What is the corpus luteum?

The Corpus Luteum

The corpus luteum is a temporary endocrine gland that forms in the ovary from the remains of an ovulated follicle, playing a critical role in producing progesterone necessary for maintaining the luteal phase of the menstrual cycle and supporting early pregnancy. 1

Structure and Formation

The corpus luteum develops after ovulation when the ruptured follicle transforms into a specialized glandular structure:

• Forms from the granulosa and theca cells of the ovulated follicle
• Located within the ovary, often visible as a hypoechoic structure on ultrasound
• Can be identified on ultrasound by a characteristic "claw sign" of partially surrounding ovarian parenchyma, confirming its ovarian origin 1

Physiological Function

The corpus luteum serves several critical reproductive functions:

• Primary function: Production of progesterone during the luteal phase and early pregnancy
• Produces large amounts of progesterone necessary for:
  • Preparing the endometrium for potential implantation
  • Supporting early pregnancy until placental function is established (approximately 6-8 weeks)
  • Maintaining menstrual cyclicity and endometrial receptivity 2
• Also produces estradiol and its metabolites that influence luteal function 3

Lifecycle of the Corpus Luteum

The corpus luteum has a defined lifecycle that varies depending on whether pregnancy occurs:

Non-Conception Cycle

• Forms immediately after ovulation
• Functions for approximately 12-14 days (luteal phase)
• Undergoes regression (luteolysis) if pregnancy does not occur
• Regression leads to declining progesterone levels, resulting in menstruation 2

Pregnancy

• If pregnancy occurs, human chorionic gonadotropin (hCG) from the developing embryo "rescues" the corpus luteum
• Continues to produce progesterone for approximately 6-8 weeks of pregnancy
• Gradually regresses as the placenta takes over progesterone production 2, 4

Regulation of Corpus Luteum Function

The corpus luteum is regulated by complex endocrine, paracrine, and autocrine mechanisms:

• Formation and maintenance: Influenced by luteinizing hormone (LH), growth hormone (GH), and various growth factors
• Key growth factors: Vascular endothelial growth factor A (VEGF A) and basic fibroblast growth factor (FGF-2) are essential for angiogenesis and development of capillary networks 5
• Local regulators: Prostaglandins, oxytocin, and progesterone itself modulate luteal cell proliferation and function 6, 5
• Regression: Triggered by prostaglandin F2alpha (PGF2alpha) in non-conception cycles, involving complex cascades including inflammatory cytokines and vasoactive peptides 5

Clinical Significance

The corpus luteum has important clinical implications:

• Luteal phase deficiency: Inadequate corpus luteum function may contribute to infertility or early pregnancy loss 2
• Ultrasound differentiation: Important to distinguish from ectopic pregnancy, as both can appear as ring-like structures in the adnexa
  • The corpus luteum is typically hypoechoic, while a tubal ring of ectopic pregnancy is more hyperechoic 1
  • The corpus luteum is located within the ovary, while an ectopic pregnancy is separate from the ovary 1
• Progesterone supplementation: Used in fertility treatments and early pregnancy to compensate for potentially inadequate corpus luteum function 2
• Early pregnancy research: The corpus luteum is being investigated as a potential factor in the pathophysiology of preeclampsia 1

Diagnostic Features on Ultrasound

On ultrasound examination, the corpus luteum typically appears as:

• A hypoechoic structure within the ovary
• May show peripheral vascularity on color Doppler, similar to ectopic pregnancy
• Distinguished from ectopic pregnancy by the "claw sign" of ovarian tissue surrounding it
• Usually less echogenic than the hyperechoic rim of an ectopic pregnancy 1

Understanding the corpus luteum's structure, function, and regulation is essential for comprehending reproductive physiology and addressing clinical issues related to fertility and early pregnancy maintenance.