D’YOUVILLE COLLEGE
BIOLOGY 108 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 26
PREGNANCY AND DEVELOPMENT
1.
Fertilization and Implantation:
a. Fertilization: union of haploid (n) ovum and haploid spermatozoon
producing diploid (2n) zygote
• Meiosis: a series of two nuclear divisions (reduction division followed by
equation division); no intervening period of DNA synthesis (chromosome
duplication)
• Special Considerations:
• Me I begins in primary oocytes of ovary prior to birth; is completed in
selected primary follicles during puberty (stimulated by FSH); produces large
secondary oocyte and small polar body (fig. 27 - 17)
• secondary oocyte ovulates & undertakes Me II following fertilization,
to produce female pronucleus and second polar body
• Union of Sperm and Egg:
• sperm undergoes changes through interaction with secretions of female
reproductive tract (capacitation)
• acrosome secretes enzymes to penetrate zona pellucida (fig. 28 - 2)
• sperm tail is shed; genetic material passes into oocyte cytoplasm
becoming male pronucleus
• changes in egg membrane prevent polyspermy
b. Cleavage: (fig. 28 - 4)
• fusion of male and female pronuclei form diploid zygote nucleus or
segmentation nucleus (fig. 28 - 3)
• Morula (morula = little mulberry) formed by rapid series of mitotic
divisions; cells become progressively smaller with each division (cleavage or
segmentation)
• Blastocyst: reorganization of cells into inner cell mass (produces embryo)
& trophoblast (membrane enclosing fluid-filled cavity)
c. Implantation: trophoblast secretions enable blastocyst to burrow into
endometrium; cleavage ends with implantation (fig. 28 - 5)
2.
Hormonal Changes During Pregnancy: (fig. 28 - 6)
a. Trophoblast: secretes human chorionic gonadotropin (HCG) for approx.
three months maintaining corpus luteum of ovary with resulting continued output
of estrogens and progesterone, continued maintenance of endometrium and
continued negative feedback inhibition of anterior pituitary gonadotropin output
b. Placenta:
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• takes over from corpus luteum for balance of pregnancy producing high
levels of estrogens and progesterone
• secretes human placental lactogen (HPL) promoting breast enlargement
and preparation for lactation
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3.
Embryonic Stage (Implantation to Eight Weeks): (fig. 28 - 7)
a. Embryonic Disc: forms from inner cell mass; consists of epiblast (future
embryonic body) & hypoblast (future extraembryonic membranes)
• organizes into three layers (primary germ layers):
i. ectoderm (= “outer skin”)
ii. mesoderm (= “middle skin”)
iii. endoderm (= “inner skin”)
• elaborate folding converts embryonic disc into cylindrical body
configuration; body separates from trophoblast except for connecting strand called
“body stalk” (future umbilical cord)
b. Extraembryonic Membranes:
• Yolk Sac: a vestigial structure in umbilical cord
• Allantois: also vestigial part of umbilical cord
• Amnion: fluid-filled shock absorber completely surrounding embryo
• Chorion: trophoblast elaborates villi (villi =finger-like folds) and is then
called the chorion; forms fetal placenta
• Placenta and Umbilical Cord: following implantation, endometrium
forms maternal contribution to placenta; secretions of the chorionic villi erode the
tissues until bathed in maternal blood (= hemochorial type placenta) (fig. 28 - 8);
vessels supplying the chorion reach it via the body stalk which thins out as the
umbilical cord
c. Organogenesis:
• most organ systems form by end of 8-week embryonic period:
• skin and nervous tissues derived from ectoderm
• linings of digestive and respiratory tracts derived from endoderm
• remaining tissues derived from mesoderm; i.e. musculoskeletal
system, blood & cardiovascular systems, kidneys & urinary system, body cavity
linings and connective tissues (fig. 28 - 13)
4.
Fetal Stage (8 Weeks to Term):
a. Maturation of Organ Systems (Highlights):
• differential growth rates (head slower, limbs & trunk faster) restore more
normal body/head proportions
• external genitalia develop
• skin progresses from thin, wrinkled to chubby, smooth condition due to
deposition of body fat
• hair and nails develop
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b. Fetal Circulation:
• Umbilical Arteries: derived from allantoic circulation, carry waste-laden
blood from fetus to placenta; reduced to small bladder arteries and umbilical
ligaments after birth
• Umbilical Vein: returns oxygen-rich and nutrient-rich blood from
placenta to fetus via ductus venosus through liver to inferior vena cava of fetus;
becomes round ligament of liver and ligamentum venosum after birth
• Foramen Ovale: opening in interatrial septum allows blood from inferior
vena (including blood from placenta) to flow directly from right to left atrium,
bypassing pulmonary circuit; becomes closed after birth (fossa ovalis)
• Ductus Arteriosus: direct connection from pulmonary trunk to aorta
shunts much of bloodflow from right ventricle to aorta, bypassing pulmonary circuit;
deteriorates after birth (ligamentum arteriosum)
5.
Parturition:
• placental estrogens sensitize oxytocin receptors in uterus
• uterine distension initiates positive feedback cycle of myometrial
contractions (labor)
• labor augmented by oxytocin from posterior pituitary; stimulates
myometrium & stimulates placenta to release prostaglandins (fig. 28 - 17)
• cervix dilates, fetus passes out vagina
• placenta delivered approx. 15 minutes later (“afterbirth”) (fig. 28 - 18)