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Embryonic development and implantation

Embryonic development and implantation. Pregnancy. Preparation of uterus Steroid hormones Fertilization Coitus Gamete transfer Capacitation of sperms Fusion of gamates. Embryonic development Preimplantation Implantation Placentation Differentiation of cells Organogenesis.

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Embryonic development and implantation

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  1. Embryonic development and implantation

  2. Pregnancy • Preparation of uterus • Steroid hormones • Fertilization • Coitus • Gamete transfer • Capacitation of sperms • Fusion of gamates

  3. Embryonic development • Preimplantation • Implantation • Placentation • Differentiation of cells • Organogenesis

  4. Must alter cyclic changes in the ovarian steroid hormones • Progesterone • High • Must maintain the CL • Most species • Some can maintain pregnancy without CL after certain stage (placental progesterone)

  5. Luteolysis • Destruction of the CL • Reinitiation of reproductive cycle • Two types • Active • Passive • Active luteolysis • Production of luteolytic agent (PGF2a) • Uterus • Passive luteolysis • Loss of luteotropic agents

  6. Progesterone • From ovary to uterus (and back to the ovary) • Positive feedback loop • Uterine production of PGF2a • Production of oxytocin by the CL • Ultimately leads to corpus luteum regression • Reinitiation of reproductive cycle Oxytocin PGF2a PGF2a

  7. Progesterone • Progesterone production by CL • Begins to decline. • Initiated by increased production of PGF2a • Increased production of PGF2a • Ablated when pregnancy has been initiated, resulting in continued Progesterone production by the CL and pregnancy maintenance Pregnancy PGF2a

  8. Maternal recognition of pregnancy • Two types • Anti-luteolytic • Diversion of PGF2a secretion • Inhibition of PGF2a secretion • Luteotropic • Maintenance of the CL by providing necessary hormone • Gonadotropin

  9. Early embryonic development Uterotubal Junction Ampullary- isthmic Junction Isthmus Ampulla • Zygote • Begins to divide as it moves through the oviduct towards the uterus • Numbers of cells increase after each division • The size of the embryo does not (cell size decreases by approximately 20 % after each division)

  10. Early embryonic development 8-cell embryos 2-cell embryo • Cells of the embryo remain within the zona pellucida as they divide • The size of the nucleus increases • All chromosomes remain intact • In cows, the embryo divides three to four times (approximately one division a day) while in the oviduct • Usually at the 16-cell or morula stage when it reaches the uterus

  11. Early embryonic development • Morula stage • All the cells of the embryo are in a tightly packed clump • Cells on the inside of the clump • Different from those on the outside • Cells inside begin to further pack themselves together and form a mass of cells called the inner cell mass (ICM), located at one end of the embryo Morula-stage embryo Blastocyst-stage embryo ICM Blastcoele

  12. Early embryonic development • The ICM • Develops into the fetus • The outer layer of cells lining the zona pellucida • Trophoblast • Placenta • Formation of a fluid-filled cavity • Blastcoele • Blastocyst Morula-stage embryo Blastocyst-stage embryo ICM Blastcoele

  13. Early embryonic development • Cells in the ICM and trophoblast • Continue to divide • Blastacoele continues to accumulate fluid • Hatching • Floats freely until it attaches itself within lumen of the uterus Hatched blastocyst Zona

  14. Attachment and establishment of pregnancy Embryo ICM ICM Placenta • After hatching • Rapid growth and development phase. • In cows, the blastocyst begins to rapidly elongate around 13 days after estrus, transforming from an ~3 mm spherical blastocyst into a long, thread-like form (around 25 cm in length) in 3 to 4 days • The elongation of the bovine embryo • Due to rapid proliferation of trophoblast cells • Cells in the ICM divide slowly during elongation

  15. Attachment and establishment of pregnancy Inner cell mass Uterine endometrium Trophoblast layer • Cattle and sheep • Attachment of trophoblast to the uterine wall • Superficial with some fusion between uterus and trophoblast cells

  16. PGF PGF PGF PGF Implantation and establishment of pregnancy Non-Pregnant • Conceptus (embryo plus placental tissue) • Produces interferon-tau (IFN-t) as it elongates • Prevents production of PGF2a by endometrium of the uterus Endometrium PGF Uterine vein PGF PGF PGF Pregnant Conceptus IFN-t IFN-t IFN-t IFN-t Endometrium Uterine vein

  17. Diversion of PGF2a secretion • Pigs • Non-pregnant • Endocrine factor • Conceptus • Divert secretion(exocrine) • Estradiol • Increased production during 11-12 days post coitus • Conceptus

  18. Diversion of PGF2a secretion • Local factor rather than systemic factor • Conceptus must be present in both uterine horns

  19. Secretion of luteotropic substances • Species with passive luteolysis • Primates • Secretion of glycoprotein hormone • Syncytiotropoblast • Human chorionic gonadotropin (hCG) • Basis of pregnancy test • Secretion begins around 10 days after ovulation

  20. hCG • Luteotropic hormone • LH-like activity • Binds to LH receptors in the CL • Maintenance of progesterone production • Increased lifespan during early stage of pregnancy • Production • Peaks around 9 to 14 weeks of pregnancy • CL loses its function during this time • Switch in steroidogenesis (placenta) • Declines gradually thereafter

  21. Neuroendocrine system • Rodents and rabbits • Coitus as stimulus • Physical contact • Physical stimulation of reproductive tract (cervix) • Release of prolactin by the anterior pituitary gland

  22. Neuroendocrine system • Prolactin • Luteotropic hormone • Switch to placental hormones • Placental lactogen • CL • Eventually dies • Steroid production by placenta

  23. Horses • Recognition of pregnancy • Movement of embryo within the uterus • 12-14 times a day during day 12-14 of pregnancy • Eventual lock-down of the embryo • Production of glycoprotein • eCG • Cause luteinization of the large follicle • Formation of secondary CL • FSH-like activity in other mammals • Loss of both CLs • Placental progestigens

  24. Placental lactogen • Some species • Type of placentation • Ruminants • Humans • Rodents • Produced by fused cells • Syncytiotropoblast • Binucleated/trinucleated cells

  25. Proteins related to pituitary GH and prolactin • Close to GH in humans • Close to prolactin in rodents • 50-50 in cows

  26. Function • Regulation of metabolism • GH-like activity • Much weaker than pituitary GH • Development of mammary gland • Prolactin-like activity • Maintenance of CL function • Rodents during early stage of pregnancy

  27. No known receptor(s) for placental lactogen • Interacts with GH receptor • Interacts with prolactin receptor

  28. Pattern of secretion • Different between cattle and sheep • Degree of fusion between tropoblasts and endometrium

  29. GH-Variant • Human only • Acts like GH • Tissue growth • Nutrient metabolism • Affects function of insulin • Pregnancy-induced diabetes

  30. Placental steroidogenesis • Cholesterol • Lipoproteins from circulation • No De Novo synthesis • Progesterone • Replace CL in some species • Maintenance of pregnancy • Precursor for fetal adrenal steroids

  31. Estrogens • Limited production • Limited 17a-hydroxylase activity • Abundant in fetal adrenal gland • Androgens from fetal adrenal gland • Converted to estrogens in the placenta • Production of estriol rather than estradiol • Secretion of estrone • Majority of placental estrogen in some species

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