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Summary - 5BBA2300 Essentials of Embryology
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EMBRYOLOGY NOTES- A 40 MCQs (best answer) 35%
- B One essay selected from a choice of 5 questions 35%
Cleavage to gastrulation
Cleavage:
- Cleavage divides the zygote up into numerous small cells, embryo does not grow in size
- Cleavage - initial cell divisions – immediately follows fertilisation. Series of rapid cell
divisions, without growth, that divides the embryo up into numerous small cells
- Cleavage forms a blastocyst
Holoblastic cleavage
- division goes all the way through the cell, complete cleavage
- e.g. mammals, flatworms, nematodes, amphibians (frogs – xenopus)(mice)
- absence of a large concentration of yolk, holoblastic cleavage is observed
- cleavage planes pass all the way through the zygotes during cytokinesis
- first cleavage always occurs along the vegetal-animal axis of the egg
- the second cleavage is perpendicular to the first
- spatial arrangement of blastomeres then follow various patterns, due to different planes of
cleavage, in various organism
- holoblastic cleavage in humans where cleavage starts when the fertilised egg passes from
the ovary into the oviduct, the resulting blastocyst enters the uterus
o 16 cell – morula
o 32 cell stage embryo blastocyst – will implant in uterus
- Number of cells increases but embryo itself does not increase in size!
, - Early cell divisions (cleavage) all occur within the oviduct, by the time embryo gets to uterus
is already a blastocyst ready to embed
Meroblastic cleavage
- Cell division cant go all the way through the cell, incomplete cleavage
- e.g. fish, reptiles, birds, insects
Superficial cleavage
Fruit fly (drosophila) as “superficial” meroblastic cleavage starting by formation of a syncytium
o get a cell that has many different nuclei within the cell but no cell membranes
separating nuclei – as cell is very yolky so nuclei migrate to periphery
o Superficial cleavage is a type of cleavage where the division of the egg is incomplete,
meaning that the nuclei divide, but the cytoplasm does not. As a result, the embryo
is made up of a syncytium, a mass of nuclei without cell membranes. This type of
cleavage is common in insects, such as fruit flies
o Only mitosis occurs, not cytokinesis – Forms Polynuclear Cell (many nuclei but no cell
membrane separating he nuclei).
o Syncytium – egg with multiple nuclei formed
Discoidal cleavage
o Seen in organisms with large yolk-rich eggs, such as birds and reptiles. In discoidal
cleavage, the cell division only occurs in the animal pole of the egg, which is the area
without yolk. The yolk-rich area of the egg remains undivided, creating a disc-shaped
blastodisc.
o Cleavage furrow doesn’t penetrate yolk
o This type of cleavage produces a flattened, disc-shaped embryo. The embryo forms a
disc of cells, called a blastodisc, on top of the yolk.
,Meroblastic (discoidal) cleavage of chick:
Summary:
- the fertilised egg (zygote) undergoes multiple rounds of division (cleavage)
- Blastomeres are individual cells that are the result of cell division – their special arrangement
can vary depending on the plane of cleavage and organism
- 2 types of cleavage: holoblastic (animals with little yolk) meroblastic (lot of yolk)
- In mammals at the end of cleavage result in a blastocyst which is a group of cells with a
hollowed out middle, inner cell mass which is what the embryo proper will develop from
- this produces a ball of cells, the blastula, with a fluid filled cavity in the centre, the blastocoel
- in mammals, blastula organises into 2 layers:
o inner cell mass (will become the embryo proper) and
o trophoblast that forms non embryonic tissue (placenta) – the blastocyst
- gastrulation – blastula folds in on itself to form 3 germ layers
Chick, human (mammals) - blastocyst
Fish, frog – blastula
Fish – blastoderm – stage of development after blastula - It is a disc-shaped group of cells that forms
on top of the yolk and gives rise to the embryo.
(all the same stage of development, juts different names for different species)
Derivatives of the germ layers:
1. Ectoderm:
- neural tube, neural crest
- Skin epidermis, hair, nails
- Nervous system including the brain and spinal cord
- Sensory organs like the eyes, ears, and nose
- Adrenal medulla
, - Neural crest cells which give rise to various structures including teeth, facial bones, and
pigment cells
2. Mesoderm:
- Somites, notochord, lateral plates
- Muscles (skeletal, smooth, and cardiac)
- Bones and cartilage
- Blood vessels
- Blood cells
- Kidneys and ureters
- Gonads
- Mesothelium lining the body cavity and organs
- Dermis layer of the skin
3. Endoderm:
- Intestinal tube
- Epithelial lining of the digestive system (including liver and pancreas)
- Respiratory system including the lungs
- Thyroid gland and parathyroid glands
- Bladder and urethra
- Notochord is made from dorsal mesoderm - The organiser will eventually become the
notochord that sits under the neural tube
- Germ layers formed during gastrulation
o Timing of germ layers different between species, in some species there is cell
specification before gastrulation begins such as in frog and fish
o Ectoderm – neural tissue, epidermis (skin)
o Mesoderm – somites, notochord, intermediate mesoderm (kidneys) and lateral plate
(body wall)
o Endoderm – respiratory tract, digestive tract
o Very important that the germ layers get into the right position within the embryo
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