| Corpus luteum | |
|---|---|
 Section of the ovary. 1. Outer covering. 1’. Attached border. 2. Centralstroma. 3. Peripheral stroma. 4. Bloodvessels. 5. Vesicular follicles in their earliest stage. 6, 7, 8. More advanced follicles. 9. An almost mature follicle. 9’. Follicle from which the ovum has escaped. 10.Corpus luteum. | |
| Details | |
| System | Reproductive system |
| Location | Ovary |
| Identifiers | |
| Latin | corpus luteum |
| MeSH | D003338 |
| TA98 | A09.1.01.015 |
| TA2 | 3484 |
| FMA | 18619 |
| Anatomical terms of microanatomy | |
Thecorpus luteum (Latin for "yellow body";pl.:corpora lutea) is a temporaryendocrine structure in femaleovaries involved in the production of relatively high levels ofprogesterone, and moderate levels ofoestradiol, andinhibin A.[1][2] It is the remains of the ovarian follicle that has released a mature ovum during a previous ovulation.[3]
The corpus luteum is coloured as a result of concentratingcarotenoids (includinglutein) from the diet and secretes a moderate amount ofestrogen that inhibits further release ofgonadotropin-releasing hormone (GnRH) and thus secretion ofluteinizing hormone (LH) andfollicle-stimulating hormone (FSH). A new corpus luteum develops with eachmenstrual cycle.
The corpus luteum develops from anovarian follicle during theluteal phase of themenstrual cycle oroestrous cycle, following the release of a secondary oocyte from the follicle duringovulation. The follicle first forms acorpus hemorrhagicum before it becomes a corpus luteum, but the term refers to the visible collection of blood, left after rupture of the follicle, that secretes progesterone. While theoocyte (later thezygote if fertilization occurs) traverses thefallopian tube into theuterus, the corpus luteum remains in theovary.[citation needed]
The corpus luteum is typically very large relative to the size of the ovary; in humans, the size of the structure ranges from under 2 cm to 5 cm in diameter.[4]
Its cells develop from the follicular cells surrounding the ovarian follicle.[5] The folliculartheca cells luteinize into small luteal cells (thecal-lutein cells) and folliculargranulosa cells luteinize into large luteal cells (granulosal-lutein cells) forming the corpus luteum. Progesterone is synthesized from cholesterol by both the large and small luteal cells upon luteal maturation. Cholesterol-LDL complexes bind to receptors on the plasma membrane of luteal cells and are internalized. Cholesterol is released and stored within the cell as cholesterol ester. LDL is recycled for further cholesterol transport. Large luteal cells produce more progesterone due to uninhibited/basal levels ofprotein kinase A (PKA) activity within the cell. Small luteal cells have LH receptors that regulate PKA activity within the cell. PKA actively phosphorylatessteroidogenic acute regulatory protein (StAR) andtranslocator protein to transport cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane.[6]
The development of the corpus luteum is accompanied by an increase in the level of the steroidogenic enzymeP450scc that converts cholesterol topregnenolone in the mitochondria.[7] Pregnenolone is then converted to progesterone that is secreted out of the cell and into the blood stream. During the bovine oestrous cycle, plasma levels of progesterone increase in parallel to the levels of P450scc and its electron donor adrenodoxin, indicating that progesterone secretion is a result of enhanced expression of P450scc in the corpus luteum.[7]
The mitochondrial P450 system electron transport chain includingadrenodoxin reductase andadrenodoxin has been shown to leak electrons leading to the formation of superoxide radical.[8][9] Apparently to cope with the radicals produced by this system and by enhanced mitochondrial metabolism, the levels of antioxidant enzymes catalase and superoxide dismutase also increase in parallel with the enhanced steroidogenesis in the corpus luteum.[7]
| Follicular structure | Luteal structure | Secretion |
|---|---|---|
| Theca cells | Theca lutein cells | androgens,[10]progesterone[10] |
| Granulosa cells | Granulosa lutein cells | progesterone,[5]oestrogen (majority),[5] andinhibin A[5][10] |
Like the previous theca cells, the theca lutein cells lack thearomatase enzyme that is necessary to produce oestrogen, so they can only performsteroidogenesis until formation ofandrogens. The granulosa lutein cells do have aromatase, and use it to produce oestrogens, using the androgens previously synthesized by the theca lutein cells, as the granulosa lutein cells in themselves do not have the17α-hydroxylase or17,20 lyase to produce androgens.[5]Once the corpus luteum regresses the remnant is known ascorpus albicans.[12]
The corpus luteum is essential for establishing and maintaining pregnancy in females. The corpus luteum secretesprogesterone, which is asteroid hormone responsible for thedecidualization of theendometrium (its development) and maintenance, respectively. It also producesrelaxin, a hormone responsible for softening of thepubic symphysis which helps in parturition.[13]
If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 10 days in humans). It then degenerates into acorpus albicans, which is a mass of fibrousscar tissue.[14]
With cessation of progesterone release, the uterine lining (functional, inner layer of the endometrium) is expelled through the vagina (in mammals that go through amenstrual cycle). Across anoestrous cycle, the functional layer regenerates to provide nourishing tissue for potential fertilisation and implantation.[15][16]
If the egg is fertilised andimplantation occurs, the syncytiotrophoblast (derived fromtrophoblast) cells of theblastocyst secrete the hormonehuman chorionic gonadotropin (hCG, or a similar hormone in other species) by day 9 post-fertilisation.[citation needed]
Human chorionic gonadotropin signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus and providing an area rich inblood vessels in which thezygote(s) can develop. From this point on, the corpus luteum is called thecorpus luteum graviditatis.[17]
The introduction ofprostaglandins at this point causes the degeneration of the corpus luteum and theabortion of thefetus. However, in placental animals such as humans, theplacenta eventually takes over progesterone production and the corpus luteum degrades into acorpus albicans without embryo/fetus loss.[citation needed]
Luteal support refers to the administration of medication (generallyprogestins) for the purpose of increasing the success ofimplantation and earlyembryogenesis, thereby complementing the function of the corpus luteum.
The yellow color and name of the corpus luteum, like that of themacula lutea of the retina, is due to its concentration of certaincarotenoids, especiallylutein. In 1968, a report indicated that beta-carotene was synthesized in laboratory conditions in slices of corpus luteum from cows. However, attempts have been made to replicate these findings, but have not succeeded. The idea is not presently accepted by the scientific community.[18] Rather, the corpus luteum concentrates carotenoids from the diet of the mammal.
Similar structures and functions of the corpus luteum exist in some reptiles.[19] Dairy cattle also follow a similar cycle.[20]
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