive System Not needed for the survival of the individual Species survival

Sexual reproduction Genes from two individual Combine at random Creates new combinations Increases chances of species survival

Sexual Reproduction

Each individual produces gametes Formed in gonads by meiosis

Male: testes produce sperm Female: ovaries produce ova

Gametes unite in process of fertilization Restores diploid number Forms zygote

Sexual Determination Each zygote inherits

23 chromosomes from mother 23 chromosomes from father 23 pairs of homologous chromosomes

Kinds of chromosomes 1-22 pairs of chromosomes: autosomal 23rd pair are sex chromosomes

Male: XY, Female: XX

Chromosomal gender of zygote determined by fertilizing sperm

Male Reproduction System

Testes Seminiferous tubules

Where spermatogenesis occurs Contain receptor proteins for FSH

in Sertoli cells

Leydig cells Secrete testosterone Contain receptor proteins for LH

Formation of Testes

First 40 days after conception: the gonads of males and females are similar in appearance

Cells that will give rise to spermatogonia and oogonia migrate from yolk sac to developing embryonic gonads Gonads could become either

TDF (testis-determining factor) promotes the conversion

to testes gene located SRY (sex determining region of Y chromosome) Found in all mammals

Leydig cells secrete testosterone Begins 8th week and peaks at 12-14th week Masculinizes embryonic structures Testosterone then declines to very low levels until puberty

Testes descend into scrotum shortly before birth

Temp below internal temp (34.4 - 35.5)

Testosterone

Responsible for development of male accessory sex organs

External genitalia identical first 6 weeks, then testosterone stimulates development of penis

Not the active agent in all cells Converted to dihydrotestosterone in some target cells Needed for penis, spongy urethra, scrotum, prostrate

Testosterone directly needed for wolfian derivatives

Epididymis, ductus deferens, ejaculatory duct, seminal vesicles

Formation of Ovaries

Absence of Y chromosome and TDF, female develop ovaries

Ovarian follicles do not appear until 2nd trimester, day 105

Chromosomal sex and development of embryonic gonads

Endocrine Regulation of Reproduction

First trimester Embryonic testes are active endocrine glands

Secrete large amounts of testosterone Embryonic ovaries not mature until third trimester

Time of birth: Gonads in both sexes relatively inactive

Before puberty: Low levels of sex steroids in both

Due to lack of stimulation

Puberty: Increased stimulation from gonadotropic hormones Induce increase in sex steroids

Hypothalamus releases GnRH into hypothalamo-hypophyseal portal vessels

Anterior pituitary secretes LH (luteinizing hormone)

In male: interstitial-cell stimulating hormone (ICSH)

FSH (follicle-stimulating hormone)

Secreted in pulsatile fashion to prevent desensitization and down regulation of receptors

Endocrine Interactions

Primary effects of LH and FSH on gonads Stimulation of spermatogenesis and oogenesis Stimulation of gonadal hormone secretion Maintenance of gonadal structure

Endocrine Regulation

Negative Feedback Inhibit GnRH from hypothalamus Inhibit anterior pituitary response to GnRH Inhibin secretion inhibit anterior pituitary release of FSH

By sertoli cells

Onset of Puberty

FSH and LH high in newborn, falls to low levels in few weeks

Puberty: driven by increased secretion of FSH and LH

FSH and LH: Brain maturation increases GnRH secretion Decreased sensitivity of GnRH to negative feedback

LH: Increased secretion triggers puberty

Late puberty, pulsatile secretion of LH and FSH increase during sleep

Stimulate a rise in sex steroid secretion

Stimulate rise in testosterone and estradiol-17β Produce secondary sexual

characteristics

Age of onset related to the amount of body fat and physical activity in the female

Leptin secretion from adipocytes may be required for puberty

Pineal Gland

Secretes melatonin Secretion influenced by light-dark cycles Inhibit gonadotropin secretion Role in humans not established

4 Phases of Human Sexual Response

Excitation phase (arousal) Myotonia and vasocongestion Engorgement of a sexual organ with blood Erection of the nipples

Plateau phase: Clitoris becomes partially hidden

Erected nipples become partially hidden by swelling of areolae

Orgasm: Uterus and orgasmic platform of vagina contract Contractions accompanying ejaculation

Resolution phase: Body return to preexcitation conditions

Refractory period

In males Erection possible, but not ejaculation

Control of LH and FSH Secretion

Negative feedback Testosterone inhibits LH and GnRH production Inhibin inhibits FSH secretion

Spermatogenesis

Spermatogonia Replicate initially by mitosis

One of the 2 primary spermatocytes undergoes

meiosis 2 nuclear divisions 1st meiotic division produces 2 secondary spermatocytes 2nd meiotic division produces 4 spermatids

Maturation of spermatozoa

Male Accessory Organs

Epididymis Maturational changes Resistance to pH changes and temperature Storage

Prostate secretes:

Alkaline fluid Citric acid Ca++ Coagulation proteins

Seminal vesicles secrete fructose

Erection, Emission, and Ejaculation

Erection: Increased vasodilation of arterioles NO is the NT Blood flow into the erectile tissues of the penis Parasympathetic

Emission: Movement of semen into the urethra

Sympathetic

Ejaculation: Forcible expulsion of semen from the urethra out of the penis Sympathetic

Female Reproductive System

Ovaries contain large number of follicles which enclose ova Extensions called fimbriae partially cover each ovary At ovulation, secondary oocyte is extruded

Fallopian (uterine) tubes: Ova drawn into the tube by cilia

Uterus: Endometrium shed during menstruation

Vagina: Cervical mucus plug

Ovarian Cycle

5 mo. gestation ovaries contain 6-7 million oogonia

Oogenesis arrested in prophase of 1st meiotic division (primary oocyte)

Apoptosis occurs 2 million primary oocytes at birth 400,000 primary oocytes at puberty

400 oocytes ovulated during the reproductive years

Menstrual Cycle

Cyclic Changes in the Ovaries Follicular Phase Ovulation Luteal Phase

Duration approximately 28 days Day 1 is the first day of menstruation

Follicular Phase FSH stimulates production of FSH receptors on granulosa cells

Follicles grow and become secondary follicle

Granulosa cells secrete estradiol

Increases sensitivity of FSH receptors

FSH and estradiol stimulate production of LH receptors in graafian follicle

Rapid rise in estradiol from granulosa cells. Negative feedback on LH and FSH

Hypothalamus increase frequency of GnRH pulses. Augments the ability of anterior pituitary to respond to GnRH to increase LH secretion

Positive feedback: LH surge begins 24 hours before ovulation Triggers ovulation

FSH increase stimulates development of new follicles

Ovulation

Under FSH, graafian follicle grows large and thin-walled, triggering LH surge

Wall of graafian follicle ruptures at day 14 Ist meiotic division is completed

Graafian follicle forms bulge on surface of ovary,

extruding secondary oocyte into the uterine tube If not fertilized, becomes corpus albicans

Luteal Phase

LH stimulates formation of the empty follicle into corpus luteum Corpus luteum secretes progesterone and estradiol

Progesterone plasma concentration rapidly rises

Negative feedback on LH and FSH Inhibin suppress FSH

Estrogen and progesterone levels fall during the late luteal phase

because the corpus luteum regresses and stops functioning

Withdrawal of estradiol and progesterone cause menstruation to occur

Menopause

Cessation of ovarian activity Age ~ 50 years Ovaries depleted of follicles Estradiol and inhibin withdrawl causes hot flashes,

and atrophy of the vaginal wall LH and FSH increase

Fertilization Ejaculation 300 million sperm, 100

reach (uterine) fallopian tube

Fertilization occurs in the uterine tubes

Acrosomal reaction: Acrosome of sperm contains hyaluronidase, an enzyme that digests a channel through zona pellucida

Sperm fuses with ovum cell membrane

As fertilization occurs, secondary oocyte completes 2nd meiotic division

Sperm enters ovum cytoplasm

Ovum nuclear membrane disappears, zygote formed

Blastocyct Formation

Cleavage: 30-36 hrs after fertilization the zygote divides by mitosis

Blastocyst develops Inner cell mass: fetus Surrounding chorion: trophoblasts form placenta

Implantation

6th day after fertilization, blastocyst attaches to uterine wall

Blastocyst secretes enzymes that allow blastocyst to burrow into endometrium

Trophoblast cells secrete hCG

hCG (Human chorionic gonadotropin)

Trophoblast cells secrete hCG Signals corpus luteum not to

degenerate until placenta secretes adequate [hormone]

Effects similar to LH Basis of pregnancy test

Placenta

Placenta Function

Gas exchange: 02 and C02

Nutrient exchange Waste exchange Synthesis of proteins and enzymes

Placental Hormones

Human placental lactogen (hPL), also called human chorionic somatomammotropin (HCS), is a polypeptide placental hormone

GH effects Diabetic-like effect

Glucose sparing effect Polyuria Lipolysis

Placental Hormones

Fetal-placental unit: Placenta must cooperate with the adrenal cortex in the fetus to produce estrogen

Estrogen stimulates endometrial growth

Inhibit prolactin secretion Growth of mammary ducts Enlargement of mother’s uterus

Progesterone

Suppresses uterine contractions Stimulates uterine growth Suppresses LH and FSH Stimulate development of alveolar tissue of the mammary gland

Parturition

Estrogen in late pregnancy Increases amount of oxytocin stored Stimulate production of oxytocin receptors in myometrium Stimulate prostaglandin production

Uterine contractions: Oxytocin, Prostaglandins

Lactation Hypothalamus releases PRH Anterior pituitary releases prolactin: Stimulate milk

production

Oxytocin needed for milk letdown