Human Ovary with Fully Developed Corpus Luteum

Maternal recognition of pregnancy is a crucial aspect of carrying a

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

to full term. Without maternal recognition to maintain pregnancy, the initial messengers which stop luteolysis and promote foetal implantation, growth and uterine development finish with nothing to replace them and the pregnancy is lost. Pregnancy maintenance relies on the continued production of

progesterone Progesterone (; P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the ma ...

which is initially produced by the

corpus luteum The corpus luteum (Latin for "yellow body"; : corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is the ...

(CL). A hormone secreting structure that develops on the

ovary The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...

after

ovulation Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and ...

. Maternal recognition of pregnancy differs between species, however they all include a signal to prevent luteolysis, which then prevents the resumption of

menstrual The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of egg ...

oestrous The estrous cycle (, originally ) is a set of recurring physiological changes induced by reproductive hormones in females of mammalian subclass Theria. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases ...

cycles. Luteolysis is the regression of the

. The process is identified by the decline of progesterone and it signifies the absence of pregnancy following ovulation. In the non pregnant uterus, the decline of progesterone allows the return of

oestrogen Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three m ...

, resulting in the upregulation of oxytocin receptors and consequently pulsatile release of PGF2α. In turn, luteolysis is induced. This regression allows the continuation of the menstrual cycle. However, if pregnancy is established, luteolysis is evaded via maternal recognition of pregnancy because high levels of progesterone are maintained by the CL and the placental hormone hCG further maintains the CL.

Mechanisms of recognition

Human

Progesterone released from the corpus luteum is promoted by human chorionic gonadotrophin (hCG) produced by the cells of the trophoblast, the outer layer of cells of the early embryo.

Sheep and cow

In most ruminant species, interferon tau has been identified as the signal for maternal recognition of pregnancy. Interferon tau is therefore also referred to as an anti luteolytic factor, essential for the maintenance of the corpus luteum. Interferon tau is secreted by the

trophectoderm The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the pl ...

of the

blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the ...

from around day 10 in

ovine Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to dom ...

species and from day 15 in

bovine Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including Bos, cattle, bison, African buffalo, Bubalus, water buffalos, and the four-horned and spiral-horned antelopes. The members of this gro ...

species. Interferon tau acts on the endometrial cells of the maternal

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

to prevent the production of the luteolytic factor, PGF2ɑ. The inhibition of PGF2ɑ production is the result of a change in gene expression. Interferon tau inhibits the transcription of the

oxytocin Oxytocin is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. Present in animals since early stages of evolution, in humans it plays roles in behavior that include Human bonding, ...

receptor gene in both sheep and cows, and also the oestrogen receptor ɑ gene in sheep. The absence of these receptors in the cells of the endometrium prevents the pulsatile release of PGF2ɑ.

References

{{Reflist, 30em Reproduction in animals Reproduction in mammals Hormones