Embryology Flashcards
First steps of cellular division after fertilisation
- D1 - fertilisation (acrosomal reaction, sperm pronucleus fuses with female pronucleus)
- D2 - zygote (2 cell stage)
- D3 - morula (16 cells stage)
- D4 - Blastocyst (blastocele cavity forms, surrounded by embryo blast and trophoblast)
- D6-7 - implantation begins (synciotrophoblast invades the decidua basalis of uterus)
Composition of a blastocyst
- Outer layer: trophoblast – contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta
- Inner layer: embryoblast – responsible for the formation of the embryo itself
- Blastocele - fluid filled cavity providing nutrients to developing blastocyt by diffusion
What does the trophoblast divide to form?
Syncitiotrophoblast and cytotrophoblast
What does the embryoblast divide to form?
Epiblast and hypoblast, forming a two-layered structure; the bilaminar disk.
The amniotic cavity forms within the epiblast
What happens in implantation?
Syncitiotrophoblast invades decidua basalis
This drives the DECIDUAL REACTION
- hypertrophy of decidual cells and become polyhedral
- Increased glycogen and lipid deposition by endometrial cells
- congestion and dilation maternal vessels
- increased endometrial oedema
Syncitiotrophoblast forms PRIMARY VILLI.
The empty spaces between primary villi are called LACUNAE. Maternal arteries and veins (sinusoids) fuse with the lacunae to form the JUNCTIONAL ZONE. This is the start of the uteroplacental circulation.
How do the 3 germ cell lines develop?
GASTRULATION
- Happens during week 3, from day 15
- Primitive streak develops in the epiblast in a caudal to cranial direction, and primitive node forms
- Primitive streak becomes the primitive groove
- Cells migrate through the primitive groove from the epiblast to the hypoblast, forming 3 layers: endoderm, mesoderm and ectoderm
What structures are derived from the endoderm?
Epithelial lining of digestive and respiratory tracts,
Lining of urethra, bladder and reproductive system
Liver and pancreas
What structures are derived from the mesoderm?
Notochord (becomes nucleus pulpous of vertebral disks, secretes SHH protein) Musculoskeletal system Connective tissues Muscular layer of stomach, intestine etc Circulatory system
What structures are derived from the ectoderm?
Epidermis of skin
Cornea and lens of eye
Nervous system
Development of the neural tube
NEURULATION occurs around day 18.
The notochord stimulates (controlled by SHH protein) formation of neural plate, groove and folds, which then fold in a caudal to cranial manner to create the neural tube (spinal cord).
Neural crest cells migrate from neural plate to to the mesodermal layer to form:
- melanocytes, sympathetic nervous system, sensory ganglia, schwann cells, glial cells, craniofacial skeleton
Anterior neural tube defect results in which condition?
Anencephaly, incompatable with life
Posterior neural tube defect results in which condition?
Spina bifida, varying severity. Due to failure to close at day 27
Folic acid reduces likelihood of NTD by how much?
50-70%
Initial development of the gonads- until indifferent gonad
During week 6
- Urogenital ridge (2 swellings of mesoderm) differentiates to nephrogenic cord (urinary system) and gonadal ridge
- Migration of primordial germ cells (endodermal origin) from the yolk sac along vitelline duct to the gonadal ridge
Forms the ‘indifferent gonad’ - from germcells (endoderm) and primitive sex cords (mesoderm)
Development of testis
- Y # contains SRY gene
- codes testis determining factor
- Stimulates primitive sex cords (mesoderm) to differentiate into straight tubules, seminiferous tubules and rete testis
- Primordial germ cells migrate from yolk sac along vitelline duct till they join the gonadal ridges
