General Embryology 3 Lecture 5 PDF

General Embryology 3 Lecture 5 Prepared by Fatma M. Abdelmaksoud Dept. of Anatomy and Embryology Fac. of Vet. Medicine Assiut University 2024-2025  Twins - The large farm anim...

General Embryology 3 Lecture 5 Prepared by Fatma M. Abdelmaksoud Dept. of Anatomy and Embryology Fac. of Vet. Medicine Assiut University 2024-2025  Twins - The large farm animals always give only one offspring. Sheep, goats, dogs, cats and pigs usually give multiple births. - Types of twins: 1. Monozygotic (Identical) twins: They develop from a single zygote. These twins are exactly identical [except for the finger prints in human]. They share a common placenta and called true twins. 2. Dizygotic twins: They develop from 2 fertilized ova. Each one of the twins has its own placenta and may be of different sex and morphological features. 3. Monsters or united twins are incompletely separated monozygotic twins. (They may be attached at the head [craniopagus], at the thorax [thoracopagus], or at the sacrum [pygopagus] Dizygotic twins Monozygotic (Identical) twins Thoracopagus Pygopagus  4. Gastrulation - It means the transformation of blastula to gastrula by formation of three germinal layers ( ectoderm, mesoderm & endoderm ) - Gastrulation occurs in the inner cell mass of the ova of mammals or the blastoderm (area pellucida) of the ova of birds. - Gastrulation takes place through 2 steps : 1. Delamination: It means the separation of the lower layer of germinal disc to form the hypoblast (endoderm). The rest of the germinal disc is known now the epiblast, form the mesoderm & ectoderm. 2. Involution: It means migration of the superficial cells of the epiblast through the primitive streak to occupy a new position between the already separated endoderm and the outer layer of the epiblast. By the end of involution these migrating cells become the mesoderm and the outer layer of the epiblast becomes the ectoderm. The mesoderm is subdivided into : 1- Paraxial mesoderm (somites): lies lateral to the neural tube. 2- Intermediate mesoderm. 3- Lateral mesoderm: The lateral mesoderm is divided by the celomic cavity into : a. Outer layer called the parietal or somatic mesoderm. b. Inner layer called the visceral or splanchnic mesoderm.  5. Organogenesis - It means formation of the organs. - By the end of the 2nd month all organs of the embryo would be formed. Some organs are developed from the ectoderm, others from the mesoderm and the rest from the endoderm. Derivatives of the Ectoderm : - The central and peripheral nervous systems (brain & spinal cord) - The nerve cells of the sense organs such as eye & ear - The epidermis of the skin, hair, nails, the sweat and sebaceous glands and also the mammary gland. - Mucous membrane lining the oral cavity, gums, Ddd enamel of teeth, salivary glands, pituitary gland, the d epithelium of the nasal cavity and paranasal sinuses. - The external auditory meatus and the outer layer of Dddddd d the tympanic membrane.- Derivatives of the Endoderm : - Most of the digestive tract & respiratory tract, the urinary bladder and the urethra. - Also gives the epithelial lining of the tympanic cavity and auditory (Eustachian) tube. - The epithelial cells of the tonsils, thyroid and parathyroid, thymus liver, pancreas and prostate gland. Derivatives of the mesoderm : - The paraxial mesoderm : forms the vertebrae & associated muscles. - The intermediate mesoderm: forms most of the urogenital system. - The lateral mesoderm forms the bones, muscles & blood vessels of the body wall and internal organs.  Choose the correct answer: 1. Gastrulation takes place in two steps, conjugation and penetration – involution – placentation – delamination – implantation 2. The first step of gastrulation is …………………………… penetration-conjugation - multiplication – invagination- involution 3. The process of formation of ectoderm, mesoderm and endoderm is called ……… gametogenesis- ovulation – gastrulation- cleavage 4. The second step of gastrulation is …………………………… penetration-conjugation - multiplication – invagination- involution Which one of the following statements is incorrect? 1. Concerning the gastrulation: a) It starts by delimination b) Involution take place through the primitive groove. c) The cells that form the notochord migrate through the primitive pit. d) It ends by formation of bilaminar embryo formed of epiplast and hypoblast Complete: 1.Gastrulation means the transformation of ………………. to ……………….. by formation of ……………………… 2. The kidney is derived from …………….. 3. Brain is derived from……………………. 4. Vertebrae are derived from……………………. 5. Epithelial lining the digestive system is derived from…………………….