3,4 Bilaminar, gastrulation and Folding.pdf

HUMAN EMBRYOLOGY Dr M. Eladl Visit the Facebook & Instagram pages for questions and discussions MOHAMED ELADL SEGMENTATION OR CLEAVAGE  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 1...

HUMAN EMBRYOLOGY Dr M. Eladl Visit the Facebook & Instagram pages for questions and discussions MOHAMED ELADL SEGMENTATION OR CLEAVAGE  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 1. THE EMBRYONIC DISC: The cells of the inner cell mass become arranged into two layers:  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 1. THE EMBRYONIC DISC: The cells of the inner cell mass become arranged into two layers: 1) Hypoblast (1ry endoderm): Consisting of small cubical cells. 2) Epiblast (1ry ectoderm): The upper thicker layer, consisting of high columnar cells.  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 2. Two cavities will appear 1) The amniotic cavity On the 7th or 8th day, a small cavity appears in the inner cell mass, which is the primordium of the amniotic cavity. 2) The primary yolk sac: After the formation of the amniotic cavity, the blastocele become modified to form the primary yolk sac.  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 3.Formation of extraembryonic mesoderm A layer of connective tissue which is THE EXTRA EMBRYONIC MESODERM appears between the trophoblast externally and the primary yolk sac and amniotic cavity internally.  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) 4.Formation of extraembryonic coelom Isolated spaces appear within the extra embryonic mesoderm. These spaces rapidly fuse to form a large isolated cavity, which is THE EXTRA EMBRYONIC COELOM.  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) As a result of the formation of the extraembryonic coelom, the extraembryonic mesoderm becomes divided into 2 layers: 1. One lining the trophoblast is called SOMATIC MESODERM. 2. The other covering the primary yolk sac is called SPLANCHNIC MESODERM. Hence, the primary yolk sac decreases in size and smaller secondary yolk sac forms.  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) The division of the extra embryonic mesoderm is NOT COMPLETE as a mesodermal mass remains connecting the two layers at the caudal end of the embryo. This mesodermal connecting mass is called the CONNECTING STALK (the future umbilical cord).  CHANGES IN THE BLASTULLA (CHORIONIC VESICLE) The trophoblast together with its lining somatic layer of the extraembryonic mesoderm form the CHORION. The chorion as well as the structures enclosed by it is called THE CHORIONIC VESICLE. CHANGES IN THE BLASTULLA (CHORIONIC VESICLE)  Third week The 3rd week is characterized by: 1) Changes in the structure of the bilaminar embryonic plate. 2) Formation of the three germ layers.  CHANGES IN THE BILAMIAR EMBRYONIC PLATE The embryonic plate changes from circular to oval to pear shaped structure.  Development of the Notochord The notochord is a flexible rodlike structure of mesodermal cells that serves as the primary longitudinal structural element of chordate. It is found in the early embryos of vertebrates and serves an organizing role in the development of the nervous system.  Development of the Notochord Formation of Prochordal Plate: About the 14th day, near the cranial end of the embryonic plate, the cubical endodermal cells in the middle line increase in height. This results in the formation of PROCHORDAL PLATE.  Development of the Notochord Formation of Primitive Streak: About the 15th & 16th days, the columnar ectodermal cells multiply and increase in number forming an opacity known as the PRIMITIVE STREAK.  Development of the Notochord Formation of Primitive Node: The cranial end of the primitive streak proliferates more to form a PRIMITIVE NODE.  Development of the Notochord Formation of Notochordal Process: On the 17th day, some cells originate from the primitive node, forming a median solid cellular cord (NOTOCHORDAL PROCESS), that grows cranially between the ectoderm and endoderm until it reaches the prochordal plate.  Development of the Notochord Formation of Notochordal Canal: On the 18th day, A primitive pit is invaginated and extends into the notochordal process, forming a NOTOCHORDAL CANAL; a cellular tube that extends cranially from the primitive node to the prochordal plate.  Development of the Notochord On the 19th day, the floor of the notochordal canal fuses with the underlying intraembryonic endoderm and both degenerate.  Development of the Notochord The transitory communication between the amniotic and yolk sac cavities is known as NEUROENTERIC CANAL. (plays a role in the maintenance and adjustment of pressure between the amniotic sac and the yolk sac).  Development of the Notochord ON THE 20TH DAY, the endodermal roof of the yolk sac is repaired and the continuity of the yolk sac is restored. The notochordal plate becomes detached from the endoderm of the yolk sac to form THE NOTOCHORD.  Fate of Notochord 1) The cranial part of the notochord is included in the basilar part of occipital bone and the posterior part of body of sphenoidal bone. 2) The parts of the notochord in the center of the bodies of the vertebrae degenerate and disappear. 3) The parts of the notochord included in the intervertebral discs undergo mucoid degeneration to form the NUCLEUS PULPOSUS.  Dr M. Eladl GASTRULATION  GASTRULATION Definition: It is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc. Time: Begins about 15 days of development and is followed by start of the development of several major organ systems. GASTRULATION  PROCESS OF GASTRULATION Some cells from the sides of the primitive streak and notochordal process migrate laterally and cranially between the ectoderm and endoderm. This third layer of embryo is known as the intra- embryonic mesoderm which is continuous with the extra- embryonic mesoderm covering the amnion and yolk sac at the margin of embryonic disc.  PROCESS OF GASTRULATION By the middle of the 3rd week, the intraembryonic mesoderm separates the ectoderm and endoderm everywhere except: I. At the oropharyngeal membrane cranially. II. At cloacal membrane caudally. III. In the median plane cranial to the primitive node where the notochordal process is located.  ROLE OF GASTRULATION 1. The epiblast and hypoblast are now known as ectoderm and endoderm respectively. 2. Gastrulation will covert the bilaminar plate into the three primary embryonic germ layers – Ectoderm: outside; this embryonic layer more or less surrounds the other germ layers – Mesoderm: middle; this germ layer lies between the ectoderm and endoderm – Endoderm: inside; this germ layer lies at the most interior of the embryo 3. The intra-embryonic mesoderm merges with the extra- embryonic mesoderm at the periphery of the embryonic disc. 4. Subsequently, neurulation will form epithelial and neural ectoderm from the ectoderm  DIFFERENTIATION OF THE 3 GERM LAYERS 1) Derivatives of Ectoderm 1) The central and peripheral nervous system. 2) Sensory epithelium of the eye, ear, and nose. 3) Epidermis of the skin and its appendages (Hair, nails & sweat and sebaceous glands) and the mammary gland. 4) The pia and arachnoid mater. 5) Suprarenal medulla and chromaffin tissue. 6) External auditory meatus and outer layer of the tympanic membrane. 7) Enamel of teeth. 8) The epithelium of the nasal cavity and the paranasal sinuses. 9) The anterior part of the buccal cavity, gums, salivary glands, pituitary gland, lower half of the anal canal & the terminal part of male urethra. 10) The conjunctiva, outer layer of the cornea, muscle of the iris, lens, the lacrimal gland, and the nasolacrimal duct.  Formation of the Neural Tube Time: At the 3rd week. Stages: As the notochord develops it stimulates the overlying ectoderm to be thickened (increase in height of cells and not due to multiplication) to form an elongated plate of thickened epithelial cells known as the NEURAL PLATE.  Formation of the Neural Tube The lateral margin of the neural plate becomes raised to form NEURAL FOLDS The median part of the neural plate becomes depressed to form NEURAL GROOVE. The neural groove has neural folds on each side, which become particularly prominent at the cranial end of the embryo and are the first signs of brain development.  Formation of the Neural Tube By the end of the 3rd week, the neural folds move & fuse, converting the neural plate into a NEURAL TUBE.  Formation of the Neural Tube The neural tube soon SEPARATES from the surface ectoderm. As the neural folds fuse to form the neural tube, some neuroectodermal cells become separated, on each side, from the cells of the NEURAL CREST and come to lie in the angle between the neural tube and the surface ectoderm.  Formation of the Neural Tube At first, the cranial and caudal ends of the neural tube are open and called the CRAINIAL & CAUDAL NEUROPORES. At the end of the 4th week, The process of formation of the neural tube is completed BY the CLOSURE OF CRAINIAL & CAUDAL NEUROPORES. The cranial end of the tube dilates to form the BRAIN VESICLE, while the remaining part forms the SPINAL CORD.  Neural Tube Defects Malformations of the spinal cord, such as spina bifida, meningocele, myelomeningocele and anencephaly 1) Spina bifida: Is a condition that affects the spine. Usually apparent at birth. occurs when the bony vertebral arches fail to form properly, thereby creating a vertebral defect, Can happen anywhere along the spine if the neural tube does not form properly or close all the way. usually in the lumbosacral region.  Types of Spina Bifida SPINA BIFIDA OCCULTA: Bony vertebral defect. The spinal cord is intact. Is evidenced by a tuft of hair in the lumbosacral region. It is the least severe variation Occurs in 10% of the population.  Types of Spina Bifida SPINA BIFIDA WITH MENINGOCELE: Occurs when the meninges protrude through a vertebral defect and form a sac filled with CSF. The spinal cord remains in its normal position.  Types of Spina Bifida SPINA BIFIDA WITH MENINGOMYELOCELE: Occurs when the meninges and spinal cord protrude through a vertebral defect and form a sac filled with CSF.  Types of Spina Bifida SPINA BIFIDA WITH RACHISCHISIS: Occurs when the posterior neuropore of the neural tube fails to close during week 4 of development. The most severe type, causing paralysis from the level of the defect caudally. Presents clinically as an open neural tube that lies on the surface of the back. Types of Spina Bifida  Neural Tube Defects 2) Anencephaly: A type of upper NTD that occurs when the anterior neuropore fails to close during week 4 of development. Failure of the brain to develop (however, a rudimentary brain is present), and failure of the bony cranial vault to form. Incompatible with extrauterine life. If not stillborn, infants survive from only a few hours to a few weeks. Common serious birth defects are seen in stillborn fetuses. Anencephaly is easily diagnosed by ultrasound, and a therapeutic abortion is usually performed at the mother’s request. Researchers have identified factors in pregnant women that might increase the risk for anencephaly like: Low folate (vitamin B9) levels during early pregnancy, Uncontrolled Diabetes, Certain medications, such as antiseizure medications and High fever Derivatives of the Neural Crest 1. NEUROBLASTS: these cells give rise to : 1. Nerve cells of SENSORY GANGLIA of 5th, 7th, 8th & 10th cranial nerve. 2. Nerve cells of the DORSAL ROOT GANGLIA. 3. Nerve cells of the SYMPATHETIC GANGLIA. 4. Nerve cells of the PARASYMPATHETIC GANGLIA OF SOME CRANIAL NERVE (III, VII, IX , and X CRANIAL NERVES). 2. SPONGIOBLASTS: these cells give rise to: 1. Pia and arachnoid matter (leptomeninges). 2. Schwann cells. 3. CHROMAFFIN CELLS: these cells give rise to: 1. Suprarenal medulla. 2. Cells of carotid and aortic bodies. 4. MELANOCYTES: Pigment cells of the skin. 5. Some bones of the face originate from the branchial arches.  DIFFERENTIATION OF THE 3 GERM LAYERS 2) Derivatives of Endoderm THE ENDODERM GIVES RISE TO: 1) The epithelial lining of 1) Gastrointestinal tract. 2) Respiratory tracts. 3) Tympanic cavity, tympanic antrum and auditory tube. 2) Urinary bladder and most of the urethra. 3) The parenchyma of the tonsils, thyroid and parathyroid glands , thymus, liver and pancreas, `  DIFFERENTIATION OF THE 3 GERM LAYERS 3) Derivatives of Mesoderm ❖ Development: A longitudinal groove appears on each side of the notochord in the intraembryonic mesoderm to divide it into: 1) Paraxial mesoderm: on each side of the notochord medial to the longitudinal groove. 2) Intermediate cell mass: in the floor of the longitudinal groove. 3) Lateral plate mesoderm: lateral to the longitudinal groove.  DIFFERENTIATION OF THE 3 GERM LAYERS 3) Derivatives of Mesoderm ❖ Development: A longitudinal groove appears on each side of the notochord in the intraembryonic mesoderm to divide it into: 1) Paraxial mesoderm: on each side of the notochord medial to the longitudinal groove. 2) Intermediate cell mass: in the floor of the longitudinal groove. 3) Lateral plate mesoderm: lateral to the longitudinal groove. DIFFERENTIATION OF THE 3 GERM LAYERS 3) Derivatives of Mesoderm  DIFFERENTIATION OF THE 3 GERM LAYERS A) PARAXIAL MESODERM ❖ Definition: Is the part of the intraembryonic mesoderm, on each side of the notochord, medial to the longitudinal groove. ❖ Development: 1) At the end of the 3rd week, the paraxial mesoderm begins to divide into a number of segments or somites (undergo segmentation) by a series of transverse grooves. 2) These somites form distinct elevations on the surface of the embryo.  DIFFERENTIATION OF THE 3 GERM LAYERS A) PARAXIAL MESODERM ❖ Fate: The cells of each somite were arranged into: 1) Ventromedial part: form the SCLEROTOME which gives rise to the axial skeleton (the vertebral column and ribs). 2) Dorsolateral part: form the DERMO-MYOTOME: The superficial part of it gives the DERMATOME, which gives the dermis of the skin and the underlying fascia of the body wall. The deep part of it gives the MYOTOME, which gives the skeletal muscles.  DIFFERENTIATION OF THE 3 GERM LAYERS B) INTERMEDIATE MESODERM ❖ Definition: Is the part of the intraembryonic mesoderm which corresponds to the floor of the longitudinal groove. ❖ Fate: It will give rise to the UROGENITAL SYSTEM. The cortex of the suprarenal gland. Testis or ovary. Kidney. Male and female duct systems.  DIFFERENTIATION OF THE 3 GERM LAYERS C) LATERAL PLATE MESODERM ❖ Definition: Is part of the intraembryonic mesoderm lateral to the longitudinal groove. ❖ Development: Small, isolated cavities appear in the lateral plate mesoderm. These cavities coalesce to form a single horse-shoe cavity, the intraembryonic coelom, which divides the lateral plate mesoderm into two layers:  DIFFERENTIATION OF THE 3 GERM LAYERS C) LATERAL PLATE MESODERM 1) Somatic or parietal layer: towards the ectoderm. This layer gives rise to: o Muscles of the chest wall and muscles of the abdominal wall. o Parietal layer of pericardium, pleura & peritoneum.  DIFFERENTIATION OF THE 3 GERM LAYERS C) LATERAL PLATE MESODERM 2) Splanchnic or visceral layer: towards the endoderm. This layer gives rise to: o Muscles of the heart, Smooth muscles of the bronchial tree and Smooth muscles of the gut. o Visceral layer of pericardium, pleura and peritoneum. 3) THE INTRAEMBRYONIC COELOM is divided into the three body cavities: pericardial, pleural and peritoneal cavities.  FOLDING Definition: It is the process of folding of the flat trilaminar embryo upon itself ventrally to form a cylindrical embryo. The embryo begins to acquire a definite shape. Time: At the beginning of the 4th week  FOLDING Definition: It is the process of folding of the flat trilaminar embryo upon itself ventrally to form a cylindrical embryo. The embryo begins to acquire a definite shape.  Causes of Folding Rapid longitudinal growth of the central nervous system is the cause of the cephalo-caudal folding (the head and tail folds). The shape of the rapidly growing somites is the cause of the lateral folding (the 2 lateral folds).  Process of Folding Folding of the ends of the embryo produces: 1. Head fold at the cephalic end. 2. Tail fold at the caudal end. Folding of the sides of the embryo produces: 1. Right lateral fold. 2. Left lateral fold. Process of Folding  Process of Folding Foregut Midgut Hindgut Tail fold Head fold Vitillo intestinal duct Umblical cord Definitive yolk sac  Results of Folding Shape: the flat trilaminar embryo becomes cylindrical embryo. Amniotic cavity: the amniotic cavity is expanded and enlarges on the expense of the yolk sac to become surrounded the embryo. Yolk sac: the yolk sac becomes constricted into two parts 1. Part enclosed within the embryo: forms the primitive gut from which most of the GIT is derived. 2. Part outside the embryo: becomes small and is called definitive yolk sac or yolk sac proper.  Results of Head Fold The part of the yolk sac, which becomes included in the head fold, is called THE FOREGUT. The forebrain comes to lies at the most cephalic end of the embryo. The buccopharyngeal membrane comes to lie on the ventral aspect of the embryo cranial to the pericardium. The septum transversum comes to lie on the ventral aspect of the embryo caudal to the pericardium.  Results of Tail Fold The part of the yolk sac which, become included in the tail fold, is called THE HINDGUT. The cloacal membrane and the connecting stalk are carried to the ventral aspect of the embryo.  Results of the Lateral Folds The part of the yolk sac, which becomes included between the lateral folds, is called THE MIDGUT. The embryo obtains a ROUND appearance. The anterior abdominal wall is formed The primitive gut becomes a tube-like structure.