Is there a phase called Late Proliferative (LP) phase of the endometrium?

The Late Proliferative Phase of the Endometrium

Yes, the late proliferative phase is a recognized subdivision of the proliferative phase of the endometrial cycle. The proliferative phase is not a uniform period but consists of distinct functional subdivisions with unique gene expression patterns and physiological characteristics.

Understanding the Proliferative Phase Subdivisions

The proliferative phase of the endometrium is commonly divided into three distinct functional subdivisions:

1. Early Proliferative Phase:

   • Characterized by tissue remodeling, angiogenesis, and modulation of inflammation
   • Higher expression of genes including transforming growth factor β2, chemokine (C-C motif) ligand 18 (CCL18), and metallothionein 2A 1

2. Mid-Proliferative Phase:

   • Marked by peak estrogen-responsive growth
   • Higher expression of heat shock proteins and implantation-associated genes
   • Expression of genes required for endometrial receptivity begins 1

3. Late Proliferative Phase:

   • Characterized by changes in immune function
   • Significant decrease in expression of genes related to natural killer (NK) cell function
   • Increased expression of angiotensin II receptor, type 2 1

Clinical and Research Significance

Research has demonstrated that the late proliferative phase has distinct characteristics:

• In studies examining FGF receptor expression, researchers specifically categorized patients into menstrual/early proliferative phase, periovulatory/early secretory phase, and late secretory phase, acknowledging these distinct phases 2

• The late proliferative phase represents the transition period before ovulation when the endometrium reaches its maximum thickness and prepares for the secretory changes that will follow 3

• Immunocompetent cell populations show significant changes between the proliferative and secretory phases, with NK cells becoming dominant during the secretory phase while CD45RO T lymphocytes are more prevalent during the proliferative phase 4

Ultrastructural and Functional Characteristics

The late proliferative phase has specific ultrastructural features:

• The endometrium reaches maximum thickness (approximately 10-14 mm) during this phase
• Endometrial glands become more tortuous and elongated
• Stromal cells continue to proliferate but at a reduced rate compared to mid-proliferative phase
• Vascular changes prepare for the upcoming secretory phase 5

Clinical Implications

Understanding the distinct phases of endometrial development is important for:

• Proper timing of endometrial sampling for diagnostic purposes
• Interpretation of histological findings in the context of the menstrual cycle
• Evaluation of endometrial receptivity for embryo transfer in assisted reproduction 3
• Assessment of postmenopausal women with proliferative endometrium, who require monitoring due to increased risk of hyperplasia or cancer 6

The recognition of the late proliferative phase as a distinct entity with unique gene expression patterns and cellular characteristics is important for accurate endometrial dating and understanding the complex physiological changes that occur during the menstrual cycle.