Embryology PDF

OVARIAN CYCLE The ovarian cycle is under the control of the Pituitary Gland. It is divided into 3...

OVARIAN CYCLE The ovarian cycle is under the control of the Pituitary Gland. It is divided into 3 phases: (FOL) 1- Follicular, (FSH) 2- Ovulatory, (LH). 3- Luteal. (LH). The ovarian cortex contains hundreds of thousands of primordial follicles (400,000 t0 500,000). FSH Each consists of one Early development of ovarian follicle is induced primary oocyte by FSH. encircled by single The simple flat follicular cells become cuboidal, layer of flat follicular then columnar then forming many layers around the cells. oocyte forming primary follicle.  1- Follicular Phase controlled by Follicle-Stimulating Hormone. It stimulates the ovarian primary follicles to develop and become mature and stimulate the production of Estrogen by the follicular cells. 2-Ovulatory Phase controlled by LH which triggers ovulation (rupture of the mature follicle), stimulates the mature follicles to produce Estrogen and stimulates corpus luteum to produce Progesterone. 3-Luteal Phase The remaining of the ruptured follicle is now called corpus luteum. It secretes Progesterone and small amount of Estrogen. These 2 hormones stimulate endometrial glands to secrete and prepare endometrium for implantation of fertilized Ovum (Blastocyst). If the oocyte is fertilized the Corpus Luteum enlarges and remains till the 4th month of pregnancy. If the oocyte is not fertilized the corpus luteum involutes and degenerates in 10- 12 days.  Phases of Menstrual Cycle 1. Menstrual Phase 2. Proliferative or Follicular Phase 3. Luteal Phase 4. Ischemic Phase It is the production of mature male & female gametes (Sperms & Ova). Spermatogenesis: It is the series of changes by which the primitive germ cells (spermatogonia) are transformed into mature sperms. Oogenesis: Sequence of events by which the primitive germ cells (oogonia) are transformed into mature oocytes.  It is the cell division that takes place in the germ cells to produce male & female gametes. It consists of two cell divisions, meiosis I & meiosis II during which the Diploid number of chromosomes (46) is reduced to Haploid number (23).  At the beginning of meiosis I, (prophase) male & female germ cells replicate their DNA so that each of the 46 chromosomes is duplicated into sister Chromatids.  By the end of the 1st meiotic division, each new cell formed (Secondary Spermatocyte or Secondary Oocyte) has haploid (half) number of chromosome. It is half number of chromosomes of the Primary Spermatocyte or primary Oocyte. WHAT IS THE DIFFERENCE BETWEEN MITOSIS & MEIOSIS? DIPLOID HAPLOID  AIM: Formation of sperms with haploid number of chromosomes. SITE: Seminiferous tubules of the testis. TIME: From puberty till old age. DURATION: About two months N.B. Sperms are stored and become functionally mature in the Epididymis.   Each daughter Spermatogonia grows to Growing give primary spermatocyte (46).  Primary spermatocyte undergoes meiotic division to give 2 secondary spermatocyte (22+ x) or (22+y). Secondary spermatocytes undergo 2nd meiotic division to form 4 haploid spermatids (half size). Spermatids are transformed into 4 mature sperms by a process called spermiogenesis.  It is change in shape (metamorphosis) through which Spermatids are transformed into mature Sperms: 1. Nucleus is condensed and forms most of the head. 2. Golgi apparatus forms the Acrosome. 3. Mitochondria forms a spiral sheath. 4. Centriole elongates to form the axial filament.  OOGENESIS AIM: Formation of secondary oocytes with haploid number of chromosomes. SITE: Cortex of the ovary TIME: Starts during fetal life becomes completed after puberty & continues until menopause. It occurs monthly Except during pregnancy.  Before Birth: (During early fetal life) , primitive ova (Oogonia). proliferate by mitotic Mitosis division and enlarge to form Primary Oocytes (46) Before and At Birth : all primary oocytes completed the prophase of the 1st meiotic division and remain arrested and do not finish their first meiotic division until puberty.  At Puberty Shortly before ovulation, the Primary Oocyte completes its first meiotic division to give Secondary oocyte (23) & First Polar Body. The Secondary Oocyte receives almost all the cytoplasm. The First Polar Body Completes its receives very little. 1st meiotic division It is small nonfunctional cell that soon degenerates. At Puberty-shortly before ovulation At Ovulation At ovulation, the secondary oocyte begins the second meiotic division but progresses only to metaphase where division is arrested. Fertilization  If the secondary oocyte is fertilized, the second meiotic division is completed otherwise it degenerates 24 hours after ovulation. Most of the cytoplasm is retained by the Mature Oocyte (Fertilized Oocyte). The rest is in the 2nd Polar Body which soon degenerates. Fertilization The egg (and corona radiata) at ovulation Corona radiata Zona pellucida (ZP-1, -2, and -3) Cortical granules Transport through the oviduct At around the midpoint of the menstrual cycle (~day 14), a single egg is ovulated and swept into the oviduct. Fertilization usually occurs in the ampulla of the oviduct within 24 hrs. of ovulation. Series of cleavage and differentiation events results in the formation of a blastocyst by the 4th embryonic day. Inner cell mass generates embryonic tissues Outer trophectoderm generates placental tissues Implantation into the uterine wall occurs ~6th embryonic day (day 20 of the menstrual cycle) Fertilization is a multi-step process whereby multiple sperm bind to the corona radiata, but only a single sperm usually fertilizes the egg 1. Acrosome Rx sperm bind to ZP proteins in the zona pellucida; this initiates the release of enzymes from the sperm allowing it to burrow through the zona pellucida. 2. Zona Rx binding of the sperm and egg plasma membranes initiates Ca+ influx into the egg and release of cortical granules from the egg that block other sperm from fertilizing the egg. This so-called cortical reaction prevents other sperm from fertilizing the egg “polyspermy” Cortical granule enzymes digest ZP proteins so other sperm can no longer bind. Hyaluronic acid and other proteoglycans are also released that become hydrated and swell, thus pushing the other sperm away. Fertilization Meiosis II complete Formation of male and female pronuclei Decondensation of male chromosomes Fusion of pronuclei Zygote Fertilized egg (zygote) Fertilized egg 2 polar bodies 2 pronuclei Day 1 0.1 mm Cleavage Cleavage = cell division Goals: grow unicellular zygote to multicellular embryo. Cleavage begins about 24h after pronuclear fusion  2 Cell Stage Individual cells = blastomeres Mitotic divisions maintain 2N (diploid) complement Cells become smaller Blastomeres are equivalent (aka totipotent). 4 cell; second cleavage 4 equivalent blastomeres Still in zona pellucida  8 Cell; third cleavage Blastomeres still equivalent Embryo undergoes compaction after 8-cell stage: first differentiation of embryonic lineages Caused by increased cell-cell adhesion Cells that are forced to the outside of the morula are destined to become trophoblast--cells that will form placenta The inner cells will form the embryo proper and are called the inner cell mass (ICM). Formation of the blastocyst  Ectopic Implantation Implantation somewhere other than upper portion of uterus “Rupture” can lead to life-threatening hemorrhage