Embryology

Question 1

Time line

Answer 1

-Overview: embryology, the embryonic and fetal periods of developments.
-Week 1: fertilization, cleavage and blastocyst formation, implantation.
-Week 2: formation of the embryonic disc, amniotic sac, and yolk sac; completion of implantation.
-Week 3: gastrulation, initial neural tube formation and mesoderm differentiation.
-Week 4: folding of the embryo, germ layer derivatives.
-Summary of major events of Weeks 5 – 38.

Question 2

Embryonic Period

Answer 2

First 8 weeks
Most birth defects caused in this period

Question 3

Fetal Period

Answer 3

Last 30 weeks
All about increasing mass and size of the fetus

Question 4

Week 1 Fertilization

Answer 4

-Release of an immature egg
-Enters the uterine tube
-Fertilization of the oocyte by the sperm generally occurs in the lateral third of the tube
-egg has to be swept into the fallopian tube by the fimbriae
-It will not be a viable pregnancy if fertilization does not happen in the lateral one third
-Fertilization takes place at the moment when chromosomes from the male and female gametes come together within the ovum, creating a zygote – the primordium of a human being.
-If the time line get through off by a few hours it can result in failed implantation

Question 5

Sperm

Answer 5

Sperm can survive for up to 5 days but the quality will diminish
the healthiest sperm last the longest
-While some sperm may be capable of surviving in the female reproductive tract for ~5 days, this is probably a maximum length of time. Generally, sperm start to wane after ~12 hours.
-Semen contains enzymes, nutrients and other compounds that promote healthy sperm and aid in the sperm’s journey to the egg.
-A male’s life choices can and will affect his fertility. Currently, community fertility (generalized male fertility) is declining. Teratogens in the environment, life choices and putting off parenthood to later ages are all to blame.
-Males prime fertal window is 16-26

Question 6

Fertility

Answer 6

Women-30
Men-30
Equal have issues-40
there could also be problems with the uterus and blockage

Question 7

Egg and Sperm meet

Answer 7

The sperm reach the oocyte and binds to receptors on the external membrane. Once contact happens the oocyte chemically changes so no other sperm can bind to it.

Question 8

Cleavage and Blastocyst Formation

Answer 8

-The zygote divides to create 2 daughter cells, called blastomeres.
-Rapid cell division continues to occur and form progressively smaller blastomeres, thus creating more cells without increasing the cytoplasmic mass
-This early cell division sequence, or cleavage, produces the large number of cells that become the building blocks of the embryo.
-The reason the cells grow and not the embryo is due to the interaction of the cells so they have the ability to create new

Question 9

Morula

Answer 9

By day 3 (72 hours after fertilization), cleavage has created a solid cluster of 12-16 cells

Question 10

Late Morula Day 4:

Answer 10

-Compaction: the tight alignment of morula cells occurs, allowing greater cell-to-cell interaction; a prerequisite for embryonic formation
-The late morula reaches the uterus on day 4 after fertilization and undergoes a series of changes:
-fluid from the uterus (maternal fluid) enters the morula, collects between blastomeres, and generates a central cavity.
-this new fluid-filled structure is named a blastocyst.

Question 11

Blastocyst Stage

Answer 11

Day 4-7
Able to see two groups of cells
-an outer cell layer surrounding the blastocyst cavity- trophoblast–>helps form the placenta
-an inner mass- the embryoblast–> forms the embryo
-The blastocyst is still free floating at this point

Question 12

Implantation

Answer 12

-Process begins on day 6 and takes about one week to complete
-At this time the trophoblast erodes the uterine lining until the entire blastocyst becomes embedded within the uterine wall
-Trophoblast will erode the endometrium and eats its wall into the maternal lining
-When this happens a release of hormones happens it allows the trophoblast to erode out finger like projections into the uterine wall

Question 13

Ectopic Pregnancy Site

Answer 13

Egg tries to embed itself somewhere that its not suppose to
Such as the fallopian tube, Ovary, Cervical

Question 14

Week 2

Answer 14

-Differentiation of the embryoblast begins
-Day 9 the inner cell mass has divided into two flat sheets of cells: the epiblast (has the amniotic sac) and hypoblast (has the yolk sac)

Question 15

Bilaminar Disk

Answer 15

Epiblast and hypoblast

Question 16

Amniotic Sac

Answer 16

formed as an extension of the epiblast (blue); the amniotic fluid that fills this outpouching acts as a buffer to protect the developing embryo and fetus from physical shock until birth.

Question 17

yolk sac

Answer 17

formed as an extension of the hypoblast (yellow); this pouch contains an insignificant amount of yolk; however, it gives rise to parts of the digestive tube and the earliest blood vessels

Question 18

Day 11

Answer 18

Implantation Complete; amniotic sac and yolk sac form

Question 19

Completion of Implantation

Answer 19

At the end of implantation, a layer of the trophoblast begins to produce hCG hormone (Human Chorionic Gonadotropin) – the basis for pregnancy tests.

Question 20

Week 3

Answer 20

Three Layered Embryo
-bilaminar disk is converted into a trilaminar embryonic disk:

-Gastrulation refers to the formation of the three primary germ layers: ectoderm, mesoderm, and endoderm.

-These layers are the precursors for all body tissues.

Question 21

Gastrulation

Answer 21

Begins with the appearance of a raised groove on the dorsal surface of the epiblast, named the primitive streak
-During this time the epiblast cells migrate inward at the primitive streak
-Cells migrate through the streak on day 14-15 and displace the hypoblast to become the endoderm
On day 16, the proceeding epiblast cells form a new layer between the epiblast and endoderm, called the mesoderm
-Any epiblast cells that remain on the dorsal surface of the embryo form the ectoderm

Endoderm-hypoblast
Ectoderm-Epiblast

Question 22

Differences between the germ Layers

Answer 22

Ectoderm and endoderm – epithelial tissues, sheets of tightly joined cells.

Mesoderm – mesenchyme tissue that consists of free cells capable of migrating within the embryo. Mesenchyme cells are star shaped cells that do not attach to one another. These cells are free to migrate freely within the developing embryo.

Question 23

Primitive Node

Answer 23

A swelling located at the cranial end of the primitive streak.

The epiblast cells that move through the node migrate cranially (anteriorly).

Question 24

Notochord

Answer 24

Stiffen rod of cells that defines the axis-creates the head and tail end of the embryo-wedged in between the mesoderm and becomes important to the vertebral column
-After birth, it persists only as the nucleus pulposus of intervertebral discs (the gelatinous substance inside the disks).

Question 25

Neurulation

Answer 25

The developing notochord signals the overlying ectoderm to form the brain and spinal cord in a process called neurulation.

Signals the ectoderm that its time to form brain and spinal cord of the embryo

Question 26

Neurulation Actions

Answer 26

As the notochord develops, the ectoderm over it thickens to form the neural plate and subsequently begins to fold inward as the neural groove.

The neural groove gradually deepens, until a hollow neural tube is pinched off into the dorsal surface of the body.

          Neural plate  → Neural groove  → Neural tube

-Neural tube touch each other pinch off and that eventually forms brain and spinal cord
-Neural Tube forms part of the Central nervous System

Question 27

Neural Tube Closure

Answer 27

Closure of the neural tube begins at the end of week 3 in the future cervical region and continues both cranially and caudally until complete fusion is reached at the end of week 4.

The cranial part of the neural tube will become the brain, and its remainder – the spinal cord.

Question 28

Neural Crest

Answer 28

As the neural tube develops, some ectoderm cells migrate along its sides.

These cells form a flattened mass called the neural crest external to the closed neural tube.

Neural crest cells give rise to the sensory nerve cells and sensory ganglia of spinal nerves.
-peripheral nervous system

Question 29

Mesodermal Differentiation

Answer 29

Middle of week 3-The mesoderm lies lateral to the notochord in a cranio-caudal direction. At the end of this period, the mesoderm has differentiated into three distinct regions:

Somites- a series of paraxial mesoderm blocks (around 40 pairs) found closest to the notochord; these are the first body segments.

Intermediate mesoderm – lateral to the paraxial mesoderm; also segmented and attached to each somite.

Lateral plate – never divides into segments, begins as a single layer that quickly splits into two and encloses a space, called a coelom (cavity).

Question 30

Coelom

Answer 30

The coelom divides the lateral mesoderm into two layers:

-somatic mesoderm – next to the ectoderm (outer, parietal, somatic = “body”)

-splanchnic mesoderm – next to endoderm (inner, visceral, splanchnic = “viscera”)

The coelom that intervenes between the somatic and splanchnic mesoderm layers will become the enclosed serous cavities of the ventral body cavity: the pericardial, pleural, and peritoneal cavities.

Question 31

Week 4

Answer 31

General body form is established by the folding of the flat trilaminar embryonic disc into a cylindrical embryo

Complex folding process occurs in multipl planes, the sides of the disk fold medial direction, while the same time the head and tail region fold under

External lateral portion around to front seal off coelom create body cavity forming cylinder or tube while tucking the head/tail in at the same time

Getting a tadpole shape by day 24

Question 32

Embryonic Folding

Answer 32

During theis the yolk sac gets gets pinch off and it becomes part of the digestive tube

The cylindrical folding of the embryo encloses a tubular part of the yolk sac, called the primitive gut.

This tube is lined with endoderm and gives rise to the future digestive tube and respiratory structures.

Mesoderm and endoderm together unite both those ends pinching off portion of yolk sac creating primitive gut. ectoderm and somatic plate mesoderm closer as that happens the abdominal wall forms and ventral body cavities at the same time

Question 33

A Day 28 Embryo

Answer 33

Somites
Uppper/lower limb bud
Tail

Adult Primordia:
Skin
Outer body wall
Body Cavities and inner tube

Question 34

Ectoderm Derivatives

Answer 34

Brain and Spinal cord
epidermis, hair, nails, skin glands
sensory nerve cells and ganglia
facial bones

Question 35

Endoderm Derivatives

Answer 35

epithelial lining and glands of the gastrointestinal and respiratory tracts.

Question 36

Notochord Derivatives

Answer 36

nucleus pulposus of intervertebral disks.

Question 37

Somite Derivatives
like rib segments

Answer 37

Sclerotome
Dermatome
Myotome

Question 38

Sclerotome

Answer 38

(“hard piece”): cells migrate medially, gather around the notochord and neural tube, and become the vertebrae and ribs.

Question 39

Dermatome

Answer 39

(“skin piece”): cells migrate to more superficial layers and become the dermis of the skin on the dorsum of the body

Question 40

Myotome

Answer 40

(“muscle piece”): remaining cells that form the trunk and limb musculature.

Question 41

Mesoderm Derivatives

Answer 41

Intermediate mesoderm: kidneys and gonads

Lateral plate mesoderm:
wall of the digestive and respiratory tubes
heart and blood vessels
dermis of the ventral body region, connective tissue of the limbs (bones, joints, ligaments)

Question 42

End of week 4

Answer 42

Tadpole shape embryo
Adult body plan
skin on outside
ectoderm components
outer body wall composed of ectoderm and somate plate mesoderm
ventral body cavity
body cavity or coelom
Yolk sac pinched off

Question 43

Month 3

Answer 43

sex is visually identifiable; blood cell formation in bone marrow

Question 44

Month 4

Answer 44

bones and joint cavities are distinct

Question 45

Month 5

Answer 45

eyelashes and eyebrows present; quickening occurs

Question 46

Month 6+7

Answer 46

eyes open, cerebrum grows; increase in body size.

Question 47

Month 8+9

Answer 47

fat accumulates in hypodermis below skin, body continues to increase in length and weight

Question 48

Birth Defects

Answer 48

Birth Defect occur in 3-6% of our live births due to genetics or exposure to teratogens

Most septable defect time is week 5 because those are when organs are develops

50/50 shot of living when born prematurely just depends on how much the respiratory organs are developed