Transcript Chapter 28 - Development

Chapter 28 - Development
SPERM MIGRATION
• In upper 1/3 of fallopian tubes
within 24 hrs of ovulation & 48 hrs
of insemination.
• Only about 3000 of original 300
million make it.
CAPACITATION
• Even after arrival at egg, sperm can’t
fertilize.
• Fluids in female soften plasma membrane
and dilute inhibitory factors that prevent the
acrosome from working
• Ca++ diffuses in and enhances tail lashing
CAPACITATION
• Timing – sperm live only 6 days, so can’t get
pregnant more than a week before
ovulation, nor more than 14 hours after [ egg
won’t be viable long enough]
Fertilization
• Several sperm needed to
enzymatically penetrate cell layers
around the ovum – acrosome
reaction – exocytosis of acrosome
• Only 1 sperm enters the ovum - tail &
midpiece disintegrate, egg releases
enzymes to destroy sperm surface
receptors – prevents polyspermy.
Fertilization
MEIOSIS
• Oocyte now completes meiosis.
• 23 male chromosomes + 23 female
chromosomes join to form zygote.
TRIMESTERS
• 1 - Fertilization through week 12 – most
sensitive time-over half die
• 2 - Weeks 13 through 24 – complete
most organ development – near end IS
possible to survive birth
TRIMESTERS
• 3 - 25 weeks – birth - growth continues to
point where survival is more likely – brain
liver & kidneys have to develop further
AFTER birth.
• Single births usually 40 weeks, twins 35
weeks
CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
• Mitotic cell divisions produce
identical copies.
• Morula - solid ball of cells.
• Blastocyst - hollow ball - cavity is
called blastocoel.
– Made of trophoblast [outer layer] &
Inner cell mass
Human Morula
Human
Blastocyst
CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
– Becomes implanted in endometrium inner cell mass facing endometrial wall.
TAKES ABOUT 1 WEEK.
• Trophoblast gives rise to chorion forms placenta with chorionic villi –
secretes hCG to maintain corpus
luteum.
CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
• Inner cell mass forms embryo and
other extra-embryonic membranes
IMPLANTATION ~ 6 days after ovulation
• Trophoblast cells facing wall fuse 
syncytiotrophoblast – grows into uterus
• Uterus responds by growing over and
burying entire blastocyst
• Ideally blastocyst implants HIGH on the
wall.
• Trophoblast grow into placenta
IMPLANTATION ~ 6 days after ovulation
• Placenta - nutritive bridge between
mother & fetus –by end of 3rd month
– Produces large amounts of hCG - peaks at
wk 8-9, drops to constant level at wk 16,
stimulates corpus luteum to continue
secretion of ES and P
– Later, the placenta secretes its own P and ES
to maintain pregnancy, and relaxin to aid in
delivery and prevent premature contractions
EXTRA-EMBRYONIC MEMBRANES
• Chorion - outer -derived from trophoblast
of blastocyst.
– Forms placenta.
– Secretes estrogens, progesterone, relaxin.
• Amnion - for protection.
– Filled with amniotic fluid.
– Shock absorber for fetus.
– Repository for fetal urine, etc.
EXTRA-EMBRYONIC MEMBRANES
• Allantois - blood vessels to placenta
and umbilicus.
– Later becomes ligament attached to
urinary bladder.
• Yolk sac - has little yolk, but source of
early blood cells & primordial
gametes.
EMBRYOGENESIS
• Embryoblast flattens into embryonic
disc in amniotic cavity – 2 layers –
epiblast [toward cavity, hypoblast –
away from]
• Primitive streak forms along midline
with primitive groove running down it
 bilateral
symmetry, front and back defined
EMBRYONIC DEVELOPMENT - Gastrulation
• Results in 3 germ layers - ectoderm,
endoderm, mesoderm.
• Epiblast will become ectoderm [skin,
etc. and CNS]
• Migration of epiblast cells into and
through the primitive groove to replace
hypoblast cells with endoderm – will be
gut lining
EMBRYONIC DEVELOPMENT - Gastrulation
• Then a third layer migrates in between
the two = mesoderm [will become
mesenchyme – basis of connective
tissues, and muscle