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Uni Biology > Human bio: embryology > Flashcards

Human bio: embryology Flashcards

1
Q

zygote

A

pronuclei = maternal.

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2
Q

embryogenesis lecture 1 learning objectives

A
  1. Know the cellular contributions of female and male gametes to the fertilised ovum
  2. Describe the processes of cleavage, blastocyst formation and implantation
  3. Describe the main characteristics of embryonic development during weeks 1 and 2
  4. Describe the extraembryonic structures and their functions during early pregnancy

You need to know the developmental events that occur during “weeks”

Note: processes are continuous - demarcation of weeks is for convenience

Days are shown in images to allow for comparison of different views and processes

You do not need to know “days”

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3
Q

week 1 of embryogenesis

A

Week 1 (Days 0 - 6) = fertilization, formation of the 1 “layered” embryo & start of implantation

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4
Q

week 2 of embryogenesis

A

Week 2 (Days 7 - 13) = formation of the bilaminar embryo (2 layers), body stalk, cavities, completion of implantation, beginning of placental development (Lecture 2)

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5
Q

zygote definition

A

Zygote = fertilized egg; single cell stage lasts for ~ 24 hours

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6
Q

embryo definition

A

Embryo = from the first mitotic (cleavage) division

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7
Q

fetus definition

A

Fetus = once all of the main body
parts are recognisable ~ 8 weeks

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8
Q

formation of the zygote

A

After fertilization spermatozoon moves close to the female pronucleus,
its nucleus swells to form the male pronuclei (tail detaches & degenerates)

Male & female pronuclei come into contact & lose nuclear membranes

Male + female pronuclei = 46
chromosomes - single strands of DNA

For the embryo to grow - mitosis

BUT

Each pronucleus must undergo DNA synthesis before mitosis can

After DNA synthesis = 46 chromosomes - double strands of DNA
joined by a centromere (sister chromatids)

Male and female pronuclei lose their nuclear membranes

Mitosis proceeds to form 2 cells = cleavage

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9
Q

Cellular contributions of the ovum to the zygote:

A

Cellular contributions of the ovum to the zygote:

  • 23 maternal chromosomes (single strands of DNA)
  • Cell membrane
  • Cytoplasm
  • Most organelles (mitochondria)
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10
Q

Cellular contributions of the sperm to the zygote:

A

Cellular contributions of the sperm to the zygote:

  • 23 paternal chromosomes (single strands of DNA)
  • Centrioles (form mitotic spindles)
  • critical to bringing maternal and paternal chromosomes together in the zygote (sperm microtubule organising centre)
  • critical for the first mitotic (cleavage)
    division in the zygote
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11
Q

In what ways can blastomeres reorganise from the 8 cell stage.

A

From 8 cell stage blastomeres reorganise into:

“inner” (embryoblast) and

“outer” (trophoblast) cells

Tropho-= nourishment; Blast = germ, sprout, bud

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12
Q

what is the morula

A

Morula (mulberry) = 16-32 cells
Trophoblast cells bring in Na+ (via active transport),

H,0 follows via osmosis to create a fluid filled cavity, the blastocyst cavity (= blastocoele)

-coele = cavity
-cyst = bladder

Blastocyst enzymatically bores its way through zona pellucida to “hatch”

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13
Q

eight cells to hatching

A

The zona pellucida gets thinner as the blastocyst cavity expands

Zona pellucida close to or over the embryoblast disintegrates to allow for hatching to occur

Embryoblast = Inner Cell Mass

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14
Q

when does implantation start?

A

Beginning of Week 2

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15
Q

Day 6

A

Implantation = result of both trophoblastic and endometrial interaction

Red cells = future cranial end of embryo

Early blastocyst stage, embryoblast cells differentiate into different cell types

Near time of implantation, cell types form two distinct layers

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16
Q

Day 7

A

Bilaminar embryo:

Epiblast (blue) = dorsal layer

Hypoblast (yellow) = ventral layer
(facing blastocyst cavity)

Trophoblast cells over the embryoblast penetrate endometrial epithelium rapidly differentiating into:

(i) Cytotrophoblast (single inner layer) &

(ii) Syncytiotrophoblast

(multinucleate invasive outer layer)
Epi - above; Hypo - below; Cyto - cell; Syn - together

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17
Q

Implantation continued & Bilaminar (2 layered) Embryo

A

8 days - bilaminar embryo is implanting
Syncytiotrophoblast develops from cytotrophoblast and erodes endometrium

10 days - bilaminar embryo is fully implanted

Extraembryonic mesoderm develops (placental development)

Syncytiotrophoblast erodes endometrial glands & blood vessels to create a lacunar network (lacuna = lake, pool)

Embryo is small enough to be nourished via diffusion

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18
Q

Chorionic Cavity & Yolk Sac Development

A

12 days

Spaces appear in extraembryonic mesoderm = early formation of a new cavity, the chorionic cavity

13 days

Chorionic cavity develops rapidly
Primary yolk sac starts to be replaced by the secondary (definitive) yolk sac

14 days

Secondary yolk sac development is complete
Connecting stalk develops

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19
Q

end of week 2

A

Connecting stalk (body stalk) has developed from extraembryonic mesoderm

Connecting stalk eventually becomes part of the umbilical cord

It suspends the bilaminar embryo, amniotic cavity and definitive yolk sac (all covered by extra-embryonic mesoderm) in the hugely expanded chorionic cavity

Chorionic cavity = space for embryonic growth;
fuses with the amniotic cavity between weeks 14-16

Amniotic cavity (will get bigger during development):
* Protection
* Freedom of movement
* Space for growth

Definitive yolk sac:

  • Contributes to formation of the
    digestive tract
  • Future gametes
  • Site of production of first blood cells
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20
Q

what happens to ectopic pregnancies

A

seldom go to term

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21
Q

ektopos means

A

out of place

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22
Q

lecture 2 learning objectives

A
  1. Describe the formation of the trilaminar embryo

= 3 germ layers that give rise to all tissues & organs (Week 3 = 14 - 20 days)

  1. Describe the early formation of placental development (up to the end of Week 3)
  2. Describe the development of the future nervous system (Week 4 = 21-27 days)
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23
Q

Beginning of Week 3 - the Primitive Streak

A

Directional Terms in Developing Embryo are equivalent to those used in quadrupeds.

  • Dorsal surface = epiblast
  • Ventral surface = hypoblast (not anterior and posterior)
  • Caudal (tail) end
    = towards the primitive streak
  • Cranial end (cephalic / rostral)
    (not superior and inferior)
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24
Q

Gastrulation - formation of the trilaminar embryo

A

3 Germ layers formed in Week 3:

  • ectoderm
  • intra-embryonic mesoderm
  • endoderm.

Germ layers give rise to all tissues and organs

No propy

14-15 days

Endoderm
16 days

Mesoderm
Definitive endoderm
Epiblast
Mesoderm
Endoderm

2 regions with absence of intra-embryonic mesoderm

Oropharyngeal
membrane
= future opening for mouth
Cloacal membrane = future opening for urethra, vagina & anus

Epiblast

Endoderm

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25
Q

the formation of the notochord

A

Day 16 - Primitive streak = half the length of the embryo

As gastrulation proceeds, becomes shorter relative to the rapidly growing embryonic disc

Day 22 - primitive streak = 10-20% of embryo’s length

Day 26 = seems to disappear

As the primitive streak gets smaller, a midline structure called the notochord develops from mesoderm

Notochord is critical to nervous system & somite (intra-embryonic mesoderm) development

Note: embryo and extra-embryonic structures

Note the position of where heart development begins

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26
Q

what is the allantois

A

The Allantois

Allantois etymology = sausage-shaped

Appears Day 16 as a small outpouching from caudal wall of the yolk sac (landmark feature

Projects into the connecting stalk

In birds serves as a respiratory organ and a repository for excreta

In humans allantois blood vessels give rise to Umbilical arteries & vein (of the umbilical cord)

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27
Q

embryonic nutrition

A

Conceptus nourished by 3 mechanisms:

  1. Uterine milk - before implantation
  2. Trophoblastic nutrition - after implantation
  3. Placenta - most of pregnancy
    Mature placenta = disc shaped organ
    Attached to the embryo / fetus by umbilical cord = source of oxygen, nutrients & means of waste removal

Diffusion suffices for the nourishment of the early embryo (small distances)

Placental development starts midway through week 2

Cardiovascular system develops quickly to support the rapidly growing embryo

Embryonic cardiovascular system is connected to the placenta

By end of week 3, the placenta is sufficiently developed to start supporting the embryo at the time its heart first starts to beat (~Day 22)

By week 12, the placenta is the sole mode of nutritional support until birth

28
Q

Early “Placental” Development

A

Placenta develops from extra-embryonic tissues:

  • Extra-embryonic mesoderm
  • Cytotrophoblast
  • Syncytiotrophoblast

By Day 21 - mesodermal core differentiates into connective tissue & blood vessels

Placenta also produces hormones

Main hormones:
estrogen (stimulates growth in fetus & mother) progesterone (suppresses follicle development & uterine contractions)

  • human chorionic gonadotrophin (HCG)

More on HCG in Embryogenesis 3 lecture

29
Q

more early “Placental” Development

A

Day 13 - end of week 2 (bilaminar embryo)

Day 20 - end of week 3 (trilaminar embryo)

By end of week 3: Tertiary villi capillaries make contact with →

Capillaries of the extra-embryonic mesoderm & connecting stalk, which make contact with →

Developing intraembryonic circulatory system of the embryo

When the primary heart tube starts beating in week 4 (day 22), the villous system is sufficiently developed to start supporting the rapidly growing embryo

30
Q

end of week 3

A
  • Trilaminar Embryo - ectoderm, intraembryonic mesoderm, endoderm
  • 3 cavities - amniotic cavity, yolk sac & chorionic cavity
  • Placenta has tertiary stem villi = starts to be functional
  • Heart tube starts to beat
  • First appearance of the future central nervous system (brain & spinal cord)
31
Q

the process of neuralation

A

formation of nervous system

4 main events:

Neural plate

  1. Formation
  2. Shaping
  3. Bending

Neural folds

  1. Apposition (bringing close together), adhesion & fusion

Terms:

  • Neural plate
  • Neural groove

Neural folds

  • Cranial neuropore
  • Caudal neuropore

Nervous system = Central & Peripheral:
Brain, spinal cord & all nerves

32
Q

Neural Crest Cells

A

Form at the tips of the neural folds

Migrate away when neural tube closure is complete

Neural crest cells migrate widely in the embryo and contribute to the formation of a diverse array of structures, such as:

  • Peripheral Nervous System
  • Connective tissue of the face and skull
  • Melanocytes of the skin and hair follicles (this is a very abbreviated list)
33
Q

Embryonic Mechanisms of Growth and Development

A
  1. Proliferation = increase in cell number (e.g. cleavage)
  2. Cavitation = formation of internal space (e.g. blastocyst formation, amniotic cavity, yolk sac, chorionic cavity)

Differentiation = alteration of developmental trajectory
(e.g. gastrulation: epiblast → endoderm, intra-embryonic mesoderm, ectoderm)

  1. Migration = cell movement from one place to another (e.g. gastrulation, neural crest cells)
  2. Induction = cells trigger neighbouring cells to differentiate (e.g. notochord induction of neural plate formation from ectoderm)

Combination = germ layers (ectoderm, intra-embryonic mesoderm, endoderm)
combine to form whole organs (Lecture 3)

34
Q

things you need to be able to recognise on an image

A
  • Neural plate
  • Neural groove
  • Neural folds
  • Neural canal
  • Region of neural crest cell development
  • Ectoderm
  • Mesoderm (intraembryonic)
  • Endoderm
  • Primitive streak
  • Amniotic cavity
  • Definitive yolk sac
35
Q

embryogenesis lecture 3 outcomes

A
  1. Describe folding of the embryo: craniocaudal (head-tail) &
    lateral (formation of a ventral body wall)
  2. Identify the major tissues / organs
    derived from the primary germ layers
  3. Explain the evolutionary significance of morning sickness
36
Q

folding of the embryo

A

End of week 3

  • Flat trilaminar disc
  • Neurulation has commenced

Mid week 4

  • Neurulation complete
  • Bent trilaminar disc = folding has commenced

End of week 4

Folded embryo
= Closed cylinder - a tube within a tube

37
Q

Head-tail folding / Sagittal plane folding

A

Day 17

Week 3

Notochord is forming

Gastrulation still occurring.

Future heart is cranial to developing brain

Intraembryonic mesoderm not visible in these images

Day 24

Rapid brain development

Position of the heart tube is caudal relative to brain

Change to the yolk sac size relative to amniotic cavity

Day 22

Early week 4

Heart tube is repositioned
- ventral to the developing brain

Heart tube starts beating

Day 27

End of week 4

Oropharyngeal membrane is open

Organs start to form (lungs, gut, liver)

Note the narrow vitelline duct of the yolk sac

Vitelline = coloured like
egg yolk

38
Q

Lateral folding / Transverse plane folding

A

Note the following:

Neurulation

Differentiating intraembryonic mesoderm - somites

The intercellular cavities created in the intraembryonic mesoderm constitutes the primitive body cavity.
I
ntraembryonic body cavity (= Intraembryonic coelom) formation creates spaces for organs to grow into:

  • Lungs
  • Heart
  • Gut tube
  • Liver
39
Q

lateral folding continued - week 4

A

Rapid growth of the amnion surrounds and “pinches” the yolk sac to form the vitelline duct

Head fold

Yolk stalk

Last region of the embryo to undergo complete lateral folding = the area occupied by the yolk sac.

In this region, small channels connecting the intraembryonic and extraembryonic coeloms (cavities) persist until the ventral body wall is completely sealed

40
Q

The Embryo at the end of week 4 = folding completed

A

End of Day 27

Embryo is no longer a trilaminar disc

Some developing organs are clearly visible

  • Brain
  • Heart
  • Liver
  • Eye
  • Limb buds
41
Q

Derivatives of the 3 Primary Germ Layers

A

List is only indicative of what develops from the 3 primary germ layers.
It is NOT comprehensive

Organs are often combinations of germ layer derivatives, e.g.

Stomach:

  • Epithelial lining is endoderm derived
  • Smooth muscle is mesoderm derived

Skin:

  • Epidermis & nerves are ectoderm derived
  • Dermis is mesoderm derived
42
Q

Morning Sickness - evolution!

A

Human chorionic gonadotrophin:

  • secreted by blastocyst & placenta
  • peaks at 10-12 weeks
  • similar to LH - stimulates growth of the corpus luteum, which doubles in size
  • corpus luteum secretes increasing amounts of estrogen & progesterone
  • Absence / lack of HCG - corpus luteum atrophies & uterus expels the conceptus (embryo / fetus + placenta + membranes)
  • HCG secretion corresponds to peak period of organogenesis & morning sickness

Morning sickness (nausea):

  • most common in the first few months of pregnancy
  • cause is unknown (many think it is due to HCG secretion)
  • may be an evolutionary adaptation to protect the developing conceptus
  • women who experience morning sickness have fewer infants with birth defects than those who don’t
  • women who experience morning sickness consume bland / “safer” foods, so less likely to ingest teratogens (environmental agents)

Teratogens cause congenital malformations (birth defects)
(teratos = marvel / monster; gen = something produced)

Examples: food poisoning (Listeriosis), alcohol, certain drugs (e.g. some antibiotics, anticoagulants, anticonvulsants), excessive caffeine, tobacco, infections, retinoic acid (vitamin A derivative for acne treatment)

43
Q

Fertilization by 2 or more sperm is called

A

polyspermy

44
Q

what is the process of implantation

A

The process of implantation, whereby the blastocyst attaches to the fundus or posterior wall of the uterus, occurs 6 days after ovulation.

45
Q

what is the trophoblast

A

The blastocyst is a hollow sphere with an outer layer of (idk what shape) cells called the trophoblast.

46
Q

what does the trophoblast form

A

The trophoblast is destined to form the placenta.

47
Q

what is the blastocoel

A

The blastocyst has an internal cavity called the blastocoel and an inner cell mass called the embryoblast which is destined to become the embryo.

48
Q

During implantation the trophoblast on the attachment side separates into 2 layers:

A

The plasma membranes of the cells of the superficial layer of the trophoblast, which is in contact with the endometrium, break down and these cells form a multinucleated mass called the syncytiotrophoblast.

The syncytiotrophoblast grows into the uterus digesting the endometrial cells.

The endometrium reacts by growing over the blastocyst and the conceptus becomes completely buried in endometrial tissue.

The deep layer of the trophoblast, closest to the embryoblast, retains individual cells and is called the cytotrophoblast.

49
Q

What are spaces is the embryonic disk flanked by and what is it flattened from?

A

During embryogenesis, the embryoblast flattens into an embryonic disc.

This disc is flanked by two spaces, the amniotic cavity and the yolk sac cavity.

50
Q

what are the two layers that the embryonic disc is composed of?

A

The embryonic disc is composed of two layers: the epiblast and the hypoblast.

the epiblast faces the amniotic cavity

the hypoblast faces away from the amniotic cavity.

The hypoblast cells multiply and form the yolk sac

51
Q

What does the thickening of the epiblast cells along the midline cause

A

The cells of the epiblast thicken along the midline and form the primitive streak with the primitive grove running down the midline.

At this stage the embryo is bilaterally symmetrical with its future right and left sides, dorsal and ventral surfaces, and cephalic (head) and caudal (tail) ends defined.

52
Q

where do the multiplying epiblast cells migrate?

A

The multiplying epiblast cells migrate medially into the primitive groove and displace the original hypoblast with a layer called the endoderm.

53
Q

what do the migrating epiplast cells form

A

The migrating epiblast cells form a third layer, the mesoderm, between the first two layers.

The mesoderm is a loosely organized tissue which differentiates into a loose fetal connective tissue called mesenchyme.

The remaining epiblast is now called the ectoderm.

54
Q

what is gastrulation

A

The process of forming the three primary germ layers, ectoderm, mesoderm and endoderm, is termed gastrulation and the embryo is now called a gastrula.

At this stage the embryo is approximately 2mm long and 16 days old.

55
Q

what is neuralation

A

NEURALATION

The nervous system develops from ectoderm.

56
Q

what occurs during the third week of development?i

A

During the third week of development, growth of the notochord, forward along the midline of the embryo, induces thickening of the overlying ectoderm to form the neural plate. As this progresses, the neural plate sinks to form the neural groove and its edges thicken to form raised neural folds on each side.

57
Q

what happens to the neural crest during neuralation

A

Cells at the crest of each neural fold separate to form the neural crest which develops as longitudinal columns on each side of the neural tube.

The neural crest is of ectodermal origin and gives rise to most of the peripheral nervous system and some structures of the skeletal, integumentary and endocrine system.

58
Q

what happens to the neural tube during neuralation

A

By day 26 the neural folds fuse creating a hollow neural tube.

The lumen of the neural tube becomes a fluid-filled space, the neural canal, which will later become the ventricles of the brain and the central canal of the spinal cord.

59
Q

what happens to the flat embryonic disc during week 4

A

During week 4 the flat embryonic disc (gastrula) grows rapidly folding around the yolk sac and becomes a C-shaped cylindrical form with the cranial and caudal ends almost touching.

60
Q

how is the ventral surface of the embryo formed

A

The lateral margins of the disc fold around the sides of the yolk sac to form the ventral surface of the embryo.

As a result of this lateral folding of the embryo, the surface of the embryo becomes covered with ectoderm (later to become the epidermis of the skin).

61
Q

what happens to the mesoderm

A

The mesoderm splits into two layers. One layer adheres to the ectoderm, the other layer adheres to the endoderm and a body cavity (coelom) develops between the two layers of mesoderm.

62
Q

what is organogenesis

A

This process, whereby the primary germ layers differentiate into organs and organ systems, is termed organogenesis.

63
Q

when are the organs are present but not fully functional.

A

At 8 weeks gestation

64
Q

what occurs due to head and tail folding

A

Head and tail folding (i.e. cranio-caudal folding) results primarily from the rapid growth of neuroectoderm.

65
Q

what has happened by the end of week 5

A

During folding the body cavity becomes divided into the thoracic cavity and the peritoneal cavity by the diaphragm.

By the end of week 5 the thoracic cavity further divides into the pleural and pericardial cavities.

Uni Biology (29 decks)

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