Formation&Development Of Reproductive System

1. Reproduction www.freelivedoctor.com

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

Each individual produces gametes

Formed in gonads by meiosis

Male: testes produce:

Testosterone

Female: ovaries produce:

Estrogens, Progesterone

Gametes unite in process of fertilization

Restores diploid number

Forms zygote

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

23 rdpair are sex chromosomes.

Male: XY

Female: XX

Chromosomal gender of zygote determined by fertilizing sperm.

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

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

Leydig cells secrete testosterone.

Begins 8 thweek and peaks at 12-14 thweek.

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

Absence of Y chromosome and TDF, female develop ovaries.

Ovarian follicles do not appear until 2 ndtrimester.

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 )

Both duct systems in both sexesbetween 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

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 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.

Primary effects of LH and FSH on gonads:

Stimulation of spermatogenesis and oogenesis.

Stimulation of gonadal hormone secretion.

Maintenance of gonadal structure.

Negative Feedback:

Inhibit GnRH from hypothalamus.

Inhibit anterior pituitary response to GnRH.

Inhibin secretioninhibit anterior pituitary release of FSH.

By sertoli cells

Female: estrogen and progesterone.

Male: testosterone.

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.

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-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.

Secretes melatonin.

Secretion influenced by light-dark cycles.

Inhibit gonadotropin secretion.

Role in humans not established.

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

Testes:

Seminiferous tubules:

Where spermatogenesis occurs.

Contain receptor proteins for FSH in Sertoli cells.

Leydig cells:

Secrete testosterone.

Contain receptor proteins for LH.

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

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.

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

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.

Spermatogonia:

Replicate initially by mitosis.

Produce two cells

One becomes a primary spermatocytes undergoesmeiosis:

2 nuclear divisions.

2 ndmeiotic division produce 4 spermatids.

Maturation of spermatozoa.

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

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.

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.

Epididymis:

Maturational changes.

Resistance to pH changes and temperature.

Storage.

Prostate secretes:

Alkaline fluid.

Citric acid.

Ca ++ .

Coagulation proteins.

Seminal vesicles secrete:

Fructose.

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

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.

5 mo. gestationovaries contain 6-7 million oogonia.

Oogenesis arrested in prophase of 1 stmeiotic division (primary oocyte).

Apoptosis occurs:

2 million primary oocytes at birth.

400,000 primary oocytes at puberty.

400 oocytes ovulated during the reproductive years.

Primary oocytes contained in primary follicles.

FSH stimulates cell growth.

Develop into secondary follicles.

Fusion of its vesicles into the antrum.

Mature graafian follicle:

1 stmeiotic division completed (secondary oocyte).

Secondary oocyte:

Under FSH stimulation:

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

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.

3 phases: Ovarian

Follicular Phase

Ovulation

Luteal Phase

Duration approximately 28 days.

Day 1 is the first day of menstruation.

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:

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.

Under FSH, graafian follicle grows large and thin-walled.

Triggers LH surge.

Wall of graafian follicle ruptures.

Day 14.

Ist meiotic division is completed.

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.

Corpus luteum regresses unless fertilization occurs:

Estradiol decreases.

Progesterone decreases.

Withdrawal of estradiol and progesterone cause menstruation to occur.

3 phases of endometrium changes:

Proliferative phase.

Secretory phase.

Menstrual 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.

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.

Progesterone cause constriction of spiral arteries.

Necrosis and sloughing of endometrium occurs.

Lasts 1-5 days.

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.

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 2 ndmeiotic division.

Sperm enters ovum cytoplasm.

Ovum nuclear membrane disappears, zygote formed.

Cleavage:

30-36 hrs. after fertilization the zygote divides by mitosis.

Blastocyst develops:

Inner cell mass: fetus.

Surrounding chorion: trophoblasts form placenta.

6 thday after fertilization, blastocyst attaches to uterine wall.

Blastocyst secretes enzymes that allow blastocyst to burrow into endometrium.

Trophoblast cells secrete 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.

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.

Decidual reaction:

Endometrial growth.

Accumulation of glycogen.

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

Maternal and fetal blood do not mix.

Envelop the embryo.

Amnionic fluid contains sloughed cells of the fetus.

Gas exchange:

0 2and C0 2 .

Nutrient exchange.

Waste exchange.

Synthesis of proteins and enzymes.

Chorionic somatomammotropin.

GH effects.

Diabetic-like effect:

Glucose sparing effect.

Polyuria.

Lipolysis.

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 mothers uterus.

Progesterone:

Suppresses uterine contractions.

Stimulates uterine growth .

Suppresses LH and FSH.

Stimulate development of alveolar tissue of the mammary gland.

Estrogen in late pregnancy:

Increases amount of oxytocin stored.

Stimulate production of oxytocin receptors in myometrium.

Stimulate prostaglandin production.

Uterine contractions:

Oxytocin.

Prostaglandins.

Hypothalamus releases PRH.

Anterior pituitary releases prolactin:

Stimulate milk production.

Oxytocin needed for milk letdown.

Formation&Development Of Reproductive System

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