Embryonic development and implantation

Pregnancy Preparation of uterus Fertilization Steroid hormones 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 Active luteolysis Passive luteolysis Reinitiation of reproductive cycle Two types Active Passive Active luteolysis Production of luteolytic agent (PGF2a) Uterus Passive luteolysis Loss of luteotropic agents

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 Progesterone Oxytocin PGF2a PGF2a

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

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

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)

Early embryonic development 2-cell embryo 8-cell embryos 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

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

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

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

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

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

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 PGF PGF Uterine vein PGF PGF

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

Diversion of PGF2a 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 Production 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 Production of glycoprotein 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 Androgens from fetal adrenal gland 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