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Embryology Revision
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Definitions:
Embryology: The study of the development of a fertilised egg into the complex
and independent system of organs that constitutes the adult animal
...
Amount of yolk: directly links to duration of development required by the
embryo
...
Microlecithal eggs: small amount of yolk
...
Macrolecithal eggs: large amount of yolk
...
Blastocyst: A fluid-filled cavity develops inside the morula and the structure
...
Gastrulation: the development of the gastrula and the formation of the germ
layers in the embryo
...
Inner cell mass: eventually becomes the embryo
Trophoblast: lines the outside of the blastocyst and becomes the
extraembryonic membranes
...
Distribution of Yolk:
Isolecithal – even distribution of yolk in the cytoplasm of eggs found in mammals and
other vertebrates
...
Telolecithal yolk:
The yolk is concentrated at one pole of the egg, separate from the developing
embryo
...
Area with more yolk is called the vegetal pole
...
Structure of sperm:
The outer acrosome of the sperm releases enzymes which break down the
zona pellucida of the ovum enabling one sperm to penetrate the ovum
...
Stages of embryology:
1
...
2
...
Cleavage
4
...
Organogenesis
...
Growth
Gamete fertilisation:
Cells in the reproductive organs (testes and ovaries in humans) divide to form
gametes
...
Male gametes are sperm (produced in the testes)
Female gametes are eggs (produced in the ovaries)
Gametes contain one set of genetic information, while body cells contain two
sets of genetic information
...
Fertilisation:
Fertilisation is the joining or fusion of a male gamete and a female gamete
...
The new cell then divides over and over again by mitosis
...
Cleavage:
Holoblastic cleavage: occurs in lightly yolked isolecithal eggs, and moderately yolked
mesolecithal eggs
...
In the absence of a large concentration
of yolk, four major cleavage types can be observed: radial, spiral, bilateral and
rotational holoblastic cleavage
...
Occurs in echinoderm, tunicates, cephalochordates,
marsupials and placental mammals
...
Occurs in birds
reptiles and most fishes, a few amphibians, and monotreme mammals
...
Meroblastic cleavage is incomplete as
cleavage furrows cannot cut through the heavy yolk
...
(Note discoidal is in vertebrates, superficial is in arthropods)
...
Gastrula is the simplest type consists of two layers of cells, the ectoderm and
entoderm, which have invaginated to form the gut
...
This becomes part of
the skeletal system
as well as the internal
organs
(Sadler,
2012)
...
The endoderm is created and formed by gastrulating cells
...
The hypoblast cells that have been pushed out of place then go on to
go inside of the primary yolk sac
...
The mesoderm germ layer is caused by two existing layers being
migrated through by gastrulating cells (Sweeney, 1998)
...
Growth:
Inner cell mass develops and curves around to form the head and trunk
...
The coelom divides into the body cavities
...
This period is known as organogenesis (development of the internal organs)
...
Germ Cell Layers – Inner Cell Mass:
The inner cell mass begins to form three germ layers, which eventually
differentiate into parts of the embryo and its membranes
...
The mesoderm (middle layer) – forms the muscoskeletal system and other
internal organs
...
Development of the Extra-embryonic Membranes:
As the inner cell mass grows, it curls around and encloses the endodermal
and mesodermal cells, which form the inner organs, leaving the yolk sac and
trophoblast to form the extra embryonic membranes
...
Cells from the endoderm form the yolk sac
...
Mesodermal cells between the yolk sac and the trophoblast split into two
layers, forming a cavity
...
The extra-embryonic membranes begin to develop as follows:
This forms a fluid filled cavity around the embryo, known as the amniotic
cavity
...
The amniotic cavity is separate from all the other cavities
...
Pushes out from the caudal end of the embryo and lies beside the yolk sac
...
Eventually the allantois & its contents surround the foetus
...
Outer surface fuses with chorion – chorioallantois
...
Implantation:
Blastocysts arrange themselves equally down the walls of the uterine horns
...
Placenta:
Develops from the allantochorion around the centre of the conceptus
...
Blood capillaries infiltrate and come into close contact of maternal capillaries
...
Placental types are species specific
...