Implantation and Placenta Development

Overview of Implantation

• Implantation involves embryo attachment to mother's endometrium and subsequent invasion.
• Species differences in implantation depth (e.g., rodents=deepest).

Key Steps in Implantation

• Timing is crucial for embryo attachment; specific window of uterine receptivity.
• Uterine preparation under progesterone's influence sets the stage for implantation.

Ruminants vs. Other Species

• Ruminants have filamentous embryos, unlike humans.
• Importance of extraembryonic membrane growth to occupy both uterine horns.

Role of Progesterone

• Progesterone crucial for preparing the uterus (high levels during days 4-10 post-ovulation).
• Activates uterine gene expression for optimal embryo attachment environment.
• Drives removal of the glycocalyx layer (inhibiting MUC1) assisting embryo attachment;
  however, paradoxically increases MUC1 prior to attachment.

Attachment Mechanics

• Requires adhesion proteins and integrins (transmembrane proteins) for embryo-mother tissue interaction.
• Integrin types: heterodimeric (combinations of alpha and beta subunits).
• Key adhesion proteins include osteopontin and yCAM-1.

Trophoblast Differentiation

• Two populations of trophoblast cells: uninucleate and multinucleate (giant cells).
• Giant cells release hormones (e.g., placental lactogen) essential for fetal growth.

Placenta Development

• Implantation marks the beginning of placenta formation.
• The placenta serves as an endocrine, digestive, circulatory, and respiratory organ.
• It mediates nutrient and hormone exchange between mother and fetus, not a passive barrier.