The Reproductive System

zygote

• is formed when sperm and egg fuse

• is first cell of new individual and all body cells arise from it

homologous structures

• male and female reproductive structures share a common origin during development

• example: male testes and female ovaries, or male penis and female clitoris

primary sex organs (gonads)

testes (male) and ovaries (female) produce two products:

1. gametes

2. sex hormones

gametes

sperm (male) and ova (female

• formed by cell division called meiosis

sex hormones (steroid hormoes)

testosterone (males) and estrogens/progesterone (females)

• vital in development and function of reproductive organs and other organs/tissues, sexual behaviours, sexual drives

accessory reproductive organs

ducts, glands, and external genitalia

hypothalamic-pituitary-gonadal (HPG) axis

• production of gametes and sex hormones is regulated by sequence of hormonal events involving hypothalamus, anterior pituitary gland, and gonads

• involves interacting hormones: GnRH, FSH, LH, testosterone, and inhibin

gonadotropin-releasing hormone (GnRH)

released from hypothalamus reaches anterior pituitary cells via hypophyseal portal system

follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

gonadotrophin released from anterior pituitary

inhibin

released from gonads of both male and female

• exerts negative feedback on FSH release from anterior pituitary

puberty

period of life when reproductive organs grow to adult size and become functional; earliest time that reproduction is possible

activation of the HPG axis at puberty

• before puberty small amounts of sex hormones stop the hypothalamus from releasing GnRH

• as puberty begins the hypothalamus becomes less sensitive to this “stop signal” and it starts released GnRH in pulse

• soon the GnRH tells the anterior pituitary to release FSH and LH; these tell the gonads (ovaries/tested) to make more sex hormones

• over time, the body needs higher hormone levels to stop GnRH and hormone release increased until the adult hormone cycle is reach

meiosis

• unique kind of nuclear division that occurs only in gonads

• same process for males and females

• sexual production cannot take place without this

• it reduces the number of chromosomes in gametes by half so zygote does not end up with twice as many chromosomes

sister chromatids

• cells DNA is replicated during interphase

• each chromosome has two sister chromatids that are identical and joined together by centromere

• DNA coils and condenses, making chromosomes visible when viewed through light microscope

homologous pairs of chromosomes

• one member of each pair is from father (paternal chromosome) and other is from mother (material chromosome)

• look alike and carry same genes that code for same traits

testes

sperm-producing male gonads that lie within the scrotum

• accessory sex glands: seminal glands, prostate, and bulbo-urethral glands

two tunics of testes

1. tunica vaginalis

2. tunica albuginea

tunica vaginalis

outer layer derived from peritoneum

tunica albuginea

inner layer forms fibrous capsule

seminiferous tubules

site of sperm production

conversion of sperm

seminiferous tubules → straight tubule → rete testis → efferent ductules → epididymis

accessory ducts carry sperm from testes to body exterior:

• epididymis

• ductus deferens

• ejaculatory ducts

• urethra

epididymis

• sperm mature in epididymis

• head contains efferent ductules that empty into highly coiled duct of epididymis

ductus deferens

~ 45 cm long

• passes through inguinal canal to pelvic cavity

• expands to form ampulla

• joins duct of seminal vesicle to form ejaculatory duct

vasectomy

cutting and ligating ductus deferens; nearly 100% effective form of brith control

seminal glands (seminal vesicles)

• on posterior bladder surface

• contains smooth muscle that contracts during ejaculation

• produces viscous alkaline seminal fluid

• duct of seminal gland joins ductus deferens to form ejaculatory duct

prostate

• encircles urethra inferior to bladder

• size of peach pit

• consists of smooth muscle that contracts during ejaculation

• secretes milky, slightly acid fluid

bulbo-urethral glands

• pea-sized glands inferior to prostate

• produce thick, clear mucus during sexual arousal

spermatogenesis

• process of forming new male gametes

• occurs in seminiferous tubules

• begins at puberty, around 14 years of age

• adult males make ~ 90 million sperm daily

sustentocytes (sertoli cells)

large columner cells act as supporting cells and play role in sperm formation

spermatogenic cells

cells that are surrounded by sustentocytes and give rise to sperm

myoid cells

smooth muscle-like cells surrounding seminiferous tubule that contract to squeeze sperm and testicular fluid though tubules

interstitial endocrine cells

produce androgens and some estrogen

three steps of spermatogenesis

1. mitosis of spermatogonia (stem cell) forms two spermatocytes

2. meiosis: spermatocytes form secondary spermatocytes, which form spermatids

3. spermiogenesis: spermatids become sperm

regions of sperm

• head: genetic region that includes nucleus and helmetlike acrosome containing hydrolytic enzymes that enable sperm to penetrate egg

• midpiece: metabolic region containing mitochondria that produce ATP to move tail

• tail: locomotor region that includes flagellum

role of sustentocytes

• large supporting cells (also called sertoli cells) extend from basal lamina to tubule lumen and surround developing spermatogonium

• contain light junctions that divide tubule into two compartments

→ basal compartment

→ adluminal compartment

• provide nutrients and signals to dividing cells

• secrete testicular fluid

• produce 2 mediators to regulate spermatogenesis:

1. androgen-binding protein (ABP)

2. inhibin

basal comparment

compartment in sustentocytes

• basal lamina to tight junctions; spermatogonia and primary spermatocytes located here

adluminal compartment

compartment in sustentocytes

• internal to tight junctions; area where meiotically active cells and tubule lumen are located

testicular fluid

rich in androgens and metabolic acid

androgen-binding protein (ABP)

keeps testosterone levels high to spermatogenesis

inhibin

inhibits spermatogenesis by inhibiting FSH release by anterior pituitary

sequence of regulatory events

1. hypothalamus releases gonadotropin-releasing hormone (GnRH)

2. GnRH binds to anterior pituitary gonadotropic cells, causing them to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

3. FSH stimulates spermatogenesis indirectly by stimulating sustentocytes to release androgen-binding protein (ABP)

male secondary sex characteristics

features induced in non reproductive organs by male sex hormones (testosterone)

• appearance of pubic, axillary, and facial hair

• enhanced growth of hair on chest on or other areas

• larynx enlargements causing deepening of voice

• skim thickens and becomes oily

• bones grow, increase in density

• skeletal muscles increase in size and mass

• boosts basal metabolic rate

• basis of sex drive (libido) in males

ovaries: female gonads

• produce gametes (ova)

• secrete female sex hormones estrogen and progesterone

• paired structures flank the uterus, are almond shaped and about twice as large

internal genitalia

located in pelvic cavity, include ovaries and duct system (uterine tubes, uterus, and vagina)

external genitalia

external sex organs

tunica albuginea

surrounds each ovary and it is fibrous

ovarian follicles

tiny saclike structures embedded in cortex

• contain immature egg (oocyte) encased by one or more layers of very different cells

female tube system includes:

• uterine tubes

• uterus

• vagina

uterine tubes

• receive ovulated oocyte and are usual sit of fertilization

• each tube ~10cm (4in) long and extends from area of ovary to surperior refion of uterus

regions of uterine tube

• infundibulum

• ampulla

• isthmus

infundibulum

• funnel-shaped opening into the peritoneal cavity

• contains ciliated projections called fimbriae that drape over ovary

ampulla

• forms half of uterine tube length

• is site where fertilization usually occurs

isthmus

• narrow medial third that empties into superolateral region of uterus

ectopic pregannay

• oocyte is fertilized in peritoneal cavity or distal uterne tube and begins developing there

• normally abort naturally with substantial bleeding

the uterus

• hollow, thick-walled muscular organ

• function is to receive, retain, and nourish fertilized ovum

uterine wall

three layers:

• perimetrium

• myometrium

• endometrium

perimetrium

layer of uterine wall

• outermost serous layer (visceral peritoneum)

myometrium

layer of uterine wall

• bulky middle layer consisting of interlacing layer of smooth muscle

• contracts rhythmically during child birth

endometrium

layer of uterine wall

• mucosal lining

• simple columnar epithelium on top of a thick lamina propria

• fertilized egg burrows into endometrium and resides thee during development

oogenesis

production of female gametes

process of oogenesis

begins in fetal period

• oogonia (diploid stem cells) divide by mitosis to produce:

• primary oocytes that undergo meiosis I to produce:

• secondary oocytes that undergo meiosis II to produce:

• ova

ovarian follicle

functional unit of ovary that encloses a single oocyte surrounded by:

• pre-granulosa cells if single layer of cells present

• granulosa cells if more than one layer present

primordial follicle

single layer of squamous epithelium pre-granulosa cells surrounding primary oocyte

primary follicles

have a single layer of cuboidal pre-granulosa cells surrounding primary oocyte

secondary follicles

have multiple layers of granulosa cells surrounding primary oocyte

vesicular (antral) follicle

• have a fluid-filled cavity called an antrum

• before ovulation primary oocyte inside vesicular follicle resumes meiosis and becomes secondary oocyte

atresia

apoptosis (programmed cell death) of oocytes and surrounding cells

• 99.9% of all follicles are never recruited

ovulation

• each month after puberty, a select few primary oocytes are activated

→ caused by high hormonal levels, especially FSH

→ one from this group is “selected” each month to become dominant follicle

first polar body

smaller cell that us almost lacking cytoplasm

secondary oocyte

large cell with almost all of mother cell cytoplasm and organelles

ovum

large cell with enough cytoplasm to nourish fertilized egg for 6-7 day journey to uterus

second polar body

small cell lacking cytoplasm degenerates and dies

ovarian cycle

monthly series of events associated with maturation of egg

• two consecutive phases, with ovulation occurring midcycle between phases

follicular phase

period of vesicular follicle growth (days 1-14)

luteal phase

period of corpus luteum activity (days 14-28)

corpus luteum

• remaining granulosa cells and internal thecal cells enlarge to form this

• secretes progesterone and some estrogen

hormonal interaction during ovarian cycle

1. GnRH stimulates FSH and LH secretion

2. FSH and LH stimulates follicles to grow, mature, and secrete sex hormones

3. negative feedback inhibits gonadotropin release

4. positive feedback stimulates gonadotropin release

5. LH surge triggers ovulation and formation of the corpus luteum

→ LH tranforms ruptured follicle into corpus luteum

6. negative feedback inhibits LH and FSH release

uterine (menstrual cycle)

cyclic series of changes in endometrium that occur in response to fluctuating ovarian hormone levels

three phases

1. days 0-4: menstrual cycle

2. days 5-14: proliferative (preovulatory) phase

3. days 15-28: secretory (postovulatory) phase

days 0-4: menstrual phase

• ovarian hormones at lowest levels

• gonadotropin levels are beginning to rise

• stratum functionalis detaches from uterine wall and is shed

• by day 5 growing ovarian follicles start to produce more estrogen

days 5-14: proliferative (preovulatory) phase

• rising estrogen levels prompt generation of new stratum functionalis layer

• estrogen also increases synthesis of progesterone receptors in endometrium

• ovulation occurs day 14

days 15-28: secretory (postovulatory) phase

• phase that is most consistent in duration

• endometrium prepares for embryo to implant

• rising proestrogen levels prompt functional alyer to become secretory mucosa and endometrial glands to enlarge and secrete nutrients

conceptus

developing offpsring

pregnancy

state of carrying a developing conceptus

gestation period

time from last menstrual cycle until birth

embryo

conceptus from fertilization through week 8

fetus

conceptus from week 9 through birth

cleavage

• occurs while zygote moves toward uterus

• rapid mitotic divisions of zygote occurs

• first cleavage occurs ~36 hours and produces two daughter cells called blastomeres, which continue to divide

• after 72 hours, cluster of cells contains 16 or more cells and is referred to as morula

blastocyst formation

• around day 4 or 5, embryo, which consists of ~ 100 cells is now reffered to as a blastocyst, reached uterus

composed of:

→ trophoblast cells

→ embryoblast

trophoblast cell

• display immunosuppresive factors

• participate in placenta formation

embryoblast

• cluster of 20-30 rounded cells

• becomes embryonic discs, which will form embryo and three or four extraembryonic membranes

• fourth extraembryonic membrane (chorin) is formed by trophoblast

implantation

• begin 6-7 days after ovulation

• trophoblast cells adhere to site with proper receptors and chemical signals

• inflammatory-like response occurs in endometrium

cytotrophoblast

inner layer od cells

syncytiophoblast

cells in outer layer lose plasma membranes, becoming multinclear mass