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