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Cleavage to gastrulation

Cleavage to gastrulation. M.A.Kai-Kai. Learning Objectives. Define major mechanisms of development. Describe the mammalian zygote . Understand the process of mitotic division/ cleavage of the zygote Understand process of blastulation

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Cleavage to gastrulation

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  1. Cleavage to gastrulation M.A.Kai-Kai

  2. Learning Objectives. • Define major mechanisms of development. • Describe the mammalian zygote. • Understand the process of mitotic division/cleavage of the zygote • Understand process of blastulation • Understand the process of gastrulation and the functions i.e. formation of the embryonic three germ layersectoderm, endoderm, mesoderm. • Overview of the adult derivatives of the germ layers.

  3. Mechanisms of Development 1.Mitosis and growth differential mitosis and growthbody form --cell proliferation e.g.in cleavageincrease in number, cells smaller in size. --growth by deposition of extracellular matrix,intracellular organelles and matrix. --spread of epithelial sheets e.g. extraembryonic membranes. 2.Restriction and determinationtotipotent blastomeresrestricted potency 3.Gene activation and differential expression of functional genes. 4.Differentiatione.g.cytodifferentiation into specific phenotype. 5.Cell/tissue interactionmediate embryonic signals e.g.primary induction-notochord. 6.Cellmovementshort or long migrations, e.g. neural crest cells. 7. Patternformationintrinsic blueprint directing development. 8.Foldinge.g.cephalic, caudal and lateral body foldsbody form 9. Morphogenesiscumulative mechanisms transform internal and external body form.

  4. CELL-CELL AND CELL CONTROL OF GENE MATRIX INTERACTIONS EXPRESSION EXTRACELLULAR CONTROL OF MATRIX CELL DIVISION STEM CELLS CELL SIGNALLING CELL MOTILITY APOPTOSIS AND SHAPE CHANGE EMBRYOLOGY REPRODUCTIVE LIMBS SYSTEM NERVOUS DIGESTIVE SYSTEM SYSTEM URINARY CARDIOVASCULAR RESPIRATORY SYSTEM SYSTEM SYSTEM Cell movement, changing cell interactions, cell division, changing patterns of gene expression are all quite restricted in the adult but are rampant in the embryo.

  5. The Zygote • The zygotea single cell formed at fertilisation. • Structure. --diploid nucleus from both parents. --cytoplasm is maternal. --surrounded by zona pellucida • Cleavagemitotic division at 12hours, 2days and 3days in pig and dog. Rate about one division/day. • Zygote period lasts from fertilisation to hatching of the blastocyst. • Nutrition/embryotroph --mammalian zygotes is provided by uterine secretions/histiotroph and the zygotes own reserve. --avian zygotes feed on the yolk.

  6. Cleavage • Cleavage is series of mitotic division of the zygote. • Rapid cell divisionsubdivide large zygote into progressively smaller cellular units--blastomeres --cells morphological and metabolical unspecialised • First cleavage synchronous. • Differences in pattern of cleavage dependent upon amount of yolk. • Totipotent early blastomeres(Hans Driesch(1893).

  7. Unique Features of Mammalian Cleavage • 1st cleavagemeriodal(2-cells) • 2nd cleavageone meriodional and one equatorialtermed rotational cleavage • Second cleavage not synchronised in blastomeres • Slow rate, first cleavage last to 24hour, subsequent divisions 10-12h. • Zygote genome switched on early at 2-cell stage • Compaction occurs at 8-cell stage. --blastomeres flatten and form intercellular connections --E-CAD(cadherin:glycoprotein) on cell surfaceadhesion. --microvilli(actin) extend on surfaces of adjacent cells and anchor cells together. --tight junctions prevent free exchange of fluid between the inside and outside allowing accumulation of fluid inside blastomeres. --the gap junctions couple all the blastomeres of the compacted embryo and permit exchange of ions and small molecules from one cell to the next.

  8. The Morula • Morula; 16-cell stage, embryo enclosed in the zona pellucida. • Late morula, first differentiation event in mammalian development. --cells aggregate into internal inner mass cells(ICM) and external trophoblast. --at 64-cell stage ICM and trophoblast form distinct populations. • Trophoblast/trophoectodermforms ectoderm of chorion/ placenta. • Inner mass cells: --internal cellspluripotent; form embryo and partly extraembryonic membranes. • Blastogenesis starts at 32-cell stage.

  9. GASTRULATION/ NEURAL TUBE FORMATION FORMATION OF BODY FOLDS AND EXTRA- EMBRYONIC SEGMENTATION/ MEMBRANES SOMITE FORMATION EARLY ORGANO- GENESIS A TIMELINE - PIG DEVELOPMENT The embryonic phase contains both series and parallel components 0 BLASTULATION In the mid to large size domestic mammals the embryonic phase lasts 30-40 days and the fetal phase varies with the size and maturity of the neonate IMPLANTATION 10 D A Y S Size of embryo 5 mm 20 10 mm 30 30 mm 50 mm 40 FETAL PHASE 114

  10. Blastulation ICM • Formation of the blastocyst --A series of rapid cell divisions produce a blastula of 64 cells with an inner cell mass and an outer layer of trophoblasts. --trophoblast secrete fluid into the morula. Creates a cavity; the blastocoele(A) --ICM/embryo proper lies to one side • Transition from morula to blastula marked by: --rapid enlargement of blastocoele --differentiation of blastomeres into ICM and trophoblast cells(B). • Trophoblast cells induce special changes in the uterine lining at implantation. • Trophoblast cells preferentially express maternal genes, and inactivate paternal genes. • The blastula hatches from the zona pellucida and implants on the uterine epithelium segregation Trophoblast

  11. Formation of hypoblastand epiblast. • Formation of hypoblast begins late in blastulation • Segregation(A) • Delamination of ICM cells.Cells expand beneath trophoblast form hypoblast(B)/extraembryonic endoderm • Hypoblast tube(blastocoele) inside tube of trophoblast(C) • Formation of hypoblast results in two-layered embryo(C) • Cells on surface ICM form epiblast(C)

  12. Blastula: Hatching and Implantation. • Blastulationembryo arrives in uterus(4) • Blastula hatches from zona pellucida and contact uterus. • Blastocyst surrounded by ZP prevents premature implantation and ectopic pregnancy. • Hatching involvesA trypsin-like protease lyses of ZP --Trophoblast cells secrete proteases which degrades the endometrial wall and blastocyst embeds(5). 3 4 4 1 5 2

  13. Stem cells Properties of stem cells. • Pluripotent embryonic stem adult stem cells. • Embryonic stem cells(ESC) from inner mass cells,totipotent can give rise to all embryonic cells except trophoblast.ectoderm, endoderm and mesoderm. • ESC formpluripotent stem cellcommitted stem cell then progenitor/precursor cellsdifferentiate into a cell lineage. Example haemangioblastmultipotent haematopoietic stem cellmyeloid progenitor cellblood cells. • Adult stem cells more lineage-restricted in their ability to differentiate • Restriction on the potency of stem cells is determined by the microenvironment • Adult stem cells important for the continual production of tissues e.g intestinal epithelium. • Inner mass cells also used for creating chimeric animals. • Expression of transcription factor e.g Oct-4 is required for the maintenance of pluripotency. • Murine embryonic stem cells useful models to elucidate the unique properties of mammalian stem cells. • Valuable therapeutic potential in treatment of degenerative diseases.

  14. Gastrulation(1) • Gastrulation transforms flat two-layered blastula(epiblast,hypoblast) into three-layered gastrulaectoderm, endoderm, mesoderm. Mechanism of gastrulation consists of: 1.Formation of primitive streak, marked by: --expansion of epiblast cells and caudal convergence(A). --Primitive streak(PS) forms as longitudinal ridge in midline(B) --PS elongates, cranial tip widened as primitive node(C)

  15. Cranial Gastrulation(2) A A A 2. Involutionof primitive streak(PS) --epiblast cells leave PS --primitive groove in midline --first group of cells form intraembryonic endoderm(A) -- --second group of epiblast cells intercalate between endoderm and ectodermform mesoderm 3. PS regressescaudally 4. Enlarged tip Of PSHensen’s node/primitive knot, moves to posterior region(C). 4.Function of PS is to form three germ layers Caudal B B B C C Hensen’s node

  16. Formation of the Notochord • The Notochord --a rod-shaped aggregate of chordamesoderm cells extending along entire length of embryo. --notochord cells formed of migrating cells from Hensen’s node • Functions of notochord. 1.Defines cranial-caudal axis of embryo. 2. Serves as primary inducer at neurulation.Induces formation of the neural tube and somitogenesis. 3. Transient, remnants in intervertebral disc as nucleus pulposus.

  17. The 3 germ layers Somatopleure Splanchnopleure Extra- embryonic FORMATION OF THE MAMMALIAN GASTRULA - 3 Migration through the primitive streak forms a three-layered embryo Transverse through primitive streak Primitive streak Ectoderm Mesoderm Embryo proper Endoderm Junctional line Splanchnopleure Trophoectoderm Primitive gut Mesoderm Endoderm/hypoblast Extraembryonic coelom Transverse section through a mammalian embryo(14days in pig)

  18. Periods of Development of Conceptus • Embryo implants by end of blastogenesis • Embryogenesis is period of differentiation and organogenesis • Major organs and systems formed, morphogenesis is later. • Period ends with formation of the primordial nervous system, circulatory system digestive and excretory systems and limb buds. • The embryo is a miniature adult of the species. • Period is most vulnerable to teratogenic malformations • High mortality rate due to nutritional deficiency, excessive ambient temperature and infections. • Embryonic deaths occur, about 15% in the queen cat, bitch and horses

  19. Summary • The zygote goes through different stages of development by specific morphogenetic processes. • Cleavage, a series of mitotic divisions produces a morula • Blastulation forms the blastula which hatches from the zona pellucida and implants in the wall of the uterus. • Gastrulation transforms the flat blastula into a three-layered gastrula • The process of gastrulation involves formation of the primitive streak by convergence of epiblast cells towards the midline and involution of the streak to form the endoderm and mesoderm. The surface epiblast cells form the ectoderm. • The notochord forms from mesoderm in Hensen’s node and acts as primary inducer of neurulation. • Organogenesis starts after neurulation.All adult organs are formed from the three germ layers. • Periods of development areblastogenesis, embryogenesis and the fetal stages.The entrance and duration of each stages in the species.

  20. References • Carlson, B. M., Foundations of Embryology (6th.Edition) 1996. McGraw-Hill inc. London. Page 151 – 226 • Gilbert, S.F., Developmental Biology (8th. Edition) 2006. Sinauer Associates Inc. Sunderland, Massachuetts. USA. Page 348 - 354 • McGeady, T.A., Quinn, P.J., Fitzpatrick, E.S., & Rayan, M.T., (2006). Veterinary Embryology. Page 25 – 38. • Noden, D.M., DeLaHunta, A., The Embryology of Domestic Animals. 1985, Williams & Wilkins. London. Page25 – 27, 33-40.

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