intro to embryology

Question 1

define oocyte

Answer 1

ovum, egg
female germ cell

Question 2

what is sperm

Answer 2

male germ cell

Question 3

define zygote

Answer 3

diploid cell resulting from union of oocyte and sperm at fertilisation

Question 4

define blastomeres

Answer 4

early embryonic cells

Question 5

define morula

Answer 5

solid Mass of 12-32 blastomeres

Question 6

define blastocyte

Answer 6

stage where morula has entered uterine cavity and blastocystic cavity

Question 7

define gastrula

Answer 7

stage after trilaminar embryonic disc formation

Question 8

define nerula

Answer 8

stage after gastrula when neural tube forms

Question 9

define embryo

Answer 9

developing human during embryonic stage (to end of 8th week)

Question 10

define fetus

Answer 10

period from 9th week to birth

Question 11

define fertilisation

Answer 11

the process by which the male sperm and female egg join together

Question 12

what happens in week 1

Answer 12

30 hours after fertilisation:
- the fertilised oocyte (egg) splits into two cells of equal size; called blastomeres

after 3 more divisions:
- there are 16 cells.
- at this point, the group of cells is referred to as the morula

the cells of the morula reorganise to form a cavity, known as the blastocyst cavity (blastocoel). From this point, the morula is known as the blastocyst

Question 13

what is the blastocyst comprised of

Answer 13

1. Outer cell mass (trophoblast) - contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta.
2. Inner cell mass (embryoblast) – responsible for the formation of the embryo itself.

Question 14

what happens in week 2

Answer 14

• the trophoblast and embryoblast divide into increasingly specialised cell types.
• The trophoblast divides into the syncytiotrophoblast and cytotrophoblast.
• The embryoblast divides into the epiblast and hypoblast, forming a two-layered structure; the bilaminar disk.
• The amniotic cavity forms within the epiblas
• After the initial rounds of cellular divisions, the embryo must implant into the endometrium of the uterus.
• During this process, the syncytiotrophoblast becomes continuous with the uterus – such that maternal blood vessels (known as sinusoids) invade the spaces within the syncytiotrophoblast (known as lacunae).
• At this point, uteroplacental circulation has begun; and further embryonic development can occur.

Question 15

what happens in week 3

Answer 15

• the cells of the bilaminar disk (epiblast and hypoblast) undergo gastrulation - the two cell layers become three germ cell layers, and the bodily axes observed in the mature adult are created
• Cells of the epiblast layer break off and migrate toward the primitive pit.
• Here, they detach and penetrate through the epiblast layer to form three new germ cell layers

Question 16

describe gastrulation

Answer 16

• a process of cellular rearrangement
• involves migration, invagination and differentiation of the epiblast.
• It is largely controlled and orchestrated by the primitive streak.

Question 17

what is the primitive streak

Answer 17

• a groove in the midline of the epiblast which appears during the third week.
• Within it there is a primitive node at the cranial end, and within the primitive node lies the primitive pit.

Question 18

what are the 3 germ layers

Answer 18

• endoderm
• ectoderm
• mesoderm

Question 19

what forms the endoderm

Answer 19

formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells.

Question 20

what forms the ectoderm

Answer 20

formed by the epiblast cells that remain in position.

Question 21

what forms the mesoderm

Answer 21

formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm.

Question 22

which structures are derived from endoderm

Answer 22

• Epithelial lining of digestive and respiratory tracts,
• Lining of urethra, bladder and reproductive System
• Liver and pancreas

Question 23

which structures are derived from ectoderm

Answer 23

• Epidermis of skin
• Cornea and lens of eye
• Nervous system

Question 24

which structures are derived from mesoderm

Answer 24

• Notochord
• Musculoskeletal system
• Muscular layer of stomach, intestine etc
• Circulatory system

Question 25

what is formed by end of week 3

Answer 25

trilaminar disc from bilaminar disc

Question 26

how is the notochord formed

Answer 26

1. tube develops from end of primitive streak extending towards cranial end
2. tube fuses with endoderm to become a groove
3. plate fold to become a tube once again - the notochord
4. notochord has a central role in further midline development

Question 27

what happens in week 4

Answer 27

neurulation

1. neuroectodermal tissues differentiate from ectoderm and thicken into neural plate, separated by the neural plate border
2. neural plate bends dorsally until the 2 plates convergence to form neural crest
3. the neural crest is disconnected from the epidermis by the closure of the neural tube. the cells of the neural crest differentiate to form most of the peripheral nervous system
4. the notochord degenerates, only persisting s the nucleus pulposus of the intervertebral discs
5. mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle

Question 28

what happens if fusion does not extend all the way to the caudal end of the embryo

Answer 28

the child is born with a spina bifida

Question 29

what happens if fusion does not extend all the way to the cranial end of the embryo

Answer 29

the child is born without the cerebral cortex - anencephaly

Question 30

what does paraxial mesoderm differentiate into

Answer 30

differentiates and divides into paired cuboidal bodies - somites

Question 31

where do somites first appear

Answer 31

in future occipital region of embryo

Question 32

what do somites give rise to

Answer 32

axial skeleton

associated musculature

adjacent dermis of skin

Question 33

what happens in week 5 to weeek 8

Answer 33

developemt of all external and internal structures ( organogenetic period)