embryology course - session 3

8/6/2019 Embryology Course - Session 3

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Ectoderm Derivatives - Neurulation

y The notochord and prechordal mesoderm induce the ectoderm to thicken and form the

neural plate (neuroectoderm) in the 3rd week of development

y By the end of the 3rd week, the plate invaginates in the midline to form the neural groove

with thickened neural folds; the folds begin fusion at the level of the 5th somite (in the

cervical region) and proceeding both caudally and cranially

y Cranial neuropore closes at day 25 (18 to 20 somites) and caudal neuropore closes at day

27 (25 somites); CNS thus represented by a close tube with narrow caudal end (spinal

cord) and broad cephalic end (brain vesicles)


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Neurulation and Neural Crest Cells

y As the neural tube is formed, it detaches from the surface ectoderm and lies deep to it;

during this detachment, some of the neuroectodermal cells at the inner border of the folds

undergo epithelial-to-mesenchymal transition and detach from the neural tube to form aflattened irregular neural crest mass, from which neural crest cells migrate; the cells can

also migrate even before the folds fuse

y The remaining ectoderm (the surface ectoderm) forms the epidermis of the skin


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Neural Crest Cells

y Neural crest cells from the trunk region leave the neural folds after closure of the tube in

this region; they migrate through one of two pathways:

y Dorsal pathway: they migrate through the dermis and enter the epidermis through holes in thebasal lamina of its cells to form the melanocytes of the skin and hair follicles

y Ventral pathway: they migrate through the anterior half of each somite (derivative of paraxial

mesoderm) and form spinal sensory ganglia, sympathetic and enteric (GIT)ganglia, Schwann

cells, and cells of the adrenal medulla (but not the cortex)


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Neural Crest Cells

y Neural crest cells from the cephalic region migrate before closure of the neural folds and

contribute to skull bones, cranial ganglia, glial cells, and melanocytes


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Mesoderm Derivatives

y The thin sheet of(intraembryonic)mesoderm differentiates into three parts around the

middle of the third week:

y Paraxial mesoderm which is thickened,

y Intermediate mesoderm which is relatively thin, and

y Lateral plate mesoderm; small spaces appear in the lateral place and join to form a cavity, the

intraembryonic coelom, thus dividing the lateral plate into somatic (parietal) and splanchnic

(visceral) layers


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The Mesoderm Layers and the Two Coeloms


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Intraembryonic Coelom (BodyCavity)

y The primordium of the body cavity begins as isolated coelomic spaces in the

lateral plate mesoderm and cardiogenic area which coalesce to form a single

horseshoe-shaped cavity that communicates laterally with the extraembryonic

cavity


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y The intraembryonic coelom can be understood by imagining a horseshoe of cavity placed

within the mesoderm of the embryo


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Folding of the Embryo Head Fold

y The flat trilaminar embryonic disc folds into a 3D embryo by four folds: head (cephalic)

fold, tail (caudal)fold and two lateral folds all stimulated by development of the CNS

y Growth of the forebrain beyond buccopharyngeal membrane results in the the head foldwhich pushes the heart, pericardial coelom, and septum transversum down and

incorporates a portion of the yolk sac forming the foregut in addition to placing the

buccopharyngeal membrane at the site of the future mouth


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Folding of the Embryo Tail Fold

y Primordium of spinal cord stimulates the tail fold by which it pushes the connecting stalk

to the ventral aspect of the embryo, incorporates a portion of the yolk sac forming the

hindgut, in addition to incorporating part of the allantois to the body of the embryo, andshifting the position of the cloacal membrane to the site of the future anus


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Folding of the Embryo Cephalocaudal Folding


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Folding of the Embryo Lateral Folding

y The spinal cord and somites stimulate lateral folding which causes incorporation of

another portion of the yolk sac forming the midgut, and ventrolateral body walls of the

embryo are formed; connection with yolk sac is reduced to yolk stalk (or omphaloentericduct) which is the site of the future umbilicus


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y As a result of the lateral folding of the embryo, the communication with the

extraembryonic cavity is narrowed to a very small area around the umbilical cord; later

when the amniotic cavity obliterates most of the extraembryonic cavity, thiscommunication is completely lost


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Paraxial Mesoderm

y Paraxial mesoderm is organized, cephalocaudally, into segments known as:

y Somitomeres: more loosely organized in the head region forming in association with

segmentation of the neural tube into neuromeresy Somites: more compact and defined regions forming from the occipital region caudally; first

somite forms on the 20th day, and last pair at the end of the 5 th week


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Fate of the Somites

y The dermomyotome is the dorsolateral portion of the somite; it can be divided into three

parts: the ventrolateral lip (VLL) and dorsomedial lip (DML) ofmuscle-forming cells, and

the remaining dorsal dermatomey The ventrolateral lip cells migrate forward and form limb and body wall musculature; the

dorsomedial lip cells migrate down the ventral aspect of the dermatome and form the

muscles of the back; the dermatome forms the dermis and subcutaneous tissue;

throughout migration, these cells retain their original segmental nerve component


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Intermediate and Lateral PlateMesoderm

y The intermediate mesoderm later forms the urogenital system

y The lateral plate mesoderm divides into the somatic and splanchnic layers; the somatic

layer contributes to the ventrolateral body wall and forms the parietal layer of the bodywhile the splanchnic layer contributes to the wall of the gut and forms the visceral layer of

these membranes


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Formation ofBlood and Blood Vessels

y Blood vessels are formed in two ways:

y Vasculogenesis: the process where blood vessels form from blood islands

y Angiogenesis: the process where new vessels are formed from existing ones

y Blood islands are composed of specialized mesenchymal cells called hemangioblasts; these

cells are derived from mesoderm; such mesoderm cells are induced to become

hemangioblasts under the effect ofVEGF released by neighboring mesoderm cells

y Cavities appear within the blood island; central cells become hematopoietic stem cells

(the ancestor of all types of blood cells), while peripheral cells become angioblasts for the

formation of the vascular endothelium


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Formation ofBlood and Blood Vessels

y The first blood vessels form in the extraembryonic mesoderm lining the yolk sac, in the

connecting stalk, and in the chorionic plate at the beginning of the third week; embryonic

vessels begin to form about two days later; the heart begins beating at the beginning ofthe forth week; the cardiovascular system is the first functional system to develop

y Embryonic vessels first form mainly in the aorta-gonad-mesonephros area; then in the

liver; then the definitive hematopoietic area, the bone marrow


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The Primitive Circulation

y The heart begins beating at the beginning of the forth week; the cardiovascular system is

the first functional system to develop


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Left-OutMaterials

y When something in the book is not mentioned in the

session, it could be due to one of two reasons:

y It is not a significant concept or structural piece of

information

y It is going to be repeated in greater detail in another

chapter

y Something not discussed in the session will not be

included for the quizzes but may be required at university


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embryology course - session 3

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