Embryo PDF - A Detailed Study of Human Development

 Table Of Content Title Page First Week of Development 4 CLEAVAGE 8 Events occur in 2nd Week of Development 16 Twins 25 Dr Amr Mosafer...

 Table Of Content Title Page First Week of Development 4 CLEAVAGE 8 Events occur in 2nd Week of Development 16 Twins 25 Dr Amr Mosafer Page : 1  General Embryology BASIC TERMINOLOGY 1 Embryology ▪ Embryology is a branch of science that deals with the study of formation and development of an organism. 2 Growth ▪ Is an increase in the number and size of cells. 3 Differentiation ▪ Is the specialization of the tissues of different organs ▪ To perform special functions. N.B: Development involves growth and differentiation. Dr Amr Mosafer Page : 2  Periods of human embryology ❖ Most of the clinicians divide human prenatal development as ↳ First, Second And Third Trimesters ↳ 3-Month Period For Each. ❖ Embryologically, ↳ Prenatal development is divided into 3 periods: 1 Germinal period : ▪ First two weeks after fertilization. 2 Embryonic period: ▪ From 3rd till the end of the 8th week (2 nd month). 3 Fetal period (organ growth): From 3 rd month till the end of the pregnancy.  Events in the first week of development Dr Amr Mosafer Page : 4  Fertilization Definition ▪ Fusion of mature sperm with an ovum ( haploid germ cells ) ▪ To form a single zygote (diploid cell ). Usual site of fertilization ▪ lateral 1/3 of uterine (fallopian) tube ( = Ampulla of fallopian tube). ▪ Preparation of sperms for fertilization 1 Capacitation of spermatozoa Capacitation ↳ Is a process of conversion of immature spermatozoa to mature one ↳ By removal of glycoprotein coat covering the head of sperm. Duration ▪ It takes 7 hrs. Site ▪ Female reproductive tract (uterus and uterine tubes). Effect of capacitation: 1) Increases sperm motility 2) Sperm becomes capable of penetrating the outer layers of the egg N.B Spermatozoa are artificially capacitated and used In Vitro Fertilization (IVF). Dr Amr Mosafer Page : 5  2 Acrosomal reaction: The acrosomal cap ↳ Release of its contents (acrosin, hyaluronidase, and trypsin like substance) ↳ Needed to pass through corona radiata and penetrate zona pellucida). Phases of fertilization: 1- Passing through corona radiata: ↳ Out of 300-500 sperms reach the site of fertilization. ↳ Only one fertilizes the ovum (the rest only help it to pass through the corona radiata). 2- Penetration of zona pellucida: ↳ By the aid of enzymes released from the acrosomal cap. 3- Fusion of oocyte-sperm cell membranes: ↳ The head of the spermatozoon enter the cytoplasm of the oocyte Dr Amr Mosafer Page : 6 1 ) Cortical and zona reactions: ↳ Cortical receptors alter their structure. ↳ Zona pellucida changes its structure & composition to become impermeable. ↳ These prevent polyspermy. 2 ) As soon as the sperm penetrate the secondary oocyte ↳ The latter undergoes second meiotic division and gives rise to an ovum. Results of fertilization: 1- First ↳ The head of sperm swells to form the male pronucleus ↳ The nucleus of the ovum swells to form the female pronucleus. 2- Second ↳ The male and female pronuclei ( each containing 23 chromosomes ) unite together to form one nucleus ( containing 46 chromosomes ) ↳ Now the fertilizaed ovum is called zygote. 3- The number of chromosomes become (46). 4- Determination of sex. 5- Initiation of cleavage: ↳ Fertilization provides energy for repetitive cell divisions of zygote. ↳ Without fertilization Oocyte degenerates. Dr Amr Mosafer Page : 7  CLEAVAGE Definition: ▪ Cleavage is a process of repeated mitotic divisions of zygote to give rise to ▪ smaller cells called blastomeres. Duration: ▪ Up to 6–7 days (after fertilization till implantation). Site ❖ Begins : In the ampulla of fallopian tube ❖ Ends : In the uterine cavity (where the conceptus reaches site of implantation). ❖ Fertilized ovum divides to form 2-cell, 4-cell stage and so on. NB About the 3rd day, A 16-cell stage is formed (known as morula) that surrounded by zona pellucida. Dr Amr Mosafer Page : 8  Blastocyst Formation ❖ Morula enters the uterine cavity around day 4. ▪ Fluid enters between the blastomeres and form a cavity (blastocele). ▪ This stage is called blastocyst. ▪ The outer cell mass flatten (now called trophoblast) ▪ Inner cell mass (now called embryoblast) lie at embryonic pole. By the end of 4th day ▪ Zona pellucida starts degeneration and disappears by the end of 5th day ↳ To allow beginning of implantation. Dr Amr Mosafer Page : 9  IMPLANTATION OF BLASTOCYST Definition: ▪ It is the process of embedding of blastocyst into the uterine endometrium. Time: ▪ Begins on 6th or 7th day after fertilization ▪ Completes by the 12th day. Site of Implantation: ▪ The normal site is the midline of the upper part of the body of the uterus in the anterior or posterior wall, near the fundus Dr Amr Mosafer Page : 10  Implantation Process ❖ Blastocyst fixes its embryonic pole to site of implantation. ❖ The trophoblastic cells rapidly proliferate  Becomes Formed Of 2 Layers: 1 An outer Syncytiotrophoblast ▪ Formed of cytoplasm with randomly dispersed nuclei and without cell membrane. 2 An inner Cytotrophoblast ▪ Formed of cells with well-defined walls. ❖ Syncytiotrophoblast ↳ Is phagocytic and makes a pit in the endometrium. ❖ Blastocyst ↳ Is gradually sinks into the pit until it sinks completely into the endometrium. ❖ Blastocyst ↳ Completely embeds in the endometrium by the 12th day of development. ❖ Site of penetration ↳ Is closed by fibrin plug at 9th day. ❖ Surface epithelium overgrows the fibrin plug at 11th day. ❖ Endometrium of pregnancy called decidua. Dr Amr Mosafer Page : 11 Dr Amr Mosafer Page : 12  Abnormal site of Implantation ▪ If conceptus does not implant at the usual site ▪ It is called abnormal or ectopic implantation and it results in ectopic pregnancy. ❖ Ectopic pregnancy or implantation are classified  According to their site of implantation as follows: 1 Uterine abnormal implantation Placenta previa: ▪ It is an implantation in lower uterine segment. ▪ Placenta previa lateralis = placenta is attached to lateral wall of body of uterus ▪ Placenta previa marginals = covers internal os partially. ▪ Placenta previa centralis = covers internal os completely. Dr Amr Mosafer Page : 13  2 Tubal implantation: ▪ It is an implantation in uterine tube. ▪ It is the most common extrauterine implantation 3 Abdominal implantation: ▪ It is a rare ectopic implantation. ▪ It usually occurs in the ovary or mesentery N.B. Implantation outside the cavity of uterus leads to ectopic pregnancy Which cannot reach full-term due to inadequate nutrition. Dr Amr Mosafer Page : 14  THE DECIDUA Definition: ▪ It is the endometrium of the pregnant uterus after implantation. Parts of the decidua: ▪ 3 parts in relation to blastocyst: - 1- Decidua capsularis: ▪ The part between blastocyst and uterine cavity. 2- Decidua basalis: ▪ The part between blastocyst and wall of uterus. 3- Decidua parietalis: ▪ The part that lines the remaining wall of the uterus. Dr Amr Mosafer Page : 15 ❖ Continuation of implantation of blastocyst Formation of Bilaminar germ disc: ❖ Embryoblast (inner cell mass) differentiates into the following two layers: 1- Hypoblast layer: Cuboidal cells lie ventrally adjacent to blastocyst cavity 2- Epiblast layer: Columnar cells lie dorsally. Dr Amr Mosafer Page : 16  Formation of Amniotic Cavity: ▪ A small cavity appears within the epiblast cells. ↳ This cavity enlarges to become the amniotic cavity. ▪ The amniotic cavity is roofed by amnioblasts (amnion forming cells), ↳ While the floor is epiblast proper. NB Amnioblasts derived from epiblast cells adjacent to cytotrophoblast. Amniotic cavity Dr Amr Mosafer Page : 17  Formation of yolk sac 1 Primary Yolk Sac ❖ The blastocele cavity is changed to primary yolk sac.  The roof of 1ry yolk sac is the hypoblast  The floor is heuser’s membrane ↳ Heuser’s membrane is derived from Hypoblast cells lay down 2 Secondary Yolk Sac: ▪ The hypoblast produces additional layer of cells that lines the inner surface of Heuser’s membrane, ↳ That converts the primary yolk sac into secondary yolk sac ↳ That becomes completely surrounded by hypoblast. 3 Definitive yolk sac: ❖ After folding of the embryo, ↳ Part of 2ry yolk sac is incorporated inside the embryo ↳ To form the gut and the remaining part bulges through the umbilical ring outside the body of the embryo ↳ To form the definitive yolk sac. 3rd Dr Amr Mosafer Page : 18  Functions of yolk sac: 1) It is the first hematopoietic organ in the embryo. 2) Absorption of nutrients (active and passive transport) 3) Synthesis of various proteins like alpha-fetoprotein 4) From 2nd to 4th week, ↳ It acts as a site of migration of the primordial germ cells 5) Its endoderm gives the mucous membrane of the gut and the respiratory tract. 6) Its surrounding extraembryonic mesoderm gives the vitelline arteries and veins. Definitive yolk sca Dr Amr Mosafer Page : 19  Congenital anomalies of yolk sac: Anomalies of the V.I.D 1 Meckel’s diverticulum: ▪ Persistence of the proximal part of v.I.D. It is 2 inches long, ▪ Incidence 2% ▪ Lies 2 feet from the ileocecal valve 2 Vitelline (umbilical) sinus: ▪ Persistence of the distal part of v.I.D umbilicus ↳ Discharges intestinal secretions 3 Vitelline fistula: ▪ Persistence of the whole duct. ▪ Umbilicus discharges intestinal contents 4 Vitelline cyst: ▪ Persistence of the middle part of the duct 5 Fibrous cord: ▪ Persistence of the duct as fibrous band Dr Amr Mosafer Page : 20  Formation of extraembryonic (primary) mesoderm By the end of the 2nd week, ↳ Extraembryonic (1ry) mesoderm appears and separates the cytotrophoblast from the yolk sac & the amnion. ❖ A single cavity develops in this mesoderm ↳ Called the extraembryonic coelom (EEC), ↳ Which splits the mesoderm into two layers: - 1 An outer layer: ▪ Lining the whole trophoblast and covering the amnion, ↳ Called somatopleuric or parietal layer of extraembryonic mesoderm 2 An inner layer: ▪ Covering the yolk sac, ↳ Called visceral (splanchnopleuric) layer of extraembryonic mesoderm Dr Amr Mosafer Page : 21 Dr Amr Mosafer Page : 22  Chorion ▪ The trophoblast & the underlying somatic extra-embryonic mesoderm is called chorionic plate. ▪ The extra-embryonic coelom is also named chorionic cavity. ▪ The blastocyst is now called chorionic vesicle (Chorion). ↳ The coelomic cavity (eec) incompletely surrounds the vesicle, ↳ As a part of the mesoderm persists ↳ Without cavitation is termed the connecting stalk (cs) ↳ At the caudal part of the embryonic disc. " Through the connecting stalk ▪ Fetal blood vessels pass between the embryo and placenta. ▪ Called chorionic villi, ▪ Develop from the syncytiotrophoblast. ▪ These surround the chorionic vesicle. Dr Amr Mosafer Page : 23  According to relations of chorionic villi to decidua, Chorionic villi are classified into two types: 1- The Chorion frondosum: ↳ Between the embryo and decidua basalis. 2- The Chorion laeve: ↳ Between the embryo and the decidua capsularis. According to the structure of the chorionic villi , 1- Primary villi (day 11-13): ↳ It is formed by a cover of Syncytiotrophoblast and a core of cytotrophoblast. 2- Secondary villi (day 16): ↳ Extraembryonic mesoderm invades into the core of 1ry villi. 3- Tertiary villus (21 day): ↳ Extraembryonic vessels appear within the mesodermal core Functions of the extraembryonic mesoderm ▪ It supports the epithelium of the amnion and yolk sac. ▪ It also is involved in the development of the fetal blood. Dr Amr Mosafer Page : 24  Twins Definition: ▪ It is the presence of more than one fetus in pregnant women. + According to their number, they are divided into: 1) Twins (most common), 1: 80. 2) Triplet: (80 x 80) 3) Quadruplet: (80x 80 x 80). Dizygotic Twins (Fraternal Twins): ▪ Two ova are fertilized by a two sperms. ▪ Not genetically similar Sex ▪ May or may not similar. ▪ Both has its own placenta, amniotic cavity, chorionic sac. Incidence: ▪ 7/1000 Births. Dr Amr Mosafer Page : 25  Mono zygotic Twins (Identical Twins): ▪ One ovum is fertilized by one sperm. ▪ The embryo splits during the early stages of development ▪ Splitting as early as two cell stage leads to two separate zygotes. ▪ The blastocysts implant separately and so the fetal membranes of these ▪ Twins resemble that of dizygotic twins. ▪ Splitting (inner cell mass), ▪ The twin has a common placenta and a common chorionic cavity ▪ But separate amniotic cavities ▪ It is genetically similar (has identical chromosomes) Sex ▪ Must be similar. Incidence: ▪ About 3 in every 1000 births. Dr Amr Mosafer Page : 26