HBL-2 Embryology PDF

HBL-2 Introduction to Embryology (Developmental Anatomy)  AIM To give an overview of embryology or the developmental anatomy of the human body.  LEARNING OUTCOMES At the end of this session students should be able to state the various events and changes that occur...

HBL-2 Introduction to Embryology (Developmental Anatomy)  AIM To give an overview of embryology or the developmental anatomy of the human body.  LEARNING OUTCOMES At the end of this session students should be able to state the various events and changes that occur during the normal development of the human body.  LECTURE OUTLINE Subdivisions of embryology General Embryology Special Embryology DEVELOPMENTAL ANATOMY (EMBRYOLOGY) Study of development of adult structure from the fertilized ovum  LEVELS OF ORGANISATION ⚫ Molecular Level >> atom, molecules ⚫ Cellular Level >> cells ⚫ Tissue Level >> Muscle, nerve, bone ⚫ Organ Level >> Heart, lungs, brain ⚫ Organ System Level >> Cardiovascular system Respiratory system ⚫ Organism Level >> Human body Recent advances in genetics, developmental biology, maternal-fetal medicine, and public health have significantly increased our knowledge of embryology and its relevance. Because birth defects are the leading cause of infant mortality and a major contributor to disabilities, and because new prevention strategies have been developed, understanding the principles of embryology is important for health care professionals (Sadler, 2010). Embryology is organized into two parts: I. General Embryology This describes the normal processes occurring during development from gametogenesis through the embryonic and foetal periods and also includes abnormalities like birth defects. II. Special Embryology This describes the normal processes of embryogenesis for each organ system. I. General Embryology ⚫ Gametogenesis ⚫ Ovulation, fertilization and implantation ⚫ Formation of germ discs ⚫ Embryonic period ⚫ Foetus and Placenta ⚫ Birth defects I. General Embryology ⚫ Gametogenesis It is the conversion of germ cells into male and female gametes. Cell division: mitosis, meiosis. Oogenesis, spermatogenesis, spermiogenesis ⚫ Ovulation, fertilization and implantation Ovulation, ovarian follicles, oocyte transport, Fertilization is the process by which male and female gametes fuse. Sites and phases of fertilization Implantation – Cleavage, blastocyst, site of implantation ◼ General Embryology ⚫ Formation of germ discs Bilaminar germ disc: Trophoblasts, embryoblasts, amniotic cavity, chorionic cavity, yolk sac. Trilaminar germ disc: Gastrulation [formation of 3 germ layers – ectoderm, mesoderm, endoderm] Primitive node, primitive pit, primitive streak ◼ General Embryology ⚫ Embryonic period Organogenesis, derivatives of 3 germ layers forming tissues, organs Neurulation [formation of neural tube], neural crest ⚫ Foetus and Placenta Foetal period is characterized by maturation of tissues and organs rapid growth of the body Foetal membranes and placenta [structure, functions] ◼ General Embryology ⚫ Birth defects These are structural, behavioral, functional, and metabolic disorders present at birth. Teratology [science that studies these disorders] – types, factors, diagnosis.  II. Special Embryology ⚫ Muscular System ⚫ Cardiovascular and Respiratory Systems ⚫ Digestive System ⚫ Urogenital System ⚫ Head, Neck, Ear, Eye, Integumentary System ⚫ Central Nervous System  Cell Division: Mitosis mitos = thread osis = condition (thread-like chromosomes in the nucleus) ◼ Mitosis = division of nuclei of somatic cells ◼ Somatic cells include: All cells in the body; Except germ cells (sperms or ova)  Cell Division: Meiosis A special type of cell division consisting of two meiotic cell divisions. ◼ 1st Meiotic Division (Reductional Division) ◼ 2nd Meiotic Division (Equatorial Division) Takes place in germ cells only Gametogenesis  Ovulation, Fertilisation and Implantation 2 3 1 1. Fertilization 2. Cleavage (Day 2-3) 4 3. Blastocyst (Day 4-5) 4. Implantation (Day 5-6) Time between ovulation and succeeding menstrual bleeding is constant (14 days) Oocyte Transport ◼ Oocyte is carried into the infundibulum by: ⚫ sweeping movements of fimbriae ⚫ motion of cilia of the tubal epithelial cells ◼ Rhythmic peristalsis of the uterine tube carry the oocyte toward the ampulla of the tube. ◼ Muscular contractions of the uterine tube push the oocyte toward uterine lumen. ◼ The fertilized oocyte will reach uterine lumen in about 3-4 days. Sperm Transport ◼ Muscular contractions of walls of uterus and uterine tube allow passage of sperm through them. ◼ Sperms move 2-3 mm per minute.: ⚫ Move slowly in acid environment of vagina ⚫ Move rapidly in alkaline environment of uterus ◼ It takes 5-45 minutes for sperms to travel from external uterine os to ampulla. ◼ Only about 200 sperms reach the fertilization site. ◼ Most sperms degenerate.  Fertilization ◼ Process by which male and female gametes fuse. ◼ Occurs in the ampulla of the uterine tube. ◼ Spermatozoa and oocyte can survive in the female reproductive tract for 24 hours. ◼ Structures penetrated by the spermatozoon during fertilization: ⚫ Corona radiata ⚫ Zona pellucida ⚫ Oocyte cell membrane  Fertilization RESULTS OF FERTILISATION ◼ Restoration of diploid number of chromosomes. ⚫ Half from father and half from mother ⚫ New combination of chromosomes different from both parents. ◼ Determination of sex of the embryo. ◼ Initiation of cleavage.  Cleavage and Blastocyst Inner embryoblast cell mass Ouier cell mass trophoblast ◼ Cleavage is the mitotic divisions of zygote resulting increased number of cells. ◼ With each cleavage the cells become smaller and are known as blastomeres. ◼ Later the blastomeres become tightly grouped cells with inner and outer layers. ◼ The embryo acquires a cavity on 3rd day after fertilization and forms a blastocyst. ◼ Inner cell mass located at one pole is called the embryoblast (future embryo). ◼ Surrounding cells that form the outer cell mass become the trophoblasts.  Implantation Implantation is the result of mutual trophoblastic and endometrial actions ◼ Zona pellucida disappears to allow implantation. ◼ Proteolytic enzymes of trophoblasts erode the uterine epithelial cells. ◼ Uterine mucosa promotes proteolytic action G of the blastocyst. D C E B X Sites of implantation A H X NORMAL site (posterior wall of uterus) F A-F Tubal pregnancy (common ectopic sites) G Abnormal pregnancy H Ovarian pregnancy  1st Week of Development Summary of Events Zygote >>> Morula >>> Blastocyst >>> Implantation 8th Day 1st Week - 2nd Week of Development 9th Day 11-12th Day 13th Day  1st Week - 2nd Week of Development 11-12th Day 13th Day 11-12th Day 13th Day End of 2nd Week  End of 2nd Week of Development ac E Epiblast Hypoblast H ys Germ disc consists of 2 apposed cell discs: ⚫ Epiblast (forms floor of amniotic cavity) ⚫ Hypoblast (forms roof of yolk sac)  Bilaminar Disc ◼ Blastocyst embedded in endometrium. ◼ Trophoblast differentiates into an inner cytotrophoblast and outer syncytiotrophoblast ◼ Embryoblast differentiates into epiblast and hypoblast (bilaminar disc) ◼ Formation of uteroplacental circulation, primary villi. ◼ Formation of amniotic and chorionic cavities. ◼ Formation of primitive yolk sac.  Embryonic Period (3rd Week – 8th Week) Characteristic features: ◼ Period of Organogenesis. ◼ Each of the 3 germ layers gives rise to specific tissues and organs. ◼ Major features of the external body form become recognizable.  Trilaminar Disc ◼ Epiblast cells migrate toward the primitive streak. and on arrival at the streak, the cells become flask-shaped and slip beneath the epiblast. ◼ A new cell layer therefore develops between the epiblast and the hypoblast. ◼ These cells then displace the hypoblast to form the definitive endoderm (embryonic endoderm). ◼ After the definitive endoderm is formed, the invaginating cells between epiblast and the newly created endoderm, will become the mesoderm  Trilaminar Disc ◼ Cells remaining in the epiblast will become the ectoderm. ◼ Epiblast therefore is the source of all 3 germ layers (ectoderm, mesoderm, endoderm) Gastrulation It is the process that establishes all 3 germ layers in the embryo.  Trilaminar Disc Mesoderm further divides into - paraxial, intermediate and lateral plate mesoderm Derivatives of Ectoderm, Endoderm and Mesoderm (Summary) Derivatives of Ectoderm, Endoderm and Mesoderm (Summary) Derivatives of Ectoderm, Endoderm and Mesoderm (Summary) Derivatives of Ectoderm, Endoderm and Mesoderm (Summary) Derivatives of Ectoderm, Endoderm and Mesoderm (Summary)  Embryonic Period (3rd Week – 8th Week) Characteristic features: ◼ Period of Organogenesis. ◼ Each of the 3 germ layers gives rise to specific tissues and organs. ◼ Major features of the external body form become recognizable e.g. Hand plates (HP), Foot plates (FP), Pericardial bulge (P) Foetal Period (3rd Month to Birth) Foetal Period (3rd Month to Birth)  Foetal Period (3rd Month to Birth) Characteristic features: ◼ Maturation of tissues and organs. ◼ Rapid growth of body. ◼ Increase in length (3rd, 4th, 5th months) ◼ Increase in weight (last 2 months of gestation)  Foetal Period (3rd Month to Birth) Relative slow down in growth of head  Foetal Period (3rd Month to Birth) Month Prenatal Developments 1 Heart starts beating. 5 Fetal movements felt by mother. Skin wrinkles. 6 Heart sound heard 8 Testes in scrotum. 10 Skin smooth and plump. Skin is wrinkled due to lack of subcutaneous fat. Fetus born before 7th month cannot survive due to insufficient differentiation of the respiratory and central nervous systems.  Foetal Period (3rd Month to Birth) Length of pregnancy / Date of Birth: ◼ 280 days (40 weeks) from 1st day of LNMP. ◼ 266 days (38 weeks) after fertilization. Age of foetus: From time of fertilization (in weeks or calendar months) Length of foetus: ◼ Sitting height: Crown-rump length (CRL) in cm. ◼ Standing height: Crown-heel length (CHL) in cm. LNMP=Last normal menstrual period Time between ovulation and succeeding menstrual bleeding is constant (14 days)  Foetal Membranes Components of Foetal Membranes: ◼ Placenta ◼ Amnion ◼ Connecting stalk (with umbilical cord, allantois and umbilical vessels) ◼ Yolk stalk (vitelline duct) ◼ Yolk sac Foetal Membranes Placenta Yolk sac Connecting stalk  Foetal Membranes Components: ◼ Placenta Amnion Placenta ◼ Amnion Yolk sac/stalk ◼ Connecting stalk Connecting ⚫ contains the allantois and umbilical vessels stalk (2 arteries, 1 vein). ⚫ Wharton jelly is a loose connective tissue in the umbilical cord protecting the umbilical vessels. ◼ Yolk stalk (vitelline duct / vitello-intestinal duct) ◼ Yolk sac: ⚫ present in the chorionic cavity. ⚫ when the amnion comes into contact with the chorion, the chorionic cavity is obliterated. ⚫ yolk sac also shrinks and become obliterated.  Foetal Membranes Components: Placenta ◼ Placenta ◼ Amnion Yolk sac/stalk ◼ Connecting stalk Connecting ⚫ contains the allantois and umbilical vessels stalk (2 arteries, 1 vein). ⚫ Wharton jelly is a loose connective tissue in the umbilical cord protecting the umbilical Amnion vessels. ◼ Yolk stalk (vitelline duct / vitello-intestinal duct) ◼ Yolk sac: ⚫ present in the chorionic cavity. ⚫ when the amnion comes into contact with the chorion, the chorionic cavity is obliterated. ⚫ yolk sac also shrinks and become obliterated.  Foetal Membranes ◼ Placenta ⚫ Discoid shape, 15-25 cm. diameter, 3 cm. thick, 500-600 gm. weight. ⚫ It is expelled from uterus 30 minutes after birth of the child. ⚫ Its maternal side show 15-20 cotyledons which are covered by the decidua basalis layer Maternal side Foetal side  Foetal Membranes ◼ Placenta ⚫ Discoid shape, 15-25 cm. diameter, 3 cm. thick, 500-600 gm. weight. ⚫ It is expelled from uterus 30 minutes after birth of the child. ⚫ Its foetal side shows the chorionic plate with chorionic vessels, umbilical cord, chorion and amnion Maternal side Foetal side  Foetal Membranes ◼ Allantoin ⚫ an endodermal diverticulum from the hind gut. ⚫ it extends from urinary bladder to umbilicus. ⚫ it then involutes to form the urachus.  Congenital Malformations: Terminologies Birth defects Congenital malformations Congenital anomalies …. are synonymous terms used to describe structural, behavioural, functional and metabolic disorders present at birth  Congenital Malformations: Causal Factors Causes of most (50-60%) anomalies are unknown; 20-25% are multifactorial.  Chromosomal abnormalities Klinefelter syndrome (Trisomy) Turner’s syndrome (Monosomy) Courtesy of Dr. F Antoniazzi and Dr. V Fanos, Dept. of Pediatrics, University of Verona, Verona, Italy.  Chromosomal abnormalities Cri du chat syndrome Down syndrome (Trisomy 21) (From Gardner EJ: Principles of genetics, 5th ed. New York, John Wiley & Sons, Inc., 1975)  Teratogens Infectious Agents Chemical Agents Rubella (German Measles) Drugs (Thalidomide)) Courtesy of Dr. Richard B, Dept. of Ophthalmology, Cornell-New York Hopital; Dr. Daniel IW, Dept. of Ophthalmology, New York University College of Medicine. Unilateral amelia Meromelia From Cooper LA et al: Am J Dis Child 110”416,1965. AMA.  Prenatal Diagnostic Investigations Ultrasonography To determine foetal age and growth parameters Detects many malformations Maternal serum screening (a-fetoprotein)* Indicates presence of neural tube defects etc. Amniocentesis (Amniotic fluid)* To assess alpha feto protein in the amniotic fluid. Chorionic villus sampling (Placental tissue)* 1=Fetus; 2=Amniotic cavity; Genetic analysis of cells to detect chromosome 3=Chorionic cavity; 4=Amnion disorders. *Higher-risk pregnancies  REFERENCES 1. Anatomy & Physiology in Health and Illness. 12th Edition. Waugh A & Grant A (2014). Churchill Livingstone Elsevier. 2. Langman’s Medical Embryology. 14th Edition. Sadler TW (2018). Lippincott, Williams & Wilkins 3. Ace Ahead STPM Text Biology 1st Term. 4th Edition Thye CL, Jacqueline RS & Nalini B (2018). Oxford Fajar Sdn. Bhd. 4. Ace Ahead STPM Text Biology 2nd Term. 3rd Edition Thye CL, Jacqueline RS & Nalini B (2018). Oxford Fajar Sdn. Bhd.  End of Introduction to Embryology (Developmental Anatomy)