Lecture Notes Menu Embryology and Development


Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation


  1. FERTILIZATION:

    1. FUSION OF THE SPERM NUCLEI AND OOCYTE COMPLETE FERTILIZATION.

        Ovum: corona radiata - granulosa cells
        zona pellucida - polysaccharide shell
        membrane changes to prevent multiple sperm
        from fertilizing ova

      1. Sperm penetrates corona radiata
      2. Attaches to zona pellucida
      3. Acrosome of sperm releases trypsin like enzyme
      4. Sperm penetrates
      5. Egg membrane depolarizes ( Na diffuses in, Ca out)
      6. Cortical granules released
      7. Zona hardens
      8. Prevents multiple fertilizations
      9. Secondary Oocyte divides
      10. Polar body formed, egg and sperm nucleus fuse

    2. THE RESULTING CELL IS CALLED A ZYGOTE
    3. 18 - 36 HOURS AFTER FERTILIZATION THE ZYGOTE CLEAVES


    Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation


  2. EARLY CELL DIVISION

    1. THE CELLS REPRESENTED IN THE CLEAVAGE STAGE HAVE THE ABILITY TO FORM ALL THE CELLS NECESSARY FOR NORMAL DEVELOPMENT.
    2. CELL NUMBER MAY BE DECREASED, INCREASED, OR ALTERED WITHOUT EFFECT TO THE EMBRYO.
    3. THE 16 CELL STAGE IS TERMED THE MORULA (3 days)
    4. THE CELLS OF THE MORULA ARE " PLURIPOTENT "
    5. MORULA ENTERS THE UTERINE CAVITY at 3 days BUT DOES NOT ATTACH
    6. AT ABOUT 7 DAYS GESTATION A BLASTOCYST IMPLANTS INTO THE UTERINE WALL
    7. THE TROPHOBLAST CELLS ERODE THE UTERINE LINING AND FORM POCKETS CALLED LACUNAE.
    8. THE TROPHOBLAST IS NOW TERMED THE CHORION
    9. THE TROPHOBLAST AND LATER THE CHORION SECRETE HCG
      1. HCG PREVENTS IMMUNE ATTACK FROM MOTHER
      2. HCG MAINTAINS THE CORPUS LUTEUM
        FSH and LH from pituitary supressed - no menses

      corpus luteum of ovulation - regresses if no pregnancy

      size : 1 - 2 cm
      corpus luteum of pregnancy - size : 2 - 3 cm


    Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation


  3. EMBRYONIC STAGE ( WEEK 2 - 8 )

    1. CELLS OF THE INNER CELL MASS FLATTEN TO FORM THE EMBRYONIC DISK ( twins develop at 10 - 14 days )
    2. AN OUTER ECTODERM AND INNER ENDODERM IS FORMED
    3. LATER THE THIRD LAYER ( MESODERM ) IS PRODUCED
    4. THESE THREE LAYERS FORM THE PRIMARY GERM LAYERS
    5. HUMANS ARE TERMED " TRIPLOBLASTIC "
    6. FOLLOWING IS A SUMMARY OF CELL TYPES PRODUCED BY EACH LAYER.

        MESODERM: DERMIS OF SKIN
        CIRCULATORY SYSTEM
        MUSCLE
        BONES ( EXCEPT FACIAL )
        ENDODERM:LINING OF GI TRACT
        LINING OF LUNGS
        KIDNEY DUCTS AND BLADDER
        THYMUS
        THYROID
        TONSILS
        ECTODERM:EPIDERMIS OF SKIN
        TOOTH ENAMEL
        LENS AND CORNEA OF THE EYE
        OUTER EAR
        BRAIN AND SPINAL CORD
        FACIAL BONES
        SKELETAL MUSCLES IN THE HEAD

    7. EMBRYONIC DISK BECOMES CYLINDRICAL AND ATTACHED TO THE DEVELOPING PLACENTA BY A STALK.
    8. EMBRYO DEVELOPS HEAD, FACIAL FEATURES, LEGS, MOUTH AND APPEARS HUMAN IN APPEARANCE.
    9. CHRIONIC VILLI DEVELOP AND ARE SURROUNDED BY MATERNAL BLOOD


    Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation


  4. PLACENTA AND AMNION

    1. PLACENTA MEMBRANE CONSISTS OF THE FOLLOWING:

      1. EPITHELIUM OF THE VILLI
      2. EPITHELIUM OF THE CAPILLARIES IN THE VILLI

    2. O2 AND NUTRIENTS DIFFUSE FROM MATERNAL TO FETAL BLOOD.
    3. CO2 AND OTHER METABOLIC WASTES DIFFUSE OUT
    4. PLACENTA DEVELOPS IN THE AREA WHERE THE CHORION REMAINS IN CONTACT WITH THE ENDOMETRIUM
        Produces:estrogen ( estradiol, esterone, estriol )
        progesterone
        human chorionic somatomammotropin
        corpus luteum produces:polypeptide hormone relaxin
        inhibits smooth muscle
        contraction of myometrium
        and relaxes ligaments in
        sacroiliac and pubis symp.

      Functions of Various Hormones

      adrenal cortex increases secretions of aldosterone

        function: renal reabsorption of sodium leading to increases fluid retention.

      parathyroid hormone secretions increase

        function: maintains higher Ca levels

      progesterone from placenta and corpus luteum: development of decidua cells important for nutrition of embryo. Manufactured from cholesterol

      decreased uterine contractions

      increased secretions by uterine tubes and uterus for developing morula/blastula
      * peak of progesterone at 6 weeks

      Estrogens from placenta and corpus luteum: increase size in uterus, breasts and external genitalia . Estrogen produced by adrenals in fetus from progesterone

      * peak of estrogens at 3 - 4 weeks

      Human chorionic somatomammotropin from placenta:

      1. partial breast development
      2. actions similar to growth hormone
      3. glucose level decreased in mother
      4. promotes release of fatty acids
        * also known as placental lactogen

      Inhibin : produced by ovaries and inhibits FSH production

      HCG: produced by chorionic cells
      reaches peak in urine at 9 weeks

    5. PLACENTA HAS TWO MAIN PARTS OR DIVISIONS

      1. EMBRYONIC PORTION WITH VILLI AND CHORION
      2. MATERNAL PORTION INCLUDING UTERINE WALL

    6. THE AMNION FORMS AROUND THE EMBRYO ENCLOSED IN THE AMNIOCHORIONIC MEMBRANE.
    7. UMBILICAL CORD IS FORMED

      1. CONTAINS 2 ARTERIES AND 1 VEIN
      2. SUSPENDS EMBRYO IN AMNIOTIC CAVITY

    8. YOLK SAC FORMS

      1. PRODUCES BLOOD CELLS AND PRECURSOR SEX CELLS
      2. AIDS IN FORMATION OF THE DIGESTIVE TUBE

    9. ALLANTOSIS

      1. EXTENDS FROM YOLK SAC INTO CONNECTING STALK
      2. FORMS BLOOD CELLS
      3. FORMS UMBILICAL VESSELS

      Placenta :

      * 10 days first evidence of placenta
      * lakes or lacuna formed by trophoblast
      * embryo derives nutrients from lacuna
      * placenta initially surrounds blastocyst
      * vessels appear at day 19
      * villi arranged in groups called cotyledona
      * 12 wk discoid type placenta- functional
      * each cotyledon supplied by 1 or 2 arterioles
      * final form 18 - 20 weeks
      * typical size at delivery App. 500 grams (1 lb )

      blood flow: 625 ml per minute
      mothers blood flow volume 30 %
      mother must increase nutrition
      Ca, Phosphorus, Protein, Iron

      * 325 mg iron needed to form fetal blood cells
      * 600 mg iron needed to increase mothers blood volume

      Fetal membranes

      Organ Composition Function
      yolk sac endoderm and splanchnic
      membrane
      Forms gut, blood cells
      vessels
      AllantoisIbidVessels connect fetal circ.
      placenta
      AmnionEctoderm and somatic
      mesoderm
      Contains fetus immersed
      fluid sole cavity of later
      pregnancy
      ChorionEctoderm (trophoblast) and
      somatic mesoderm
      Placental area is organ for
      resp. nutrition digestion
      Umbilical CordAmnion wraps about the
      yolk sac and body stalk
      Vascular path between
      mother and fetus

      2nd week - 2 layer embryo:

      1. gastrulation inner cell mass becomes 3 layers
      2. at 7 days trophoblast connects with endometrium
      3. cells lose boundry and become syncytium
      4. inner cell mass cleft produces amnion
      5. 9 days embryo well within uterine wall
        syncytial trophoblast - thick and spongy
        cellular trophoblast forms
      6. 11 days primitive mesoderm appears
      7. 13 day yolk sac appears
      8. splanchnic mesoderm around yolk sac
        somatic mesoderm covers amnion and lines chorion
      9. Heart pumps at 20 days
      10. Fetal RBC formed at 21 days. Fetal RBC are
      11. Fetal vessels appear throughout body and body stalk
      12. Connect allantoic sac to fetal abdomen
      13. Umbilical cord forms : blood vessels are connected and urachus connected to fetal bladder.
      14. Allantoic sac atrophies rapidly
      15. Organ development complete by 12th week
      16. Most malformations occur during first 12 weeks gestation
      17. medications and immunizations should be avoided until after 12th week

    Organs of Pregnancy :

    OrganCompositionFunction
    Decuida - that part lost at pregnancy or at menses. Also the part around ectopic pregnancy
    D. Parietalis (vera)Non implantion area of uterus
    Pro gravid endo.potential but unused site of implantation
    D. Capsularis
    (reflexa)
    Outer ply of amnion
    superficial part of endometrium of pregnancy
    D. Basalis
    (serotina)
    Maternal Placenta
    Endometrium of pregnancy beneath chorionic sac.
    Supplies maternal blood to placenta
    Chorionic Tissue
    C. Laeve Smooth area of chorion
    Lacks villi
    middle ply of amnionic sac
    C. Frondosum Chorion attached to D. Basalis
    Fetal portion of placenta


    Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation


  5. FETAL CIRCULATION

    1. AFTER 8 WEEKS GESTATION EMBRYO IS CALLED A FETUS
    2. FETAL CIRCULATION IS MUCH DIFFERENT THAN ADULT
    3. THE MAIN DIFFERENCES ARE:

      1. UMBILICAL VEIN TRANSPORTS O2 RICH BLOOD TO THE FETUS
      2. FETAL BLOOD HAS MUCH MORE CAPACITY TO CARRY O2
      3. VEIN CARRIES BLOOD TO THE LIVER AND IS PARTIALLY SHUNTED BY THE DUCTUS VENOSUS
      4. THIS ALLOWS ABOUT HALF OF THE BLOOD TO BYPASS THE LIVER
      5. THE DUCTUS JOINS THE INFERIOR VENA CAVA
      6. BLOOD FROM THE LOWER PART OF THE FETAL BODY IS MIXED WITH O2 RICH BLOOD FROM PLACENTA
      7. BLOOD IS FORCED INTO THE RIGHT ATRIUM
      8. THE FETAL HEART HAS AN OPENING BETWEEN THE RIGHT AND LEFT ATRIUM THAT SHUNTS BLOOD AWAY FROM THE PULMONARY CIRCULATORY SYSTEM.
      9. THE OPENING IS CALLED THE FORAMEN OVALE
      10. THE DUCTUS ARTERIOSUS CONDUCTS BLOOD AWAY FROM THE PULMONARY TRUNK TO THE AORTA ( LUNG BYPASS )


    Fertilization | Early Cell Division | Embryonic Stage | Placenta and Amnion | Fetal Circulation



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