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Nursing School

The Embryo
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from class notes...



Development after fertilization is divided into 3 phases:

I.          Cellular Multiplication - (pre-embryonic period/period of the ovum)

II.         Cellular Differentiation - (embryonic membranes)

III.       Development of Organ Systems



I.         Cellular Multiplication (first 2 weeks)

      -begins as a zygote moves through the fallopian tube into the uterine cavity-which takes 3 or more days

      -the zygote period - when sperm fuses w/the ovum (approx 24 hrs - 14 days)   

      -Life of a zygote = the 1st 2 wks.

      -the zygote enters a period of rapid division called cleavage; during which it divides into 2 cells, 4 cells, 8 cells etc…  these cells are called blastomeres

      -blastomeres eventually form a solid ball of cells called morula

      -upon reaching the uterus, the morula floats freely for a few days b4 a cavity forms within the cell mass;  this inner-solid mass of cells is called the blastocyst

      -the blastocyst develops into the embryo & into one of the embryonic (fetal) membranes called amnion

      -Trophoblasts are the outer cells that surround the blastocyst which develop into the other embryonic membrane called the chorion; trophoblasts also give way to the fetal placenta & production of the hormone HCG; also, chorionic villi will develop from trophoblasts

      -HCG- Human Chorionic Gonadotropin - the hormone that allows pregnancy to be sustained & for endometrium to grow; (endomerium is the lining of the uterus)

      -chorionic villi- obtains O2 & nutrients from the maternal bloodstream & disposesof CO2 & waste products into the maternal blood stream (mimics the duties of the placenta)


A.  Implantation

            -occurs in 6-7 days (approx. 1 week)

            -while floating in the uterine cavity, the blastocyst is nourished by the uterine glands; glands secrete a mixture of lipids, glycogen & mucopolysaccharides

            -the blastocyst attaches itself to the surface of the endometrium for further nourishment-this is the site of implanataion where the placenta attaches and develops.

            -normally, the placenta attaches in the upper portion of the uterus where there’s a rich blood supply; implanting at this site prevents the placenta from attaching too deeply into the uterine muscle (which can lead to a lot of bleeding during delivery)

            -the endometrium thickens, the cells enlarge, and after implantation the endometrium is called the decidua

            -Decidua- means to “cast off” or “discard”; after birth, the decidua is discarded as vaginal discharge/bleeding called lochia

            3 Portions of the Decidua:

    Decidua basalis - where nidation (aka implantation) takes place

      -where the maternal part of the placenta will develop

    Decidua capsularis - covers the blastocyst

    Decidua vera - lines the rest of the uterine cavity & doesn’t come into direct contact with the embryo



            -from the end of the zygote stage; 2 weeks to 8-10 weeks

            -the most critical developmental phase d/t establishment of the principle organs which are vulnerable to environmental agents called teratogens

            -teratogens- may cause malformations and include: alcohol, prescription meds, illegal drugs, air pollutants, radiation, viruses

            - early in the pregnancy (often b4 a woman knows she's pregnant, usually first 2 weeks) either they don’t affect the embryo at all, or, they are so severe they may result in miscarriage (aka spontaneous abortion)

            - IF malformation does occur because teratogen affected the embryo, 2 things can happen:  teratogen can cause cellular necrosis (death) or it destroys or alter cell function


II. Cellular Differentiation  - (embryonic & fetal membranes)

            - Amnion & Chorion-protect & support the embryo as it grows & develops; they begin to form at the time of implantation


            A. Amnion = inner membrane; originates from the ectoderm (primary germ layer); inside the amnion membrane is amniotic cavity filled w/fluid called amniotic fluid

            *at term (38-42 wks) it may contain 500-1500 ml fluid which has a pH of 7-7.25 (neutral to slightly alkaline)*


            Amniotic fluid

                        -derived initially from maternal serum & the amnion cells

                        -after the 5th mo. Of gestation the fetus swallows the fluid and urinates which helps increase the amount of amniotic fluid

                        -composed of albumin, urea, uric acid, creatinine, bilirubin, fat, inorganic salt, epithelial cells, leukocytes, enzymes & lanugo



                        -protect fetus from direct trauma by distributing & equalizing the impact

                        -separates fetus from fetal membrane

                        -allows for freedom of mov’t & permits musculoskeletal growth

                        -protects from loss of heat & maintains a constant fetal body temp.

                        -serves as a source of oral fluids (they drink it, it also enters lungs)

                        -prevents adhesions of the skin & cord & umbilical cord pressure

                        -acts as an excretion & collection system


            More amniotic fluid facts:

                        -fluid is replaced every 3 hours

                        -volume may be associated w/fetal death or abnormalities:


                                    *polyhydraminos = greater than 2 L of fluid; associated w/congenital conditions such as hydrocephaly, Down Syndrome, malformation of the GI tract  **these conditions CAUSE the increased fluid


                                    *oligohydraminos = less than 400 ml fl.; assoc. w/poor fetal lung devel, GI defect, defect in renal system


                                    *normal amount of fluid = 500-1500 ml


            FYI- A fetus doesn’t NEED kidneys to survive in-utero because the placenta takes over this function.


            B. Chorion = outer membrane & first to develop - thick w/many fingerlike projections called Chorionic Villi on its surface

            - genetic test - CVS - Chorionic Villus Sampling - removes tissue from the chorionic villi



Embryonic Structures:

            Yolk Sac- small; function in early embryonic life

            -develop as a 2nd cavity in the blastocyst about 8-9 days after conception

            -forms primitive RBC during 1st 6 wks of devel.

            -transfers oxygen & nutrients to the embryo while placental circulation is being developed

            -as embryo develops, the yolk sac is incorporated into the body as the primary digestive system (aka. Primitive gut) where it degenerates

            -failure to degenerate leads to Meckel’s Diverticulitis (which may not be discovered until adulthood)


            Body Stalk-attaches the embryo to the yolk sac; elongates to become the umb. Cord

            -4 vessels initially but 1 of them atrophies leaving 3 vessels = 2 arteries & 1 large vein

            -1-2% of newborns are born w/only 2 vessels which usually indicates a cardiac or GI defect


III.  Development of Organ Systems


About 3 wks after conception, the mass of blastocyst cells differentiate into the primary germ layers: -from these germ layers, all tissues, organs & organ systems will develop by this DNA directed activity

a)   Ectoderm - outer = CNS

b)   Mesoderm - middle = muscles

c)   Endoderm - inner = organ linings



Major function: provides nutrients & O2 from mother to baby; takes away waste & CO2 from baby to mother for excretion by the lungs & kidneys


-has 2 portions: maternal=red, fleshy, disc-like & fetal=grey, shiny & has umb cord

-serves as the kidneys, lungs, endocrine system & GI tract of the fetus/embryo

-by the 3rd month, it’s fully formed, functioning & has developed

-weights about 1 lb. at birth

-connected to the fetus by the umbilical cord



-contains 2 arteries & 1 vein twisted upon each other & protected from pressure by a blue-ish/white substance called Wharton’s jelly; also protected by high volume of blood pulsating through vessels in utero (400mL of blood/min through cord)

- arteries take deoxygenated blood away from the heart of the embryo

-sometimes cord is loosely wrapped around the head, neck or shoulders

            this is called --- nuchal cord

-nuchal cord is described by the # of times it is wrapped around (nuchal x1, x2)


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