Lecture 7 - Embryo Transport and Implantation

What structure helps trap the oocyte?

Infundibulum

Where does fertilization occur?

The oviduct/ampulla

The oocyte enters the uterine cavity as...

An early blastocyst

What features of the uterus help transport of the embryo?

-Ciliary beating

-Oviduct fluid flow

-Smooth muscle contraction

What are the steps in attachment of the embryo to the uterine wall?

1. Development of the embryo within the confines of the zona pellucida

2. Hatching of the blastocyst from the zona pellucida

3. Acquisition of uterine receptivity

4. Maternal recognition of pregnancy

What is the acquisition of uterine receptivity?

-In response to steroid hormones, the uterus becomes ready to accept the blastocyst

-This must happen for attachment to occur

What is an embryo?

-An organism in the early stages of development

-Not distinguishable between species

What is a fetus?

-A potential offspring that is still within the uterus

-Generally recognizable as a member of a given species

What is a conceptus?

The product of conception (embryo/fetus)

What are trophoblasts?

-Cells surrounding the blastocyst

-Give rise to the fetal part of the placenta

-Secrete proteases that help hatching

What is necessary for implantation?

-Synchronized development of the preimplantation embryo and establishment of the receptive uterus

-Regulated by ovarian hormones estrogen and progesterone

What are the 2 uterine cell types?

-Epithelial cells

-Stromal cells

What is the function of estrogen in implantation?

Epithelial cell proliferation (mitogenic)

What is the function of progesterone in implantation?

-Stromal cell differentiation/decidualization

-Promotes embryo attachment to epithelium

What is the function of decidual cells?

-Form in species where implantation is invasive

-Support embryo development, immodulate immune response, form vasculature

Where is progesterone mitogenic?

-In the mammary gland

-Can contribute to cancer in dogs especially because of the long luteal phase

What are the 3 stages of implantation and what occurs in each?

1. Apposition: lumen closes around the embryo

2. Adhesion: attachment of blastocyst to epithelium

3. Invasion: embryo goes through epithelium into stroma

What is superficial versus invasive implantation?

Superficial only undergo the apposition and adhesion stages of implantation

Maternal recognition of pregnancy must occur prior to...

Luteolysis

What happens during maternal recognition?

-PGF2a secretion does not occur

-Endometrial oxytocin receptor synthesis is blocked

What is required for maintenance of pregnancy? What is its source?

-Progesterone

-CL is the source during early pregnancy, and in some species the placenta takes over

The life-span of the CL is extended by what 2 strategies?

-Production of anti-luteolysin (ruminants and pigs)

-Production of luteotropin (humans, primates, rodents)

Explain the maternal recognition of pregnancy in ruminants.

-The blastocyst secretes IFN-t

-This binds to oxytocin receptors in the uterus so that oxytocin can't bind and secrete PGF2a

Explain the maternal recognition of pregnancy in pigs.

-The blastocyst secretes estrogen

-This helps push PGF2a to the lumen so it can be degraded instead of entering circulation

Explain the maternal recognition of pregnancy in horses.

The blastocyst travels through the uterine lumen for 3-4 days and secretes a factor that maintains the CL

Explain the maternal recognition of pregnancy in primates.

A luteotropic signal from CG (chorionic gonadotropin)

Explain the maternal recognition of pregnancy in rodents.

A luteotropic signal from prolactin and placental lactogens

Explain the maternal recognition of pregnancy in cats and dogs.

-There is no pregnancy recognition signal

-In dogs, the CL is maintained the entire 2 months whether there is pregnancy or not

-In cats, they are induced ovulators