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The following diagrams illustrate the events of Meiosis in both sexes. It names the cells at each level. Remember, Meiosis has all the same phases as Mitosis in each cycle (PMAT = Prophase, Metaphase, Anaphase, Telopahse). The major events of Meiosis are as follows:
- Meiosis I: Separation of homologous chromosomes (the butterflies)
- Meiosis II: Separation of sister chromatids (The wings of the butterfly)
In Prophase I, you still have both homologous butterflies, so this is the only time you can make the tetrad, and have crossing over.
The following mnemonics can help you bind cell names of the various stages of gametogenesis to the genetic events about to happen:
- oogonium is going to begin Meiosis
- 1o oocyte - is about to undergo Meiosis I.
- 2o oocyte - is about to undergo Meiosis II.
Get used to talking about the events of Meiosis in a simple manner, and this diagram won’t need to be memorized.
For oogenesis, it is useful to bind the knowledge of the the meiotic state to the stage of life of the enclosing female body in which they occur. (I know that sounds awkward, but the AAMC effectively asks at what stage of life these events happen for the woman whose eggs they are).
Look at the diagram below, and read the discussion below it.
- By the end of the first trimester of prenatal life, a female body has all 400,000 ova she can make waiting in her ovaries in Prophase I, with the egg’s chromosomes wrapped up in the tetrad.
- Meiosis I isn’t conpleted until the menstrual cycle. Meiosis II begins and stops at Metaphase II by ovulation.
- This is why nondisjunction errors increase with increasing maternal age
- (Not tested, but for your sense of unfortunate cosmic balance: certain diseases, like Fragile X are related to the age of the father)
- During fertilization, Meiosis II finally completes.
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By definition, the menstrual cycle starts with the first day of menses. Ovulation always happens on day 14. Thus, the follicular phase’s length is constant. The luteal phase (ovulation to menses) has a variable length. In the beginning of the cycle, all the hormones are at their lowest. FSH rises, causing the maturation of a group of follicles, only one of which matures the fastest, suppressing the others. FSH stimulates the stromal (supporting) cells of the follicle to produce Estrogen (oestrus-maker). A small rise in LH is seen as the menses end. Estrogen continues to rise, until it gets high enough to flip a switch in the pituitary, causing a positive-feedback loop with LH. This produces the LH surge, which causes the follicle to burst, expelling the ovum. This ovum floats free in the fluid surrounding the ovary, but gets sucked into the ampulla (wide part) of the fallopian tubes by their peristaltic action. (See below for the egg’s fate.)
In addition to leading to ovulation, Estrogen makes the uterine glands grow tall and straight, by replication of the stem cells at the base of the glands left after menstruation. After ovulation, the follicular stromal cells become the corpus luteum (yellow body). This produces progesterone (pro-gestation steroid), which makes those uterine glands curly and secretory, which makes the uterine lining juicy and ready to support implantation. If there is no implantation (as in the diagram at upper right), then the corpus luteum degrades into a scar (corpus albicans/white body). Without progesterone, the endometrial arterioles go into spasm (constrict), and the uterine lining gets ischemic (low O2), necrotic (dead), and sloughs off (menses). The other outcome of the menstrual cycle is discussed in the next section.
Image from Wikimedia Commons
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If a sperm is present, it climbs through the cervix, uterus, and fallopian tube to meet the ovum in the ampulla of the fallopian tubes. From there, the conceptus travels to the uterus. This takes about 3 days. By the time the zygote has made this journey, it has already progressed (zygote → 16-cell morula (L.: raspberry) → blastula (cells with an inner cell mass and a spherical shell of syncytiotrophoblasts (together+cell+feeding+seeds),
separated by a fluid-filled cavity called the blastocoel (seed+cavity). The inner cell mass (yellow in the diagram) becomes the embryo, and the syncytiotrophoblasts (blue in the diagram) becomes the placenta. This implants by day 3. The placenta then begins to make Beta-HCG (Human chorionic [placental] gonadotrophin), which supports the corpus luteum, so that it continues to make progesterone for 3 months. After those 3 months, the placenta has grown big enough to make its own progesterone to keep the uterus “juicy” enough to feed the placenta.
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Differentiation: a cell becomes a different type of cell by expressing different proteins.
Determination: a cell “decides” to differentiate because it has mRNAs floating in its cytoplasm that commit the cell to producing either proteins that either are the proteins made by the final cell it will turn into, or proteins that are transcription factors for genes that will make it the next-most differentiated cell on the path to becoming that final cell (e.g.: morula cell → Inner cell mass cell → ectoderm → neuroectoderm → hypothalamic cell.)
Vocab: coelom = hollow
Blast = seed.
Blastocoel = hollow in the middle of seeds.
Blastula = stage of development defined by the blastocoel.
Creation of the germ layers: After the zygote develops into the blastula, some of the cells form an invagination that make a “little stomach”. At this point, the embryo is called a gastrula (L.: little stomach). The cells on the outside will make ectoderm (attractoderm), and the cells on the inside will make endoderm (inner-skin), or the mucosal surfaces of the body, and anything in contact with them (e.g: the ducts of the pancreas, liver, lung lining, bladder lining, etc.)
The mesoderm (“meat-o-derm”) forms everything else, which, as noted, has blood vessels running through it: (pancreas cells except for the duct, the body of the liver, heart ,etc…) The fate of the ectoderm is the only one that gets more complicated. After the three germ layers have formed, the ectoderm forms two parallel ridges on the back that rise up and fold in towards each other. This is called neurulation. The diagrams at right and below show the view of the back and a cross-section respectively.
It is the notochord, a piece of mesodermal tissue, that induces the overlying ectoderm become the neural plate (purple), fold, pinch off, and disconnect from the rest of the ectoderm, which in this region makes the skin of the back. The crest of the neural folds also pinches off and gets buried (green tissue), becoming neural crest cells. The buried tube is called the neural tube and becomes the CNS (brain & spinal cord). The hole in the center of the neural tube forms the central canal of the spinal cord and the ventricles of the brain. The neural crest forms the actual nerves of the PNS (somatic - motor/sensory, and autonomic - sympathetic/ parasympathetic). The notochord forms the nucleus pulposus of the vertebral column.
Once the notochord has induced the neural tube to form, the neural tube takes over the job of boss, directing the development of the rest of the body by induction.
The other frequently tested example of induction is the formation of the eye. The forebrain sends out a pseudopod called the optic stalk, which induces the overlying regular ectoderm to invaginate and form a pearl of tissue that becomes the lens.