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

Cleavage to gastrulation

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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 - PowerPoint PPT Presentation

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Page 1: Cleavage to gastrulation

Cleavage to gastrulationCleavage to gastrulation

M.A.Kai-KaiM.A.Kai-Kai

Page 2: Cleavage to gastrulation

Learning Objectives.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 layersectoderm, endoderm, mesoderm.

Overview of the adult derivatives of the germ layers.

Page 3: Cleavage to gastrulation

MechanismsMechanisms of Developmentof Development1.Mitosis and growth differential mitosis and growthbody form--cell proliferation e.g.in cleavageincrease 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 determinationtotipotent blastomeresrestricted potency3.Gene activation and differential expression of functional genes.4.Differentiatione.g.cytodifferentiation into specific phenotype.5.Cell/tissue interactionmediate embryonic signals e.g.primary induction-notochord.

6.Cell movementshort or long migrations, e.g. neural crest cells.7. Pattern formationintrinsic blueprint directing development.8.Foldinge.g.cephalic, caudal and lateral body foldsbody form9. Morphogenesiscumulative mechanisms transform internal and external body form.

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EMBRYOLOGYCELL MOTILITY AND SHAPE CHANGE

CELL SIGNALLING

EXTRACELLULAR MATRIX

CELL-CELL AND CELL MATRIX INTERACTIONS

CONTROL OF GENE EXPRESSION

CONTROL OF CELL DIVISION

STEM CELLS

APOPTOSIS

LIMBS

NERVOUS SYSTEM

CARDIOVASCULAR SYSTEM RESPIRATORY

SYSTEM

URINARY SYSTEM

DIGESTIVE SYSTEM

REPRODUCTIVE 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.

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The ZygoteThe Zygote The zygotea single cell formed at fertilisation.Structure.

--diploid nucleus from both parents. --cytoplasm is maternal.

--surrounded by zona pellucidaCleavagemitotic division at 12hours, 2days and 3days 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.

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Unique Features of Mammalian CleavageUnique Features of Mammalian Cleavage 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 yolkmeroblastic/incomplete and holoblastic/complete Totipotent early blastomeres(Hans Driesch 1st cleavage is meriodal 2nd cleavageone 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.

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The MorulaThe MorulaMorula; 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/trophoectodermforms ectoderm of chorion/ placenta.Inner mass cells are pluripotent; form embryo and partly extraembryonic membranes.

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0

10

20

30

40

SEGMENTATION/ SOMITE FORMATION

BLASTULATION

GASTRULATION/ NEURAL TUBE FORMATION

FORMATION OF BODY FOLDS AND EXTRA- EMBRYONIC MEMBRANES

EARLY ORGANO-GENESIS

FETAL PHASE

DAYS

IMPLANTATION

5 mm

10 mm

30 mm

50 mm

114

A TIMELINE - PIG DEVELOPMENT

Size of embryo

The embryonic phase contains both series and parallel components

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

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BlastulationBlastulation 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.

segregation

Trophoblast

ICM

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Stem cellsStem cellsProperties 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 cellcommitted stem cell then progenitor/precursor cellsdifferentiate into a cell lineage.Example haemangioblastmultipotent haematopoietic stem cellmyeloid progenitor cellblood 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.

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Formation of hypoblastFormation of hypoblastand epiblastand epiblast..

Formation of hypoblast begins late in blastulation

1. Segregation(A)

2. 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)

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Blastula: Hatching and Implantation.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 involvesA trypsin-like protease lyses of ZP

--Trophoblast cells secrete proteases which degrades the endometrial wall and blastocyst embeds(5).

1

2

3 4 4

5

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Gastrulation(1)Gastrulation(1) Gastrulation transforms flat two-layered blastula(epiblast,hypoblast) into three-layered gastrulaectoderm, endoderm, mesoderm.

Mechanism of gastrulation consists of:

1.Formation of primitive streak(PS)

2.Involution of PS

3.Regression of PS.

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 node

Primitive streak

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Gastrulation(2)Gastrulation(2)

22. . 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 ectodermform mesoderm

3. PS regresses caudally

Enlarged tip Of PSHensen’s node/primitive knot, moves to posterior region(C).

4.Function of PS is to form three germ layers

A

B

A

B

C

AA

B

C Hensen’s node

Cranial

Caudal

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Formation of the NotochordFormation of the NotochordThe Notochord--a rod-shaped aggregate of chordamesoderm cells extending along entire length of embryo.--notochord cells formed of migrating cells from Hensen’s nodeFunctions 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..

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Embryo proper

Transverse through primitive streak

FORMATION OF THE MAMMALIAN GASTRULA - 3

Migration through the primitive streak forms a three-layered embryo

Primitive streak

Primitive gut

Ectoderm

Mesoderm

Endoderm

The 3 germ layers

Extra-embryonic

Splanchnopleure

Somatopleure

Transverse section through a mammalian embryo(14days in pig)

Splanchnopleure

Trophoectoderm

Mesoderm

Endoderm/hypoblast

Extraembryonic coelom

Junctional line

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Periods of Development of ConceptusPeriods 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

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SummarySummary 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 areblastogenesis, embryogenesis and the fetal stages.The entrance and duration of each stages in the species.

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ReferencesReferences1. Carlson, B. M., Foundations of Embryology (6th.Edition) 1996.

McGraw-Hill inc. London. Page 151 – 226

2. Gilbert, S.F., Developmental Biology (8th. Edition) 2006. Sinauer Associates Inc. Sunderland, Massachuetts. USA. Page 348 - 354

3. McGeady, T.A., Quinn, P.J., Fitzpatrick, E.S., & Rayan, M.T., (2006).

Veterinary Embryology. Page 25 – 38.

4. Noden, D.M., DeLaHunta, A., The Embryology of Domestic Animals.

1985, Williams & Wilkins. London. Page25 – 27, 33-40.