Female reproductive system

I.  Introduction

- female reproductive system far more complex than male:

• production of sex cells
• production of sex hormones
• receive penis during sexual intercourse
• create environment for implantation
• create environment for embryo development

- sex organs:  ovaries;  produce eggs and sex hormones

- accessory ducts:  oviducts (uterine tubes), uterus, vagina

- external sex organs:  external genitalia, vulva

- mammary glands

II.  Anatomy of female reproductive system

A.  Ovaries

- paired, flank uterus on each side, almond-shaped

- X-section:  outer layer with forming gametes at different stages of development; inner layer region has blood vessels, nerves

- in cortex see ovarian follicles:  one or more layer(s) of cells surrounding an immature egg, oocyte

• surrounding cells called follicular cells
• at birth woman has many follicles each containing an oocyte
• every month at least one follicle and associated oocyte stimulated to develop
• follicle matures, oocyte "released" through ovulation
  • fertilization -- completes maturation
• "leftover" follicle -- corpus luteum -- becomes endocrine gland

B.  Uterine tubes

- receive ovulated oocyte, provide site where fertilization occurs; very muscular walls

- fimbrae with cilia draw egg into tubes; peristalsis moves egg to uterus

- ectopic pregnancy -- fertilization and implantation in peritoneal cavity

C.   Uterus

- hollow, thick walled organ:  receives, retains, nourishes fertilized ovum

- size/shape of inverted pear; during pregnancy up to 60X original size

- layers:

• perimetrium: outermost, visceral peritoneum
• myometrium:  bulky, interlacing bundles of smooth muscle
• endometrium:  mucosal lining of uterine cavity, undergoes cyclical changes in response to blood levels of ovarian hormones

D.  Vagina

-  thin-walled tube, 8-10 cm long, extends from vestibule to cervix

• receives penis/semen during intercourse
• provides passageway for delivery during childbirth
• highly distensible wall
• very well vascularized
• acidic mucosa to prevent infection

E.  External genitalia

-  collectively known as vulva

• mons pubis 
  • rounded area overlying pubic symphysis
• labia majora
  • two hair-covered skin folds surrounding vaginal and urethral openings
  • homologous to male scrotum
• labia minora
  • two hairless skin folds
  • enclose space called vestibule
  • homologous to ventral penis
• vestibule
  • space within labia minora that contains external openings of urethra and vagina
  • flanking vagina are vestibular glands that release mucous into vestibule
    • moisten, lubricate, facilitate intercourse
    • homologous to bulborethral glands in male
• clitoris:  anterior to vestibule
  • erectile tissue, homologous to penis
  • covered by thin fold, prepuce

F.  Mammary glands:  

• present in both sexes only functional in female; modified sweat glands
• internally divided into 15-20 lobes by CT
• lobes divided into lobules that contain secretory units or alveoli that empty secretion into lactiferous duct system
• unite at nipple

III.  Physiology of female reproduction

A.  Oogenesis

- note that in male gamete production begins at puberty; in female preparation for gamete production begins in fetus

• stem cells --  oogonia
• oogonia divide mitotically (several million) enlarge, mature -- before birth
• cells become primary oocytes when they begin first meiotic division (DNA replicated)
  • division is stalled prior to birth
• primary oocyte enclosed by follicular cells
• at puberty under appropriate stimulation
  • follicle growth
  • completion of first meiotic division 
    • secondary oocyte and polar body
  • ovulation
  • when fertilized second meiotic division takes place
    • ovum and polar body

-  thus at birth all of women's potential eggs formed:  of about 2 million follicles, at birth about 700,000 remain; by puberty about 300-400,000 left

B.  Ovarian cycle

-  follicular phase:  period of follicular growth, d1-d14

-  luteal phase:  period of corpus luteum activity, d14-d28

1.  follicular phase

• starting at puberty a group of  follicles stimulated to continue development every 28 days
• follicle cells divide
  • follicle cells begin to produce estrogen
• complete meiosis I
• ovulation
• meiosis II

2.  luteal phase:

1. remaining follicular cells begin producing  progesterone, estrogen -- corpus luteum (CL)
2. if fertilization occurs, the embryo produces human chorionic gonadotrophin (HCG) which maintains CL until placenta can kick in its own progesterone/estrogen production
3. if fertilization does not occur, CL degenerates as no HCG

C.  Hormonal regulation of ovarian cycle

-  feedback inhibition patterns:

• extremely low levels of estrogen stimulate FSH
• moderate levels of estrogen inhibit FSH/LH
• high level of estrogen stimulate LH/FSH release

D.  Uterine cycle:  menstrual cycle, regulated by ovarian hormones

• d1 of cycle, est/prog very low -- shed endometrium
  • FSH level begin to increase due to lack of feedback inhibition
• FSH stimulates follicle growth
  • follicles produce estrogen
  • uterine build up
• as estrogen levels increase, LH/FSH spike
  • ovulation
  • estrogen levels begin to drop
  • corpus luteum develops -- secretes progesterone/some estrogen 
    • builds up endometrium and prepares it for implantation
    • depending if HCG present or not, CL maintained or degenerates and likewise endometrium

E. Extra-uterine estrogen effects

- female secondary sex characteristics:  increased deposits of subcutaneous fat, widening, lightening of pelvis; growth of axillary and pubic hair.

- maturation of reproductive structures

F.  Female sexual response

1.  excitement or arousal:

• sexual arousal results from:
  • psychological stimuli
  • physical stimuli
    • tactile stimulation of clitoris and external genitalia
• stimuli trigger neural reflexes 
  • parasympathetic induced vasodilation of arterioles throughout vagina and external genitalia
  • swelling of labia
  • erection of clitoris -  more sensitive to stimulation
  • increased flow through vaginal capillaries
    • fluid forced out of vessels into vaginal lumen
    • lubricant for intercourse
  • increased vestibular gland activity
  • breast enlarge and nipples erect as result of vasocongestion
    • nipples very sensitive -- stimualtion increases neural influences on vagina, external genitalia

2.  Plateau  

- changes initiated during excitement phase intensify

• lower third of vagina swells with blood
  • tighten around penis
• simultaneously uterus contracts upwards, lifts cervix, enlarges upper two thirds of vagina
  • space for ejaculate
• systemic effects 
  • increased BP, respiratory rate, muscle tension

3.  Orgasm

- at certain level of arousal, massive, whole body neural discharge

• lower third of vaginal canal and uterus contract rhythmically
• pelvic musculature contracts rhythmically
• increased HR/BP
• endorphin/enkephalin release
• sexual intensity can diminish to plateau  -- brought back to peak again
  • no refractory period
  • no ejaculate