Key Features of Fertilization
- Fertilization in 1st 1/3 of oviduct
- Cumulus mass / granulosa cells (?)
- sperm penetrates through this by wriggling.
- Cumulus mass holds sperm in place when binding to the zona pellucida.
- Binds to ZP3 receptor
- Acrosome reaction (between sperm head and ZP); digests zona pellucida & matures the sperm
- Sperm head (inner acrosomal membrane) fuses to oocyte membrane. Protein receptors on both membranes are important for fusion as well as the calcium.. exocytosis.. alters ZP to prevent polyspermy.
- Parthenogenesis- an embryo without sperm contribution, is called parthenogenesis
Key features of Week 1 Development
- Look at summary diagram
- Identify names/changes over time
- Zygote --> blastomere --> blastocyst --> uterine body; via cilia & floating in fluid (secreted by uterine body)
- Zona Pellucida
- Pale purple around early conceptors
- "pale zone" synthesised by oocyte;
- extracellular matrix;
- made out of glycoproteins 'ZP proteins 1, 2 and 3'.
- Each species has its own type of ZP proteins = species specificity.
- Zp function:
- 1) Protection of oocyte and blastocyst (like shell around egg).
- 2) Provides a structure for the blastocyst. Patterns the development of the blastocyst. As the cells proliferate they squish against the ZP and become squamous.
- 3) Sperm receptor (allows sperm to bind for it).
- 4) Prevents implantation.
- 5) Modified by granulosa molecules to prevent polyspermy.
- Adplantation: egg roles and slowly increases adhesion to surface epithelium (has not yet implanted).
- Implantation: takes 1 week
Key Features of Week 2 Development
- By the end of the week the blastocyst is fully within the uterine wall.
- Trophoblast layer;
- ICM (forms the embryo);
- Placenta comes from the trophoblasts as well as contributions from the ICM;
- Carnegie Stages
- 23 stages of embryonic development
- Stages 1-5 are 1st week of development
- Refer to features on the embryos, rather than the size
- Look at Time line
- Trophoblast cells form 2 populations of cells:
- cytotrophoblasts (single nuclues near ICM)
- Dividing rapidly mitotically and fused together; multinucleated - synsidiotrophoblasts
- Trophoblast cells form 2 populations of cells:
- Stromal area of the uterus:
- Spiral arteries are held open by trophoblast cells; maintains leaking of maternal blood into the conseptus
- Uterine glands in epithelium; secrete into surrounding spaces
- 2 layers of ICM = bilamina embryo
- ICM mass = epiblast & hypoblast (carnegie stage 4)??
- stage 5 (invaded the uterine wall)
- Gastrolation = trilamina embryo
Main features and events going on; what is / is not there anymore
- First 2 weeks: Abnormalities = genetic, spontaneously aborted.
- After 2nd week, abnormalities = developmental abnormalities, less genetic.
Folding events and development of coelums
- 3 intraembryonic coelums = PPP - pericardial, peritoneal and _
- 3 extraembryonic coelums = Amnionic, yolk and coleolic
- Endoderm lines the yolk sac; ectoderm lnies the emnionic sac. these two cavities are completely independent of each other.
- The third cavity is the large space outside the embryo
- Amnionic cavity has an overcoat of mesoderm (outer surface)= "extraembryonic mesoderm" --> includes the embryonic stalk as well as the lining of the coleolic sac.
- Buccopharyngeal membrane. Where oral cavity will form. Ectoderm and endoderm in close proximity, no mesoderm in between.
- Cloecal membrane = lower end of gastrointestinal tract. Ectoderm and endoderm in close proximity, no mesoderm in between.
- Site of placental blood vessels? - stalk brought around to ventral surface?
- Transverse septum lies beneath the heart (beneath the buckapharyngeal membrane) and represents the site of where the emnionic cavity meets the yolk sac. Later contributes to development of the liver.
- End of 4th week: neural tube (NT) folds - now two layers of ectoderm.
- Key events of folding is that it brings the amnionic sac from the ventral to the dorsal side (?); therefore the anionic sac is wrapped around the head and caudal end of the embryo, and the yolk sac is squashed in the middle in the midgut region. Therefore the embryo is developing within the amnionic sac.
- Amnionic cavity increases in volume, filling the amnionic space. The yolk sac gets compressed at the midgut region; lies next to the emnyionic stalk.
- The amnionic membrane then fuses with the chorionic membrane. From that stage on we have a single cavity in which the embryo develops. The yolk sac is essentially lost.
- Amnionic fluid space allows the embryo to develop with equal pressure on all surfaces. Structures are not restricted with growth. Fetal stage of development the fetus swallows the fluid allowing development of...
- Fluid is tightly regulated.
- Feature: overall growth of the embryo.
- Carnegie Stage 9 = end of week 3
- Next feature to see is the somites
5 = implantation 6 = bilamina embryo 7 = flat embryonic disk - http://embryology.med.unsw.edu.au/embryology/index.php?title=Carnegie_stage_7
- Don't need to memorise, just have an idea of the events that are going on
- Generation of neural plate
- Neural arches (cover later)
- Seonsory plaquodes in _
- Heart forming (cardiogenesis) - heart forms beneath cranial region
- Begining of development of limbs (NOT arms and legs!)
- Covers stages 10-13
- The first somite forms early on and then spreads into the head region.
- Form on either side of the neural groove.
- At this time the neural plate is not fused to form the neural tube.
- The more somites, the older the embryo
- 2.5-4.5 mm in length
- ~ 13-20 somites
- Stamedium = indentation on the surface. The buccapharengeal membrane lies on the floor. Then heart. Then Transverse septum (beneath where amnionic sac and yolk sac fuse.
- Embryo has begun to form a C-shape as the cranial and tail end curve. Embryo is measured from the crown-rump length(CRL)
- Major differences now
- NT is closed, leaving an opening of the neural pores (caudal, cranial etc)
- TS beneath heart - differentiating within that is the liver. Will be the major vascular tissue in the embryo (all blood vessels go into the lievr and out into the heart)
- The brain has a thin layer of ectoderm on the surface. Head is formed by the pharengeal arches. (No head yet).
- Surface features unique to stage 12:
- 4 Pharengeal arches = head and neck strctures. Form in a rostro-caudal sequence, labelled by numbers 1st pharengeal arch etc... The 4th one fuses with the 6th.
- Vesicles - bulges in the head region(forebrain, midbrain and hindbrain)
- Otic plaqode is visible in the head region(hole by head) - simple ectodermal epithelium makes the cochlear and _ canals (ie the inner ear). Sinks beneath the surface and is eventually pinched off completely.
- Late stage 12:
- Can see developing upper and lower limb buds. Note relative position of limb bud to the embryo - upper limbs developed low down near liver. As trunk grows the limbs change their relative position.
- Look at diff in size of the two limbs: Upper limb is bigger because it develops first. Always a stage ahead.
- Can no longer see odic plaqode
- Lensic plaquode forming; also gets pinched off surface to form lens
- Nasal plaquode (very large compared to the others) - forms the inner epithelium of the nasal cavity.
- Ventral view = yolk sac side / anterior surface
- Dorsal view = ectoder / back of embryo
- Ventrolateral / dorsolateral = angled view from side, 45 degrees
- Rostrocaudal / craniocaudal = from head to tail
- Wk 2: large chorionoic space (smaller yolk sac and amnionic space) allows development of the embryonic disk without any attachments
- Amniotic space expands and the chorionic space is eventually lost
- Villi are always called "chorionic villi" because they extend from the chorionic layer.
- Syncytiotrophoblasts release HCG as they proliferate and extend into the maternal dicidua. As they do so the interaction with the stromal cells cause the stroma cells to transform = deciduation
- 2 folds of 2 layers - cytotrophoblasts and syncytiotrophoblasts
- extra-embryonic mesoderm thought to arise gastrulation. Forms distinct layers; over yolk sac, over amnion, over stalk. Will eventually form the placental cord.
- placental half vs chorion levea
- Placental half: lots of uterine glands. Wk 3 = passive diffusion of nutrients from the blood filled spaces, through the mesoderm and into the embryo. Never physical contact between foetal and maternal blood (mother may develop antibodies against it, prevent further pregnancies).
- Primary villi = 1st villi that form
- As the mesoderm folds into these extending fingers, they become secondary villi
- primordia of the foetal placenta is on the decidua vasalis on the maternal side
- Other side facing away from uterus = decidua capsularis
- decidua vasalis = everything else in the uterus(?)
- Umbilicus contains blood vessels and _
- Yolk sac lies adjacent to the umbilicus
- Fusion of amniotic membrane with the chorionic membrane. Now called the chorionic plate = continuous structure. Within the chorionic plate at the site of decidua vasalis = large blood vessels (arteries and veins that have left the cord and spread out into the villi)
- 3 main villi classifications: primary (trophoblasts), secondary (+ mesoderm), tertiary (blood vessels)
- Tertiary: blood islands fuse to form vessels.
- Role of syncytiotrophoblasts: implantation and endocrine role (secrete HCG)
- Cytotrophoblasts - single cell and single cytoplasm. Form cyto.. column of the anchoring villi.
- CT surrounding blood vessels
- Features in an anchoring villi: extra-embryonic mesoderm (in the core of the villi); cytotrophoblasts cells on the outer surface(single layer of cells; trophoblastic column (outside the extra-embryonic mesoderm; maternal blood-filled space (surrounding the villi - diffusion/exchange of nutrients); clusters of syncytiotrophoblasts outside the cytotrophoblasts.
- Hofbauer cells: remodel mesoderm continuously as well as gobble up RBC nuclei
- missed the beginning...
- respiratory & GI have common endodermal origin
- allantois is endodermal and extends into the connecting stalk. It represents the upper end of the developing bladder eventually
- Hindgut: will ofrm more than just the GI tract. In wk4 it forms the trachea.
- From week 4 onwards the oral cavity is open - can swallow amniotic fluid
- Stomach in the foregut is the most rapidly forming structure
What to know:
- Respiratory development:
- 5 structures that contributed to the development of the diaphragm (use the images in his theory questions)
- 1x FNP (white purple and green bits in picture)
- 2x Maxillary prominences (1st and second arches are the most prominent)
- 2x Mandibular prominences
Head development (first movie thing), Good summary:
- Lying between the maxillary arches is the stomedium (indentation on the anterior surface of the head). At its floow is the buccopharyngeal membrane, breaking down in the 4th week of development. Then amniotic fluid can enter the foregut region of the head.
- The lower jaw is formed by the fusion of the two lateral mandibular processes. It is rare to find an abnormality in the fusion of the mandible.
- The smaller maxillary prominences never fuse in the midline (anterior surface of the head). It fuses to the FNP to form the upper jaw. Therefore the epithelium that forms these structure has to transform to mesenchymal. Wk 5.
- On going growth of underlying NT (brain is expanding - increasing size of the cranial area). No skull yet, just a bit in ectoderm (?). Develpment of bony plates occurs much later.
- Nasal epithelium began on the surface as placodal patches on the side of the head. As the FNP grows this area becomes internalised. (Not like otic placode, which get pinched off the surface).
- Growth of the eyes laterally; located on the lateral surfaces of the developing head. Growth of the head brings it around anteriorly.
- Substantial growth of the cranial vault.
- Eyes are held anteriorly via ligamentous attachment.
- Upper and lower jaws are fused, however inside the foetus the nasal and oral cavity have not been separated yet. (Palate not developed yet) (Neo-natal face)
- Post-natal process = ossification of the suture regions
- 8 weeks of embryonic development
Trilamina Embryo: Wk 3 Gastrolation key features
- Presence of Buccopharyngeral membrane (BPM) at the top of the embryonic disk = marker for folding events in development of the head. The top end folds over, bringing the BPM to the anterior area of the face.
- As it continues to fold, formation of GI tract establishes the pharynx.
- Folding brings endoderm on the ventral side, folding over, forming a hollow tube on endoderm = fore-gut. Ectoderm on the surface.
- Fore-gut runs from the BPM down through the embryo
- Brain & hearth on ventral surface; pharyngeal arches (PA) on lateral surfaces
- 1st PA is split into superior and inferior portions.
- Endoderm on inside, mesoderm in core of arch, ectoderm on surface
- Contributions from neural crest (NC)
- Ectodermal component key to head development = sensory placodes
- Stage 11, day 25, early in development - development of placodes
- Dorsal side of the embryo, are circular patches of surface ectoderm called 'placodes' - Otic placodes.
- Otic = hearing, optic = vision. Don't mix them up!!
- Forms columnar epithelium (surrounding material is cuboidal)
- Placode folds inwards as it proliferates
- Stage 12, day 26:
- Otic placode is very indented; epithelium is still intact. Still surface ectoderm lining.
- Presence 1st and 2nd PA
- Otic placode is at the level of the 2nd PA
- On the ventral surface is a large region, the nasal placode. It later wraps around the FNP.
- Can see Pharyngeal groove/cleft #1 - lies beneath the 1st PA
- Surface ectoderm has folded in; the endoderm is coming out to meet it, forming a thin membranous region. Difference from BPM is that it never breaks down.
- Mandibular processes are fusing in the center
- Stage 13
- Surface ectoderm has thickened in these areas due to transformation of epithelium.
- Loss of surface Otic placode; pinched off and lost its connection with the surface. Now called an otic vesicle or oto cyst.