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).

Historic Embryology - Menstrual
1839 Corpus Luteum Structure \| 1851 Corpus Luteum \| 1933 Pap Smear \| 1937 Corpus Luteum Hormone \| 1942 Human Reproduction Hormones \| 1951 Corpus Luteum \| 1969 Ultrastructure of Development and Regression \| 1969 Ultrastructure during Pregnancy

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.

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

Ovulation in the rabbit

Human Oocyte Numbers

The graph shows the changes in human germ cell numbers in the ovary with age, peaking at about 7 million (occuring in early fetal development) and then decreasing by apopotic cell death.
At puberty there remain only about 400,000 and only about 10% of these will be released through reproductive life.

Graph based on data from: Hassold, etal., Environ Mol Mutagen 1996. 28: 167-175

Oocyte Development

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