Female Reproductive System

 

Meiosis YouTube - Meiosis; Meiosis An Interactive Animation
     The sex cells, or gametes, develop from sex stem cells by the process of meiosis. The stem cell is diploid which means that it has a pair of each of the 22 autosomal (non-sex) chromosomes and 2 sex chromosomes for a total of  46 chromosomes. The stem cell then undergoes two divisions:
     The first division, called the reduction division, results in each new cell having only one of each autosomal chromosome and one sex chromosome for a total of 23 chromosomes. These cells are now haploid.
     The second division, called the replication division, results in replicates of each chromosome separating and being distributed to two new cells.

 

Ovaries Diagnostic Pelvic Laparoscopy; laparoscopic ovarian cystectomy
     The ovaries are the female sex organs. The ovaries are the size and shape of almonds and are secured to the lateral walls of the pelvis by suspensory ligaments. The medial side of the ovaries is attached to the wall of the uterus by the ovarian ligament.
File:Gray1161.png
     The production of female gametes that occurs in the ovary is called oogenesis.

 

Oogenesis 
     The process by which female gametes develop is called oogenesis. The female stem cells that produce the gametes are called oogonia (sing. oogonium). During development of the embryo these stem cells migrate into the ovary.
     In the developing female fetus, oogonia become primary oocytes that begin the first division of meiosis. However, this division is not completed and the primary oocytes remain “frozen” in the prophase stage of the first meiotic division.
     At birth, oogonia are no longer present. Each primary oocyte is surrounded by a single layer of squamous epithelial cells called follicular cells. The primary oocyte together with its follicular cells is called a primordial follicle. There are about two million primordial follicles with their primary oocytes in the ovaries at birth suspended in the first division of meiosis.
     As the female grows, primary oocytes begin to die and disappear with their follicular cells. This process continues until puberty when there are only about 400,000 primordial follicles left in the ovaries. The primary oocytes continue the process of oogenesis after puberty begins.

     

Puberty
     At puberty, the pituitary gland begins to secrete follicle stimulating hormone (FSH). Under the influence of FSH the female begins the monthly ovarian and uterine cycles.  FSH causes a limited number of primordial follicles with their primary oocytes to develop further on a monthly basis as described in the ovarian cycle.

 

Ovarian Cycle
  Primary follicle
     The ovarian cycle begins when a select number of primary oocytes start to grow bigger. The follicular cells of the follicle divide and become cuboidal. The follicle with the larger primary oocyte and its surrounding cuboidal follicular cells is now called a primary follicle. The primary oocyte continues to grow and the follicular cells continue to divide until there is more than one layer of follicular cells surrounding the primary oocyte. When there is more than one layer of cells surrounding the primary oocyte the cells are called granulosa cells instead of follicular cells. The granulosa cells produce the female sex hormones called estrogens.
  Secondary follicle
     The granulosa cells of the primary follicle continue to divide and begin to produce a fluid. As the fluid produced by the granulosa cells builds up it comes to occupy a central region in the follicle called an antrum. When the follicle has an antrum it is called a secondary follicle. The secondary follicle continues to grow as the fluid continues to build up until the follicle produces a noticeable bulge on the surface of the ovary. The follicle is now ready to burst and is called a mature follicle.
  Ovulation Human ovulation captured on film 
     The mature follicle bursts when there is a sudden upsurge in the secretion of luteinizing hormone (LH) by the pituitary gland. Just prior to bursting the primary oocyte completes the first division of meiosis. However, the division is unequal and produces a large secondary oocyte and a smaller polar body that contains primarily the nucleus. The secondary oocyte, with granulosa cells that immediately surround it called the corona radiata, is ejected into the peritoneal cavity in the process called ovulation.
  Corpus luteum
     After the follicle ruptures the granulosa cell become transformed into cells dedicated to producing steroid hormones, in particular, progesterone. The follicle has now become transformed into the corpus luteum (“yellow body”). Progesterone is the hormone that maintains the pregnancy if the secondary oocyte is fertilized by a sperm cell.

 

Uterine (Fallopian) Tubes
     The uterine tubes form the initial part of the duct system through which the gametes travel. It is an open-ended tube whose open end, the infundibulum, flares like a funnel. The edges of the infundibulum have finger-like projections called fimbriae which partially surround the ovary. The infundibulum with its fimbriae help to ensure that during ovulation the secondary oocyte is swept into the uterine tube.
     Contractions of smooth muscle in the wall of the uterine tube along with the movement of cilia on the epithelium lining the lumen of tube move the secondary oocyte toward the uterus.

 

Uterus
     The uterus, or womb, is a hollow organ located in the pelvis between the urinary bladder and rectum. The uterus receives and nourishes the growing embryo and fetus.
     The uterus can be divided into the following regions:
  Body - the major portion of the uterus.
  Fundus – the rounded superior region above the entrance of the uterine tubes.
  Cervix – the inferior barrel-shaped region that protrudes into the vagina.
     The wall of the uterus has three layers:
  Endometrium – the inner mucosa.
  Myometrium – the middle muscular layer.
  Perimetrium – the outer serous membrane or serosa.

 

Uterine Cycle
     The uterus undergoes a uterine cycle that is coordinated with the ovarian cycle. The cycle is designed to prepare the endometrium of the uterus for implantation of the developing embryo after fertilization occurs. If fertilization does not occur the innermost layer of endometrium is sloughed and the uterus is prepared for the next ovulation.
     The cycle occurs on a monthly (28 day) basis approximately as follows:
  Menstrual phase (days 1-5)
     During this phase the superficial functional layer of the endometrium is sloughed off. The detached tissues and blood passes through the vagina as the menstrual flow.
  Proliferative phase (days 6-14)
     The estrogen levels in the blood rises and this stimulates renewal of the functional layer of the endometrium. Ovulation occurs at the end of this stage.
  Secretory phase (days 15-28)
     Progesterone released by the corpus luteum stimulates further development of the endometrium. Blood supply to the endometrium increases and the endometrial glands increase in size and begin secreting nutrients into the uterine cavity.