Cleavage to gastrulation M A KaiKai Learning Objectives

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 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.

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 inductionnotochord. 6. Cell movement short or long migrations, e. g. neural crest cells. 7. Pattern formation 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.

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. CELL-CELL AND CELL MATRIX INTERACTIONS CONTROL OF GENE EXPRESSION EXTRACELLULAR MATRIX CONTROL OF CELL DIVISION STEM CELLS CELL SIGNALLING CELL MOTILITY AND SHAPE CHANGE EMBRYOLOGY APOPTOSIS REPRODUCTIVE SYSTEM LIMBS NERVOUS SYSTEM CARDIOVASCULAR SYSTEM DIGESTIVE SYSTEM RESPIRATORY SYSTEM URINARY SYSTEM

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 12 hours, 2 days and 3 days in mouse, 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. The Zygote

Unique Features of Mammalian Cleavage is series of mitotic division of the zygote into progressively smaller cellular units; blastomeres First cleavage synchronous. Differences in pattern of cleavage dependent upon amount of yolk meroblastic/incomplete and holoblastic/complete Totipotent early blastomeres(Hans Driesch 1 st cleavage is meriodal 2 nd cleavage one meriodional and one equatorial/termed rotational cleavage Second cleavage not synchronised in 2 blastomeres 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.

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 fo rms ectoderm of chorion/ placenta. Inner mass cells are pluripotent; form embryo and partly extraembryonic membranes. The Morula

A TIMELINE - PIG DEVELOPMENT The embryonic phase contains both series and parallel components 0 BLASTULATION 10 D A Y S 20 30 GASTRULATION/ NEURAL TUBE FORMATION SEGMENTATION/ SOMITE FORMATION OF BODY FOLDS AND EXTRAEMBRYONIC MEMBRANES Size of embryo 5 mm 10 mm EARLY ORGANOGENESIS 30 mm 50 mm 40 FETAL PHASE 114 IMPLANTATION 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

Blastulation Formation of the blastocyst --A series of rapid cell divisions produce a blastula of 64 cells with an inner cell mass(ICM) 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 in the uterus. Blastogenesis starts at 32 -cell stage. ICM segregation Trophoblast

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 form pluripotent 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.

Formation of hypoblast and epiblast. 1. 2. 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 twolayered embryo(C) Cells on surface ICM form epiblast(C)

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

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(PS) 2. Involution of PS 3. Regression of PS. Primitive node 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) Primitive streak

Gastrulation(2) 2. Involution of 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 regresses caudally Enlarged tip Of PS Hensen’s node/primitive knot, moves to posterior region(C). 4. Function of PS is to form three germ Cranial A AA Caudal B B B C C Hensen’s node

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. Formation of the Notochord

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

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

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.

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