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functions of placenta
-primary site nutrient and gas exchange
-metabolism (syn of glycogen)
5 functions of placenta and fetal membranes
"a falling off"
-functional layer of endometrium
-part that separates from the remainder of the uterus after childbirth
superficial part of decidua overlying conceptus that forms the maternal part of the placenta
fetal part of placenta
develops from part of the chorionic sac when chorionic villi project into the intervillous space containing maternal blood
meeting point of fetal part/villous chorion of placnta and maternal part/decidua basalis of the placenta
-attached by the cytotrophoblastic shell(external layer of trophoblastic cells on maternal surface of placenta).
-chorionic villi anchor the chorionic sac to the decidua basalis, and endometrial arteries and veins pass freely through gaps in the shell into the intervillous space
primary chorionic villi
formed at the end of Week 2 by proliferation of cytotrophoblastic cells producing cellular extensions that grow into the syncytiotrophoblast
secondary chorionic villi
develop in Week 3 as the primary villi begin to branch and mesenchyme grows into the villi, forming a loose mesenchyme connective tissue core; villi now cover the entire surface of the chorionic sac
tertiary chorionic villi
some secondary villi differentiate into capillaries and blood cells and form tertiary villi once the vasculature is visible. The capillaries fuse to form networks and become connected with the embryonic heart; by the end of Week 3 embryonic blood flows through the tertiary villi capillaries and oxygen and nutrients begin to diffuse.
bushy part of the chorionic sac where chorionic villi persist (as opposed to the smooth chorion, where the villi are compressed and degenerate producing an avascular bare area). The villous chorion will form the fetal part of the placenta.
layers of placental membrane
Early/Until week 20
- Connective Tissue of villus, -Endothelium of fetal capillaries
Late/after week 2— -Syncytiotrophoblast,
hCG (human chorionic gonadotropin-maintains corpus luteum)
hPL (human placental lactogen)
hCT (human chorionic thyrotropin)
hACT (human chorionic corticotropin)
progesterone (essential for maintenance of pregnancy)
beneficial substances that cross placenta
Nutrients—glucose, Amino Acids, Free fatty acids, vitamins
Electrolytes—H2O, Na, K, Cl, Calcium Phosphate
Waste Products—CO2, urea, uric acid, bilirubin
Cells—fetal and maternal RBCs
Immunoglobins—IgG (7S class)
detrimental substances that cross placenta
Viruses—Rubella (causing severe congenital anomalies), etc
Immunoglobins—IgG (anti Rh antibodies)
Drugs—most drugs, thalidomide, cocaine, alcohol, caffeine, nicotine, warfarin
Micro-organisms—Treponema Pallidum (causes syphilis), Toxoplasma Gondii (destructive changes in the brain and eyes)
substances that do not cross the placenta
Nutrients—maternally derived triglycerides, cholesterol
Hormones—All protein hormones, including insulin
amnionic fluid components
proteins (hormones/enzymes), desquamated fetal cell,
production of amnionic fluid
-Initial secretion by amniotic cells;
-most fluid derived from maternal tissue fluid through diffusion across the amniochorionic membrane and later through the chorionic plate with dialysis of maternal and fetal blood.
-Fetus contributes by secreting fluid from the respiratory tract, and excreting absorbed fluid as fetal urine.
reabsorption of amnionic fluid
-Swallowed by the fetus and reabsorbed into its respiratory and GI tracts and bloodstream;
-excess fluid and waste products removed via placenta.
-During the final stages of pregnancy, the fetus swallows up to 400 mL of fluid per day
amount of amnionic fluid
At week 12—50 mL
At week 38/full term—1000 mL (gradually increases throughout pregnancy)
functions of amnioic fluid
-Permits symmetric external growth of embryo;
-acts as a barrier to infection; -permits normal lung development; -prevents adherence of the amnion to the embryo;
-cushions embryo against injury by distributing impacts received by the mother;
-helps control embryo body temperature by maintaining a constant temperature;
-enables fetus to move freely aiding in muscular development;
-involved in maintaining homeostasis of fluid and electrolytes
functions of the yolk sac
-Plays a role in the transfer of nutrients in the 2nd and 3rd weeks before uteroplacental circulation is established;
-well-vascularized extraembryonic mesoderm covering the yolk wall first develops blood in week 3 until week 6;
-becomes incorporated into the embryo as the primordial gut in week 4 with endoderm giving rise to epithelium of the respiratory and digestive tract;
-lining of the yolk sac develops primordial germ cells in week 3 which migrate to the developing sex glands and differentiate.
fate of the yolk sac
At 32 days, the yolk sac is large.
By 10 weeks, is has shrunk to about 5mm and lies in the chorionic cavity between the amnion and chorionic cavity,
-yolk stalk detaching around week 6.
-By week 20 the yolk sac is very small and will eventually be no longer visible.
fraternal twins that originate from two different zygotes.
There are always 2 amnions and 2 chorions, although the chorions and placentas may be fused.
identical twins that originate from one zygote.
-Fetal membrane formation can depend on when the twinning process occurs, but usual cases involve separation of inner cell mass into two embryoblasts during the blastocystic stage. Each embryo will have its own amnion, but share a chorion and a common placenta (although it is possible to have the same fetal membrane structure as DZ twins, especially if twinning occurs at a later stage).
Attachment—usually near the center of the fetal surface of the membrane, although can be anywhere, epithelium is continuous with amnion adhering to the chorionic plate of the placenta
Size—1-2 cm in diameter, 30-90 cm in length
Vasculature—easily seen umbilical vessels branch on fetal surface to form the chorionic vessels, which enter the chorionic villi and radiate to and from the cord; two arteries and one vein surrounding by Wharton's Jelly run within the cord. Umbilical arteries carry deoxygenated blood for exchange with placenta/mother, and umbilical veins receive oxygen rich blood from placenta/mother.
progressive dilation of cervix mediated by changes in hormones, ending with complete dilation. Lasts approximately 12 hours for first pregnancies and 7 hours for women who have already had a child
Begins with a fully dilated cervix and ends with delivery of the baby, when the fetus descends through the cervix and vagina. Lasts approximately 50 minutes (first time)/20 minutes (second pregnancy on)
Begins with birth and ends when placenta and membranes are expelled, lasting about 15 minutes. Caused by a hematoma forming deep to the placenta, separating it from the uterine wall allowing for expulsion. Pressure on the abdomen can assist the hematoma to form.
Post-expulsion of placenta. Lasts approximately 2 hours while contractions of uterus constrict spiral arteries to prevent excessive uterine bleeding
- process during which the fetus, placenta, and fetal membranes are expelled from the mother's reproductive tract.
hormones of parturition
Corticotropin-releasing hormone—secreted by fetus hypothalamus, stimulating... Adrenocorticotropin Hormone—from the anterior pituitary, causing secretion of...
Cortisol—from the adrenal cortex; synthesizes estrogens, stimulates contraction.
Oxytocin—stimulates peristaltic contractions of uterine smooth muscle; stimulates...
Prostaglandins—released from the decidua; stimulates myometrial contractility
Estrogens—increase myometrial contractile activity; stimulate release of Oxytocin and prostagladins
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