The Menstrual Cycle

Menstrual Cycle Background:

• Female infant is born with approximately 1 million oocytes in each ovary

• Approximately 400 oocytes are ovulated during a lifetime

• Each month the endometrium prepares for the implantation of a fertilized ovum

• If fertilization does not occur, the endometrial lining is shed as a menstrual cycle

Physiology:

• Endocrine system controls ovarian and uterine cycles through hormones

  • Gonadotropin releasing hormone (GnRH) is secreted by the hypothalamus

  • Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH) are secreted by the anterior pituitary gland

  • Estrogen, progesterone, and inhibin are secreted by the ovaries

The Hypothalamus

• Called the “control center of the endocrine system”

• Located in the center of base of the brain → below thalamus and above pituitary gland

• Composes the floor of third ventricle

Functions of the Hypothalamus:

• Nerve centers at the anterior and posterior portions of the hypothalamus monitor blood temperature and adjust for abnormal body temperature

• Works to balance food intake

• Regulates the menstrual cycle and other reproductive functions

  • secretes hormones into blood that travels directly to the pituitary gland

  • pituitary gland receives releasing and inhibitory hormones via blood from the hypopophyseal portal system

The Hypophyseal Portal System

• AKA: hypothalamo-hypophyseal portal system

• group of blood vessels that link the hypothalamus to the anterior pituitary gland

The Pituitary Gland AKA:

Hypophysis

The Pituitary Gland

• Size of pea

• Two separately functioning lobes

  • Anterior → adenohypophysis - hormone producing

  • Posterior → neurohypophysis

Adenohypophysis

• Growth hormone

• Prolactin

• Adrenocorticotropic hormone (ACTH)

• Thyroid Stimulating Hormone (TSH)

• Follicle stimulating hormone (FSH)

• Luteinizing hormone (LH)

Neurohypophysis

• Anti-diuretic hormone (ADH)

• Oxytocin

The Ovaries

• Function as both gonads and endocrine glands

  • Oogenesis

  • Production and secretion of estrogen, progesterone, and small amounts of testosterone

  • Ovarian endocrine functions are responsible for:

    • female characteristics

    • regulation of menstrual cycle

    • pregnancy

Gonadotropin Releasing Hormone (GnRH) AKA:

Luteinizing hormone releasing hormone (LHRH)

Gonadotropin Releasing Hormone (GnRH)

• Secreted by hypothalamus

• Stimulates the release of FSH and LH from the anterior pituitary gland

Follicle Stimulating Hormone (FSH)

• Glycoprotein secreted by the anterior pituitary gland in response to GnRH

• Responsible for promoting follicle development within the ovary

  • Promotes maturation of egg, the cells surrounding the egg that produces hormones that support pregnancy (estrogen and progesterone) and fluid around the egg

  • As follicles grow, cells in follicle produce an increasing amount of estrogen that is released into the blood stream

  • Estrogen stimulates the endometrium to thicken prior to ovulation

  • The higher estrogen levels signal the hypothalamus and pituitary gland to slow FSH production

Luteinizing Hormone (LH)

• a gonadotropin secreted by the anterior pituitary gland

• needed for follicles to grow and ovulation of the dominant follicle

• controls progression of the menstrual cycle

Process of lutenizing hormone:

• surge of LH occurs just prior to ovulation that actually triggers ovulation

• after ovulation, the remaining cells of the released ovum remain as the corpus luteum

• Corpus Luteum produces estrogen and progesterone that support implantation of pregnancy

  • Progesterone matures endometrium in preparation for implantation of pregnancy

• If pregnancy does NOT occur, levels of estrogen and progesterone fall and menstruation occurs

Estrogen AKA:

Oestrogen

Estrogen

• CLASS of natural steroid hormones

  • Estradiol, estriol, estrone

• Regulates the growth, development, and function of reproductive system

• Ovaries → main source

  • Corpus luteum and placenta both produce small amounts

• Stimulates the endometrium to thicken prior to ovulation

• Promotes blood clotting and loss of calcium from bones

Progesterone

• Belongs in a class of steroids known as progestogens

• Production stimulated by corpus luteum (produced by ovaries)

  • small amounts also released from the adrenal glands

• Prepares body for pregnancy

• During pregnancy the placenta begins secreting progesterone at end of 1st trimester

How does progesterone prepare the body for pregnancy?

• prepares endometrium for implantation

• decreases the contractility of the uterine smooth muscle

• blocks the development of new follicles

• makes the vaginal secretions thicker to block sperm

• increases basal body temperature

• stimulates the development of the alveolar system

• inhibits lactation during pregnancy

• decreases maternal immune response

• stimulates growth of uterine wall and blood vessels

Phases of the Ovarian Cycle:

• Follicular phase (Day 1-13)

• Ovulatory phase (Day 14)

• Luteal phase (Day 15)

The Menstrual Cycle

• Reproduction begins at around age 11-13 and ends around age 50

• Typical menstrual cycle is 28 days

  • May range from 21-35 days

  • If longer or shorter than 21-35 days:

    • Polymenorrhea - too frequent

    • Oligomenorrhea - not frequent enough

Polymenorrhea

Menstruation occurring more frequent than every 21 days

Oligomenorrhea

Menstruation occurring less frequent than every 35 days

Phases of Menstrual Cycle:

• Menstrual (Days 1-5)

• Proliferative (Days 6-13)

  • Early proliferative

  • Late proliferative

  • Ovulation on day 14

• Secretory (days 15-28)

Endometrium

• Specialized mucosa that varies in thickness and composition through the menstrual cycle

• Divided into 2 layers:

  • Functional

  • Basal

Functional Layer of Endometrium

• Innermost layer of endometrium

  • Glands and stroma

• Appearance changes constantly throughout all phases of menstrual cycle

• Endometrial cavity is a potential space between functional layers

Basal Layer of Endometrium

• Outermost layer of the endometrium

• Thin and hyperechoic

Measuring the Endometrium:

• Measure in the midline, sagittal plane with the widest area of the fundus

  • Stretch out the uterus

    • Align endometrial cavity with the endocervical canal

  • Includes all layers of the endometrium

  • Thickness depends on phase of menstrual cycle

Endometrial thickness during menstrual phase:

2-4 mm

Endometrial thickness during early proliferative phase:

4-6 mm

Endometrial thickness during late proliferative / peri-ovulatory phase:

6-8 mm

Endometrial thickness during secretory phase:

8-15 mm

Menstruating Phase endometrium U/S appearance:

• Variable appearance

• Thin broken hyperechoic central interface

  • 2-4mm

Early Proliferative Phase endometrium U/S appearance:

• Relatively thin, hyperechoic line

  • Composed of echogenic basal layer next to the central endometrial interface

• 4-6 mm

Late Proliferative Phase endometrium U/S appearance:

• Becomes more hypoechoic during later proliferative phase

• Has a triple layer or “halo appearance”

• 3 layers:

  • central thin echogenic line of endometrial interface

  • hypoechoic functional layer

  • hyperechoic basal layer

• 6-8 mm

Secretory Phase endometrium U/S appearance:

• Thick, uniformly hyperechoic central band

• All 3 layers mentioned in late proliferative phase are all hyperechoic

• Maximum thickness of 8-15 mm

Secretory Phase of Menstrual Cycle:

• Endometrial glands become tortuous and filled with glycogen and mucin

• Uterine artery has a lower pulse rate

• Progesterone and Estrogen is present