Week 1 Ovulation to Implantation PDF

WE MAKE DOCTORS FIRST WEEK OF DEVELOPMENT: OVULATION TO IMPLANTATION DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the: Ovarian Cycle Ovulation Fertilization Process Identify the cleavage process Know the most important features of the implantation process 01 Ovarian...

WE MAKE DOCTORS FIRST WEEK OF DEVELOPMENT: OVULATION TO IMPLANTATION DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Understand the: Ovarian Cycle Ovulation Fertilization Process Identify the cleavage process Know the most important features of the implantation process 01 Ovarian Cycle At puberty, the female begins to undergo regular monthly cycles. Gonadotropin-releasing hormone (GnRH), produced by the hypothalamus, acts on the adenohypophysis of the pituitary gland, which in turn secrete gonadotropins. These hormones, follicle- stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and control cyclic changes in the ovary.  What happens? 1 4 When a follicle becomes aatretic, 15 to 20 primary stage follicles the oocyte and surrounding are stimulated to grow under the follicular cells degenerate and are influence of FSH replaced by connective tissue, forming a corpus atreticum 2 FSH rescues these cells from a pool of continuously forming primary follicles. 5 FSH also stimulates maturation of follicular (granulosa) cells surrounding the oocyte. 3 6 Under normal conidtions, only 1 In cooperation, theca interna and of these follicles reaches full granulosa cells produce maturity and only one oocyte is estrogens. discharged. As a result of this estrogen production: The uterine endometrium enters the follicular or proliferative phase. Thinning of the cervical mucus occurs to allow passage of sperm. The anterior lobe of the pituitary gland is stimulated to secrete LH. At midcycle there is an LH surge that: Elevates concentrations of maturation promoting factor, causing oocytes to complete meiosis I and initiate meiosis II. Stimulates production of progesterone by follicular stromal cells (luteinization) Causes follicular rupture and ovulation 02 Ovulation  In the meantime, the surface of the ovary begins to bulge There is an increase in LH locally, and at the apex, an that causes the primary avascular spot, the stigma, oocyte to complete meiosis I appears. and the follicle to enter preovulatory mature vesicular stage. Meiosis II also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovulation. Prostaglandin levels also increase in response to the LH surge and cause local muscular contractions in the ovarian wall.  Granulosa cells remaining in the wall of the ruptured follicle develop a yellowish pigment and change into lutein cells, which form the corpus luteum and secrete estrogens and progesterone. CORPUS LUTEUM Progesterone along with some estrogen, causes the uterine mucosa to enter the secretory stage in preparation for implantation of the embryo. Once the oocyte is in the uterine tube, it is propelled OOCYTE TRANSPORT by muscular contractions of the tube with the rate of transport regulated by the endocrine status during and after ovulation. Corpus Albicans IF FERTILIZATION DOES NOT OCCUR, THE CORPUS LUTEUM REACHES MAXIMUM DEVELOPMENT APPROXIMATELY 9 DAYS AFTER OVULATION. Subsequently, the corpus luteum shrinks because of degeneration of lutein cells (luteolysis) and forms a mass of fibrotic scar tissue, the corpus albicans. Simultaneously, progesterone production decreases, precipitating menstrual bleeding. If the oocyte is fertilized, degeneration of the corpus luteum is prevented by human chorionic gonadotropin. 03 Fertilization  Fertilization, the process by which male and female gametes fuse, occurs in the ampullary region of the uterine tube. Only 1% of sperm deposited in the vagina enter the cervix. Sperm movement from the cervix to the uterine tube occurs by muscular contractions. The trip from cervix to oviduct can occur as rapidly as 30 minutes or as slow as 6 days. Reaching the isthmus, sperm become less motile and cease their migration. Spermatozoa are NOT able to fertilize the oocyte upon arrival in the female genital tract but must ungergo: 1. Capacitation 2. Acrosome reaction  PENETRATION OF THE CORONA PHASE 1: RADIATA Of the 200 to 300 million spermatozoa normally deposited in the female genital tract, only 300 to 500 reach the site of fertilization. Only one of these fertilizes the egg. It is thought that the others aid the fertilizing sperm in penetrating the barriers protecting the female gamete. Capacitated sperm pass freely through corona cells  PENETRATION OF THE ZONA PHASE 2: PELLUCIDA The zona is a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction. Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby coming in contact with the plasma membrane of the oocyte. Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte surface. This contact results in release of lysosomal enzymes. In turn, these enzymes alter properties of the zona pellucida (zona reaction) to prevent sperm penetration.  FUSION OF THE OOCYTE AND PHASE 3: SPERM CELL MEMBRANES CORTICAL AND RESUMPTION OF THE METABOLIC ZONA REACTIONS: SECOND MEIOTIC ACTIVATION OF THE DIVISION: EGG: As a result of the release of cortical oocyte The oocyte finishes its The activating factor is granules, which contain second meiotic division probably carried by the lysosomal enzymes, (1) immediately after entry spermatozoon. Activation oocyte membrane of the spermatozoon. Its encompasses the initial becomes impenetrable chromosomes (22 plus X) cellular and molecular to other spermatozoa, (2) arrange themselves in a events associated with the zona pellucida vesicular nucleus known early embryogenesis. prevent sperm binding as the female and penetration. pronucleus. The main results of fertilzation are: Restoration of the diploid number of chromosomes: half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents. Determination of the sex of the new individual. An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. Therefore, the chromosomal sex of the embryo is determined at fertilization. Initiation of cleavage. Without fertilization, the oocyte usually degenerates 24 hours after ovulation. Cleavage Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, these cells, which become smaller are known as blastomeres. Until the eight-cell stage, they form a loosely arranged clump. After the 3rd cleavage maximize their contact. This process, COMPACTION, segregates inner cells, approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16 cell morula (mulberry). 30 hrs 40 hrs 4 days Blastocyst Formation When the morula enters the uterus, fluid begins to penetrate through the zona pellucida into the intercellular spaces this become confluent, and finally, a single cavity, the Blastocele, forms. Currently, the embryo is a BLASTOCYST. Inner cell mass, are now called the Embryoblast, are at one pole, and those of the outer cell mass, or trophoblast, flatten and form the epithelial wall of the blastocyst. 04 Uterus at time of Implantation The wall of the uterus consists of three layers: 1. Endometrium or mucosa lining the inside wall 2. Myometrium, a thick layer of smooth muscle 3. Perimetrium, the peritoneal covering lining the outside wall During this menstrual cycle, the uterine endometrium passes through three stages: 1. Follicular or proliferative phase 2. Secretory or progestational phase 3. Menstrual phase At the time of implantation, the mucosa of the uterus is in the secretory phase, during which time uterine glands and arteries become coiled and the tissue becomes succulent. As a result, three distinct layers can be recognized in the endometrium: a superficial compact layer, an intermediate spongy layer, and a thin basal layer. Normally, the human blastocyst implants in the endometrium along the anterior or posterior wall of the body of the uterus, where it becomes embedded between the openings of the glands. References: Chapter 3. Week 1: Ovulation to Implantation Bevis, R. (1992). Langman’s Medical Embryology, 6th edition T W Sadler Langman’s Medical Embryology, 6th Edition Williams & Wilkins 412pp £20.50 0-683- 07473-8. Nursing Children and Young People, 4(3), 14. https://doi.org/10.7748/paed.4.3.14.s14 Thank You!