Midterm Embryo

7/31/2019 Midterm Embryo

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Implantation of the Blastocyst

Completed during the second weekEmbryoblast

That produce a bilaminar embryonic discEmbryo disc

Composed of epiblast & hypoblast

Gives rise to the germ layers that form all the tissues and organs of the embryoExtraembryonic structures

Amniotic cavity Amniom Yolk sac Connecting stalk Chronic sac

Synctiotrophoblast

Invades the endometrium connective tissue Supports the endometrial capillaries and glands Cells displace endometrial cells in the central part of the implantation site Produces a hormone human choronic gonadotrophin hCG

o Maintains the hormonal activity of the corpus luteum in theovary during pregnancy and forms the basis of pregnancy test

Endometrial cells

Under goes apoptosis (Programmed cell death)Proeolytic enzymes

Produced by the SynctiotrophoblastDecidual cells

Degenerate adjacent to the penetrating synctiotrophoblast Synctiotrophoblast engulfs these degenerating cells providing a rich source of embryonic

nutrition

Cytotrophoblast

A mononucleated layer of cells Mitotically active Increasing mass of synctiotrophoblast They fuse an lose their cell membranes

Synctiotrophoblast

Rapidly expanding Multinucleated mass

Amniotic cavity

A small space appears in the embryoblast Primordium of the amniotic cavity

Amnioblast

Separate from the epiblast and line the amnion which encloses the amniotic cavity


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Embryonic disc

Circular bilaminar plate of cells Two layers

o Epiblast Thicker layer Consisting of high columnar cells related to the amniotic cells Forms the floor of the amniotic cavity

Continuous peripherally with the amniono Hypoblast

Consisting of small cuboidal cells adjacent to the exocoelomic cavity Forms the roof of the exocoelomic cavity Contionous with the exocoelomic membrane

Primary yolk sac

Membrane together with the hypoblastExtraembryonic mesoderm

Cells from the yolk sac endoderm form a layer of connectiveLacunae

Isolated cavityCorpus Luteum

An endocrine glandular structures that secretes estrogen and progesterone to maintain thepregnancy

Embryotroph

The fluid in lacunar spacesExtraembryonic somatic mesoderm

Lining the trophoblast Covering the amnion

Extraembryonic splanchnic mesoderm

Surrounding the yolk sac


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Summary of implantation

Implantation of the blastocyst begins at the end of the first week and is completed by the end ofthe second week

The molecular event relating to human implantation are just beginning to emerge Cytokines, steroid hormones and various growth factors are involved in implantation

The zona pellucida degenerates (DAY 5)

Resulting from enlargement of the blastocyst and

degeration caused by enzymatic lysis

The lytic enzymes are released from the acrosomes of

the sperms that surround and partially penetrate the

zona pellucida

The Blastocyst adheres to the endometrial epithelium (DAY 6)

The trophoblast differentiates into two layers

synctiotrophoblast and cytotrophoblast (DAY 7)

The syncytiotrophoblast erodes endometrial tissues and theblastocyst starts to embed in the endometrium (DAY 8)

Blood Filled lacunae appear in the syncytiotrophoblast (DAY

9)

The blastocyst sinks beneath the endometrial epithelium and

the defect is filled by a closing plug (DAY 10)

Lacunar networks form by a fusion of adjacent lacunae (DAYS

10 and 11)

The syncytiotrophoblast erodes endometrial blood vessels ,

allowing maternal blood to seep in and out of lacunar

networks therby establishing a uteroplacental circulation (

DAYS 11&12)

The defect in the endometrial

epithelium gradually

disappears as the epithelium

is repaired (DAYS 12 & 13)

Primary chronic villidevelop (DAYS 13 & 14)


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Summary of Second week

Rapid proliferation and differentiation of the trophoblast are important features of the secondweek

These processes occur as the blastocyst completes its implantation in the endometrium The changes results from the adaptation of these tissues for implantation are known as the

decidual reaction

Concurrently the primary yolk sac forms and extraembryonic mesoderm develops.

The extraembryonic coelom later becoes the choronic cavity The primary yolk sac becomes smaller and gradually disappears as the secondary yolk sac

develops

CHANGES

The amniotic cavity appears as a space between the cytotrophoblast and theembryoblast

The embryoblast differentiation into a billaminar embryonic disc consisting of epiblastrelated to the amniotic cavity and hypoblast adjacent to the blastocyst cavity

The perichordial plate develops as a localized thickening of the hypoblast, whichindicates the future cranial region of the embryo and the future site of the mouth, the

prechordial plate is also an important organizer of the head region.

THIRD WEEK

Rapid development of the embryo from the embryonic disc during the third week is characterized by

Appearance of primitive streak Development of notochord Differentiation of three germ layers

The third week of embryonic development occurs during the week following the first missed

menstrual period

Gastrulation

Formative process by which the three germ layers and axial orientation are established in theembryos

Converts bilaminar to trilaminar embryonic disc Beginning of morphogenesis (development of body form) Significant event occurring in the third week Begins with formation of primitive streak

Bone morphogenetic proteins

Essential role in this processThree Germ Layers

Embryonic ectodermo Gives rise to the epidermis, central and peripheral nervous system, retina

Embryonic endodermo Source of epithelial linings of the respiratory passage of gastrointestinal tract

Embryonic mesodermo Gives rise to smooth muscular coats, connective tissues and organso Forms most of the cardiovascular system


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o Source of blood cells and bone marrow, the skeleton, striated muscles, and thereproductive and excretory organs

Important processes during gastrulation

Formation of primitive streak Formation of germ layers Formation of notochord

Primitive streak

First sign of gastrulation An opacity formed by a thickened band of epiblast As the streak elongates by addition of cells to its caudal end , its cranial end proliferates to form

Primitive node

Concurrently, a narrow groove Primitive groove develops in the primitive streak Primitive pit small depression in the primitive node

Mesenchyme

Tissue consisting of loosely arranged cells suspended in a gelatinous matrix Formed shortly after primitive streak appears Forms the supporting tissues of the embryo Connective tissue framework of glands

Mesenchymal Cells

Ameboid Actively phagocytic

Mesoblast Undifferentiated mesoderm Forms the intraembryonic or embryonic mesoderm

Cells from the epiblast displace the hypoblast forming the intraembryonic or embryonic endoderm

Remaining cells form the intraembryonic or embryonic ectoderm

Normally the primitive streak undergoes degenerative changes and disappears by the end of the

fourth week

Notochordal process and notochord

Notochordal process

Some mesenchymal cells migrate cranially from he primitive node and pit Forming a median cellular chord

Notochordal cord

Acquired lumen through the processPrechordal plate

The notochordal process gorws craniallt between the ectoderm and endoderm until it reachesit

The primordium of the oropharyngeal membrane


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Nueral tube formation

Neural plate appears as a thickening of the embryonic ectoderm cranial to the primitive node The nueral plate is induced to form by the developing notochord A longitudinal nueral groove develops in the nueral plate, which is flaked by neural folds Fusion of the folds forms the nueral tube, the primordium of the central nervous system Neuralation is the process of the neural plate formation and its infolding to form the neural

tube

Nueral Crest Formation

As the neural folds fuse to form the nueral tube, neuroectodermal cells migrate dorsolaterallyto form a nueral crest between the surface ectoderm and the neural tube

The neural crest soon divides into two masses that give rise to the sensory ganglia of thecranial and spinal nerves

Other nueral crest cells migrate from the neural tube and give rise to various other structuresSomite formation

The mesoderm on each side of the notchord thickens to form longitudinal columns of paraxialmesoderm

Division of these paraxial columns into pairs of somites begins cranially by the end of the thirdweek

The somites are compact aggregates of meenchymal cells from which cells migrate to give riseto the vertebrae, ribs and axial musculature

During the third week the number of somites present is an indicator of the age of the embryoFormation of intraembryonic coelom

the coelom (cavity) within the embryo arises as isolated spaces in the lateral mesoderm andcardiogenic mesoderm

the coelomic vesicles subsequently coalesce to form a single, horseshoe-shaped cavity thateventually gives a rise to the body cavities, the peritoneal cavity

Formation of blood vessels

blood vessels first appear in the wall of the yolk sac, allantoris and chorion. They develop within the embryo shortly thereafter Spaces appear within aggregations of mesenchyme blood islands The spaces soon become lined with endothelium derived from the mesenchymal cells These primordial tubules sprout and unite with other vessels to form a primordial

cardiovascular system

Toward the end of the third week, the heart is represented by paired endocardial heart tubesthat are joined to blood vessels in the embryo and in the extra embryonic membranes

By the end of the third week, the heart tubes hve fused to form a tubular heart that is joined tovessels in the embryo, yolk sac, chorion and connecting stalk to form a primordial blood cells

hemangioblasts are derived mainly from the endothelial cells of the blood vessels in the walls

of the yolk sac and allantoris

Fetal and adult erythrocytes probably develop from different hematopoietic percursors


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Completion of chrionic villi formation

Primary chronic villi become secondary chrionic villi as they acquire mesenchymal cores Before the end of the third week, capillaries devlop in the secondary chrionic villi,

transforming them into tertiary chrionic villi

Cytotrophoblasic extensions from these stems villi join to form a cytotrophoblastic shell thatanchors the chrionic sac to the endometrium

The rapid development of chrionic villi during the third week greatly increases the surface areaof the chorioin for the exchange of oxygen and nutrients and other substances betweenmaternal and embryonic circulations


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Embryoblast

Epiblast

Trilaminar Embryonic Disc

Endoderm

Epithelial parts of

Trachea Bronchi Lungs

Epithelium of G.I Tract Pancreas Liver Urachus Urinary Bladder

Epithelial parts of

Pharynx Thyroid Tympanic cavity Pharyngotympanic

Cavity

Tonsils Parathyroid Glands

Mesoderm

Head

Cranium Connective tissue of the

head

DentinParaxial Mesoderm

Muscles of the head Striated skeletal muscles Skeleton except cranium Dermis Connective tissue

Intermediate Mesoderm

Urogenital systemLateral Mesoderm

Connective tissue of themuscle of viscera

Serous membrane ofpleura

Primordial heart Blood & lymphatic cells Spleen Supra (adrenal) cortex

Ectoderm

Surface Ectoderm

Epidermis Hair Nails Cutaneous Mammary glands Ant. Part of pituitary

glands

Enamel of teeth Internal ear Lens of eye

Nuero Ectoderm

Nueral Crest

Cranial & sensory ganglia& nerve Medulla of suprarenal

gland

Pigment cells Pharyngeal arch

cartilages

Head mesenchyme &connective tissue

Bulbar & conal ridges inheart

Nueral Tube

CNS Retina Pineal body Post. Part of pituitary

gland


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Midterm Embryo