General Embryology (Embryonic and Fetal Period) PDF

GENERAL EMBRYOLOGY By. Solomon Demissie (Radiology, MSc CLINICAL ANATOMY)  DEVELOPMENT  sexual reproduction is the process by which organisms produce offspring by making sex cells called gametes.  Male gametes are sperm in males and secondary oocytes in females.  The organs that...

GENERAL EMBRYOLOGY By. Solomon Demissie (Radiology, MSc CLINICAL ANATOMY)  DEVELOPMENT  sexual reproduction is the process by which organisms produce offspring by making sex cells called gametes.  Male gametes are sperm in males and secondary oocytes in females.  The organs that produce gametes are called gonads  testes in the male and the ovaries in  CONT.….  Once developed sperm have been deposited in the female reproductive tract and a secondary oocyte has been released from the ovary, fertilization can occur.  This process initiates a cascade of developmental events that, when completed properly, produces a healthy newborn baby.  CONT…….  From fertilization through the eighth week of development, the embryonic period and the developing human is called an embryo.  The fetal period begins at week nine and continues until birth and the developing human is called a fetus EMBRYONIC PERIOD First Week of Development  The first week of development is characterized by several significant events including Fertilization Cleavage of the zygote, Blastocyst formation, and implantation. FERTILIZATION  Fertilization is the process by which the genetic material from a haploid sperm cell and a haploid secondary oocyte merges into a single diploid nucleus.  Occurs in the uterine (fallopian) tube within 12 to 24 hours after ovulation.  Sperm live 48 hrs. and secondary oocyte only 24 hrs. after release. Sperm pathway for fertilization i. Sperm swim (flagella) to uterus ii. uterus and tubes (contraction) iii. capacitation (a series of functional changes) For fertilization to occur, a sperm cell must 1. Penetrate corona radiata; the cells that surround the secondary oocyte, 2. Penetrate zona pellucida; the clear glycoprotein layer between the corona radiata and the oocyte’s plasma membrane. 3. Acrosomal reaction the release of the contents of the acrosome. The acrosome is a helmet like structure that covers the head of a sperm, contains several enzymes.  DURING FERTILIZATION  The nucleus in the head of the sperm develops into the male pronucleus, and the nucleus in the fertilized ovum develops into the female pronucleus.  After the pronuclei form, they fuse, producing a single nucleus with 23 chromosomes from each pronucleus. FERTILIZATION  Fusion of the two haploid (n) pronuclei restores the diploid number (2n) of 46 chromosomes in the fertilized ovum, now called a zygote.  Fertilization takes approximately 24 hours. TWIN FORMATION  Dizygotic (fraternal) twins are produced from the independent release of two secondary oocytes and the subsequent fertilization of each of them by different sperm.  They are the same age and in the uterus at the same time, but genetically they are as dissimilar as any other siblings.  Dizygotic twins may or may not be the same sex. TWIN  Monozygotic (identical) twins develop from a single fertilized ovum, they contain exactly the same genetic material and are always the same sex CLEAVAGE OF THE ZYGOTE  After fertilization, the zygote undergoes mitotic cell divisions called cleavage that initially increase the number of cells without increasing the overall size of the cell mass.  The progressively smaller cells produced by cleavage are called blastomeres.  MORULA  Successive cleavages eventually become more rapid and at 16 cell stage they produce compaction a solid sphere of cells called the Morula.  Inner cells of the morula constitute the inner cell mass which gives rise to tissues of the embryo proper  Outer cells compose the outer cell mass which forms the trophoblast, which later BLASTOCYST FORMATION  About the time the Morula enters the uterine cavity, fluid begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass.  Gradually, the intercellular spaces forms a single cavity, the blastocele.  CONT.….  At this time  Embryo is called a blastocyst.  Cells of the inner cell mass- embryoblast  the outer cell mass- trophobiast, flatten and form the epithelial wall of the blastocyst  IMPLANTATION  The blastocyst remains free within the uterine cavity for about 2 days and nourished by secretions from the endometrium.  Then the blastocyst loosely attaches to the anterior or posterior part of endometrium in a process called implantation.  The endometrial glands in the vicinity enlarge, and the endometrium becomes more vascular by forming new blood vessels.  DEVELOPMENT OF THE TROPHOBLAST  The trophoblast develops into two layers;  Syncytiotrophoblast:  Cytotrophoblast:  The two layers of trophoblast become part of the chorion (the fetal membranes) as they undergo further growth.  Syncytiotrophoblast  invades the myometrium and the blastocyst becomes implanted in the endometrium and inner one-third of the myometrium.  The trophoblast secrete human chorionic gonadotropin (hCG).  HCG is the key hormone in the  Syncytiotrophoblast  HCG is used to detect pregnancy in clinical parctice  Its presence in the maternal urine or blood is an indication that an implanted embryo is present in the uterus.  DEVELOPMENT OF THE EMBRYO BLAST  The embryoblast also differentiate into;  Hypoblast (primitive endoderm) and  Epiblast (primitive ectoderm).  Cells of the hypoblast and epiblast together form a flat disc referred to as the bilaminar embryonic disc.  a small cavity appears within the layers of epiblast cells and enlarges to form the  DEVELOPMENT OF THE AMNION  As the amniotic cavity enlarges, a single layer of squamous cells forms a dome- like roof above the epiblast cells called the amnion.  Thus, the amnion forms the roof of the amniotic cavity and the epiblast forms the floor.  CONT. …..  As the embryonic disc increases in size and begins to fold, the amnion eventually surrounds the entire embryo creating the amniotic cavity that becomes filled with amniotic fluid.  Most amniotic fluid is initially derived from maternal blood. CONT. …..  Later, the fetus contributes to the fluid by excreting urine into the amniotic cavity.  Amniotic fluid serves as oa shock absorber for the fetus, o helps regulate fetal body temperature, ohelps prevent the fetus from drying out oprevents adhesions between the skin of the fetus and surrounding tissues. CONT. ….  The amnion usually ruptures just before birth; it and its fluid constitute the “bag of waters.”  Embryonic cells are normally sloughed off into amniotic fluid.  THIRD WEEK OF DEVELOPMENT (GASTRULATION)  The gastrulation is the process the bilaminar embryonic disc (epiblast and hypoblast) is transformed into a trilaminar embryonic disc.  During the third week, the three primary germ layers are established. These are:  Endoderm  Mesoderm  FOURTH WEEK OF DEVELOPMENT  The fourth through eighth weeks are very significant in embryonic development because all major organs appear during this time.  The term organogenesis refers to the formation of body organs and systems.  By the end of the eighth week, all major body systems have begun to develop, although their functions for the most part are minimal.  DID YOU THINK IT? It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert"  EMBRONIC CHANGES  The embryo undergoes dramatic changes in shape and nearly triples its size.  It is essentially converted from a flat, two-dimensional trilaminar embryonic disc to a three-dimensional cylinder, via a process called embryonic folding. FETAL PERIOD  The period from the beginning of the ninth week to birth is known as the fetal period.  It is characterized by maturation of tissues and organs and rapid growth of the body.  During the fetal period, the fetus is less vulnerable to the damaging effects of drugs, radiation, and microbes than it was as an embryo. CRL  The length of the fetus is usually indicated as the crown- rump length (CRL) (sitting height).  the measurement from the vertex of the skull to the rump (buttock).  These measurements, expressed in centimeters, are correlated with the age of the fetus in weeks BASIC CHANGES SEEN DURING FETAL PERIOD  Primary ossifícation centers are present in the long bones and skull by the 12th week.  Also by the 12th week, external genitalia develop to such a degree that the sex of the fetus can be determined by external examination (ultrasound).  The weight of the fetus increases during this period and by the end of the fifth month is still < 500 g.  The fetus is covered with fine hair, called lanugo hair; eyebrows and head hair are also visible.  During the fifth month, movements of the fetus can be felt by the mother.  A fetus born early in the sixth month has great difficulty of surviving.  Although several organ systems are able to function, the respiratory system and the central nervous system have not differentiated sufficiently and coordination between the two systems is not yet well established  If born at 7th month the infant has a 90% chance of surviving.  AGE DETERMINATION  The last menstrual period (LMP)  Gestational sac (GS) (4wk-7wk)  CRL (7th -14th wk)  Biparietal diameter (BPD),  Head circumference (HC)  Abdominal circumference (AC)  Fémur length (FL)  An accurate determination of fetal size and age is important for managing pregnancy NAEGELE'S FORMULA  The Naegele's formula is simple arithmetic method for calculating the EDD (estimated date of delivery) based on the LMP (last menstrual period).  Last date MP  Add seven days  Subtract 3 months  Add one year e.g. LMP is July 23, 2020 calculate EDD ? FETAL MEMBRANES AND PLACENTA  The placenta is the organ that facilitates nutrient and gas exchange between the maternal and fetal compartments.  As the fetus begins the ninth week of development, its demands for nutritional and other factors increase, causing major changes in the placenta.  Foremost among these is an increase in surface area between maternal and fetal components to facilítate exchange.  The disposition of fetal membranes is also altered as production of amniotic fluid increases.  Main functions of the placenta are (1) exchange of metabolic and gas between maternal and fetal bloodstreams and (2) production of hormones. AMNIOTIC FLUID  The amniotic cavity is filled with a clear, watery fluid that is produced in part by amniotic cells but is derived primarily from maternal blood.  The amount of fluid increases from approximately 30 mL at 10 weeks of gestation to 1,000 mL at 37 weeks.  During the early months of pregnancy, the embryo is suspended by its umbilical cord in this fluid, which serves as a protective cushion.  The fluid (1) absorbs jolts, (2) prevents adherence of the embryo to the amnion, and (3) allows for fetal movements. BIRTH DEFECT  Birth defect, congenital malformation, and congenital anomaly are terms used to describe structural, behavioral, functional, and metabolic disorders present at birth.  Malformations occur during formation of structures (organogénesis).  They may result in complete or partial absence of a structure or in alterations of its normal configuration.  Malformations are caused by environmental and/or genetic factors acting independently or in concert.  Most malformations have their origin during the third to eighth weeks of gestation CASES 1. Ectopic pregnancy 2. Polyhydramnions and Oligohydrmnious 3. Premature Rupture of Membrane (PROM),placental previa 4. Neural tube defect ( spina bifida and anecephaly (cranioschiasis)) 5. Gastroschisis, fetal hydrops and Omphalocele 6. Craniosynostosis (abnormal sutural closure) 7. Achondroplesia , Acromegaly, Microcephaly, and Hydrocephalus GREAT THANKS