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
      Allantois	Ibid	Vessels connect fetal circ. placenta
      Amnion	Ectoderm and somatic mesoderm	Contains fetus immersed fluid sole cavity of later pregnancy
      Chorion	Ectoderm (trophoblast) and somatic mesoderm	Placental area is organ for resp. nutrition digestion
      Umbilical Cord	Amnion 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 :

   Organ	Composition	Function
   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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