Embryology From Fertilization to Gastrulation. 2 Fertilization.

EmbryologyFrom Fertilization to Gastrulation

2

Fertilization

3

Fertilization

Steps of fertilization (Conception). Several sperm penetrate corona radiata. Several sperm attempt to penetrate

zona pellucida. One sperm enters egg and nuclei fuse,

producing a zygote. Egg’s plasma membrane and zona

pellucida change to prevent polyspermy.

Fertilization

Compaction starts

First Days of Development Day 0-5

First Days of Development Day 6

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Occurrence of Pregnancy

When a zygote begins dividing, it is termed an embryo (pre-embryo). Developing embryo travels down

oviduct and eventually implants in endometrium. (Implantation / pregnancy) Presence of human gonadotropic

hormone (HCG) in the blood confirms pregnancy. If implantation does not occur, a

woman never knows fertilization took place.

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Human Development before Implantation

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Pre-Embryonic and Embryonic Development Processes of development.

Cleavage - Cell division without growth. Growth - Increase in size of cells. Morphogenesis - Shaping of embryo. Differentiation - Cells take on specific

structure and function.

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Extraembryonic Membranes

Membranes that extend out beyond the embryo. Amnion - Provides fluid environment for

developing embryo and fetus. Yolk sac - First site of red blood cell

formation. Allantois - Contributes to cardiovascular

system. Chorion – outermost membrane,

develops from the trophoblast, contributes to the placenta.

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Extraembryonic Membranes

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Pre-Embryonic and Embryonic Development Stages of development.

Morula - Solid mass of cells resulting from cleavage.

Blastocyst (Blastula) - Ball of cells formed from morula. Embryonic disk - Inner mass of cells of

blastocyst. Gastrula - Embryo composed of three tissues.

Ectoderm, mesoderm, endoderm. These are the foundation of the body

systems (Germ Layer Theory).

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Early Developmental Stages

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Pre-Embryonic and Embryonic Development Stages of development.

Neurula - Nervous system develops from ectoderm located just above the notochord. Involves induction as one tissue influences

the development of another tissue.

Review Question.

Which of the following is the origin of the mitochondrial DNA of all human Adult Cells?

A) Paternal onlyB) Maternal onlyC) A combination of paternal and maternalD) Either paternal or maternalE) Unknown origin

Review Question.

Which of the following is the origin of the mitochondrial DNA of all human Adult Cells?

A) Paternal onlyB) Maternal onlyC) A combination of paternal and maternalD) Either paternal or maternalE) Unknown origin

First Days of Development Day 7

Week TwoDays 8

Day 9

Day 10

Day 11

Day 12

Days 12-13

Chorion membraneconsists of?

Days 13

Day 14-15

Primary ectoderm:primordial germ cells, endothelial cells, andhematopoietic stem cells.

Extraembryonic mesoderm of the yolk sac wall is a major site of hematopoiesis.

Chorion membraneconsists of?

Origin of the germ lineA. Migration of the primordial germ cells.

B. Migration into posterior body wall.

Clinical ApplicationHuman chorionic gonadotropin (hCG) hCG is a 57,000 MW glycoprotein with two subunits

(alpha and beta) produced by the syncytiotrophoblast.

Enters the maternal blood circulation.

Prevents degeneration of the corpus luteum.

Stimulates production of progesterone in the corpus luteumand chorion, which sustains the placenta.

Can be assayed in maternal blood at day 8 after fertilizationAnd in maternal urine at day 10. This is the basis of early diagnosis of pregnancy.

Clinical ApplicationComplete hydatidiform mole

Persistent trophoblastic diseaseMetastatic choriocarcinoma

IV. Clinical Applications.F. Genomic Imprinting.1. Cytogenetic analysis of hydatidiform moles suggests

Paternal genetic complement->placental development.Maternal genetic complement->embryo development.

2. Methylation of DNA is a mechanism that leads to independentexpression of maternal and paternal genomes duringearly development. Female germ line highly methylated.

3. Example of pattern of inheritance:Father->Prader-Willi SyndromeMother->Angelman Syndrome

4. Severity and age of onset of several genetic diseases also differon inheritance pattern.

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 2 – The Two-Layered Embryo Bilaminar embryonic disc – inner

cell mass divided into two sheets Epiblast and the hypoblast

Together they make up the bilaminar embryonic disc


Week 2 – The Two-Layered Embryo Amniotic sac – formed by an

extension of epiblast Outer membrane forms the amnion Inner membrane forms the amniotic

sac cavity Filled with amniotic fluid


Week 2 – The Two-Layered Embryo Yolk sac – formed by an extension of

hypoblast Digestive tube forms from yolk sac NOT a major source of nutrients for

embryo Tissues around yolk sac

Gives rise to earliest blood cells and blood vessels


Figure 3.4 (1 of 3)


Figure 3.4 (2 of 3)


Figure 3.4 (3 of 3)


Week 3 – The Three-Layered Embryo Primitive streak – raised groove on

the dorsal surface of the epiblast Gastrulation – a process of

invagination of epiblast cells Begins at the primitive streak Forms the three primary germ layers


Week 3 – The Three-Layered Embryo Three Germ Layers*

Endoderm – formed from migrating cells that replace the hypoblast

Mesoderm – formed between epiblast and endoderm

Ectoderm – formed from epiblast cells that stay on dorsal surface

*All layers derive from epiblast cells!


The Primitive Streak

Figure 3.5e–h


The Notochord

Primitive node – a swelling at one end of primitive streak Notochord forms from primitive node

and endoderm Notochord – defines body axis

Is the site of the future vertebral column Appears on day 16


Formation of the Mesoderm and Notochord

Figure 3.6


Neurulation

Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal

midline thickens Neural groove – ectoderm folds inward


Neurulation

Neurulation (continued) Neural tube – a hollow tube pinches off

into the body Cranial part of the neural tube becomes

the brain Maternal folic acid deficiency causes

neural tube defects


Neurulation

Neural crest Cells originate from ectodermal cells Forms sensory nerve cells

Induction Ability of one group of cells to influence

developmental direction of other cells


The Mesoderm Begins to Differentiate Somites – our first body segments

Paraxial mesoderm Intermediate mesoderm – begins

as a continuous strip of tissue just lateral to the paraxial mesoderm


The Mesoderm Begins to Differentiate Lateral plate – most lateral part of

the mesoderm Coelom – becomes serous body cavities

Somatic mesoderm – apposed to the ectoderm

Splanchnic mesoderm – apposed to the endoderm

Copyright 2009, John Wiley & Sons, Inc.

Third week of development Begins 6 week period of rapid development

and differentiation Gastrulation

1st major event of 3rd week – about 15 days Bilaminar embryonic disc transforms into

trilaminar embryonic disc Ectoderm (skin and nervous system), mesoderm

(muscle, bones, connective tissues, peritoneum), and endoderm (epithelial lining of GI tract, respiratory tract, and several other organs)

Involves rearrangement and migration of epiblast cells Primitive streak establishes head (primitive node) and

tail ends


Gastrulation


Third week of development (cont.)

Gastrulation (cont.) 16 days after fertilization notochord forms –

induces tissue to become vertebral bodies 2 depressions form

Oropharyngeal membrane will later break down to connect mouth to pharynx and GI tract

Cloacal membrane will later degenerate to form openings of anus, urinary and reproductive tracts

When cloacal membrane appears, wall of yolk sac forms allantois Extends into connecting stalk In most other mammals used for gas exchange and

waste removal – human placenta does this instead Does function in early formation of blood and blood

vessels and urinary bladder


Development of the notochordal process


Third week of development (cont.) Neurulation

Notochord also induces formation of neural plate Edges of plate elevate to form neural fold Neural folds fuse to form neural tube Develop into brain and spinal cord Neural crest cells give rise to spinal and cranial nerves

and ganglia, autonomic nervous system ganglia, CNS meninges, adrenal medullae and several skeletal and muscular components of head

Head end of neural tube develops into 3 primary brain vesicles Prosencephalon (forebrain), mesencephalon

(midbrain), and rhombencephalon (hindbrain)


Third week of development (cont.) Development of somites

Mesoderm adjacent to notochord and neural tube forms paired longitudinal columns of paraxial mesoderm

Segment into paired, cube-shaped somites Number of somites can be correlated to age

of embryo Each somite has 3 regions

Myotome – develops into skeletal muscles of neck, trunk and limbs

Dermatome – develops into connective tissue Sclerotome - develops into vertebrae and ribs

Development of intraembryonic coelom Splits lateral plate mesoderm into

Splanchnic mesoderm – forms heart, blood vessels, smooth muscle and connective tissues of respiratory and digestive systems

Somatic mesoderm – gives rise to bones, ligaments, dermis of skin


Neurulation and the development of somites


Third week of development (cont.) Development of cardiovascular system

Angiogenesis – formation of blood vessels Spaces develop in blood islands to form lumens of blood

vessels Pluripotent stem cells form blood cells By end of 3rd week, heart forms and begins to beat

Development of chorionic villi and placenta Chorionic villi – fingerlike projections of chorion

projecting into endometrium Blood vessels in chorionic villi connect to embryonic

heart through body stalk (becomes umbilical cord) Maternal and fetal blood do not mix – diffusion only


Development of chorionic villi


Placentation Process of forming placenta

By beginning of 12th week has 2 parts Fetal portion formed by chorionic villi of chorion Maternal portion formed by decidua basalis of

endometrium Functionally allows oxygen and nutrients to diffuse

from maternal to fetal blood while carbon dioxide and wastes diffuse from fetal to maternal blood

Not a protective barrier – allows microorganisms, drugs, alcohol to pass

Connection between embryo and placenta through umbilical cord 2 umbilical arteries carry deoxygenated fetal blood to

placenta 1 umbilical vein carries oxygenated blood away from

placenta Afterbirth – placenta detaches from uterus


Placenta and umbilical cord


Fourth week of development

4th -8th week - all major organs develop Organogenesis – formation of body organs and

systems Embryo triples in size this week Converted from flat disc to 3D cylinder through

embryonic folding Main force is different rates of growth for different

parts Head fold brings heart and mouth into eventual

adult position Tail fold brings anus into eventual adult

position Lateral folds for primitive gut – forerunner of GI

tract


Embryonic folding

I. somite/Nt development.

I

Axial skel. Vol muscl. Dermis

Urinary systemGenital system

SplanchnopleuricSomatopleuricmesoderm


Week 3 – The Three-Layered Embryo Primitive streak – raised groove on

the dorsal surface of the epiblast Gastrulation – a process of

invagination of epiblast cells Begins at the primitive streak Forms the three primary germ layers


Week 3 – The Three-Layered Embryo Three Germ Layers*

Endoderm – formed from migrating cells that replace the hypoblast

Mesoderm – formed between epiblast and endoderm

Ectoderm – formed from epiblast cells that stay on dorsal surface

*All layers derive from epiblast cells!


The Primitive Streak

Figure 3.5e–h


The Notochord

Primitive node – a swelling at one end of primitive streak Notochord forms from primitive node

and endoderm Notochord – defines body axis

Is the site of the future vertebral column Appears on day 16


Figure 3.6


Neurulation

Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal

midline thickens Neural groove – ectoderm folds inward


Neurulation

Neurulation (continued) Neural tube – a hollow tube pinches off

into the body Cranial part of the neural tube becomes

the brain Maternal folic acid deficiency causes

neural tube defects


Neurulation

Neural crest Cells originate from ectodermal cells Forms sensory nerve cells

Induction Ability of one group of cells to influence

developmental direction of other cells


The Mesoderm Begins to Differentiate Somites – our first body segments

Paraxial mesoderm Intermediate mesoderm – begins

as a continuous strip of tissue just lateral to the paraxial mesoderm


The Mesoderm Begins to Differentiate Lateral plate – most lateral part of

the mesoderm Coelom – becomes serous body cavities

Somatic mesoderm – apposed to the ectoderm

Splanchnic mesoderm – apposed to the endoderm

76

Germ Layer Theory of Organ –System Development

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Embryology From Fertilization to Gastrulation. 2 Fertilization.

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