Pregnancy
Conceptus
Period of gestation
Preembryo
Embryo
Fetus
Neonate
oöcyte viability
sperm viability
fertilization
fates of ejaculated sperm include:
leak out of the vagina immediately after deposition
destroyed by the acidic vaginal environment
fail to make it through the cervix
dispersed in the uterine cavity or destroyed by phagocytic leukocytes
reach the uterine tubes
Spermmust undergo capacitation before they can penetrate the oocyte
ovulated oöcyte is encapsulated by
corona radiata
zona pellucida
sperm binds to the zona pellucida and undergoes the acrosomal reaction
acrosomal enzymes
perm contacts oöcyte membrane:
fertilin binds to integrin
only one sperm is allowed to penetrate the oöcyte
mechanisms to ensure monospermy
fast block to polyspermy
slow block to polyspermy
cortical granules release enzymes that destroy sperm receptors
these enzymes cause sperm already bound to receptors to detach
upon entry of sperm, the secondary oöcyte:
completes meiosis II
casts out second polar body
oöcyte nucleus swells, and the two pronuclei approach each other
fertilization
first cleavage produces two daughter cells called blastomeres
morula– the 16 or more cell stage (72 hr old)
by fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)
single layer of trophoblast cells
inner cell mass
trophoblast takes part in placenta formation
inner cell mass becomes the embryonic disc
six to seven days after ovulation
trophoblast proliferates
cytotrophoblast
syncytiotrophoblast
implanted blastocyst is covered over by endometrial cells
implantation completed by d14 following ovulation
corpus luteum is maintained
corpus luteum to continue to secrete progesterone and estrogen
chorion continues hormonal stimulus
by second or third month, the placenta becomes functional
placenta formed from
embryonic trophoblastic tissues
maternal endometrial tissues
chorion develops fingerlike villi
decidua basalis
part of the endometrium that lies between the chorionic villi and stratum basale (s.b. endometrii)
decidua capsularis
portion of the decidua directly overlying the chorionic vesicle and facing the uterine cavity
placenta fully formed and functional by the end of third month
embryonic placental barriers
chorionic villi
endothelium of embryonic capillaries
placental hormones
blastocyst develops into gastrula with three primary germ layers
ectoderm
endoderm
mesoderm
before becoming three-layered, the inner cell mass subdivides into
upper epiblast → ectoderm
lower hypoblast → endoderm
amnion
provides a buoyant environment that protects the embryo
helps maintain constant homeostatic temperature
amniotic fluid
yolk sac
forms part of gut tube
produces earliest blood cells and vessels
source of primordial germ cells
allantoïs
structural base for umbilical cord
becomes part of urinary bladder
chorion
encloses embryonic body and all other membranes
during 3rd week, two-layered embryonic disc becomes a three-layered embryo
primary germ layers
primitive streak
as cells begin to migrate
first cells that enter the groove form the endoderm
cells that follow push laterally between the cells forming the mesoderm
cells that remain on the embryo’s dorsal surface form the ectoderm
notochord
serve as primitive tissues from which all body organs will be derived
ectoderm → structures of nervous system and skin epidermis
endoderm → epithelial linings of the digestive, respiratory, and urogenital systems
mesoderm → all other tissues
cells of endoderm and ectoderm are securely joined and form almost all epithelia
gastrulation sets the stage for organogenesis
by 8th week, all organ systems are recognizable
neurulation
formation of neural plate
neural plate folds inward as a neural groove
by 22nd day, neural folds fuse into a neural tube
anterior end becomes the brain; the rest becomes the spinal cord
neural crest cells give rise to cranial, spinal, and sympathetic ganglia
embryonic folding begins with lateral folds
next,head and tail folds appear
endodermal tube forms the epithelial lining of the GI tract
organs of GI tract become apparent, and oral and anal openings perforate
endoderm forms epithelium linings of the hollow organs of digestive and respiratory tracts
first evidence is appearance of the notochord
three mesoderm aggregates appear lateral to the notochord
40 pairs of somites
sclerotome → vertebrae and ribs
dermatome → forms the dermis of skin on dorsal part of body
myotome → forms skeletal muscles of the neck, trunk, and limbs
intermediate mesoderm → gonads and kidneys
lateral mesoderm
somatic mesoderm
dermis of skin in ventral region
parietal serosa of ventral body cavity
bones,ligaments, and dermis of limbs
splanchnic mesoderm
heart and blood vessels
most connective tissue
by end of the 3rd week:
embryo has system of paired vessels
vessels forming heart have fused
unique vascular modifications in prenatal development
two umbilical arteries
one umbilical vein
three vascular shunts
ductus venosus → ligamentum venosum
foramen ovale → fossa ovalis
ductus arteriosus → ligamentum arteriosum
Chadwick’s sign
breasts enlarge; areolae darken
uterus expands
lordosis
pelvic ligaments and pubic symphysis “relax”
weight gain
placenta secretes chorionic somatomammotropin (hCS)
promotes growth of the fetus and exerts a maternal glucose-sparing effect
stimulates the maturation of the breasts
chorionic thyrotropin (hCT)
increases maternal metabolism
parathormone levels are high
ensures positive calcium balance
GI tract
morning sickness occurs due to elevated levels of estrogen and progesterone
urinary tract
urine production increases to handle the additional fetal wastes
respiratory system
nasal congestion may occur
dyspnea may develop late in pregnancy
Cardiovascular system
blood volume increases 25-40%
venous pressure from lower limbs is impaired, resulting in varicose veins
estrogen reaches a peak during the last weeks of pregnancy increasing myometrial irritability
weak Braxton-Hicks contractions may take place
as parturition nears, oxytocin and prostaglandins cause uterine contractions
emotional and physical stress
activates hypothalamus
sets up positive feedback mechanism, releasing more oxytocin
from onset of labor until the cervix is fully dilated (10 cm)
initial contractions are 15–30 minutes apart and 10–30 seconds in duration
cervix effaces and dilates
amnion ruptures, releasing amniotic fluid (the so-called “breaking of the water”)
infant’s head enters the true pelvis
from full dilation to delivery of the infant
strong contractions occur every 2–3 minutes and last about 1 minute
urge to push increases
crowning occurs when the largest dimension of the head distends the vulva
delivery of the placenta accomplished within 30 minutes of birth
afterbirth
placenta fragments must be removed to prevent postpartum bleeding
at 1–5 minutes postpartum, the infant’s physical status is assessed based on five signs
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8–10indicates a healthy baby
as carbon dioxide is no longer removed by the placenta, central acidosis occurs
this excites the respiratory centers to trigger the first inspiration
requires tremendous effort
once lungs inflate, surfactant in alveolar fluid helps reduce surface tension
umbilical aa and umbilical v constrict; become fibrosed
fates of other fetal vessels
proximal umbilical aa become superior vesical arteries and distal parts become medial umbilical ligaments
umbilical v becomes the ligamentum teres
ductus venosus becomes the ligamentum venosum
foramen ovale becomes the fossa ovalis
ductus arteriosus becomes the ligamentum arteriosum
unstable period lasting 6-8 hours after birth
first 30 minutes the baby is alert and active
heart rate increases (120-160)
respiration is rapid and irregular
temperature falls
activity diminishes, and infant sleeps about three hours
second active stage follows in which the neonate regurgitates mucus and debris
after this, infant sleeps, with waking periods occurring every 3–4 hours
production of milk by the mammary glands
estrogens, progesterone, and lactogen stimulate the hypothalamus to release PRH
anterior pituitary responds by releasing prolactin
Colostrum
solution rich in vitamin A, protein, minerals, and IgA antibodies
released the first 2–3 days
followed by true milk production
after parturition, milk let-down becomes a conditioned reflex
advantages for the infant
fats and iron are better absorbed
its amino acids are metabolized more efficiently than those of cow’s milk
contains beneficial chemicals
IgA other immunoglobulins
complement
lysozyme
interferon
lactoperoxidase
interleukins and prostaglandins
prevent overzealous inflammatory responses
natural laxatives help cleanse the bowels of meconium
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[ Anatomy & Physiology 3 syllabus ] [ Page created 2006-05-23 ][ Last update 2010-08-25 ] [ Questions about this lecture? E-mail me ] |
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