embryonic development and implantation
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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
• 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)
Luteolysis• Destruction of the CL
– Reinitiation of reproductive cycle– Two types
• Active• Passive
• Active luteolysis– Production of luteolytic agent (PGF2)
• Uterus
• Passive luteolysis– Loss of luteotropic agents
• From ovary to uterus (and back to the ovary)– Positive feedback loop
• Uterine production of PGF2
• Production of oxytocin by the CL
– Ultimately leads to corpus luteum regression
• Reinitiation of reproductive cycle
PGF2
Progesterone
Oxytocin
PGF2
• Progesterone production by CL – Begins to decline. – Initiated by increased
production of PGF2
– Increased production of PGF2
• Ablated when pregnancy has been initiated, resulting in continued Progesterone production by the CL and pregnancy maintenance
PGF2
Progesterone
Pregnancy
Maternal recognition of pregnancy
• Two types– Anti-luteolytic
• Diversion of PGF2 secretion
• Inhibition of PGF2 secretion
– Luteotropic• Maintenance of the CL by providing necessary
hormone– Gonadotropin
Early embryonic development
• 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)
AmpullaIsthmus
Ampullary-isthmic Junction
Uterotubal Junction
Early embryonic development
• 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
2-cell embryo 8-cell embryos
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
ICM
Blastcoele
Morula-stageembryo
Blastocyst-stageembryo
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
ICM
Blastcoele
Morula-stageembryo
Blastocyst-stageembryo
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 blastocystZona
Attachment and establishment of pregnancy
• 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
ICM
ICM
Embryo
Placenta
Attachment and establishment of pregnancy
• Cattle and sheep– Attachment of trophoblast to the uterine wall
• Superficial with some fusion between uterus and trophoblast cells
Inner cell mass
Trophoblast layer
Uterine endometrium
Implantation and establishment of pregnancy
• Conceptus (embryo plus placental tissue)– Produces interferon-tau
(IFN-) as it elongates• Prevents production of
PGF2 by endometrium of the uterus
IFN-IFN-IFN-
IFN-
PGF PGFPGF PGF
PGF PGFPGF
PGF
Uterinevein
Non-Pregnant
PregnantConceptus
Endometrium
Uterine vein
Endometrium
• Diversion of PGF2 secretion
– Pigs• Non-pregnant
– Endocrine factor
• Conceptus– Divert secretion(exocrine)
– Estradiol• Increased production during 11-12 days post
coitus– Conceptus
• Diversion of PGF2 secretion– Local factor rather than systemic factor
• Conceptus must be present in both uterine horns
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
• 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
• Neuroendocrine system– Rodents and rabbits– Coitus as stimulus
• Physical contact• Physical stimulation of reproductive tract
(cervix)• Release of prolactin by the anterior pituitary
gland
• Neuroendocrine system– Prolactin
• Luteotropic hormone• Switch to placental hormones
– Placental lactogen
– CL• Eventually dies
– Steroid production by placenta
• 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
Placental steroidogenesis
• Cholesterol– Lipoproteins from circulation
• No De Novo synthesis
• Progesterone– Replace CL in some species
• Maintenance of pregnancy• Precursor for fetal adrenal steroids
• Estrogens– Limited production
• Limited 17-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