early pregnancy

where does fertilization typically occur?

in the ampulla of the oviduct

acrosomal reaction

sperm releases hydrolytic enzymes to penetrate zona pellucida

zona reaction

“hardening” of zona pellucida prevents binding of new sperm

what processes must be triggered in order to complete fertilization?

• zona reaction (block entry of additional sperm)

• completion of oocyte meiosis

• fusion of sperm + egg pronuclei to form zygote

• initiate developmental program of embryo

inner cell mass

cell population that will become the embryo

trophoblast

• cell population that will become the outer layer of the fetal membranes (contributes to the placenta)

• produce maternal recognition hormone/factors

• cells play key roles in fetal-maternal communication

what conditions must be present in order for implantation to occur?

• activated embryo (blastocyst stage) + receptive uterus: progesterone-dominant environment with a little bit of estrogen (ratio important; diestrus)

• timing is extremely important

blastocyst hatching

• destruction of zona pellucida allows for increased growth, access to uterine nutrients, and adhesion to uterine lining

• results from cooperation between embryo + proteases secreted by the endometrium

what two challenges must a developing embryo overcome after hatching?

• maintaining progesterone-dominant environment

• obtaining nutrients

what are different strategies for maintaining progesterone?

• maintain the corpus luteum (maternal recognition of pregnancy)

• placental progesterone production

what are the different strategies for obtaining nutrients?

• (early) implantation

• development of extra-embryonic membranes (late implanters)

what is maternal recognition of pregnancy?

• embryo-induced prolongation of CL lifespan

• typically occurs a few days before luteolysis, except in the dog

  • in dogs: CL lasts length of gestation

how can an embryo prevent production of PGF2a?

• embryonic products

  • ruminants: interferon-τ (tau)

  • pigs: estrogen

  • horses: unidentified compound that promotes uterine PGE2 synthesis over PGF2a

how can an embryo provide gonadotropic support (to CL)?

• LH support

  • primates: “human” chorionic gonadotropin (hCG)

  • equids: equine chorionic gonadotropin (eCG) after day 36

• prolactin

  • rodents: placental lactogen to supplement pituitary prolactin

what is unique about horses’ maternal recognition of pregnancy?

horses have 2 recognition mechanisms: prevent PGF2a production + luteotrophic support (eCG)

are maternal recognition of pregnancy and implantation dependent processes?

• no: they are two distinct (independent) processes

• hormonal signal (progesterone) can persist without implantation

how do late implanters obtain nutrients?

• absorb histotroph (nutrition derived from maternal tissue other than blood) secreted by uterus

• develop extra-embyronic membranes → high surface area for nutrient absorption

where do invasive blastocysts end up after implantation?

embedded in uterine stroma

where do non-invasive blastocysts end up after implantation?

adhered to uterine epithelium

displacement penetration

• trophoblast induces apoptosis of uterine epithelium

• invasive implantation

• rodents, some primates

intrusion penetration

• trophoblast invades between uterine epithelial cells

• invasive implantation

• carnivores

fusion penetration

• trophoblast fuses with uterine epithelium

• invasive implantation

• rabbit