Maternal Recognition of Pregnancy - Anatomy & Physiology

Overview

Species	Cow	Ewe	Sow	Mare	Queen	Bitch
Preganacy Recognition Factor	Bovine Interferon tau	Ovine Interferon tau	Oestrogen and Pituitary Prolactin	Unidentified	Pituitary Prolactin	None Required

The free-floating blastocyst produces specific proteins that signal to prevent luteolysis.
Sheep blastocysts produce ovine interferon tau (OIFN-τ)
Cattle blastocysts produce bovine interferon tau (BIFN-τ)
Interferon tau is a small protein produced by trophoblastic cells of the blastocyst.
It is present in the uterus at day 13-21 after ovulation.
It is not luteotrophic, so does not enhance progesterone production by the corpus luteum.
Instead, it binds to the uterine endometrium and inhibits the synthesis of oxytocin receptors.
In addition to preventing the upregulation of oxytocin receptors, interferon tau binds to the apical portion of uterine glands to promote synthesis of proteins that are critical for embryonic survival pre-implantation.
As a result, there is no production of the luteolytic factor PGF2α and the corpus luteum is maintained.
The corpus luteum then produces progesterone to maintain the pregnancy.

The ruminant conceptus undergoes rapid elongation between day 14 and 16.
During this period, IFN-τ is secreted at high levels before decreaseing.
Small embryos produce much less IFN-τ than larger embryos.
There is a hige proportion of embryo loss in the first 3 weeks of gestation, as small embryos fail to secrete enough IFN-τ in this critical period. Thus they do not block luteolysis.
- This may be related to poor nutritional status of the mother.

The pig conceptus produces oestradiol as the signal for maternal recognition of pregnancy.
Also, PGF2α is produced in significant quantities, but is re-routed into the uterine lumen.
Oestradiol is produced 11-12 days after ovulation. This does not inhibit secretion of PGF2α , but causes it to be secreted in a different direction than in a cycling sow.
It is thought that prolactin production in the uterine endometrium increases under the influence of oestradiol. This acts to change the ionic flux for calcium, promoting exocrine secretion of PGF2α.
It is secreted away from submucosal capillaries, towards the uterine lumen.
Luminal PGF2α has little access to the circulation, and thus cannot cause luteolysis.
Oestadiol also inactivates PGE2-9 oxoreductase,preventing the conversion of PGE2 to PGF2α.
Production of oestradiol not only prevents luteolysis, but serves to stimulate contractions of the uterine myometrium. This ensures distribution of the conceptuses with proper spacing along the uterine horns.

The secretion of oestradiol occurs in the period of conceptus elongation.
There must be at least two conceptuses in each uterine horn for pregnancy to be maintained.
If they are not present in one uterine horn, PGF2α will be secreted in an endocrine fashion and luteolysis will occur, thus terminating the pregnancy.