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HISTOLOGY & EMBRIOLOGY II > LECTURE 10 (Third week of development) > Flashcards

LECTURE 10 (Third week of development) Flashcards

1
Q

What happens in the third week of development?

A
  • Appearance of primitive streak
  • Development of notochord
  • Differentiation of three germ layers
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2
Q

What is Gastrulation?

A

When the three germ layers (precursors of all embryonic tissues) and the axial orientation are established in embryos

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3
Q

During the third week, what is the embryo referred to as?

A

Gastrula

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4
Q

What are the three germ layers that arise from the Epiblast?

A
  • Ectoderm (outer)
  • Mesoderm (middle)
  • Endoderm (inner)
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5
Q

What does Embryonic Ectoderm give rise to?

A
  • Epidermis
  • CNS and PNS
  • Eyes and internal ears
  • Neural crest cells
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6
Q

What does the Embryonic Mesoderm give rise to?

A
  • All skeletal muscles
  • Blood cells & lining of blood vessels
  • Visceral smooth muscle coats
  • Kidney
  • Reproductive system
  • Bone
  • Spleen
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7
Q

What does the Embryonic Endoderm give rise to?

A
  • GI tract
  • Liver
  • Endocrine system
  • Urethra
  • Bladder
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8
Q

What is the first morphologic sign of gastrulation?

A

The formation of the PRIMITIVE STREAK on the surface of the epiblast of the bilaminar embryonic disc

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9
Q

What happens during gastrulation?

A

Epiblast cells move towards the PRIMITIVE STREAK -> enter the primitive streak -> migrate away from primitive streak as individual cells

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10
Q

Describe the formation of the three germ layers

A

ENDODERM = Epiblast cells move inside primitive pit by INGRESSION -> Populate inside of embryo -> Replace cells in HYPOBLAST

MESODERM = Some epiblast cells diverge into space between EPIBLAST and DEFINITIVE ENDODERM -> forming INTRAEMBRYONIC MESODERM

ECTODERM = Last layer to form

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11
Q

Describe the formation of the Notochordal process

A

Some MESENCHYMAL CELLS migrate cranially from the PRIMITIVE NODE and PIT forming the NOTOCHORDAL PROCESS -> The process acquires a lumen, the NOTOCHORDAL CANAL -> Notochordal process grows cranially between the ectoderm and endoderm until it reaches the PRECHORDAL PLATE

(Prechordal plate = a small circular area of columnar endodermal cells where the ectoderm and endoderm are fused)

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12
Q

What is caudal to the primitive streak?

A

A circular area, THE CLOACAL MEMBRANE, which indicates the future site of the anus

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13
Q

Describe the instructive signals that induce notochord precursor cells to form the notochord

A
  • From the primitive streak region
  • Molecular mechanism involves Shh signalling from the floor plate of the NEURAL TUBE
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14
Q

What is the function of the Notochord?

A
  • Defines the primordial longitudinal axis of the embryo and gives it some rigidity
  • Provides signals for development of MUSCULOSKELETAL STRUCTURES and CNS
  • Contributes to the intervertebral discs between the bodies of adjacent vertebrae
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15
Q

What is the Notochordal process?

A

A cellular tube that extends cranially from the primitive node to the prechordal plate

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16
Q

Describe the formation of the Neural tube

A

NEURAL ECTODERM folds to form NEURAL FOLD -> fold forms NEURAL GROOVE (with NEURAL FOLDS on each side) which invaginate and closes forming NEURAL TUBE -> NEURAL CREST CELLS are detached from NEURAL ECTODERM and undergo an epithelial to mesenchymal transition

17
Q

The notochord functions as the _____________________ in the early embryo

A

Signalling center

18
Q

What does the developing notochord induce?

A

Overlying embryonic ectoderm to thicken and form the NEURAL PLATE (primordium of the CNS)

19
Q

What happens to the Notochordal process to form the Nucleus Pulposus

A

1) NOTOCHORDAL PROCESS becomes a cellular tube that extends cranially from the PRIMITIVE NODE to the PRECHORDAL PLATE
2) As the openings merge, the floor of the NOTOCHORDAL CANAL disappears and the remains of the NOTOCHORDAL PROCESS for the flattened, grooved NOTOCHORDAL PLATE
3) Beginning at the cranial end of the embryo, the NOTOCHORDAL PLATE cells proliferate and undergo infolding creating the NOTOCHORD -> Notochord detaches from endoderm
4) NOTOCHORD degenerates as the bodies of the vertebrae form -> small portions of it form the NUCLEUS PULPOSUS of each intervertebral disc

20
Q

What is the Allantois?

A
  • Appears on day 16
  • A small expansion of the mesoderm and forms blood vessels that will become the PLACENTA
  • Blood vessels of the allantoic stalk become UMBILICAL ARTERIES
21
Q

When is neuralation complete?

A

At the end of the fourth week

[when closure of the caudal neuropore occurs]

22
Q

What are Neural Tube Defects (NTDs)?

A

NTDs result when neural tube closure fails to occur

23
Q

What is Anencephaly?

A

When the neural tube fails to close in the cranial region so most of the brain fails to form

[It’s a lethal defect -> most cases are diagnosed prenatally and the pregnancies are terminated]

24
Q

What is Spina Bifida?

A

When the neural tube fails to close from the cervical region caudally

[more susceptible to genetic/environment factors + children lose a degree of neurological function]

25
Q

Describe the formation of the Neural crest

A

Neural crest cells detach from the ectoderm and form the NEURAL CREST between the NEURAL TUBE and the OVERLYING SURFACE ECTODERM -> Neural crest separates into right and left parts forming the SENSORY GANGLIA of the spinal and cranial nerves

26
Q

What do Neural crest cells give rise to?

A
  • Spinal ganglia
  • Ganglia of the autonomic nervous system
  • Ganglia of cranial nerves V, VII, IX and X
  • Neurolemma sheaths of peripheral nerves
  • Contribute to formation of the LEPTOMENINGES , THE ARACHNOID MATTER and PIA MATER
27
Q

Describe the development of Somites

A

Cells form the PRIMITIVE NODE form PARAXIAL MESODERM which differentiates, condenses and begins to divide into SOMITES (week 3) which form in a CRANIOCAUDAL SEQUENCE -> First appear in the future occipital region and develop CRANIOCAUDALLY -> Gives rise to most of the AXIAL SKELETON, ASSOCIATED MUSCULATURE and SKIN

28
Q

The Intraembryonic coelom divides the lateral mesoderm into which layers?

A
  • SOMATIC/PARIETAL LAYER OF LATERAL MESODERM = located beneath the ectodermal epithelium -> form embryonic body wall
  • SPLANCHNIC/VISCERAL LAYER OF LATERAL MESODERM = located adjacent to the endoderm -> form embryonic gut
29
Q

During the second month, the Intraembryonic coelom is divided into which body cavities?

A
  • Pericardial cavity
  • Pleural cavities
  • Peritoneal cavity
30
Q

Describe the development of the Cardiovascular system

A

1) At the end of the 2nd week, Embryonic nutrition is obtained from MATERNAL BLOOD by DIFFUSION through the EXTRAEMBRYONIC COELOM and UMBILICAL VESICLE
2) At the beginning of the 3rd week, blood vessel formation begins in the EXTRAEMBRYONIC MESODERM of the UMBILICAL VESICLE, CONNECTING STALK and CHORION
3) During the 3rd week, a PRIMORDIAL UTEROPLACENTAL circulation develops

31
Q

What is the difference between Vasculogenesis and Angiogenesis?

A

Vasculogenesis = formation of new vascular channels by assembly of individual cell precursors (ANGIOBLASTS)

Angiogenesis = formation of new vessels by budding and branching from preexisting vessels

31
Q
A
32
Q

Describe the process of blood vessel formation

A

During the third week, blood vessel formation begins when MESENCHYMAL CELLS differentiate into ENDOTHELIAL CELL PRECURSORS/ANGIOBLASTS -> ANGIOBLASTS aggregate to form isolated angiogenic cell clusters -> Blood cells develop from HEMANGIOGENIC EPITHELIUM of vessels as they grow on the umbilical vesicle

33
Q

Blood formation does not begin in the embryo until the ________ week

A

Fifth

34
Q

Describe the formation of the Cardiovascular system

A

1) Heart and great vessels form from MESENCHYMAL CELLS in the CARDIOGENIC AREA
2) Paired, longitudinal endothelial-lined channels/Endocardial heart tubes develop during the 3rd week and fuse to form a PRIMORDIAL HEART TUBE
3) Tubular heart joins with blood vessels in the embryo forming a PRIMORDIAL CARDIOVASCULAR SYSTEM (first organ to reach a functional state)
4) Embryonic heartbeat can be detected during the FOURTH WEEK

35
Q

Describe the development of Chorionic Villi

A

1) PRIMARY CHORIONIC VILLI appear at the end of the second week and begin to branch to form SECONDARY CHORIONIC VILLI (cover entire surface of chorionic sac)
2) Some mesenchymal cells in the villii differentiate into CAPILLARIES and BLOOD CELLS
3) TERTIARY CHORIONIC VILLI have blood vessels visible in them. Capillaries in Chorionic villi fuse to form ARTERIOCAPILLARY NETWORKS which connect with the EMBRYONIC HEART
4) By the end of the third week, embryonic blood flows through the capillaries in the chorionic villi

36
Q

_____________ cells of the chorionic villi proliferate and extend through the synctiotrophoblast to form an extravillous cytotrophoblastic shell which surrounds the chorionic sac and attaches to the endometrium

A

Cytotrophoblastic cells

37
Q

Where does the main exchange of material between the blood of the mother and embryo take place?

A

Branch Villi

HISTOLOGY & EMBRIOLOGY II (13 decks)

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