Reproductive cycle, Pregnancy and labor

Reproductive cycle, Pregnancy and labor

Objectives

At the end of this lecture, student will be able to

• Discuss the process of oogenesis in ovaries

• Describe the major events of each phase of the reproductive cycle

• Correlate the events of uterine cycle with the events of the ovarian cycle

• Prepare a labelled diagram of the major hormonal changes that occur during the uterine and ovarian cycles

• Describe the functions of various hormones involved in pregnancy

• Explain the events associated with the three stages of labor

• Describe the structural and functional changes which occur in mother during pregnancy

• Explain on sex differentiation

Content

• Oogenesis

• Reproductive cycle

• Hormonal changes during reproductive cycle

 • Preganacy, Hormones, Structural and functional changes in mother

• Labor – Stages

• Sex determination

OOGENESIS

• Formation of gametes in the ovaries

• Begins in females before they are even born

During early fetal development, primordial germ cell in ovaries

â

Differentiate into oogonia (diploid, 2n)

             â Mitosis

Produce millions of germ cells

â                                           â

Germ cells degene(Process Atresia)            Develop into larger cells(Primary oocytes)

• At birth 200,000 - 2,000,000 primary oocytes in each ovary

• About 40,000 at puberty

• Around 400 will mature and ovulate

THE FEMALE REPRODUCTIVE CYCLE

It encompasses the ovarian and uterine cycles

Ovarian cycle

- Series of events in the ovaries that occur during and after the maturation of an oocyte

- Develop a secondary oocyte

Uterine (menstrual) cycle

- Concurrent series of changes in the endometrium of uterus

- Prepare the endometrium each month to receive a fertilized egg

Phases of the Female Reproductive Cycle

• Encompasses 4 phases

- The menstrual phase

- The preovulatory phase

- Ovulation

- The postovulatory phase

• Cycle typically ranges from 24 to 35 days

• The pre ovulatory phase is more variable in length than the other phases

Menstrual Phase

• Also called menstruation or menses

• lasts for roughly the first 5 days of the cycle

Events in the ovaries

• FSH, several primordial follicles develop into primary, secondary follicles

Events in the uterus

• Stratum functionalis of the endometrium is shed

• Discharges blood, tissue fluid, mucus, and epithelial cells

• Declining levels of progesterone and estrogens à stimulate release of prostaglandins à uterine spiral arterioles constrict. (Cells they supply become oxygen-deprived and start to die)

Preovulatory phase

• Time between the end of menstruation and ovulation

• This phase is more variable in length than the other phases

• It lasts from days 6 to 13 in a 28-day cycle

Events in the ovaries

• A single secondary follicle will outgrow & become dominant follicle

• Dominant secondary follicle becomes the mature (graafian) follicle

• Continue to enlarge and produce estrogen

• With reference to the ovarian cycle,

The menstrual + preovulatory = follicular phase, as ovarian follicles are growing and developing

Events in the uterus

• Estrogens liberated into the blood by growing ovarian follicles

• Stimulate the repair of the endometrium

• Cells of the stratum basalis undergo mitosis, produce a new stratum functionalis

• Endometrium thickens, straight endometrial glands develop

• Arterioles coil and lengthen, penetrate the stratum functionalis

• Also termed the proliferative phase because the endometrium is proliferating

Ovulation

• Rupture of the mature (graafian) follicle, Release of the secondary oocyte into the pelvic cavity

• Occurs on day 14 in a 28-day cycle

• Secondary oocyte remains surrounded by its zona pellucida and corona radiata

• Brought about by a surge of LH

High levels of estrogens exert a positive feedback effect (green arrows) on the hypothalamus and anterior pituitary, thereby increasing secretion of GnRH and LH

• Signs and symptoms include

- Increased basal body temperature

- Clear, stretchy cervical mucus

- Changes in the uterine cervix

- Abdominal pain

Postovulatory phase

• Time between ovulation and onset of the next menses

• Most constant part of the female reproductive cycle

• It lasts for 14 days in a 28-day cycle, from day 15 to day 28

Events in one ovary

• The mature follicle collapses

• A blood clot forms from minor bleeding of the ruptured follicle, the follicle becomes the corpus hemorrhagicum

• Theca interna cells mix with the granulosa cells, transforms into corpus luteum cells under the influence of LH

• Corpus luteum secretes progesterone, estrogen, relaxin, & inhibin

• The luteal cells also absorb the blood clot

• Also called the luteal phase

• If the oocyte is not fertilized,

- Corpus luteum has a lifespan of only 2 weeks

- Secretory activity declines

- Degenerates into a corpus albicans

- Levels of progesterone, estrogens, and inhibin decrease

- Release of GnRH, FSH, and LH raise

- Follicular growth resumes, a new ovarian cycle begins

• If the secondary oocyte is fertilized and begins to divide

- Corpus luteum persists past its normal 2-week lifespan.

- “Rescued” from degeneration by human chorionic gonadotropin (hCG)

- Produced by the chorion of the embryo, 8 days after fertilization

- LH, hCG stimulates the secretory activity of the corpus luteum

• Presence of hCG in maternal blood or urine - indicator of pregnancy hormone detected by home pregnancy tests

Events in the uterus

• Progesterone and estrogens from corpus luteum promote

- Growth, coiling of the endometrial glands

- Vascularization of the superficial endometrium

- Endometrium thickens in readiness for implantation

• Endometrial glands secrete glycogen

• Also called the secretory phase of the uterine cycle

• If fertilization and implantation do not occur

- Corpus luteum degenerates

- Discharge of the endometrium

- Initiation of another reproductive cycle

Female reproductive cycle

Summary of hormonal interactions in the ovarian and uterine cycles

Pregnancy- its maintenance and parturition

• Corpus luteum can only survive for about 2 weeks in the absence of a pregnancy

• Corpus luteum will survive for the first 6-10 weeks of a pregnancy

• Secrete progesterone

• Progesterone secretion is required for the maintenance of a pregancy

• After implantation, embryo secretes hCG

• Placenta also secretes increasing amounts of estrogen (mainly estriol)

Function of progesterone during pregnancy

• Inhibits the contractions of the uterine myometrium

• Helps to keep the embryo/fetus in the uterus

• Maintains the uterine endometrium in a secretory, nutrient rich state to support the developing embryo/fetus

Function of estrogen during pregnancy

• Maintains the secondary sexual characteristics of the mother

• Stimulate mammary gland growth in preparation for lactation

• Contractions of the myometrium at the time of birth (parturition)

Human chorionic gonadotropin (hCG)

• Secreted by chorion in to blood

• Stimulates the corpus luteum to continue production of progesterone and estrogens

• Placenta is fully established, the secretion of hCG is greatly reduced

Relaxin

• Hormone produced first by the corpus luteum of the ovary and later by the placenta

• Increases the flexibility of the pubic symphysis

• Helps dilate the uterine cervix during labor

Human chorionic somatomammotropin (hCS)

• Also known as human placental lactogen (hPL)

• Rate of secretion of hCS increases in proportion to placental mass

• Help prepare the mammary glands for lactation

• Enhance maternal growth by increasing protein synthesis

• Regulate certain aspects of metabolism in both mother and fetus

Hormones during pregnancy

Changes during Pregnancy

• Near the end of the third month of pregnancy, the uterus occupies most of the pelvic cavity

• Fetus continues to grow

• End of a full-term pregnancy, the uterus fills nearly the entire abdominal cavity

• Pregnancy-induced physiological changes also occur, including

• Weight gain due to the fetus, amniotic fluid, the placenta, uterine enlargement, and increased total body water

• Increased storage of proteins, triglycerides, and minerals

• Breast enlargement in preparation for lactation

• Lower back pain due to lordosis (hollow back)

• The maternal blood volume increases by over 30%

• Respiratory activity increases by about 20%

• Increased urine output

• Constipation

• Pregnancy amenorrhea

Parturition/ Labor

• Process by which the fetus is expelled from the uterus through the vagina, also referred to as giving birth

• True labor

- Dilation of the cervix

- Expulsion of the fetus

- Delivery of the placenta

• False labor

- Pain in abdomen at irregular intervals

- No cervical dilation

• Control of labor contractions during parturition occurs via a positive feedback cycle

True labor can be divided into three stages

1.   Stage of dilation

• Time from the onset of labor to the complete dilation of the cervix

• Lasts 6–12 hours

• Regular contractions of the uterus

• Usually a rupturing of the amniotic sac, and complete dilation (to 10 cm) of the cervix

2. Stage of expulsion

• The time (10 minutes to several hours) from complete cervical dilation to delivery of the baby

3. Placental stage

• The time (5–30 minutes or more) after delivery until the placenta or “afterbirth” is expelled by powerful uterine contractions

• Constrict blood vessels that were torn during delivery, reducing the likelihood of hemorrhage

• Labor lasts longer with first babies, about 14 h

• After delivery of the baby, maternal reproductive organs and physiology return to the prepregnancy state (Puerperium, about 6 weeks)

• Tissue catabolises, the uterus undergoes a remarkable reduction in size, Involution

• For 2–4 weeks after delivery, women have a uterine discharge called lochia

• Consists initially of blood and later of serous fluid derived from the placenta

SEX DIFFERENTIATION

• There are 46 human chromosomes (of 23 different pairs) in a normal somatic cell

• Karyotype- An entire set of chromosomes arranged in decreasing order of size and according to the position of the centromere

• 22 pairs are called autosomes- homologous chromosomes look alike and have the same appearance in both males and females

Human karyotype showing autosomes and sex chromosomes

• Two members of the 23rd pair are the sex chromosomes

• They look different in males and females

• In females, the pair consists of two chromosomes called X chromosomes

• In males, X chromosome + Y chromosome (smaller)

• Y chromosome has only 231 genes

• Spermatocyte undergoes meiosis, gives rise to

- Two sperm that contain an X chromosome

- Two sperm that contain a Y chromosome.

• Oocytes have no Y chromosomes, produce only X-containing gametes

• If the secondary oocyte is fertilized by an X-bearing sperm, the offspring normally is female (XX).

• Fertilization by a Y-bearing sperm produces a male (XY)

• Individual’s sex is determined by the father’s chromosomes

• Sex is determined at the time of fertilization by the presence or absence of a Y chromosome in the sperm

Summary

• Oogenesis is the formation of gametes in the ovaries

• Female reproductive cycle comprises of uterine and ovarian cycle

• Function of the ovarian cycle is to develop a secondary oocyte

• Function of the uterine (menstrual) cycle is to prepare the endometrium each month to receive a fertilized egg

• Uterine and ovarian cycles are controlled by GnRH from the hypothalamus

• If fertilization and implantation do not occur, the corpus luteum degenerates

• If fertilization and implantation do occur, the corpus luteum is maintained by hCG

• Pregnancy is maintained by human chorionic gonadotropin (hCG), estrogens, and progesterone

• During pregnancy, several anatomical and physiological changes occur in the mother

• During pregnancy, some joints become less stable, and certain physical activities are more difficult to execute

• Labor is the process by which the fetus is expelled from the uterus through the vagina to the outside

• Oxytocin stimulates uterine contractions via a positive feedback cycle

• Each somatic cell has 46 chromosomes—22 pairs of autosomes and 1 pair of sex chromosomes

• In females, the sex chromosomes are two X chromosomes

• In males, they are one X chromosome and a much smaller Y chromosome