Lecture 3: Reproduction 1 (Meiosis, Sex Determination and Differentiation, Basic Steroidogenesis, Puberty, and Reproductive Capacity, Male Reproductive Structures, Spermatogenesis, Erection, and Ejaculation

how many autosomes and sex chromosomes in sperm?

22 autosomes + X or Y sex chromosome

how many autosomes and sex chromosomes in ovum?

22 autosomes + X sex chromosome

how many autosomes and sex chromosomes in fertilized egg

22 pairs of autosomes and 1 pair of sex chromosomes (male XY, female XX)

how many chromosomes in somatic cells

46 chromosomes (by mitosis)

how many chromosomes in germ cells

23 chromosomes

steps of meiosis

1. interphase: DNA replicates, each chromosome has 2 identical sister chromatids

2. homologous chromosomes pair up with each other

3. crossing over occurs (exchange of alleles)

4. independent assortment (random alignment and separation of homologous chromosomes)

5. meiosis 1 occurs

6. 2 daughter cells with 23 chromosomes, each chromosome still containing 2 sister chromatids (nx2)

7. 23 chromosomes line up along the equator, and the sister chromatids separate = meiosis II

8. have 4 haploid cells (n, or 23 chromosomes/cell nucleus)

male sex determination

Y chromosome (XY) → SRY gene expression → testes develop

female sex determination

absence of Y chromosome (XX) → no SRY gene → ovary determining (OD) genes expression → ovaries develop

male sex differentiation

• sertoli cells → MIS secreted → Mullerian ducts regress → no female internal structures

• leydig cells → testosterone → wolffian ducts develop → development of male internal structures

• testosterone → 5-alpha-reductase (enzyme) → dihydroxytestosterone (DHT) → development of external male structures

female sex differentiation

• no MIS → development of female internal structures

• no testosterone → wolffian ducts regress and development of female external genitalia

HPG axis

• GnRH = gonadotropin releasing hormone

• FSH = follicle stimulating hormone

• LH = luteinizing hormone

• ** sex hormones are typically inhibitory at hypothalamus and anterior pituitary. however, estrogens can be stimulatory to anterior pituitary during a particular time of the menstrual cycle.

steroidogenesis

where and when is testosterone produced

• males: leydig cells of testes

• females: theca cells of ovary in follicular phase, large luteal cells of ovary in luteal phase, and during pregnancy by placenta

• DHEA (an androgen) is produced by adrenal gland of males and females → testosterone derived

where and when is estrogen produced

• females: granulosa cells of ovary in follicular phase, large luteal cells of ovary in luteal phase, and during early pregancy by large luteal cells and placenta

• some male tissues (ex. breast tissue and brain) produce estrogen

• DHEA → estrogen

where and when is DHT produced

• requires 5alpha-reductase and testosterone

• male (in utero): development of penis, scrotum, prostate

• male (after puberty): in prostate, also in hair follicles (associated with male pattern baldness)

where and when is progesterone produced

• female: large luteal cells in ovary during luteal phase, and placenta during pregnancy

• can be converted to testosterone and then estrogens

puberty in female

GnRH secretion increases → FSH and LH secretion increases → estrogen secretion increases

puberty in males

increased GnRH secretion → increased LH and FSH → stimulates seminiferous tubules and testosterone secretion

why is pulsatile pattern in GnRH release important

• continuous GnRH → LH and FSH desensitize

• pulsitate GnRH → LH and FSH constant

male reproductive capacity

• continuously active after puberty

• reproduce at least into their 70s and 80s, possibly longer

female reproductive capacity

• a single oocyte is released per 28 day cycle

• after ovulation, egg remains viable for 24-48 hours

• menopause occurs around age 50 and females lose reproductive capacity

testis

(located in scrotum) — where sperm is produced and stored

epididymis

where sperm matures.

vas deferens

where sperm travels during ejaculation

semniferous tubules

primary site of spermatogenesis in testes

leydig cells

secrete testosterone

role of sertoli cells in spermatogenesis

• transports androgen binding protein which binds and transports testosterone from Leydig to Sertoli cells where it can be released to bathe the developing sperm

• generate other secretions that nourish and guide sperm development

• phagocytize defective sperm

circualting androgens can be converted to

1. DHT by 5alpha-reductase (particularly important in utero for metal development of male external genitalia)

2. beta-estradiol by aromatase in some tissues (like adipose, liver, testes, and brain)

steps of spermatogenesis

primary spermatocytes is produced by

spermatogonia during and after puberty due to FSH and LH actions

relationship of sertoli cells and leydig cells in testis

1. LH stimulates Leydig cells → produce testosterone

2. FSH stimulates sertoli cells + testosterone from leydig cells move into sertoli cells → support spermatogenesis

3. differentiation as cells move toward the lumen

mechanism of erection

A drug that targets what molecule or enzyme would improve initiation and maintenance of erection?

• PDE5 inhibitors (ex. Viagra, Levitra, Cialis) cause maintained elevation in cGMP

• PDE5 (a phosphodiesterase) breaks down cGMP → erection ends.

ejaculation mechanism

increased sympathetic activity

1. strong contractions of epididymis, vas deferens, ejaculatory ducts, prostate, and seminal vesicles

2. increased secretion of fluids by seminal vesicles and prostate

3. emission: sperm and semen move to urethra

4. ejaculation: expulsion of sperm and semen via rapid contractions of urethral smooth muscle and skeletal muscle at base of penis

5. urethral sphincter at bladder closes so that urine doesn’t mix with semen and so that sperm doesn’t enter the bladder

Explain why there is decreased functioning of the reproductive tract (decreased fertility) with anabolic steroid use?

Increased plasma testosterone→

• Decreases GnRH and LH directly →Decreased testosterone production by Leydig cells

• Decreases FSH indirectly via decreased GnRH→ in combination with the decreased local [testosterone] in the testes →decreased Sertoli cell function→

• Decreased spermatogenesis and decreased fertility

• In females: decreased GnRH and LH and FSH also occurs, which decreases female fertility as well, plus high circulating testosterone has masculinizing effects.

prostate fluid secretion

30% seminal fluid

vas deferens fluid secretion

10% seminal fluid

seminal vesicles fluid secretion

60% seminal fluid

bulbourethral gland fluid secretion

lubricating mucus