Anatomy Lecture 4

Front 1

What occurs during the first week following fertilization?

Back 1

Zygote progresses to the blastocyst stage and begins implantation into the endometrium

Front 2

What occurs during the second and third week following fertilization?

Back 2

Inner cell mass becomes disc-shaped
Fetal membranes begin to form
Placenta continues formation

Front 3

Define trophoblast

Back 3

Outer cell mass

Front 4

What happens to the trophoblast during the 2nd week?

Back 4

Blastocyst settles on its embryonic pole and trophoblast differentiates into two layers:
1) Cytotrophoblast
2) Syncytiotrophoblast

Front 5

What forms the fetal side of the placenta during the 2nd week?

Back 5

Cytotrophoblast, syncytiotrophoblast, and extraembryonic sematopleuric mesoderm

Front 6

Define embryoblast

Back 6

Inner cell layer

Front 7

Define hypoblast (primary endoderm)

Back 7

First recognizable germ layer to form and gives rise to extraembryonic endoderm that lines primary yolk sac

Front 8

Define epiblast (primary ectoderm)

Back 8

Develops concurrently with hypoblast and is immediately adjacent to trophoblast
Eventually gives rise to three definitive germ layers of embryo (ectoderm, mesoderm, endoderm) as well as extraembryonic mesoderm

Front 9

Define the extraembryonic mesoderm

Back 9

Derived from primary yolk sac cells and cells from epiblast which ultimately surround germ disc, amniotic vesicle, and primary yolk sac
Forms loose meshwork between embryo and trophoblast
Secondary cavity forms- chorionic- which splits mesoderm except in one area which suspends the embryonic disc into the cavity
Uncleaved mesoderm is connecting stalk which will become umbilical cord

Front 10

What forms the secondary, or definitive, yolk sac?

Back 10

New endodermal cells and surrounding extraembryonic splanchnopleuric mesoderm

Front 11

What imporant event occurs during the third week following fertilization?

Back 11

Gastrulation

Front 12

What occurs as a result of gastrulation?

Back 12

Formation of three definitive germ layers (endoderm, mesoderm, ectoderm), three structures (primitive streak, notochord, neural plate), and a third fetal membrane (allantois)

Front 13

What signals the beginning of gastrulation?

Back 13

Apprearance of primitive streak

Front 14

Where does the primitive streak appear?

Back 14

At the caudal end of the disc

Front 15

What does the primitive streak impart on the developing embryo?

Back 15

Bilateral symmetry and craniocaudal aspects

Front 16

Describe the development of the intraembryonic mesoderm

Back 16

Epiblast cells move toward primitive streak then invaginate and assume a new position below the epiblast and above the hypoblast
Cells spread in all directions and become continuous with extraembryonic mesoderm at the periphery of the disc
More epiblast cells displace the hypoblast and form the definitive endoderm
Remaining cells become definitive ectoderm

Front 17

Describe the formation of the notochord

Back 17

1) Prenotochordal cells move from primitive pit of the primitive node and extend to prechordal plate. Briefly intercalate with hypoblast
2) Hypoblast is replaced with endoderm and prenotochordal cells lift from hypoblast and form definitive notochord

Front 18

What is the neurenteric canal?

Back 18

Small canal that brings the amniotic and yolk sacs into continuity for a brief time

Front 19

What are some roles of the notochord?

Back 19

Plays major inductive role in formation of neural plate
Serves as axis for the formation of the axial skeleton and associate musculature
Eventually undergo regressive changes and persist in adult as nucleus pulposus

Front 20

What becomes of the primitive streak?

Back 20

Usually disappears after the end of 4th week
Occasionally remnants will persist and may give rise to multiple tissue tumors called teratomas

Front 21

What is the intraembryonic mesoderm derived from?

Back 21

Primitive node and primitive streak

Front 22

What is the notochord derived from?

Back 22

Cranial portion of primitive node

Front 23

What is the paraxial mesoderm derived from?

Back 23

Node and cranial portion of primitive streak

Front 24

What is the intermediate mesoderm derived from?

Back 24

Mid primitive streak

Front 25

What is the lateral plate mesoderm derived from?

Back 25

Caudal primitive streak

Front 26

What is the extraembryonic mesoderm derived from?

Back 26

Most caudal primitive streak

Front 27

What is the role of the anterior visceral endoderm (AVE)?

Back 27

Express genes essential for head formation

Front 28

What is the role of the nodal?

Back 28

Initiates and maintains primitive streak

Front 29

What is the role of the bone morphogenetic protein-4 (BMP-4) and fibroblastic growth factor (FGF)?

Back 29

Work in concert to ventralize mesoderm that contributes to kidneys, blood, body wall mesoderm

Front 30

What is the role of noggin, chordin, and follistatin?

Back 30

Expressed in primitive node and antagonize action of BMP-4 that results in dorsalization of notochord and paraxial mesoderm

Front 31

What is the role of the bracyury (T) gene?

Back 31

Regulates mesoderm formation in the middle and caudal regions of the germ disc
Abscence of this gene's expression results in sirenomelia (caudal dysgenesis)

Front 32

What is the role of the hepatic nuclear factor-3B (HNF-3B)?

Back 32

Maintains primitive node and is necessary for forebrain and midbrain differentiation

Front 33

What is the role of FGF-8?

Back 33

Induces expression of nodal and lefty-2 genes on left side of germ disc
Results in upregulation of PITX2 which establishes left side of embryo

Front 34

What is the role of lefty-1?

Back 34

Expressed from left side of floor plate of neural tube and acts as a barrier preventing left-sided factors going to right side
Sonic Hedgehog (Shh) has similar fxn and supresses left-sided genes on the right and expressed from notochord

Front 35

What happens during growth of the amniotic sac?

Back 35

Eventually expands and obliterates the chorionic cavity and completely surrounds the developing embryo and umbilical cord

Front 36

How is amniotic fluid acquired?

Back 36

During early pregnancy, simple diffusion from the endometrium
Later pregnancy, fetal kidney produces urine to increase volume

Front 37

List 7 fxns of the amniotic fluid

Back 37

1) Symmetrical growth of embryo
2) Prevents adherence of amnion
3) Maintains temperature
4) Prevents mechanical trauma
5) Allows for movement
6) Fluid wedge at partuition
7) Allows for estimation of fetal maturity

Front 38

List 4 characteristics of the embryonic yolk sac

Back 38

1) May play limited role in metabolism of embryonic nutrients
2) First site of hematopoiesis (embryonic wave) and may produce serum proteins
3) Gives rise to germ cell layer that later populates gonads
4) Normally disappears before birth, but its connection to the developing gut may persist as Meckel's diverticulum

Front 39

List 3 characteristics of the allantois

Back 39

1) Develops from the caudal wall of yolk sac around day 16
2) Rudimentary in humans but may induce the formation of vitelline and umbilical vessels
3) Persists in the adult as the median umbilical ligament

Front 40

List 6 derivatives of the ectoderm

Back 40

1) Central nervous system
2) Peripheral nervous system (autonomic ganglia and DRG)
3) Sensory epithelium (ear, eye, nose)
4) Epidermis of skin (hair, nails, sweat glands, mammary glands)
5) Pituitary gland
6) Enamel of teeth

Front 41

Name 8 derivitives of the mesoderm

Back 41

1) Supporting tissues (CT, bone, cartilage)
2) Muscle (striated, smooth)
3) Blood and lymph
4) Heart, blood, and lymph vessel walls
5) Kidney
6) Gonads
7) Suprarenal gland cortex
8) Spleen

Front 42

List 5 derivitives of the endoderm

Back 42

1) Lining of gut and parenchyma of associated organs (liver, gall bladder, pancreas)
2) Lining of respiratory system
3) Lining of urinary bladder
4) Parenchyma of thyroid and parathryoids
5) Lining of tympanic cavity and auditory tube

Front 43

Where does the first meiotic division of the ovum occur?

Back 43

Ovary

Front 44

30 hours post fertilization, the zygote is called what?

Back 44

Blastomere

Front 45

4 days post fertilization, the zygote is called what?

Back 45

Morula

Front 46

What happens around 5 days post fertilization?

Back 46

Zygote is called blastocyst
Trophoblast is outer cell layer, which will later become the placenta

Front 47

What is morula contained within?

Back 47

An amorphous membrane, the zona pellucida

Front 48

What is the embryoblast?

Back 48

Inner cell mass of the morula

Front 49

What is a blastocoele?

Back 49

Cavity within the morula
At this point becomes blastocyst

Front 50

When does implantation of the blastocyst occur?

Back 50

5 to 6 days post fertilization

Front 51

What are L-selectins?

Back 51

Carbohydrate binding proteins on the trophoblastic cells that bind to carbohydrate receptors on the endometrial lining cells which initiates "capture" of the blastocyst to the endometrium

Front 52

What is the relationship between integrin and laminin in the trophoblast?

Back 52

Integrin molecules expressed by the trophoblast attach to the endometrium via receptor sites on laminin molecules in the extracellular matrix of the endometrium

Front 53

What is the role of fibronectin?

Back 53

Responsible for migration of the blastocyst into the endometrium

Front 54

What two components of the placenta does the trophoblast give rise to?

Back 54

Syncytiotrophoblast- amorphous multinucleated layer that erodes into the endometrium and maternal blood vessels
Cytotrophoblast- mononucleated cell layer that gives rise to the syncytiotrophoblast

Front 55

What two other components (besides syncytiotrophoblast and cytotrophoblast) form the placenta?

Back 55

Lacunar spaces- form within syncytiotrophoblast and will fill with maternal blood
Extraembryonic mesoderm- added to inner surface of cytotrophoblast to complete third layer

Front 56

What nurishes the developing placenta and embryo?

Back 56

Passive diffusion from maternal blood and uterine gland secretions

Front 57

Where does the syncytiotrophoblast begin to erode?

Back 57

Maternal sinusoidal capillaries

Front 58

At what point does maternal blood begin to circulate through lacunar spaces?

Back 58

10-11 days

Front 59

What are the two layers of the bilaminar germ disc and what do they give rise to?

Back 59

Hypoblast- primitive endoderm that gives rise to exocoelemic membrane and forms the primary yolk sac
Epiblast- adjacent to trophoblast and gives rise to all three definitive germ layers of the embryo

Front 60

Describe amnioblast

Back 60

Derived from epiblast and form the amniotic membrane over the disc

Front 61

Describe extraembryonic mesoderm

Back 61

Derived from primary yolk sac and epiblast and fills the blastocyst cavity

Front 62

Describe chorionic cavity

Back 62

Forms within the extraembryonic mesoderm and divides it into splanchnopleuric and somatopleuric extraembryonic mesoderm

Front 63

What occurs during the second week post fertilization?

Back 63

Two extraembryonic membranes form (amnioblasts and Hueser's membrane)

Front 64

Describe the primitive streak

Back 64

1) Appears at the caudal end of the germ disc within the epiblast and signals the beginning of gastrulation
2) Consists of the primitive groove, primitive node, and primitive pit
3) Cells that move through the primitive streak will replace the hypoblast to form the definitive endoderm and also establish the intraembryonic mesoderm
4) Cells that remain in epiblast become definitive ectoderm
5) Cells that move through primitive node form the prechordal plate, notochord, and paraxial mesoderm

Front 65

What is gastrulation?

Back 65

Event in the third week of development that results in formation of the definitive germ layers (ectoderm, mesoderm, and endoerm), three structures (primitive streak, primitive node, and neural plate), and a third fetal membrane (allantois)

Front 66

When does gastrulation begin?

Back 66

Appearance of primitive streak

Front 67

What occurs during invagination?

Back 67

Epiblast cells move to the primitive streak and beneath the epiblast

Front 68

What two areas of the germ disc are not infiltrated by mesoderm?

Back 68

Buccopharyngeal membrane- future opening of oral cavity
Cloacal membrane- future anal membrane

Front 69

What is the process of formation of the notochord?

Back 69

1) Dense cluster of midline mesodermal cells that migrated through the primitive pit. Responsible for induction of the forebrain in the overlying ectoderm
2) They form the hollow notochord process which extends to the prechordal plate from the primitive pit
3) Notochord process fuses with hypoblast
4) Definitive notochord lifts from hypoblast as it is being replaced with endodermal cells
5) Neurenteric canal brings amniotic and yolk sacs into continuity for a brief time
6) Notochord has an inductive role in neural plate formation and forms the center axis for axial skeleton and associated musculature

Front 70

What is a teratoma?

Back 70

Primitive streak disappears, but if remnants remain, they may give rise to multiple tissue tumors called sacrococcygeal teratomas
Teratomas may also arise from primordial germ cells that do not migrate to gonads
May arise within gonads as well

Front 71

What is the fate map of mesoderm?

Back 71

1) Intraembryonic mesoderm- derived from primitive node and primitive streak
2) Prechordal plate- from cranial portion of node
3) Notochord- from cranial portion of node
4) Paraxial mesoderm- from node and cranial portion of primitive streak
5) Intermediate mesoderm- from mid primitive streak
6) Lateral plate mesoderm- from caudal primitive streak
7) Extraembryonic mesoderm- from most caudal primitive streak

Front 72

What is the normal volume of amniotic fluid?

Back 72

800-1000 cc

Front 73

What is polyhydramnios?

Back 73

Over abundance of amniotic fluid
Can cause anencephalia, gastrointestinal atresia (no development), or diabetes

Front 74

What is oligohydramnios?

Back 74

Too little amniotic fluid
Can cause renal atresia, hypoplasia, or polycystic kidneys