2. HEADACHE DUE TO CAUSES INSIDE THE BRAIN-
a. Brain tumors
b. Bleeding in the space around the brain
c. High blood pressure.
These kinds of headaches will be discussed in the forthcoming posts hopefully.

1) ENDODERM

2) ECTODERM

3) MESODERM

These are three germ layers. All tissues of the body are derived from one or more of these layers.

a) Some cells of the inner cell mass diffrentiate into flattened cells that to line its free surface. These constitute the ENDODERM- the FIRST GERM LAYER to be formed.

b) The remaining cells of inner cell mass become columnar cells from the SECOND GERM LAYER- ECTODERM. The embryo is in the form of a disc with 2 layers.

c) A space appears between the ectoderm and the trophoblast, this is the “AMNIOTIC CAVITY” which is filled by amniotic fluid or “LIQUOR AMNEU”. The roof of this cavity is formed by amniogenic cells derived from trophoblast and the floor from ectoderm.

d) “PRIMARY YOLK SAC” a cavity is developed from the flattened cells arising from the endoderm or from trophoblast which spread and line the inside of the blastocystic cavity by the cells of endodermal origin.

Soon after the formation of the prochardal plate some of the ectodermal cells lying along the central axis near the tail end of the disc begin to proliferate and form an elevation that bulges into the amniotic cavity. This elevation is called primitive streak. The primitive streak is at first a rounded or oval swelling, but with elongation of embryonic disc it becomes a linear structure lying in the central axis of the disc.

The intraenbryonic mesoderm spreads through out the disc except in the region of prochardal plate. The mesoderm extends on cranial to the prochordal plate, and here mesoderm from the two sides becomes continous across the midline. In the region of the prochordalplate the ectoderm and endoderm remain in contact. In later development the ectoderm and endoderm mostly persists as a living epithelium. On the other hand the bulk of the tissues of the body is formed predominantly from mesoderm. As there is no mesoderm in the prochardal plate, this region remains relatively thin and later forms ” BUCCOPHARNGEAL MEMBRANE”.

The prmitive streak gradually elongates along the central axis of the embryonic disc. The disc also elongates and becomes pear shaped.

Hence this layer of trophoblast is called the syncitio trophoblast or “PLASMODIO TROPHOBLAST”. Deep to the syncitium the cells of the trophoblast retain their cell walls and form second layer called the “CYTOTROPHOBLAST” also called Langhan’s Layer.

2) The syncitiotrophoblast grows rapidly and become thick small cavities called “lacunae” appear in this layer, gradually the lacunae increase in size. At first they are irregularly arranged but gradually they come to lie radially around the blastocyst. The lacunae are seperated from one another by partitions of syncitium which are called “TRABECULAE”. The lacunae gradually communicate with each other so thet eventually one large space is formed. Each trabeculus is now surrounded all round by this lacunar space.

3) The syncitiotrophoblast grows into the endometrium. As the endometrium is eroded some of its blood vessels are opened up and blood from them fills the lacunar space.

4) Each trabeculae is initially made up entirely of the syncitiotrophoblast. Now the cells of the cytrophoblast begin to multiply and grow into each trabeculus. The trabeculus thus comes to have a central core of cytotrophoblast covered by an outer layer of syncitium. It is surrounded by the maternal blood filling the lacunar space. The trabeculus is now called “PRIMARY VILLUS” and the lacunar space is now called “INTERVILLOUS SPACE”.

5) The extraembryonic mesoderm lining the inner side of the trophoblast, now invades the centre of each primary villus. The villus thus comes to have a core mesoderm. Outside this there is a layer of cytrophoblast which is inturn surrounded by syncitium. This structure is called “SECONDARY VILLUS”.

6) Blood vessels are seen in the mesoderm forming the core of each villus, with their appearance the villus is fully formed and is called “TERTIARY VILLUS”. The blood vessels of the villus establish connections with the circulatory system of the embryo, fetal blood now circulates through the villi, while maternal blood curculates through intervillous space.

7) The cytotrophoblast, that grows into the trabeculus does not penetrate the entire thckness of syncitium and therefore does not come in contact with the decidua. At a later stage however the cytotrophoblast emerges through the syncitium,of each villus the cells of trophoblast now spread out to form a layer that completely cuts off the syncitium from the decidua. This layer of cells is called as “CYTOTROPHOBLASTIC SHELL”. The cells of this shell multiply rapidly and placenta increases in size.

The villi that are first formed are attached on the fetal side to the embryonic mesoderm and on thematernal side to the cytotrophoblastic shell. They are therefore called “ANCHORING VILLI”. Each anchoring villi consists of a stem (truncus chorii) this divides into a number of branches (rami chorii) which inturn divides into finer branches (ramuli chorii). The ramuli are attached to the cytotrophoblastic shell, the anchoring villi gives off numerous branches which grow into intervillous space as free villi. New villi also spread from the chorionic side of intervillous space. Ultimately almost the whole intervillous space becomes filled with villi. As a result the surface area available for the exchange between maternal and fetal circulations becomes enormous.

These newly formed villi at fist consists only of the syncitiotrophoblast, they are subsequently invaded by the cytotrophoblast, mesoderm and the blood vessels and pass through the stages of primary, secondarty and tertiary villi.

b) Small cavities appear in extraembryonic mesoderm, gradually these join together to form larger spaces and ultimately one larger space is formed. This cavity is called “EXTRA-EMBRYONIC COELOM”. With its formation the extramembryonic mesoderm splits into two layers, the part lining the inside of the trophoblast and the outside of the amniotic cavity is called ” PARIETAL or SOMATOPLEUORIC EXTRAEMBRYONIC MESODERM “. It will be seen that the extraembryonic coelom does not extend into that part of the extraembryonic mesoderm which attaches the wall of the amniotic cavity to the trophoblast. The developing embryo along with the amniotic cavity and the yolk sac is now suspended in the extraembryonic coelom and is attached to the wall of the blastocyst only by this unsplit part of the extramembryonic mesoderm. This mesoderm forms a structure called ” CONNECTING STALK”.

a) WHARTON’S JEELY- It is a thick extraembryonic mesoderm.

b) RIGHT and LEFT UMBLICAL ARTERIES- these carries the deoxygenated blood from the foetus to the placenta. After birth the two umbilical arteris are converted into MEDIAL UMBILICAL LIGAMENT.

c) LEFT UMBILICAL VEIN- It carries the oxygenated blood from the foetus to the placenta. After birth the Umbilical vein is converted into LIGAMENTUM TERES of the liver.

d) Remanents of vitelli intestinal duct.

e) Remanents of Allantois diverticulum

Dorsal pancreatic bud

VENTRAL PANCREATIC BUD- It develops on the ventral side caudal to the hepatic bud and communicates with it.

DORSAL PANCREATIC BUD- It develops on the dorsal side and grows dorsally into the dorsal mesentry.

After the rotation of the duodenal loop, the ventral and dorsal buds come in contact with one another and lies within the concavity of duodenum.

The ventral bud gives rise to

a) Lower end of the head.

b) Uncinate process of the pancreas.

The dorsal bud gives rise to

a) Upper part of the head, whole of the body and tail of the pancreas. Secretory elements of pancreas are formed by the proliferation of primitive duct. Islets of Langerhans are also derived from the primitive duct system.

DUCT SYSTEM- The main pancreatic duct of wirsing is formed by the wholes of- Duct of ventral bud.

Distal part of the duct of dorsal bud along with bile duct forms “Hepatic Pancreatic Ampulla” which opens into major duodenal papilla at the junction of foregut and midgut.

Accessory pancreatic duct of Santotini- It is formed by the proximal part of duct of dorsal bud , usually disappears. If present it opens onto the minor duodenal papilla which is present 2cm above the major duodenal papilla.

HEPATIC BUD- Divides into right and left branches which divides and redivides and gives rise to cords of liver cells. Later these cords of liver cells are invaded by liver sinusoids which are formed by the breaking of the umblical and vitelline veins and few sinusoids are developed inside. One liver sinusoid which grows bigger in size and connects left umblical vein with the Inferior venacava is called “Ductus Venosus”. Because of this channel blood can pass freely from the left umblical vein in the Inferior venacava. Because of this reason the right umblical vein disappears.

CYSTIC BUD- It gives rise to gall bladder, the cystic duct. The mesenchymal cells give rise to “Von-Kupffer’s Cells”

The lower most part of the hepatic bud forms the bile duct which opens directly on the ventral side but later after the rotation of duodenal loop it comes to open along with pancreatic duct on the postero-medial aspect of duodenum.

The mesoderm of septum transversum gives rise to

a) Glisson’s capsule

b) Falciform ligament

c) Triangular ligament

d) Coronary ligament.