Chapter 20 Reproduction. Copyright © The McGraw-Hill Companies, Inc. Permission required for...

Chapter 20

Reproduction

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Testosterone

Female: ovaries produce: Ova Estrogens, Progesterone

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.

alleles 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.


Formation of Testes

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

During this time: Spermatogonia and oogonia migrate from yolk

sac to developing embryonic gonads Gonads could become either.

TDF (testis-determining factor): hypothetical promotes the conversion to testes:

gene located on short arm of Y, called SRY (sex determining region of Y chromosome)

Found in all mammals


Formation of Testes

Structures in the testes: Seminiferous tubules: 43 to 50 days post

conception Germinal cells: sperm. Nongerminal cells: Sertoli cells

(sustentacular cells). Leydig cells (interstitial cells):

Appear about day 65. Endocrine function: secrete androgens

Main: Testosterone


Formation of Testes

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. Decline occurs by end of second trimester

Testes descend into scrotum shortly before birth. Temp about 3 degrees below internal temp 35 degrees C


Formation of Ovaries

Absence of Y chromosome and TDF, female develop ovaries.

Ovarian follicles do not appear until 2nd trimester.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Development of Accessory Sex Organs and Genitalia

Presence or absence of testes determines the accessory sex organs and external genitalia.

Male accessory organs derived from wolffian ducts.(mesonephric)

Sertoli cells secrete MIF(mullerian inhibition factor).

Female accessory organs derived from mullerian ducts. (paramesonephritic)


Development of Accessory Sex Organs and Genitalia Both duct systems in both sexes between

days 25 and 50 Regression of mullarian ducts begins about

day 60 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 (DHT) in some

target cells Needed for penis, spongy urethra, scrotum,

prostrate Testosterone directly needed for wolfian

derivatives: Epididymis, ductus deferens, ejaculatory duct, SV


Development of Accessory Sex Organs and Genitalia


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


Endocrine Regulation of Reproduction

Hypothalamus releases LHRH (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

Female: estrogen and progesterone.

Male: testosterone.


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


Onset of Puberty

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.


Onset of Puberty Stimulate rise in

testosterone and estradiol-17. 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


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.


Control of LH and FSH Secretion

Negative feedback: Testosterone

inhibits LH and GnRH production.

Inhibin inhibits FSH secretion.

Aromatization reaction producing estadiol in the brain is required for the negative feedback effects of testosterone on LH.

Brain is a target organ for testosterone

Converted to derivatives


Testosterone Secretion Responsible for

initiation and maintenance of body changes in puberty.

Stimulate growth of muscles, larynx, and bone growth until sealing of the epiphyseal discs.

Promote hemoglobin synthesis.

Acts in paracrine fashion and is responsible for spermatogenesis.


Testosterone Secretion

Negative feedback of testosterone and inhibin Keep relatively constant levels of

gonadotropins Results in relatively constant levels Different in female

At menopause: no more sex steroids In males, gradual decrease


Endocrine function: testes

Testosterone: main androgen Sertoli and Leydig cells secrete small

amounts of estradiol. Have receptors for estradiol (as do other

male structures) May be needed for spermatogenesis

Estradiol may be responsible for: Negative feedback in brain. Sealing of epiphyseal plates. Regulatory function in fertility.


Spermatogenesis

Spermatogonia: Replicate initially by mitosis. Produce two cells

One becomes a primary spermatocytes undergoes meiosis: 2 nuclear divisions. 2nd meiotic division produce 4

spermatids.


Spermiogenesis

Maturation of spermatozoa.

Cytoplasm is pinched off and ingested by the Sertoli cell cytoplasm.


Sertoli Cells Blood-testes barrier:

Prevents autoimmune destruction of sperm. Produce FAS ligand which binds to the FAS

receptor on surface to T lymphocytes, triggering apoptosis.

Secretes inhibin. Phagocytize residual bodies:

Transmit information molecules from germ cells to Sertoli cells.

Secrete ABP (androgen-binding protein): Binds to testosterone and concentrates

testosterone in the tubules.


Hormonal Control of Spermatogenesis

Testosterone required for completion of meiosis and spermatid maturation.

Testes secrete paracrine regulators:

IGF-1. Inhibin.

FSH necessary in the later stages of spermatid maturation.


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.


Female Reproductive System

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

Uterus: Womb. 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.


Ovarian Cycle

Primary oocytes contained in primary follicles. FSH stimulates cell growth.

Develop into secondary follicles. Fusion of its vesicles into the antrum. Mature graafian follicle: 1st meiotic division completed

(secondary oocyte).


Ovarian Cycle

Secondary oocyte: Under FSH stimulation:

Theca cells secrete testosterone. Granulosa cells: contain the enzyme aromatase to convert testosterone into estrogen.


Ovulation Graafian follicle forms bulge on

surface of ovary. Extrudes secondary oocyte into

the uterine tube. Empty follicle becomes corpus

luteum and secretes: Progesterone. Estrogen. If not fertilized becomes corpus

albicans.


Menstrual Cycle

3 phases: Ovarian 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.


Follicular Phase

Rapid rise in estradiol: 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.


Follicular Phase

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. Triggers LH surge.

Wall of graafian follicle ruptures. Day 14. Ist meiotic division is completed.


Luteal Phase

LH stimulates formation of the empty follicle into corpus luteum.

Corpus luteum secretes: Progesterone: Plasma concentration rapidly rises. Estradiol.

Negative feedback on LH and FSH. Inhibin: suppress FSH.


Luteal Phase

Corpus luteum regresses unless fertilization occurs: Estradiol decreases. Progesterone decreases.

Withdrawal of estradiol and progesterone cause menstruation to occur.


Endometrial Changes

3 phases of endometrium changes: Proliferative phase. Secretory phase. Menstrual phase.


Proliferative Phase

Ovary is in follicular phase. Estradiol stimulate growth of

endometrium. Spiral arteries develop. Stimulate production of receptor

proteins for progesterone. Cornification of vaginal epithelium

occurs.


Secretory Phase

Ovary is in luteal phase. Progesterone stimulates

development of uterine glands, which become engorged with glycogen.

Endometrium becomes thick, vascular and spongy.

Cervical mucus thickens and becomes sticky.


Menstrual Phase

Progesterone cause constriction of spiral arteries.

Necrosis and sloughing of endometrium occurs.

Lasts 1-5 days.


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.


Fertilization

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 Syncytiotrophoblast

secretes enzymes that create blood filled cavities in the maternal tissue.

Cytotrophoblast then from projections (villi) that grow into the venous blood.

Producing chorion frondosum on the side that faces the uterine wall.

Other side of chorion bulges into the uterine cavity.


Placental Changes

Decidual reaction: Endometrial growth. Accumulation of glycogen.

Decidua basalis: maternal tissue in contact with the chorion frondosum.

Maternal and fetal blood do not mix.


Amnion

Envelop the embryo.

Amnionic fluid contains sloughed cells of the fetus.


Placenta Function

Gas exchange: 02 and C02.

Nutrient exchange. Waste exchange. Synthesis of proteins and

enzymes.


Placental Hormones

hCS: Chorionic somatomammotropin. 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.


Placental Hormones

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”.

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