Urinary System Embryology PDF

THE EMBRYOLOGY OUrogenital SystemF









DEPARTMENT OF ANATOMY,
UNIVERSITY OF MEDICAL SCIENCES, ONDO.  Presentation outline Introduction of the Urogenital System URINARY SYSTEM Kidney Sy...

THE EMBRYOLOGY OUrogenital SystemF









DEPARTMENT OF ANATOMY,
UNIVERSITY OF MEDICAL SCIENCES, ONDO.  Presentation outline Introduction of the Urogenital System URINARY SYSTEM Kidney Systems Bladder and Urethra GENITAL SYSTEM Testis Ovary Genital Ducts Indifferent Stage External Genitalia Indifferent Stage Descent of the Testes Descent of the Ovaries Clinical Correlates References  Learning objectives By the end of this section, you will be able to: Describe the development of the Collecting and excretory systems of the Kidneys and the derivatives. Discuss the formation of the Bladder and Urethra. Discuss the development of the testis and ovary Explain the Genital ducts indifferent stage and external Genitalia indifferent stage. Elucidate the Descent of the testis and ovary Discuss the clinical correlates associated with the Development of the Urogenital system.  Introduction of the Urogenital System  Functionally, the urogenital system can be divided into two entirely different components:  the urinary system and  the genital system.  Both develop from a common mesodermal ridge (intermediate mesoderm) along the posterior wall of the abdominal cavity,  initially, the excretory ducts of both systems enter a common cavity, the cloaca. Introduction of the Urogenital System  URINARY SYSTEM Kidney  kidneys are formed in a cranial-to-caudal sequence during intrauterine life in humans:  The kidneys develop in 3 stages which are  the pronephros  , mesonephros, and . Metanephros Pronephros At the beginning of the fourth week, the pronephros is represented by 7 to 10 solid cell groups in the cervical region. These groups form vestigial excretory units, nephrotomes. By the end of the fourth week, all indications of the pronephric system have disappeared. Mesonephros The mesonephros and mesonephric ducts are derived from intermediate mesoderm from upper thoracic to upper lumbar (L3) segments. @ week 4 The length of the tubules lengthen rapidly, to form an S-shaped loop, and acquire a tuft of capillaries that will form a glomerulus at their medial extremity.  Mesonephros Around the glomerulus, the tubules form Bowman’s capsule, and together these structures constitute a renal corpuscle. Laterally, the tubule enters the longitudinal collecting duct known as the mesonephric or Wolffian duct. Mesonephros


Metanephros: The Definitive Kidney  Appears in the fifth week.  Its excretory units develop from metanephric mesoderm in the same manner as in the mesonephric system 
Collecting System of the Kidneys
 Collecting ducts of the permanent kidney develop from the ureteric bud, an outgrowth of the mesonephric duct close to its entrance to the cloaca.  The bud penetrates the metanephric tissue, which is molded over its distal end as a cap.  Subsequently, the bud dilates, forming the primitive renal pelvis, and splits into cranial and caudal portions which will form the future major calyces.  Each calyx forms two new buds while penetrating the metanephric tissue.  The ureteric bud gives rise to the ureter, the renal pelvis, the major and minor calyces, and collecting tubules.  Function of the Kidney The definitive kidney formed from the metanephros becomes functional near the 12th week. Urine is passed into the amniotic cavity and mixes with the amniotic fluid. The fluid is swallowed by the fetus and recycles through the kidneys. During fetal life, the kidneys are not responsible for excretion of waste products because the placenta serves this function. Development of the Bladder and Urethra  Bladder and Urethra During the fourth to seventh weeks of development, the cloaca divides into the urogenital sinus anteriorly and the anal canal posteriorly. The urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus. The tip of the septum will form the perineal body, a site of insertion of several perineal muscles.  Initially, the bladder is continuous with the allantois, but when the lumen of the allantois is obliterated, a thick fibrous cord, the urachus, remains and connects the apex of the bladder with the umbilicus. The next part is a rather narrow canal, the pelvic part of the urogenital sinus, which in the male gives rise to the prostatic and membranous parts of the urethra. Bladder and Urethra cont’d During differentiation of the cloaca, the caudal portions of the mesonephric ducts are absorbed into the wall of the urinary bladder. Consequently, the ureters, initially outgrowths from the mesonephric ducts, enter the bladder separately. As a result of ascent of the kidneys, the orifices of the ureters move farther cranially; those of the mesonephric ducts move close together to enter the prostatic urethra and in the male become the ejaculatory ducts. The epithelium of the urethra in both sexes originates in the endoderm; the surrounding connective and smooth muscle tissue is derived from visceral mesoderm.  References 1. T.W. Sadler. (2015). Langman's Medical Embryplogy. 13th Editlon. Wolters Kluwer Health. ISBN 978-1-4511 -9164-6 2. Vishram Singh. (2012). Textbook of Clinical Embryology. Elsevier. ISBN: 978-81-312-3048-0 3. Larry Cochard. Netter’s Atlas of Human Embryology  ASSIGNMENT Discuss the congenital anormalies of the development of each of the organs below Kidney Bladder Urethra Describe in detail the ascesion of the kidney CENTRAL NERVOUS SYSTEM



















DEPARTMENT OF ANATOMY, UNIVERSITY OF MEDICAL SCIENCES, ONDO.  Presentation outline The Central Nervous System Neural Tube formation Formation of Neural Crest Cells Formation of Ectodermal Placodes Development of Spinal Cord Histological Differentiation of the spinal cord Brain Formation Derivatives of Brain Vesicles Development of Ventricular System Clinical Correlates Review questions References  Learning objectives By the end of this section, you will be able to: Write on the constituents of the central nervous system Describe the process of formation of neural tube, neural crest cells and ectodermal placodes Discuss the development of the brain and spinal cord Give accounts of different developmental anomalies associated with the central nervous system  The Central Nervous System  The whole of the nervous system is derived from ectoderm except its blood vessels and some neuroglial elements. The specific cell population of early ectoderm, which gives rise to entire nervous system and special sense organs, is termed neural ectoderm. The neural ectoderm later differentiates into three structures: neural tube, neural crest cells, and ectodermal placodes.  The neural tube gives rise to the central nervous system (CNS), the neural crest cells form nearly entire peripheral nervous system, and ectodermal placodes contribute to cranial sensory ganglia, hypophysis, and inner ear The Central Nervous System  Neural Tube formation The central nervous system (CNS) appears at the beginning of the third week as a slipper-shaped plate of thickened ectoderm, the neural plate in the mid-dorsal region in front of the primitive node. Its lateral edges soon elevate to form the neural folds.  Neural Tube formation With further development, the neural folds continue to elevate, approach each other in the midline, and finally fuse, forming the neural tube. Neural Tube formation A and B. Transverse sections of embryos Neural Tube formation A. Dorsal view of a late presomite embryo at approximately 18 days.. B. 20 days.  Neural Tube formation cont’d Fusion begins in the cervical region and proceeds in cephalic and caudal directions. Once fusion is initiated, the open ends of the neural tube form the cranial and caudal neuropores that communicate with the overlying amniotic cavity. Final closure of the cranial neuropore occurs at the 18- to 20-somite stage (25th day); closure of the caudal neuropore occurs approximately 3 days later. Diagrammati c sagittal section of the b  Formation of Neural The surface ectoderm of one side becomes continuous with Crest Cells the surface ectoderm of opposite side over the neural As the neural folds come tube. together and fuse, cells at the Thus cells at the tips of neural tips of neural folds break away folds (neural crest cells) do not from the neurectoderm to form participate in the neural tube the neural crest cells. formation. The neural crest cells at first remain in the midline between the dorsal surface of the neural tube and the surface ectoderm, and then forms two-cell clusters dorsolaterally, one on either side of the neural tube. The neural crest cells differentiate to form cells of dorsal root ganglia, sensory ganglia of cranial nerves, autonomic ganglia, adrenal medulla, chromaffin tissue,  Formation of Ectodermal Placodes Prior to the neural tube closure, the neural fold contains two types of cell populations: neural crest cells and neuroepithelial cells. During neurulation, the neural crest cells are detached and neuroepithelial cells get incorporated into the surface ectoderm. These areas of neuroepithelium within the surface ectoderm are termed ectodermal placodes.  Development of Spinal The lateral walls Cord The spinal cord develops from the caudal part of the neural of the neural plate and the caudal eminence. tube thicken, gradually The neural tube caudal to the fourth pair of reducing the size somites develops into the spinal cord. of the neural canal until only a minute central canal of the spinal cord is present at 9 to 10 weeks (C). Illustrations of the development of the spinal cord. A, Transverse section of the neural tube. D, Section of the wall of the neural tube shown in A. E, Section of the wall of the developing spinal cord  Development of Spinal Cord cont’d Neuroepithelial, Mantle, and Marginal Layers The wall of a recently closed neural tube consists of neuroepithelial cells. These cells extend over the entire thickness of the wall and form a thick pseudostratified epithehum. During the neural groove stage and immediately after closure of the tube, they divide rapidly, producing more and more neuroepithelial cells. Collectively, they constitute the neuroepithelial layer or neuroepithelium. Left, Cross section through the early neural tube. Right, Higher magnification of a segment of the wall of the neural tube.  Development of Spinal Cord cont’d Once the neural tube closes, neuroepithelial cells give rise to another cell type, the primitive nerve cells, or neuroblasts. They form the mantle layer (gray matter), a zone around the neuroepithelial layer. The outermost layer of the spinal cord, the marginal layer, contains nerve fibers emerging from neuroblasts in the mantle layer. As a result of myelination of nerve fibers, this layer takes on a white appearance and therefore is called the white matter of the spinal cord Section of the neural tube A,B- Two successive stages in the development of the spinal cord  Development of Spinal Cord cont’d Basal, Alar, Roof, and Floor Plates As a result of continuous addition of neuroblasts to the mantle layer, each side of the neural tube shows a ventral and a dorsal thickening. The ventral thickenings, the basal plates, which contain ventral motor horn cells, form the motor areas of the spinal cord; The dorsal thickenings, the alar plates, form the sensory areas. A longitudinal groove, the sulcus limitans, marks the boundary between the two. The dorsal and ventral midline portions of the neural tube, known as the roof and floor plates, respectively, do not contain neuroblasts; they serve primarily as pathways for nerve fibers Crossing from one side to the other. A,B- Two successive stages in the development of the spinal cord  Nerve Cells : Neuroblasts, or Histological Differentiation of primitive nerve cells, arise the spinal cord exclusively by division of the neuroepithelial cells. With further differentiation, becomes the adult nerve cell or neuron. Glial Cells: The majority of primitive supporting cells, the glia blasts, are formed by neuroepithelial cells after production of neuroblasts ceases. they differentiate into protoplasmic astrocytes and fíbrillar astrocytes. These cells are situated between blood vessels and neurons where they provide support and serve metabolic functions. Neural Crest Cells: Crest cells migrate laterally and give rise to sensory ganglia (dorsal root ganglia) of the spinal nerves and other cell types Spinal Nerves:  The cephalic end of the neural tube Brain Formation shows three dilations, the primary brain vesicles: (1) the prosencephalon, or forebrain; (2) the mesencephalon, or midbrain; and (3) the rhombencephalon, or hindbrain. Simultaneously, it forms two flexures; (1) the cervical flexure at the junction of the hindbrain and the spinal cord and (2) the cephalic flexure in the midbrain region.  Brain Formation cont’d A deep furrow, the rhombencephalic isthmus By five weeks of development, the separates the primary brain vesicles have mesencephalon from the differentiated into five secondary metencephalon and the pontine flexure marks the vesicles: boundary between the  The prosencephalon forms the metencephalon and telencephalon and diencephalon, myelencephalon  the mesencephalon remains, and The main derivatives of  the rhombencephalon forms the these vesicles are; metencephalon and telencephalon (cerebral hemispheres), myelencephalon. diencephalon (optic vesicle, thalamus, hypothalamus, pituitary), mesencephalon (anterior [visual] and posterior [auditory] colliculi), metencephalon (cerebellum, pons), and myelencephalon (medulla oblongata).  The lumen of the spinal cord, the central canal, is Brain continuous with that of the brain vesicles. The Formatio cavity of the rhombencephalon is the fourth n cont’d ventricle, that of the diencephalon is the third ventricle, and those of the cerebral hemispheres are the The lumen of the lateral ventricles. mesencephalon connects the third and fourth ventricles. This lumen becomes very narrow and is then known as the aqueduct of Sylvius. Each lateral ventricle communicates with the third ventricle through the interventricular foramina of Monro Derivatives of Brain Vesicles  Development of Ventricular System The cavities of brain vesicles form the ventricular system of adult brain. The hindbrain cavity becomes the fourth ventricle. The narrowed mesencephalic cavity becomes the cerebral aqueduct (aqueduct of Sylvius). The diencephalic cavity becomes the third ventricle. The twin telencephalic cavities become lateral ventricles. The two lateral ventricles communicate with the third ventricle via interventricular foramina (of Monro). The third ventricle communicates with the fourth ventricle through the cerebral aqueduct. The fourth ventricle is continuous below with central canal of the spinal cord. Cerebrospinal fluid (CSF) is formed in the ventricles, mainly in lateral ventricles by choroid plexuses.  Hydrocephalus: I Clinical it is a clinical condition characterized by dilatation of Correlates ventricles due to excess accumulation of CSF within them. It occurs either due to overproduction of CSF or obstruction to its circulation. Dandy–Walker syndrome: It occurs due to atresia and blockage of apertures in the roof of fourth ventricle (e.g., foramen of Magendie and foramina of Luschka). This syndrome consists of dilatation of fourth ventricle, agenesis of cerebellar vermis, occipital meningocele, and often agenesis of splenium of corpus Hydrocephalus callosum.  Clinical Correlates cont’d Exencephaly (A) is characterized by failure of the cephalic part of the neural tube to close. As a result, the vault of the skull does not form, leaving the malformed brain exposed. Later, this tissue degenerates, leaving a mass of necrotic tissue. This defect is called anencephaly (B), although the brain stem remains intact  Review questions Discuss the formation of the neural tube Briefly describe the development of the spinal cord Write on the derivatives of the brain vesicles  References 1. T.W. Sadler. (2015). Langman's Medical Embryplogy. 13th Editlon. Wolters Kluwer Health. ISBN 978-1-4511 -9164-6 2. Vishram Singh. (2012). Textbook of Clinical Embryology. Elsevier. ISBN: 978-81-312-3048-0