Introduction to General Embryology PDF

Introduction to general embryology Dr. Yara Hassan  What is meant by embryology? It is the science which deals with the development of an individual from a single cell into complex organism within the uterus. It begins with fertilization of an ovum and culminates with the birth of the baby. a pro...

Introduction to general embryology Dr. Yara Hassan  What is meant by embryology? It is the science which deals with the development of an individual from a single cell into complex organism within the uterus. It begins with fertilization of an ovum and culminates with the birth of the baby. a process called morphogenesis, which includes cell division, transformation or specialization, migration, and even programmed cell death (apoptosis).  Divisions of Prenatal Period Embryonic Period fetal period from fertilization to the end of eight week 9th week(third month) until the birth (a) pre- (b)embryonic embryonic period proper. period (fertilization) to the third week to the the end of second end of eighth week week of of IUL intrauterine life (IUL)  Subdivisions of Embryology General Embryology Systemic Embryology It deals with the development of an It deals with the functional individual during first eight weeks after maturation of various fertilization (embryonic periods) organs and systems  Cell Division and Gametogenesis The body cells are divided into two types GERM CELLS has a haploid (half) number somatic cells has diploid of chromosomes 23 (double) number of chromosomes 46 46 chromosomes are arranged in 23 pairs. The 22 pairs are called autosomes whereas the 23rd pair is called sex chromosomes. The sex chromosomes are of two types: X and Y germ cells are essential to produce gametes. The female gametes are secondary oocytes, which are released from ovary in a cyclic manner. each secondary oocyte has 22 autosomes and one X chromosome (22X). two types of sperms—one containing X (22X) and the other containing Y (22Y) the male gametes are sperms, which are produced by testis from puberty onward.  Cell Division MITOSIS MEIOSIS Occurs in the reproductive organs to occurs in somatic cells produce gametes one cell divides into two consists of two phases daughter cells that are (a) First meiotic division genetically identical to the (b) The second meiotic division: parent The cell.between the two period It is the mitotic except that there is no mitotic divisions is called duplication of DNA during short interphase interphase. During interphase before Daughter cells have half the number o mitosis begins, each chromosomes chromosome replicates its deoxyribonucleic acid (DNA) Crossing over of chromatids takes pla  Spermatogenesis The spermatogenesis is the process of formation of spermatozoa from primordial germ cells (PGCs) spermatogonia present in the walls of the seminiferous tubules of the testis Process of Spermiogenesis The spermatid is transformed into the sperm. Structure of Sperm  Oogenesis  The oogenesis is the process of formation of female gametes—the oocytes from PGCs  The process of oogenesis begins long before birth in the cortex of the ovary.  The primary oocyte enters the prophase of first meiotic division before birth  this division is arrested till puberty due to the presence of an oocyte maturation inhibitor (OMI) factor secreted by the follicular cells surrounding the oocyte  The secondary oocyte, at the time of ovulation is in metaphase stage of second meiotic division, which continues till fertilization.  The secondary oocyte completes its meiotic division only when it is fertilized ovulation fertilization Development of Ovarian Follicles and Ovulation Structure of the Female Gamete (Secondary Oocyte)  THE FERTILIZATION  the process of fusion of male and female gametes (pronuclei) to form a zygote  It takes place within 24 hours of ovulation, in the most dilated part of the uterine tube—the ampulla  The results of fertilization are: (a) Determination of genetic sex of embryo (b) restoration of diploid number of chromosomes (c) initiation of cleavage Before fertilization, the sperm must gain the ability to Penetrate the various oocyte barriers. 1. First barrier is formed by cells of corona radiata. 2. Second barrier is formed by zona pellucida made of glycoproteins. 3. Third barrier is formed by vitelline membrane of oocyte itself. This ability is achieved by two processes: (A) Capacitation (B) Acrosome Reaction.  Capacitation  It is a process of removal of the glycoprotein coat and seminal plasma proteins covering the plasma membrane of sperm in the acrosomal region  in the female genital tract; it lasts about 7 hours. Acrosome Reaction  occurs when capacitated sperm meets the zona pellucida (acrosome reaction is induced by zona proteins). The acrosomal reaction leads to release of enzymes from acrosomal cap such as hyaluronidase..  Steps of Fertilization The fusion of pronuclei of male and female gametes  Penetration of corona radiata  Penetration of zona pellucida  Fusion of sperm and oocyte cell membranes  Completion of second meiotic division of oocyte and formation of female pronucleus  Formation of male pronucleus  Formation of zygote What are the Results of Fertilization?  Cleavage  The cleavage consists of repeated mitotic divisions of the zygote into smaller units  a rapid increase in the number of cells  These cells are called blastomeres  The zygote divides to form two cells and continuous divisions occurs till it becomes 16-cells resemble a mulberry fruit  and is termed morula What is the role of zona pellucida?  IMPLANTATION  Definition: It is the process by which the blastocyst becomes embedded in the endometrium.  Time: It starts at the 7th day and is completed at the 11th day.  Site?  Mechanism of implantation: It starts by adhesion of the blastocyst by its embryonic pole to the endometrium at the implantation site. Trophoblast proliferates at the embryonic pole to form a new layer of cells that has no cell walls and this layer is called syncytiotrophoblast. This new layer secretes proteolytic enzymes that erode the endometrium and form the implantation cavity.  At the 9th day, the site of penetration by the blastocyst becomes blocked by fibrin clot. Two days later (11th day), endometrial epithelium overgrows and covers the fibrin clot.  What happens to the blastocyst during implantation? ▪ Changes of blastocyst during implantation: (1 outside &2 inside) 1. Trophoblast differentiates into outer syncytiotrophoblast and inner cytotrophoblast, starting at the embryonic pole then extends all over the blastocyst. 2. Formation of amniotic and yolk sac cavities. 3. Formation of double layered embryonic disc (epiblast and hypoblast). Rule of 2   Abnormal sites of implantation: 1. Placenta Previa: Implantation occurs at the lower segment of uterus. a.Placenta praevia (centralis): covers the internal os completely. b.Placenta praevia (marginalis): covers the internal os partially. c.Placenta praevia (lateralis): Above the internal os. 2. Ectopic pregnancy: Blastocyst is abnormally implanted outside the uterus in the following sites:  Tubal in uterine tube which may occur in ampulla, isthmus or intramural parts. In this case, rupture of the tube is expected at the 8th week of pregnancy leading to severe internal hemorrhage. ▪ Ovarian on the surface of the ovary. ▪ Omental: in peritoneum (abdominal or pelvic). TIME FOR QUIZ During the pre-embryonic period, the fertilized egg undergoes cleavage, resulting in a solid ball of cells. This structure is known as: A. Morula B. Blastocyst C. Zygote D. Trophoblast Which of the following events occurs at the start of the fetal period? A. Major organ systems begin to form from the germ layers B. The embryo is fully formed, and growth becomes the main focus C. The fertilized egg implants into the uterine wall D. The zygote divides to form a blastocyst An embryo at the end of the 8th week shows a well-defined head, trunk, and limb buds, with the beginning of digit formation. Question: Based on the developmental timeline, which phase is the embryo likely transitioning into? A. The pre-embryonic period, as cell differentiation is just beginning B. The fetal period, as major organs are now formed and will grow and mature C. Gastrulation, where primary germ layers are still forming D. The implantation stage In meiosis, the critical event that increases genetic diversity by exchanging DNA between chromatids is known as: A. Segregation B. Crossing-over C. Synapsis D. Random fertilization In spermatogenesis, which cells are located within the seminiferous tubules and serve as the precursor for sperm cells? A. Spermatocytes B. Sertoli cells C. Spermatogonia D. Leydig cells The transformation of a spermatid into a mature sperm cell is known as: A. Spermatogenesis B. Spermiogenesis C. Gametogenesis D. Oogenesis At which stage does the developing embryo typically enter the uterus, ready for implantation? A. Zygote B. Morula C. Blastocyst D. Gastrula The trophoblast layer of the blastocyst plays an important role in which of the following processes? A. Differentiating into the embryo proper B. Forming the inner cell mass C. Attaching to and invading the endometrium for implantation D. Undergoing cleavage to increase cell number A couple experiences difficulty with conception, and lab analysis reveals an issue with the acrosome reaction in the sperm cells. Question: What is the acrosome reaction, and why is it critical for successful fertilization? A. Fusion of sperm and oocyte membranes; it triggers embryo cleavage B. Release of enzymes that penetrate the zona pellucida; it allows sperm entry into the oocyte C. Division of the sperm cell’s chromosomes; it prevents polyspermy D. Formation of the blastocyst; it facilitates implantation