Embryology

Question 1

When does embryology happen?

Answer 1

from conception to birth- 38 weeks

Question 2

What are the 3 periods during embryology ?

Answer 2

(1) Pre- embryonic period, 1st week
- Fertilisation in ovary to implantation in uterus- single cell
(2) - Embryonic period, weeks 2-8
- Forming a embryo
- Multicellular blastocyst to recognisable vertebrate
- Organogenesis- most organs produced
(3) Fetal period, week 9-38 (until birth) (3 trimesters)
- MAJOR growth of existing structures plus a few new ones/organs.

Question 3

What are the general cellular events in embryonic development?

Answer 3

1- Cell addition by mitosis or cleavage- from a single cell to more than 5x10^12 cells
2- Cell movement and migration- in order to from layered, folded and or tubular structures
3- Programmed cell death or apoptosis- regression and elimination of tissues that are no longer required after it has served its function.

Question 4

What happens at day zero?

Answer 4

The zygote

• Successfully fertilisation- sperm implanted inside ovary- creating a single cell (male sperm release nucleus)
• This one cell is called Zygote
• Then forms diploid chromosomes (23pairs)

Question 5

What is the appearance of the zygote?

Answer 5

maternal and paternal pronuclei + cytoplasm

surrounded by a transparent capsule = the Zona Pellicuda

Question 6

What happens at Day 1?

Answer 6

• Start to get mitosis- splitting of cell
• Divides to form 2 blastomeres (Cell)
• Still surrounded by capsule - splits inside capsule
• Doesnt double in size yet as no replication etc

Question 7

What happens now during Week 1: Pre Embryonic period?

Answer 7

Cleavage: daily series of binary cell divisions: Both blastomeres divide and increase to 4 then to 8 and 16 etc
- Ocytes moves down oviduct (lined with ciliated columnar epithelia) to uterine cavity , wafted by cilia (on oviduct epithelium ) , as they wanna move the egg down the oviduct to the uterine cavity

Question 8

What happens at 3 days?

Answer 8

Produced a mass of ball of cells called Morula

Question 9

What happens at 6 days?

Answer 9

Get rearrangement and migration of cells which are forming structures
-get a blastocyst structure- created this internal cavity which is filled with fluid.

Question 10

What does the blastocyst do?

Answer 10

In uterus

And starts to implant itself on the uterine wall

Question 11

What happens at Day 5& 6 of Blastocyst?

Answer 11

Starts to segregate into 2 separate regions of cell
1-Inner cell mass= Embryoblast
forms the embryo (blast means maker)
2- Outer layer of cells= Trophoblast
- forms the placenta (tropho means feeder) - separated by fluid (blastocele)
- placenta - organ that interphase between the maternal blood supply and the embryo blood supply -takes nutrients and oxygen from mother’s blood supply to embryo

Question 12

What happens at days 7-10?

Answer 12

The membrane around blastocyst ruptures- Zona Pellucida

-This then allows the trophoblast cells to begin invading the epithelium and stroma of the maternal uterine wall.

Question 13

What happens between days 5 and 7?

Answer 13

: The blastocyst and inital stage of its implantation in the uterine wall
Trophoblast attach and start to migrate between the columnar epithelium of the uterine wall.
The embryo does not develop in the uterus cavity

Question 14

Where does the embryo develop?

Answer 14

in the mothers tissue
-blastocyst migrates and works its way through the uterine epithelium and embeds in uterine stroma - inside the connective tissue

Question 15

What happens in week 2?

Answer 15

Deep impact and Division of the blasts
- The trophoblasts cells go further into uterine wall until whole blastocyst is inside the epithelium.
The embryoblast splits into 2 layered disc:
1 -Epiblast, outer layer of columnar cells
- Hypoblast- inner layer of cuboidal cells
2The trophoblast splits into 2 divisions:
-Cytotrophoblast- inner cells remain as single layer around the blastocyst
- Syncytiotrophoblast- outer cells lose their membranes, coalesce and infiltrate deeper forms syncitium

Question 16

What else happens during week 2?

Answer 16

Amniotic and Yolk sacs

• formation of future sacs
• developing embryo is encased in a amniotic sac- fluid filled bag
• Cavities develop each side of the embryo
• In the epiblast secrete Amnioblast cells
  
  • form the amniotic sac, fluid and membranes
• Function: homeostatic environment, shock absorption, shock-absorption, buoyancy, low resistance to movement , protection from intrauterine infection

From the Hypoblast - they also form a sac- Yolk sac- Cells migrate
Trophoblast penetrating into the maternal tissue- to find supply of oxygen and nutrients and then develop placenta
- Need another nutrient supply which is from YOLK SAC
- hypoblast sends out cells which wrap around the cytotrophoblast to form YOLK SAC (aka Heusers membrane)
- Function: nutrients for early embryo, replaced later by placenta

Baby is in amniotic sac

Question 17

What happens in week 3?

Answer 17

Gastrulation
- Primitive streak (midline groove) appears in caudal (tail) end of the epiblast-

Cells in the streak proliferate and ingress into hypoblast
- Replacing orginial occupants ( which die by apoptosis) - hypoblast and form endoderm

• 1st wave of cells migrate down through the midline and start to invade the layer filled by hypoblast. When hypoblast cells are dead and leave space and epiblast cell reform this layer making continous layer of the endoderm
• The epiblast cell continue to go down, they start to fill the space between the endoderm and epiblast.- Mesoderm (A 2nd wave of ingression by epiblast cells occurs between, forming a middle layer- Mesoderm)
• Remaining epiblast cells convert into the Ectoderm

Question 18

What are the significance of the Germ layers?

Answer 18

• Are no longer pluri potent stem cells and are no longer capable of generating any cell type in the body due to going through gastrulation-
• NOW - they have become committed to producing only a restricted set of cells and tissues known as derivatives
• This commitment to produce cells with a particular fate involves inactivation of different sets of genes in the cells of each germ layer.

Question 19

What are the 3 germ layers?

Answer 19

Outer - Ectoderm- columnar epithelial cell
Middle, Mesoderm- Fibroblast- like cells and fibres (mesesnchyme)
Inner , Endoderm: cuboidal epithelial cells

Question 20

What can your ectoderm create?

Answer 20

• All your nervous tissue
• Outer epithelia
• Epidermis of skin
• Cornea and lens of eye
• Epithelium of oral and nasal cavity of paransal sinuses
• Tooth enamel
• Epithelium of pineal amnd pituitary gland and adrenal medulla
• Melanocytes

Question 21

What can your mesoderm create?

Answer 21

-Everything inbetween : diverse
Includes connective tissue

• Skeletal, smooth, cardiac muscle
• Cartilage, bone and other connective tissues.
• Blood, bone marrow and lymphoid tissues
• Endothelium of blood vessels and lymphastics

Question 22

What can your endoderm create?

Answer 22

-Inner epithelia and some organs

• Epithelium of digestive trat
• Glandular derivatives of digestive tract
• Epithelium of respiratory tract and tonsils
• Thryoid, parathryroid and thymus glands
• epithelium of reproductive ducts and glands
• epithelium of urethera and bladder

Question 23

What happens in week 4?

Answer 23

Becoming cylindrical- Germ disc grows and folds: tubular embryo
-Germ layers form different tissues and folds

1- Ectoderm: midline thickens and folds above- forms neural tissue ( brain, spinal cord)

2- Mesoderm: cells aggregate in groups; Midline, compact: forms somites develop into forerunners of the axial skeleton.-form into vertebrate
Lateral migrate: forms scleratomes (bones ), myotomes (muscle), dermatomes (dermis / connective tissue)

3- Endoderm: folds under
- Forms the inner gut tube

Question 24

What happens at the end of week 3?

Answer 24

The disc is folding

• form the vertebrate of spinal cord
• Get folding downwards from the midline and what will form the neural tube

Question 25

What happens at Week 4?

Answer 25

Somites form vertebrate
Ectoderm: folding above to from neural tube
Mesoderm: midline cells condense to form somites and lateral cells migration to various tomes foreruners of bone (scleratomer- bone, myotomes-muscle, dermatomes-tissue)

Question 26

What happens at week 5?

Answer 26

A recognisable vertebrate

Question 27

What is the placenta?

Answer 27

Maternal blood supply meets embryonic blood supply
A vascular network with 2 separate parts:
1-Maternal blood vessels- space appear in syncytio-trophoblast
Invaded by capillaries in uterine wall

2- Embryonic blood vessels- finger like villi project from cyto-trophoblast
Inner cells at core of villi differentiate into capillaries
Capillaries come out of placenta and run close to maternal blood supply

Note that the maternal vessels supply oxygen and nutrients and remove embryonic co2 and waste products:
exchange is by diffusion, maternal-embryonic/ fetal blood does not mix.

Question 28

What happens in week 9?

Answer 28

Early fetus, an incipient human

-FETAL DEVELOPMENT
Week 12: 10cm long
external genitalia- determine gender
-disproportionate head growth

Week 24: 20cm long

• Face developed
• increased body growth
• limb movement

Week 27/28: 25cm long
-may survive premature birth but hypothermia and lung would collapse

Week 38L 35-40cm long
- Birth

Question 29

What is the importance of taking birth histories when examining infants and children?

Answer 29

1- Ask was delivery normal and without injury
- Respiratory distress, risk factor cerebral palsy

2- Was the child premature?

• a risk factor for specific vision problems
• mild (high myopia); moderate ( eye movement squnit, nystagmus); severe (retinopathy:ROP)

3- Specific risks for retinopathy of prematurity - extreme prematurity less than 28 weeks

• duration of oxygen given on ventilator
• blood transfusion required; occurence of seizure

Question 30

What are abnormal development?

Answer 30

1- spontaneous embryonic abortion- major malformations incompatible with further development terminate- 50% of pregnancies

2- Birth defects : affects 5-10% live births

• embryonic defects can result in gross malformations, some preclude extra uterine life
• fetal defects : usually cause under development of exisiting tissues/organs - so cause disabilities

Cause: genetic and environmental teratogens.

Question 31

What are the environment teratogens?

Answer 31

Maternal exposure to toxic agents:

1-Infectious: e.g viruses and parasites

• rubella, herpes, HIV and toxoplasmosis
• micro-opthalmia and heart defects

2-Physical e.g ionizing radiation
-blindness, craniofacial and CNS defects

3- Chemical: e.g too many to list

• thalodmide: limb malformation
• alcohol: mental retardation.