The menstrual cycle Flashcards
The menstrual cycle
Occurs roughly every 28 days.
Includes events in the ovaries (ovarian cycle) and the uterus (uterine cycle).
The Ovarian Cycle
Follicular Phase: Lasts about 13 days. Primary follicle matures and secretes estrogen. Under control of FSH.
Ovulatory Phase: Day 14. A secondary oocyte (note this is only after Meiosis I) is released from the ovary, as a result of increased LH from anterior pituitary. (ie. Ovulation)
Luteal Phase: Lasts about 14 days. Full formation of corpus luteum (exists for about 2 weeks) in the ovary that secretes estrogen and progesterone.
Uterine Cycle
Note that the hormones released during the ovarian cycle direct the procedure in the uterus.
Menstruation: Lasts about 5 days. Result of degradation of corpus luteum annd drop in estrogen and progesterone. Causes sloughing of the endometrial lining.
Proliferative Phase: Lasts 9 days. Estrogen from the follicle drives proliferation of the endometrium. Ovulation occurs here.
Secretory Phase: Lasts about 14 days (occurs after ovulation), estrogen and progesterone further develop the endometrium. Includes the secretion of glycogen, lipids and some other materials. If no pregnancy, menstruation will be triggered again. Secondary oocyte is traveling down the uterine tube at this point.
Hormone cascade in the menstrual cycle
GnRH from hypothalamus > LH and FSH release from Pituitary
FSH Stimulates granulosa cells and thecal cells to develop during follicular phase and secrete estrogen. Estrogen initially inhibits GnRH, LH, and FSH, but when it peaks at the end of the phase it has a positive effect on LH.
This positive effect on LH, causes a surge in LH that stimulates ovulation and then causes the follicle to become the corpus luteum.
When there is no pregnancy, estrogen and progesterone act to heavily inhibit GnRH, FSH, LH. When LH secretion drops it causes degradation of corpus luteum and leads to menstruation.
If estrogen and progesterone levels are kept artificially high what is the consequence?
There would be no ovulation.
If these artificially high levels are suddenly removed it results in menstruation.
Endometrial shedding
Result of drop in estrogen and progesterone
Why do estrogen and progesterone drop?
Corpus luteum degenerates
Why does the corpus luteum degenerate?
Drop in LH
Why is there a drop in LH?
Inhibitory effect of estrogen and progesterone.
Ovulation should be inhibited when…
Pregnancy occurs.
This is done by maintaining high estrogen and progesterone levels, that inhibit LH and therefore prevent ovulation.
Note though that the corpus luteum only degenerates if fertilization has not occured.
How does pregnancy proceed if LH hormone cannot remain high, but LH is necessary for the maintenance of the Corpus Luteum?
If an egg is fertilized, it is implanted in the endometrial lining. A placenta forms, with the CHORION, as part of it developing from the zygote.
hCG is secreted by the chorion and takes the place of LH in preventing degradation of the corpus luteum.
Estrogen and progesterone stay elevated.
*hCG is used to confirm pregnancy.
Secretion of the Chorion
Human Chorionic Gonadotropin. hCG which takes the place of LH in maintaining the corpus luteum.
Hormonal state during menopause
High LH and FSH, but low estrogen and progesterone as a result of the lack of follicle cells.
During the secretory phase of the endometrial cycle what is occuring?
Estrogen and progesterone are being secreted from the corpus luteum.
Secondary oocyte is in the uterine tube.
Initial process of fertilization
A secondary oocyte is ovulated and enters the uterine tube (has corona radiata and zona pellucida). It can remain fertile for about a day like this.
Sperm if deposited near the cervix will be diluted and activated of CAPACITATED (loss of inhibiting factors in the sperm). Will swim to secondary oocyte.
Fertilization
Fusion of the spermatozoan with the secondary oocyte (still only having undergone one round of meiosis).
Acrosome reactoin
Sperm penetrates corona radiata and binds and penetrates the zona pellucida.
Acrosome
Head of sperm with many hydrolytic enzymes.
Acrosomal process
After penetration of the corona radiata, the acrosomal process penetrates the zona pellucida.
Bindin in the acrosomal process, helps bind the receptors in the zona pellucida.
How quickly does meiosis occur post-insertion of sperm nucleus enters the secondary oocyte?
Twenty minutes.
Produces an Ootid and a polar body. Ootid rapidly matures into an ovum.
After fertilization
Sperm and ovum nuclei fuse, and the cell is now considered a Zygote.
Fast block to polyspermy
depolarization of the egg plasma membrane that prevents fusion of other spermatozoas.
Slow block to polyspermy
Influx of Ca2+; also known as the cortical reaction.
results in:
Swelling of the zona pellucida and plasma membrane
Hardening of the zona pellucida
Egg activation - increases metabolic activity and protein synthesis
Slow block to polyspermy
Influx of Ca2+; also known as the cortical reaction.
results in:
Swelling of the zona pellucida and plasma membrane
Hardening of the zona pellucida
Egg activation - increases metabolic activity and protein synthesis
Embryogenesis
Occurs within 36 hours of fertilization.
However, proceeds slowly.
Cleavage
The first stage of embryogenesis, results in a ball of cells called a MORULA.
Note that the cell divisions that occur in this initial stage are primarily in S and Mitotic stages, as the size does not get much larger than the zygote was intitially.
Blastulation
The Morula progresses to become a blastocyst at which point it embeds in the endometrium wall (roughly a week after fertilization).
Which has a ring of cells (TROPHOBLAST) surrounding a cavity and an inner cell mass.
Trophoblast
Becomes the Chorion.
Also secretes proteases that lyse the endometrial cells, allowing it t burrow into the wall and absorb nutrients. This is the primary source of nutrients for the developing embryo over the initial weeks.
The placenta only develops after about 3 months.
Inner cell mass
Inside the blastocyst, the inner cell mass becomes the embryo.
** if two inner cell masses form, identical twins result.
Also produces the:
Amnion - surrounds a fluid filled cavity that contains the developing embryo. Note that amniotic fluid is the “water” that breaks.
Yolk Sac - First site of red blood cells in the embryo
Allantois - develops from embryonic gut and forms blood vessels of the umbilical cord that transports blood between embryo and placenta.
Loss of the corpeus luteum in the first trimester results in…
Shedding of the endometrial lining, and the loss of the baby.
Is the Corpeus Luteum essential throughout pregnancy?
No. Once the placenta develops, it supplies sufficient estrogen and progesterone.
Placenta development
Result of placental villi that are chorionic projections extending into the endometrium. Fetal capillaries extend into the placental villi and sinuses fill with the mothers blood.
Note that no blood is passed between mother and child during development of the embryo.
Gastrulation
When the three primary germ layers develop:
Endoderm, mesoderm, ectoderm
Gastrulation in primitive organisms
Blastula (akin to the blastocyst), undergoes invagination in which the inner layers become the endoderm, mesoderm forms from the endoderm, and ectoderm is the outer layer.
The cavity that remains is the Archenteron, and is the primitive gut.
The blastopore formed at the site of invagination becomes the anus.
Gastrulation in humans
Gastrula develops from the embryonic disk (a double layer of cells)
ie. not a spherical blastula.
Ectoderm
Entire nervous system
Pituitary gland
Cornea and lens
Epidermis and derivative (skin, hair, sweat glands, sensory receptors)
Nasal, oral, anal epithelium
Mesoderm
Muscle, bone and connective tissue
Cardivascular system and lymphatic system, including blood
Urogenital organs (kidney, ureters, gonads, reproductive ducts)
Dermis of skin
Endoderm
GI tract epithelium (except mouth and anus)
GI glands (Liver, pancreas etc)
Respiratory epithelium
Epithelial linings of urogenital organs and ducts
Urinary bladder.
Epithelium as ectoderm and endoderm
Note that epithelium outside the body is ectoderm and epithelium inside the body is endoderm.
Steps after fertilization
Embryogenesis > Blastulation > Gastrulation > Nuerulation
Neurulation
Formation of the nervous system.
Initiates when the ectoderm forms a portion known as the neural plate.
At the edge of the plate are neural crest cells, which thicken and fold upwards to create the neural folds. The bottom of the plate forms the neural tube.
Neural tube
Forms the central nervous system.
Induced by the underlying notochord (gives rise to the vertebral column that is mesoderm in nature.
Neural crest cells
Separate from neural tube as it finishes forming and migrate to other parts of the body to form:
melanocytes, glial cells, adrenal medulla, some peripheral neurons, some facial connective tissue.
Organogenesis
Development of Organs. Neurulation is included in this.
Fetus
Eighth week of gestation.
All major organ systems are present.
Fetus is still only 5 cm long.
What is the most vulnerable stage of pregnancy to toxins?
The first trimester, as this is when organ formation is occuring.
Consequence of low folic acid for the fetus?
abnormal formation of the neural tube and CNS
What can illnesses in the mother cause for the embryo later on in life?
Influenza has been linked to schizophrenia and German measles, also deafness, eye abnormalities, and heart defects.
Smoking by a mother…
Can result in hypoxia for a baby, as a result of vasoconstriction of uterine blood vessels.
This can lead to a reduction in grey matter.
Alcohol consumption
Can lead to stunted growth, brain defects, and other behavioural/physical problems.
Totipotent
Cells of the trophoblast and the inner cell mass up until the blastocyst stage.
Can become any cell type.
Pluripotent
Can differentiate into any of the three primary germ layers.
Potential to be any of the 220 cell types that make up an adult human.
Cannot contribute to the trophoblast.
Multipotent
Cells after gastrulation. Can become any cell type within one of the three germ layers.
Determination v differentiation
Determination precedes differentiation, but fixes the fate of a cell.
Dedifferentiation
A specialized cell becomes unspecialized.
Induced Pluripotent Stem Cells (iPS)
Cancer cells
Two exceptions to the rule that every cell has the same genome and that no genetic changes normally take place during development.
- B and T cells
- Gametes
Fetal stage
Second trimester to birth
Second Trimester
Organ systems continue to develop. Grows to an average weight of 0.6 kg. Develops distinctly human appearance.
Third Trimester
Rapid growth and significant adipose tissue deposition.
Most organ systems are fully developed.
A baby born 1 - 2 months early has a good chance of survival.
Body changes in a mother that is pregnant
Increased respiratory rate.
50% increase in blood as a result of a drop in oxygen levels.
Increased release of erythropoietin and renin.
Increased glomerular filtration rate (50%)
Demand for nutrients and vitamins increased by 30%
Uterus and mammary glands increase exponentially.
Parturition
Birth.
Driven by contraction of the uterine wall.
High levels of progesterone during pregnancy inhibit contractions. But uterine excitability increases near the end of pregnancy. Results from change in ratio of progesterone and estrogen, oxytocin secreted by posterior pituitary, and mechanical stretching of uterus and cervix.
Weak contractions
Occur throughout pregnancy
Labour contractions
Occur when the pregnancy reaches full term.
Potentially positive feedback. increased pressure on cervix, results in release of oxytocin, oxytocin causes uterine contractions with greater intensity and therefore greater pressure on cervix wall.
Note that contractions of the uterus after birth can help minimize blood loss.
Stages of labour
Stage 1: Dilation of the cervix.
Stage 2: birth
Stage 3: expulsion of the placenta after it separates from the uterus.
Estrogen and progesterone
Increased breast development during puberty and this is increased with adipose and and grandular tissue during pregnancy.
These processes however, inhibit prolactin and the production of milk.
After birth with the drop of estrogen and progesterone, milk production begins.
