Lecture 34 Fertilization; Cleavage Implantation - Structure of Sperm & Ovum PDF

34 Fertilization; Cleavage & Implantation ILOs By the end of this lecture, students will be able to 1. Correlate the structure of sperm and ovum with the process of fertilization. 2. Evaluate role of mitochondria & flagellum in sperm motility. 3. Justify inheritance of...

34 Fertilization; Cleavage & Implantation ILOs By the end of this lecture, students will be able to 1. Correlate the structure of sperm and ovum with the process of fertilization. 2. Evaluate role of mitochondria & flagellum in sperm motility. 3. Justify inheritance of mitochondria from the mother in view of sperm structure. 4. Interpret steps and results of normal fertilization. 5. Distinguish the cascade of cleavage till implantation. 6. Correlate the mechanisms and site of implantation to clinical conditions resulting from their dysregulation. Fertilization Definition: it is the process by which mature sperm and mature ovum meet and fuse forming the zygote. Site: At the ampullary part (lateral third) of the uterine tube. Stages: Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the female genital tract but must undergo capacitation and the acrosome reaction to acquire this capability. Parts of the seprmatozoa ⮚ Spermatozoa are composed of a head, housing the nucleus, and a tail that is divided into four regions: neck, middle piece, principal piece, and end piece. (Fig 1) ⮚ Head of the spermatozoa; It is occupied by the condensed electron-dense nucleus, containing only 1 member of the 23 pairs of chromosomes (22 autosomes + the Y chromosome-or 22 autosomes + the X chromosome), and the acrosome, which partially surrounds the anterior aspect of the nucleus. ⮚ Middle piece; contains a mitochondrial sheath that provides energy needed for sperm motility. Thus, mitochondria are present outside the head. Mitochondria are inherited from the mother only. ⮚ Tail (flagellum); houses the axoneme that has a structure similar to the cilia and responsible for sperms motility. (Fig. 1) ⮚ Role of head in fertilization; the acrosome comes into contact with the cell membrane of the spermatozoon anteriorly. It houses various enzymes, including neuraminidase, hyaluronidase, acid phosphatase, aryl sulfatase, and a trypsin-like protease known as acrosin. Fertilization steps Page 1 of 7  Capacitation: is a period of conditioning in the female reproductive tract; in human it lasts approximately 7 hours. During this time, a glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa. Acrosome reaction: which occurs after binding of the head to the zonapellucida, is induced by zona proteins; enzymes needed to penetrate the zonapellucida are released including acrosin- and trypsin-like substances. Figure 1. Parts of the spermatozoa Phases of fertilization: ⮚ Phase 1, penetration of the corona radiate. ⮚ Phase 2, penetration of the zona pellucida. ⮚ Phase 3, fusion of the oocyte and sperm head cell membranes. Results of fertilization: ⮚ Restoration of the diploid number of chromosomes. ⮚ Determination of the sex of the new individual.(How?) ⮚ Initiation of cleavage: without fertilization, the oocyte usually degenerates 24 hours after ovulation. Page 2 of 7  Figure 2. Structure of the ovum Cleavage Definition:is a series of mitotic divisions that results in an increase in cells. Site: The uterine tube, medial to the ampulla. Stages of cleavage: A. Morula stage: Page 3 of 7 About 30 hours after fertilization, the zygote divides into 2 cells (blastomeres), then into 4 blastomeres at 40 hours. - Twelve cell stage is reached after 3 days of fertilization, while the 16 cell stage is reached at the 4th day. The developing embryo of 12 – 16 blastomeres is called morula that enters the uterus nearly 3 days after fertilization. The morula gets its nutrition from its own cytoplasm. B. Blastocyst formation: As the morula enters the uterus, fluid from the uterine cavity penetrates through the zona pellucida and coalesces to form a single cavity (the blastocele) and the embryo is called blastocyst. The blastocele divides the blastomeres into inner cell mass which will form the embryo proper. The surrounding cells of the periphery form the outer cell mass which will form the trophoblast that will form the fetal part of the placenta. The cells of the inner cell mass are now called the embryoblast and are located at one pole of the blastocyst. The zona pellucida disappears immediately before implantation. Events during the first week of human development. 1, oocyte immediately after ovulation; 2, fertilization,approximately 12 to 24 hours after ovulation; 3, stage of the male and female pronuclei; 4, spindle of the first mitotic division;5, two-cell stage (approximately 30 hours of age); 6, morula containing 12 to 16 blastomeres (approximately 3 days ofage); 7, advanced morula stage reaching the uterine lumen (approximately 4 days of age); 8, early blastocyst stage (approximately 4.5 days of age; the zonapellucida has disappeared); and 9, early phase of implantation (blastocyst approximately 6 days of age). The ovary shows stages of transformation between a primary follicle and a preovulatory follicle as well as a corpus luteum. The uterine endometrium is shown in the progestational stage. Implantation Page 4 of 7 Definition: It means penetration of the blastocyst into the superficial (compact) layer of the uterine endometrium.(What happens if the implantation extends deeper than this?) Time: Starts by the end of the first week (6th day), and is completed about the 11th day. Normal site of implantation: is the endometrium of the anterior or posterior wall of the body of the uterus in or near the middle line. Mechanism of implantation: While floating freely in the uterus, the embryo gets nourishment from secretions of the uterine glands. About 6 days after fertilization, the blastocyst at the inner cell mass side (embryonic pole) attaches to the endometrium after disappearance of zonapellucida. The trophoblast proliferates rapidly and becomes differentiated into two layers: A. An inner cellular layer of cytotrophoblast. B. An outer multinucleated protoplasmic mass with no cell boundaries called syncytiotrophoblast. -The syncytiotrophoblast forms finger like processes that extend into the endometrium of the uterus. By the end of the first week, the blastocyst is superficially implanted in the compact layer of the endometrium. - The syncytiotrophoblast at the embryonic pole expands quickly. It produces enzymes that erode the uterine maternal tissues, so enabling the blastocyst to burrow into the endometrium. This implanted blastocyst gets its nourishment from the eroded maternal uterine tissues. After complete blastocyst implantation, the endometrium is called "decidua". THE DECIDUA Definition: It is the compact layer of the endometrium after implantation of the blastocyst. Types of the decidua: 1. Decidua basalis: It is the part of endometrium lying at the base of the implanted blastocyst, between it and the myometrium of the uterus. It will form the maternal origin of the placenta. 2. Decidua capsularis: It is the part of endometrium covering the surface of the implanted blastocyst; it lies between the blastocyst and the uterine cavity. 3. Decidua parietalis: It is the part of endometrium lining the rest of the uterine cavity. 4. Decidua marginalis: It is the part of endometrium lying at the junction between decidua capsularis and parietalis (It lies at the margins of the implanted blastocyst). - The last three types of decidua will ultimately fuse and degenerate with the advance of pregnancy due to growth of the fetus. Page 5 of 7 The early stages of human placental development. Diagram depicting the early steps in placenta formation following blastocyst implantation. (A,B) The pre-lacunar stages. (C) The lacunar stage. (D) The primary villous stage. 1° ys, primary yolk sac; ac, amniotic cavity; cs, cytotrophoblastic shell; eec, extra-embryonic coelom; exm, extra-embryonic mesoderm; GE, glandular epithelium; ICM, inner cell mass; lac, lacunae; LE, luminal epithelium; mn. tr, mononuclear trophoblast; pr. syn, primary syncytium; TE, trophectoderm; vs, blood vessels. Development. CLINICALCORRELATES ❖ The syncytiotrophoblast is responsible for hormone production, including human chorionic gonadotropin (hCG). By the end of the second week, quantities of this hormone are sufficient to be detected by radioimmunoassays.(What is its significance?) ❖ Abnormal implantation (Ectopic pregnancy): It is implantation of the blastocyst in any place other than the normal site. It might result in death of the embryo or early abortion with severe internal haemorrhage. Abnormal sites of implantation: 1. Uterine ectopic pregnancy: a. At the cornu of the uterus (the site of attachment of the uterine tube) leading to early abortion. b. At the lower uterine segment or cervix leading to placenta previa. 2. Extra uterine ectopic pregnancy: a. The commonest site is in the fallopian tube. (What will be the results?) b. Ovary (ovarian pregnancy). Page 6 of 7  c. Peritoneum of Douglas pouch (abdominal pregnancy), broad ligament of the uterus, or omentum of the stomach (rare). ❖ Placenta previa: means implantation of the blastocyst in the lower segment of the uterus close to the internal os of the cervix. Thus the placenta will precede the foetus at delivery (normally, the foetus delivered first, followed by the placenta). (What will be the result of that?) Page 7 of 7