Menstrual Cycle

Introduction

The human reproductive cycle, an cyclic endocrine regulated change in female anatomy and physiology that occur over 28 days (4 weeks, a lunar month) during reproductive life (between puberty and menopause). Endocrine changes during pregnancy block the menstrual cycle, which normally would shed the functional layer of the uterine lining each cycle.

This cycle differs from other non-primate female vertebrates (eg rats, mice, horses, pig) that have a reproductive cycle called the estrous cycle (oestrous, British spelling).

• The average menstrual cycle is 28 days with ovulation (egg release) occuring approximately the middle of the cycle.
• The last menstrual period is used clinically in determining developmental ages. (More Birth Date)
• Menstruation phase (period) is the loss of the uterus epithelial functional layer and occurs if fertilization and implantation has not occurred before the end of the current cycle.

Links original Human Menstrual Cycle page

Oocyte Development and Release

Primary Oocyte

• arrested at early Meiosis 1
  • diploid: 22 chromosome pairs + 1 pair X chromosomes (46, XX) autosomes and sex chromosome

• Oogenesis- pre-antral then antral follicle (Graafian follicle is mature antral follicle released)

Secondary Oocyte

• 1 Day before ovulation completes (stim by LH) Meiosis 1
• haploid: 22 chromosomes + 1 X chromosome (23, X)
• nondisjunction- abnormal chromosome segregation
• begins Meiosis 2 and arrests at metaphase
• note no interphase replication of DNA, only fertilization will complete Meiosis 2

Ovulation (HPG Axis)

• Hypothalmus releases gonadotropin releasing hormone (GRH, luteinizing hormone–releasing hormone, LHRH) -> Pituitary releases follicle stimulating hormone (FSH) and lutenizing hormone (LH) -> ovary follicle development and ovulation.
  • release of the secondary oocyte and formation of corpus luteum
  • secondary oocyte encased in zona pellucida and corona radiata
• Ovulation associated with follicle rupture and ampulla movement

Additional Images

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  Simplified cycle drawing

• 

  Human ovary and tube as viewed by laparoscopy

• 

  Human Hypothalamus - Pituitary - Gonad axis

Internet Links

Women’s Health Queensland Wide - Understanding Your Menstrual Cycle

Glossary Links

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Cite this page: Hill, M.A. (2024, June 17) Embryology Menstrual Cycle. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Menstrual_Cycle

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G