Organogenesis / Embryonic Period / 3 rd to 8 th weeks
Formation & development of body organs from embryonic tissue is
Each of the 3 germ layers gives rise to a number of specific tissues &
Formation of the Neural tube
The neuro-ectoderm (neural plate) is derived from the ectoderm
overlying the notochord and is induced by it during the third week.
Then neural plate begins to fold. It is 1st converted into neural groove .
The neural groove deepens and eventually forms a neural tube .
Two masses of ectoderm at edges of neural plate, form neural crest .
Initially the neural crest separates neuro-ectoderm from skin ectoderm.
As folding of the neural tube occurs, the neural crest cells detach from
the ectoderm and form clusters that migrate into the mesoderm.
There are three derivatives of the ectoderm in this region:
Skin ectoderm – gives rise to the epidermis of the skin
Neural crest ectoderm – are cells that migrate widely and give
rise to a large variety of structures, to be listed later.
Neural ectoderm – gives rise to the central nervous system,
including the neurones and neuroglia.
Derivaties of Ectoderm
1. Surface Ectoderm
A. Lining epithelia
Mucous membrane of lips ,cheeks , gums, part of floor of
mouth , part of the palate, nasal cavity & sinuses
Lower part of anal canal ( below pectinate line)
Terminal parts of male urethra
Outer surface of labia minora & whole of labia majora
Anterior epithelium of cornea, epithelium of conjunctiva,
epithelial layers of ciliary body & iris
Outer layer of tympanic membrane, epithelial lining of
membranous labyrinth including the special end organs
Lacrimal canaliculi, sac, nasolacrimal duct
B. Glands :
Sweat glands, sebaceous glands parotid gland, mammary
glands, pars anterior of pituitary gland
C. Other derivatives
1. Neural tube
Retina & Optic nerve & musculature of iris
Pineal & pituitary gland
Neuroglia – Ependymal cells , Macroglia(Astrocytes,
2. Neural crest
Chromaffin cells (adrenal medulla)
Dorsal root ganglia & dorsal root of spinal nerve
Sensory ganglia of V, VII, IX & X cranial nerves
Melanocytes of skin
Bones & connective tissues of cranio-fascial structues
Parts of heart
3. Ectodermal Placode:Ectodermal thickenings which have important
roles in development of special sensory systems.
Otic placode – gives rise to structures needed for hearing &
Lens placode – forms lens
Nasal placode – nasal cavities & para nasal sinus
APUD-Amine-precursor uptake and decarboxylation cells
Mesoderm forms several distinct masses :
Mesoderm in the lateral part of the embryo is divided into three distinct
- a longitudinal column of cells that lies next to the notochord
- it gives rise to the axial skeleton and skeletal muscle
- Intermediate cell mass
- it gives rise to the genitourinary system
Lateral plate mesoderm
- gives rise to body wall structures
- is continuous with the extra-embryonic mesoderm
- splits into two layers enclosing the intra-embryonic coelom
Two important masses of mesoderm
The trilaminar embryo has two important masses of mesoderm:
At the cranial end is the transverse mesoderm, in which are
situated the pericardial cavity and the cardiogenic
At the caudal end is the connecting stalk that contains the
allantoic diverticulum (allantoises) , a small outgrowth from
the roof of the yolk sac and projecting into the connecting stalk. The
allantoic diverticulum will later give rise to the greater part of the
Folding of the Embryo.
Folding occurs by differential growth of tissues. Neural ectoderm
grows faster than the surrounding skin ectoderm and consequently
fold to form a neural tube. Similarly, skin ectoderm grows faster
than the underlying mesoderm and endoderm, and this differential
growth causes folding of the trialminar disc and gives shape to the
Folding occurs mainly at the edges of the embryonic disc and forms
three main folds:
Lateral folds - convert the embryo into a tubular structure.
These are not three separate folds but occur simultaneously and
merge into one another.
The notochord, neural tube and somites stiffen the dorsal axis of the
As a result of the formation of the head fold:
The foregut is formed by folding of the endoderm
The stomodaeum is an invagination of ectoderm, and has the
buccopharyngeal membrane separating it from the foregut It opens
into the amniotic cavity.
The pericardial cavity and cardiogenic mesoderm are shifted to
the ventral aspect of the embryo and lie ventral to the foregut.
The part of the transverse mesoderm between the pericardial cavity
and the yolk sac is the septum transversum proper. In it the liver
The amniotic cavity extends ventral to the cranial end of the
The yolk sac is constricted from the cranial aspect.
As a result of the formation of the tail fold:
the hindgut is formed
The cloaca is an invagination of ectoderm and has the cloacal
membrane separating it from the hindgut.
The connecting stalk is shifted ventrally
The allantoic diverticulum is shifted ventrally. It is an invagination of
hindgut endoderm into the yolk sac.
The amniotic cavity extends ventral to the caudal end of the embryo.
The yolk sac is constricted from the caudal end
Transverse folding of the embryo
Converts the endoderm into a primitive gut tube
The intra-embryonic coelom surrounds the gut tube (Transverse
The communication between the intra- and extra- embryonic
coeloms becomes constricted and eventually obliterated
Note that drastic and important changes occur in the embryonic
cavities as a consequence of folding:
The amniotic cavity surrounds the embryo completely on all
aspects and becomes the predominant cavity. It enlarges
The yolk sac becomes constricted on all sides, and becomes a
small sac connected to the midgut by a narrow vitelline duct. It
becomes progressively smaller.
The extra-embryonic coelom is gradually obliterated by the
expanding amnion and eventually disappears completely
During the fourth week the embryo is segmented. Each segment
consists of a somite innervated by a segmental nerve derived from a
segment of the neural tube.
A somite is divided into two parts:
The sclerotome is the ventro-medial part of the somite. It
contains a “cavity” of loose cells. Cells from the sclerotome migrate
medially to surround the notochord and neural tube and form the
The dermomyotome is the dorso-lateral part of the somite. Cells
from the dermomyotome migrate laterally and, as its name implies,
gives rise to (i) skeletal muscle and (ii) the dermis of the skin. The
concept of the myotome in gross anatomy is an embryological
concept. Each anatomical myotome is derived from the
embryological dermomyotome that is innervated by a segmental
nerve and forms a goroup of skeletal muscle cells and the dermis of
the corresponding segment of ectoderm.
The neural tube induces the formation of the neural
arches and their fusion across the midline.
Def ects of closure of the neural tube will also cause failure of fusion
of the overlying neural arches. This anomaly is termed a
meningomyelocoele . As illustrated in the adjacent diagram.
The vertebral bodies are formed from two adjacent
Note that the segmental spinal nerves emerge at the level of the
corresponding somite, between adjacent vertebrae. The
intervertebral discs correspond to the original somites and
Blood vessels develop throughout the mesoderm
Mesodermal cells differentiate into endothelial cells surrounding a
central group of erythroblasts. These are blood islands that
coalesce to form blood vessels. Almost all parts of the mesoderm
gives rise to blood vessels.
Differentiation of blood vessels
Blood islands and eventually blood vessels appear:
in the extra-embryonic mesoderm in the second week
in the intra-embryonic mesoderm in the third week
the primitve heart tube develops in the cardiogenic mesoderm
(in the transverse mesoderm) at the beginning of the fourth week
and a primitive circulation is established
Three structures develop in the transverse mesoderm:
cardiogenic mesoderm – in which the primitive heart tubes form
pericardial cavity into which the heart tubes invaginate
the septum transversum forms part of the diaphragm, fibrous
pericardium and connective tissue of the liver.
Note that the cardiogenic mesoderm and septum transversum are
situated in the cervical region and so are innervated from cervical