Embryology Lecture Notes - First Week PDF

Summary

This document provides lecture notes on embryology, focusing on the first week of development. It covers key stages such as capacitation, acrosomal reaction, and fertilization, as well as abnormal fertilization types and concepts of cleavage, morula and blastocyst formation.

Full Transcript

Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Embryology Lecture Notes "First Week" By the end of this lecture, you should be able to: Identify the steps of capacitation Identify the steps of acro...

Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Embryology Lecture Notes "First Week" By the end of this lecture, you should be able to: Identify the steps of capacitation Identify the steps of acrosomal reaction Identify the steps (phases) of fertilization Define the normal & abnormal fertilization Know the steps of blastocyst formation Capacitation: It is a process that takes place inside the female genital tract mostly in the cervix. It takes approximately 7 hours. During this time a glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that covers the acrosomal region of the sperm. As a result the sperm becomes more active and motile. Only capacitated sperm can pass through the corona cells and undergo the acrosome reaction. Acrosomal reaction: Definition: it is the process by which the sperms release the contents of the acrosomal cap. Only capacitated sperms can do this process once they get in contact with the corona radiata surrounding the oocyte. Mechanism: the following substances are released: a) Hyaluronidase enzyme. b) Trypsin-like substances. c) Zona lysine. Fertilization: It takes place in the ampulla of the uterine tube. Nearly at the midcycle (time of ovulation). The male and female pronuclei fuse to form a zygote. The phases of fertilization include: o phase 1, penetration of the corona radiata. o phase 2, penetration of the zona pellucida. o phase 3, fusion of the oocyte and sperm cell membranes. 1 Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh Results of fertilization: Resumption of 2nd meiotic division of the oocyte. Restoration of diploid number of chromosomes. Sex determination. Zygot formation and start of the cell division. Cortical and zona reaction: The zona pellucida is a glycoprotein shell surrounding the oocyte. Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby it gets into contact with the plasma membrane of the oocyte. This contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte (cortical reaction). In turn, these enzymes alter the zona pellucida structure and properties and alter the permeability of the oocyte plasma membrane to prevent other sperm penetration (polyspermy). What are the types of abnormal fertilization? Superfecundation: o Fertilization of two or more ova released during the same menstrual cycle by sperms from separate acts of coitus. Parthenogenesis (virgin birth) o Growth and development of an embryo without fertilization by a male! o It occurs naturally in some plants, invertebrates and some fish. 2 Level 1 semester 1 (Embryology Lecture Notes) Dr. Dalia Saleh o The offspring are all-females. Triploidy: o Occurs when there is double fertilization of an ovum (dispermy). o This results in zygot with 69 chromosomes (3n). o The extra set of chromosomes results in the formation of different congenital anomalies. What is cleavage? Cleavage is a series of mitotic divisions of the zygote. Zygote cytoplasm is successively cleaved (2-cells, 4-cells, 8-cells stage, etc.) resulting in a rapid increase in the number of cells. These cells (blastomeres) became smaller with each cleavage division. Cleavage occurs as the zygote passes along the uterine tube toward the uterus and is still surrounded by the zona pellucida. What is morula? It is formed about 3 days after fertilization. When there are 16 to 32 blastomeres, it is called a morula. The blastomeres of morula undergo repeated mitotic division within zona pellucida. The zona pellucida keeps the cells together during cleavage and prevents adhesion of these cells to the uterine tubes. The morula enters the uterus. What is blastocyst? It is formed when fluid secreted within the morula forms a cavity. The inner cell mass or the embryoblast (will form the embryo). The outer cell mass or the trophoblast (will form part of the placenta). 3

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