intro to embryology Flashcards
define oocyte
ovum, egg
female germ cell
what is sperm
male germ cell
define zygote
diploid cell resulting from union of oocyte and sperm at fertilisation
define blastomeres
early embryonic cells
define morula
solid Mass of 12-32 blastomeres
define blastocyte
stage where morula has entered uterine cavity and blastocystic cavity
define gastrula
stage after trilaminar embryonic disc formation
define nerula
stage after gastrula when neural tube forms
define embryo
developing human during embryonic stage (to end of 8th week)
define fetus
period from 9th week to birth
define fertilisation
the process by which the male sperm and female egg join together
what happens in week 1
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
what is the blastocyst comprised of
- Outer cell mass (trophoblast) - contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta.
- Inner cell mass (embryoblast) – responsible for the formation of the embryo itself.
what happens in week 2
- 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.
what happens in week 3
- 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
describe gastrulation
- a process of cellular rearrangement
- involves migration, invagination and differentiation of the epiblast.
- It is largely controlled and orchestrated by the primitive streak.
what is the primitive streak
- 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.
what are the 3 germ layers
- endoderm
- ectoderm
- mesoderm
what forms the endoderm
formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells.
what forms the ectoderm
formed by the epiblast cells that remain in position.
what forms the mesoderm
formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm.
which structures are derived from endoderm
- Epithelial lining of digestive and respiratory tracts,
- Lining of urethra, bladder and reproductive System
- Liver and pancreas
which structures are derived from ectoderm
- Epidermis of skin
- Cornea and lens of eye
- Nervous system
which structures are derived from mesoderm
- Notochord
- Musculoskeletal system
- Muscular layer of stomach, intestine etc
- Circulatory system
what is formed by end of week 3
trilaminar disc from bilaminar disc
how is the notochord formed
- tube develops from end of primitive streak extending towards cranial end
- tube fuses with endoderm to become a groove
- plate fold to become a tube once again - the notochord
- notochord has a central role in further midline development
what happens in week 4
neurulation
- neuroectodermal tissues differentiate from ectoderm and thicken into neural plate, separated by the neural plate border
- neural plate bends dorsally until the 2 plates convergence to form neural crest
- 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
- the notochord degenerates, only persisting s the nucleus pulposus of the intervertebral discs
- mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle
what happens if fusion does not extend all the way to the caudal end of the embryo
the child is born with a spina bifida
what happens if fusion does not extend all the way to the cranial end of the embryo
the child is born without the cerebral cortex - anencephaly
what does paraxial mesoderm differentiate into
differentiates and divides into paired cuboidal bodies - somites
where do somites first appear
in future occipital region of embryo
what do somites give rise to
axial skeleton
associated musculature
adjacent dermis of skin
what happens in week 5 to weeek 8
developemt of all external and internal structures ( organogenetic period)
