Human Reproductive Cycle: Ovulation and Menstruation

Ovulation Cycle

• The ovulation cycle is a 28-day reproductive cycle that ends in the release of a single mature ovum (ovulation) under the influence of anterior pituitary gonadotrophic hormones (FSH and LH).
• Follicular development is characterized by:
  • Growth and differentiation of primary oocyte
  • Proliferation of follicular cells
  • Formation of zona pellucida
  • Development of theca folliculi
• FSH and LH secretion from the anterior pituitary gland is stimulated by GnRH from the hypothalamus.
• During each cycle, FSH promotes growth of several primordial follicles, but only one follicle develops into a mature follicle.

Follicular Development

• Primordial follicles:
  • Formed during fetal life
  • Consist of a primary oocyte enveloped by a single layer of flattened follicular cells
  • Protect the ova and form different types of follicle with progress in development
• Primary follicles:
  • First indication of development is the change of flattened follicular cells to columnar
  • Consist of primary oocyte surrounded by a layer of cuboidal or low columnar follicular cells
  • Growing follicle forms a homogenous layer of glycoprotein in between follicular cells and primary oocyte (zona pellucida)
• Secondary/multilaminar follicle:
  • Follicular cells undergo mitotic division
  • Form several layers of follicular cells surrounding the primary oocyte (stratum granulosum)
  • Primary oocyte increases in size
  • Follicular cells are now called granulosa cells
• Preantral follicle:
  • Fluid-filled spaces appear between granulosa cells
  • Fluid is secreted by granulosa cells
• Tertiary/antral follicle:
  • Fluid-filled spaces coalesce
  • Form a bigger cavity (follicular antrum)
  • Primary oocyte with surrounding granulosa cells is pushed to one side of the follicle
  • Fluid in antrum is called liquor folliculi
• Mature/Graafian follicle:
  • Around 7th day of sexual cycle, one of the tertiary follicles increases in size
  • Forms the largest mature follicle
  • Remaining follicles degenerate and become atretic
  • Reaches the periphery of the cortex and starts projecting on to the surface of the ovary

Ovulation

• Just before ovulation, the primary oocyte of the mature Graafian follicle completes the first meiotic division and forms a secondary oocyte and first polar body.
• LH elevates concentrations of maturation-promoting factor, causing primary oocytes to complete meiosis I and initiate meiosis II.
• LH stimulates production of progesterone by follicular stromal cells (luteinization).
• LH causes follicular rupture and ovulation.

Structure of the Ovum at Ovulation

• The ovum is not fully mature
• Secondary oocyte is undergoing division to shed off the second polar body
• It is surrounded by the zona pellucida and cells of the corona radiata
• No nucleus is seen, and a spindle is present

Corpus Luteum

• After ovulation, the remaining part of the follicle undergoes changes to form the corpus luteum.
• The corpus luteum:
  • Is an important structure
  • Secretes progesterone
  • Is derived from the ovarian follicle after it has ruptured to shed the ovum
  • Undergoes changes to form a yellowish body due to the presence of lutein
• The corpus luteum:
  • Secretes progesterone for about 14 days
  • Persists for about 14 days
  • If fertilization does not occur, the granulosa cells produce inhibin, which inhibits the secretion of gonadotrophins, leading to regression of the corpus luteum
  • It remains relatively small and is called the corpus luteum of menstruation
  • At the end of its functional life, it degenerates and becomes converted into a mass of fibrous tissue called the corpus albicans

Menstrual Cycle

• The uterus consists of three layers: perimetrium, myometrium, and endometrium.
• The endometrium is divided into two layers: stratum functionalis (functional layer) and stratum basalis (basal layer).
• The stratum functionalis is shed during menstruation, while the stratum basalis is permanent and gives rise to a new stratum functionalis after each menstruation.
• The menstrual cycle is under the influence of oestrogens and progesterone.
• The cycle can be divided into three phases:
  • Menstrual phase (days 1-7)
  • Proliferative phase (days 6-14)
  • Secretory phase (days 15-28)

Clinical Applications

• Endometriosis: fragments of endometrial tissue are present in the uterine tubes, on the ovary, and in the peritoneum of the pelvic cavity.
• Ovarian cysts: fluid-filled sacs in or on an ovary.
• Abnormal menstrual cycles:
  • Amenorrhea: the absence of menstruation
  • Dysmenorrhea: painful menses
  • Hypomenorrhea: scanty blood flow during the menstrual cycle
  • Menorrhagia: profuse blood flow during the menstrual cycle
  • Metrorrhagia: bleeding between menstrual cycles
  • Oligomenorrhea: reduced frequency of menstrual cycles
• Use of hormones in birth control (contraceptive) pills:
  • Estrogen and progesterone are used to inhibit ovulation
  • These hormones act on the hypothalamus and pituitary gland, resulting in the inhibition of secretion of GnRH, FSH, and LH