Female Menstrual Cycle

The Menstrual Cycle Overview

  • Menstruation involves the monthly shedding of the uterine lining.

  • Known by various terms: menses, menstrual period, or simply period.

  • Menstrual blood consists of blood and tissue from the uterus, flowing through the cervix and out of the vagina.

Menstrual Cycle Phases

1. Cycle Initiation

  • Begins with an egg maturing in one of the ovaries.

  • The endometrium (uteral lining) thickens in anticipation of a possible pregnancy.

2. Egg Release

  • Mature egg is released from the ovary, while the uterine lining continues to thicken to accommodate a fertilized egg.

3. If No Fertilization Occurs

  • If fertilization does not occur, the endometrium breaks down, leading to menstruation (discharge of blood and tissue).

Cycle Duration

  • The average cycle lasts 28 days, but can be shorter (21 days) or longer (35 days) and still be considered normal.

Role of the Endocrine System

  • The menstrual cycle is regulated by the endocrine system, particularly hormones from the pituitary gland and ovaries.

Hormonal Changes During the Cycle

  • Days 1-4: Follicle Stimulating Hormone (FSH) stimulates the maturation of an egg while the endometrium thickens due to estrogen secretion from the developing egg.

  • Around Day 14: Luteinizing Hormone (LH) rises, triggering ovulation, which is when the mature egg is released into the fallopian tube.

  • It takes approximately seven days for the egg to travel through the fallopian tube to the uterus.

Post-Ovulation Phase

  • After ovulation, progesterone levels rise to maintain endometrial thickness.

  • If no fertilization occurs, progesterone and estrogen levels drop, leading to the breakdown of the endometrium.

Phases of the Menstrual Cycle

A. Proliferative Phase

  • The endometrium rebuilds after menstruation.

  • The ovarian follicle matures in preparation for ovulation.

B. Ovulation

  • Secondary oocyte is released from the ovary.

C. Luteal Phase

  • Formation of the corpus luteum; thickening and continued development of the endometrium.

Stages of Early Embryonic Development

1. Gametogenesis

  • Formation of gametes (sperm and egg).

2. Fertilization

  • The sperm penetrates the egg, initiating the fusion of their nuclei.

3. Cleavage

  • Fertilized egg undergoes mitotic divisions, forming a solid mass of cells (morula) progressing to a blastocyst.

4. Gastrulation

  • Formation of primary germ layers (ectoderm, mesoderm, and endoderm) through organized cell migration.

5. Organogenesis

  • Differentiation of germ layers into major organ systems.

6. Growth and Tissue Specialization

  • Organs achieve unique structures and functions, helping in the development from embryo to fetus.

Human Embryonic and Fetal Development

  • From fertilization to implantation, the developing embryo connects to the uterus with the placenta.

  • In the fetal stage, organs mature with the support of the placenta through the umbilical cord.

  • As the fetus grows, developing distinct features such as eyelashes and recognizable human form by month two, culminating in preparation for birth by month nine.