Chapter 19:  Human Reproduction

 

I.    Mammalian Reproduction

A.     Functional anatomy of the human male:

1.   Gonads:  testes – the gamete and androgen producing organ

    1. seminiferous tubules
    2. epididymis

2.   accessory reproductive organs:

a.       Glands:  seminal vesicles; the prostate gland; bulbourethral or Cowper’s glands:  see below for functions

b.      Ducts:  passage from the testes to accessory organs; Vas or ductus deferens; ejaculatory duct and urethra.

c.       External genitalia:  scrotum (sac of skin that contains the testes) and penis.

B.     What’s in semen?  Sperm and accessory gland fluids.

1.   epididymis:  carnitine.

2.   seminal (vesicle) fluid:  fructose, fibrinogen and prostaglandins

3.   prostate gland secretions:  bicarbonate, fibrinogenase and fibrinolysin

4.   bulbourethral gland:  mucus and buffers

5.   all these ingredients are mixed only in the event of ejaculation.

6.   Fertility index notes:

    1. at least 20 million sperm/ml
    2. 40% of the sperm must show vigorous swimming
    3. 60% of the sperm must be of normal shape and size
    4. pH needs to be approx. 7.3-7.5

C.     Male reproductive physiology

1.   spermatogenesis

a.       seminiferous tubules:  spermatogonium cells and primary spermatocytes

b.      diploid primary spermatocyte à four haploid spermatids.  buried in nuturing sertoli cells.  spermatozoa are released from the sertoli cells into the lumen of the tubules.  From here they will make their way to the epididymis.

2.      regulation of testes function

a.       hypothalamus (GnRH) à anterior pituitary (gonadotropins: LH and FSH)

b.      Role of LH:  Leydig (or interstitial) cells

c.       Role of FSH:  stimulates the sertoli cells; ABP; spermiogenesis

D.    Functional anatomy of the human female

1.   gonads:  ovaries

    1. follicles:  where oogenesis occurs; estrogen production.
    2. Corpus luteum:  produces progesterone, estrogens, relaxin, and inhibin

2.  accessory reproductive organs

a.       accessory ducts:  uterine tubes (aka: oviducts or fallopian tubes), uterus, cervix and vagina

b.      External genitals or vulva:  mons pubis (hair covered mound above clitoris);  labia majora;  labia minora;  clitoris; Bartholin’s glands

E.     Female reproductive physiology

1.   oogenesis:  A summary of some of the differences between oogenesis and spermatogenesis follows:

    1. It can take from approx. 12 to more than 50 years for a female germ cell to complete meiosis whereas the completion of spermatogenesis takes 60-70 days.
    2. Females are born with the all the potential egg cells, approx. 500,000, they will ever have.  At puberty there will be about 85,000 left, more than enough to last through menopause.  After puberty spermatogenesis of all stages is taking place on a daily basis.
    3. For every primary oocyte that completes meiosis one egg or oocyte will have been formed.  For every primary spermatocyte that enters meiosis four sperm cells will form.  Although the mechanics of the meiotic nuclear divisions are identical in females and males, in females cytokinesis is unequal.

2.   regulation of the ovarian  and menstrual cycle:  Similar to the male, in the female, the hypothalamus produces and secretes gonadotropin releasing hormone (GnRH).  However at puberty the GnRH is released in pulses rather than in steady amounts as with the male.  The following is a brief summary of the events and regulation of the menstrual cycle:

a.       increase in FSH secretions:  stimulates growth; LH receptor development; estrogen secretion.

b.      Increasing estrogen secretions:  stimulates endometrium;  stimulates LH surge;  inhibits FSH secretions.

c.       LH surge and smaller FSH spike:  stimulates ovulation and corpus luteum development

d.      Corpus luteum produces and secretes estrogens, progesterone, and inhibin.

e.       Increasing levels of estrogens and progesterone:  inhibits FSH and LH secretions by negative feedback;  stimulates secretory phase of uterine endometrium.

f.        Corpus luteum degenerates if fertilization fails to occur

g.       Decreases in estrogen and progesterone:  endometrium breaks down and menstrual flow begins

h.       Continued decrease in estrogen diminishes negative feedback inhibition of FSH, which in turn will increase and begin the cycle anew.

 

F.      Fertilization

1.   sperm acrosome (sac of hydrolytic enzymes)

2.   fast block to polyspermy

3.   Entrance of the sperm into the egg cytoplasm

a.       completion of meiosis II by the egg nucleus

b.      the cortical reaction: slow block to polyspermy

4.   Sperm can survive in the female reproductive tract for up to 5 days

5.      Coitus can induce ovulation at almost any point in the month

6.      fertilization takes place in the fallopian tube.  The zygote begins to divide and develop as it travels toward the uterus, a trip that can take 3-4 days, where it will implant