Animal development Flashcards
Fertilisation
Internal or external
Activates egg - meiotic division -reinitiates transcription and translation - prevent polyspermy
Establishes pattern - point of fertilisation - defines plane of first cell division in amphibians - nematodes - anterior pole
Process of fertilisation
Sperm makes contact with egg and discharges contents of acrosome by exocytosis
Hydrolytic enzymes released to penetrate eggs jelly coat
Plasma membrane of egg and sperm fuse - sperm contents enter cytoplasm
Polyspermy has to be prevented - fast and slow block
Fast block to polyspermy
A neuron-like electrical response (depolarisation) of the egg plasma membrane
This prevents a second sperm from fusing with the membrane - 2 second timescale
Slow block to polyspermy
Cortical reaction
After membrane fusion - cortical granules fuse with the egg’s plasma membrane - releasing their contents
Enzymes separate the plasma and vitelline membranes
Mucopolysaccarides cause water (by osmosis) to enter the peri-vitelline space - physical separation of two membranes - contents harden
Vitelline membrane known as the fertilisation membrane
20 seconds
Cleavage
Rapid mitotic divisions
1-2-4-8-16
divide fertilised egg into nucleated blastomeres
No increase in embryo size
32 cell stage called a morula
Hollow ball of blastomeres - blasatula - cavity - blastocoel
Controlled by maternal proteins and RNAs
Gastrulation
Cell and tissue movements rearranging the blastula
Establishes multilayered body plan
Cells that will form endodermal and mesodermal tissue move inside the embryo - ectoderm remains external
Varies between species but has a common core process
Gastrula
two cell layer thick hollow embryo
Created by invagination of cells with endoderm fate
Blastopore
Point of invagination in gastrulation
Protostomes - forms mouth first - annelids, molluscs, arthropods
Deuterostomes - forms anus first - echinoderms, chordates
Blastocoel
Cavity of blastula
Archenteron
Interior space of gastrula
Endoderm
Forms gut and associated organs
Ectoderm
Forms epidermis and central nervous system
Mesoderm
Forms muscle
Forms from pouches of cells that pinch off from the endoderm
Diploblastic
2 tissue layers - ecto and endoderm
Triploblastic
3 tissue layers - ecto, meso and endoderm
Chicken gastrulation
occurs on small plate of tissue on a huge yoke
Only cells on top of the yolk cleave
The morula is a disk of cells on the yolk - blastodisc
Gastrulation occurs by separation of upper ectoderm from lower endoderm and not invagination
Mesoderm tissue forms by invagination of cells in line of ectoderm
Mammalian gastrulation
Blastocyst is rougly 100 cell hollow ball containing inner cell mass
Inner cell mass surrounded by trophoblast
endoderm forms from lower side of inner cell mass
Ectoderm forms from upper cell mass
Mesoderm froms by invagination of upper ectoderm
Protostomes
Mouth forms first from point of invagination
Spiral cleavage - diagonal to vertical axis
Determinate cleavage - fate set early
Deuterostomes
Anus forms first from point of invagination
Radial cleavage - parallel to vertical axis
Indeterminate cleavage - fate set late
Neurulation
Notochord:
Feature of all chordates
Longitudinal flexible rod between gut and nerve cord
Longitudinal skeletal support
Not the vertebral column
Nueral tube:
Nervous system
Forms from plate of dorsal ectoderm:
Neural plate
Neural groove
Neural crest
Neural crest
Anteriorly forms the brain and parts of sensory organs
Posteriorly it will form other structures - including ganglia
Notocord
mesodermal origin
Embryo maintenance in birds
Aqueous environment - egg
Extra embryonic membranes are formed from the blastodisc:
The yolk sac
The amnion
The chorion
The allantois
As embryo develops - pinches itself off from these membranes - remains attached to the yolk at mid body position
Embryo maintenance in mammals
Amnion - space above ectoderm
Chorion develops from trophoblast
Allantois - out pocket of embryo gut - later incorporated into umbilical cord
