DEVELOPMENT OF BRAIN UPTO

72-HOURS OF INCUBATION IN CHICK EMBRYO

Dr. N.G.Kotadiya

There are complex changes observed information of nervous system and otherorgan systems during the developmentof 72 hours chick embryo. After the 48hours of incubation the process becomefaster. During this changes occurs ininternal structure of embryo henceexternal or morphological changes arealso observed.

About 18 hours of incubation theectodermal cell differentiate intoneural plate, Neural groove andNeural fold.About 27 hours of incubation Neuraltube and Neuropore develop. At 33hours of incubation neuroporeclosed.

Development of brain in 33-hours chick embryo

The neural tube is completed in theanterior half of the embryo. It opensanteriorly by the anterior neuropore.Sinus rhomboidalis and primitivestreak still remain. The threedivisions of the brainprosencephalon, mesen­cephalonand rhombencephalon are marked.The prosencephalon is produced intotwo large optic vesicles.

48­hour old Chick Embryo

The forty eight hour old chick embryo is obtainedafter 48 hours (two days) of incubation. It has thefollowing salient features:

Flexure: The bending of the embryo to the right sideis called flexure. The bendings turn the anterior endto the right. Two flexures are noticeable in the 48-hour old chick embryo. One flexure is seen at thelevel of mid­brain and this flexure turns the fore-brain backwards. This flexure is called cranialflexure. The second flexure appears at the level ofhind-brain and it is called cervical flexure. Thecervical flexure turns the head towards the right.

Torsion: This process of the twisting of the entireembryo along the antero-posterior axis is calledtorsion. In 48 hours it has progressed posteriorly asfar as the posterior end of the cervical flexure i.e.,approximately to the level of the 13th somites.

Eye: The prosencephalon develops twooutgrowths called optic vesicles. The opticvesicle invaginates to form a double--walledcup called optic cup. The two walls of thecup develop into the retina and the openingof the cup develops into the pupil. The opticcup is attached to the brain by a stalkcalled optic stalk.Lens: The lens develops from the ectodermlying opposite to the optic cup. Theectoderm in this region thickens to formlens placode. The placode invaginates toform a lens vesicle.

72­hour old Chick Embryo

Toraion: Torsion is the twisting of the embryo in such away that the left side of the embryo comes to lie on theyolk. This twisting starts from the anterior end. In the 72hour old chick embryo it reaches upto the level ofomphalomesenteric arteries. As a result of torsion only thetail region lies flat with its ventral side on the yolk. Theremaining portion of the embryo rests on its left side.Flexures: A 72-hour old chick embryo has four flexures.They are as follows:1. Cranial flexure: It appears at the level of mid-brain.

(38-hours of incubation)2. Cervical flexure: It appears at the level of hind-brain.

(48-hours of incubation)3. Dorsal flexure: It appears in the region of 10th to 12th

somites, (60-hours of incubation). It continues withcervical flexure anteriorly. The cervical and dorsalflexures give a semicircular shape to the embryo.

4. Caudal flexure: It develops in the tail region. (72-hoursof incubation).

Brain: The brain has three main divisions,namely the prosencephalon, themesencephalon and the rhombencephalon.Further, the prosencephalon is divided into ananterior telence­phalon and a posteriordiencephalon. Similarly, therhomben-cephalon is divided into an anteriormetencephalon and a post-eriormyelencephalon. The brain develops a pair ofoptic vesicles, optic stalks, an optic recess, opticchiasma, infundi-bulum, epiphysis and all thefour ventricles. In addition, the brain developstelencephalic vesicles, corpora quadrigeminaand crura cerebri.

Spinal cord: The germinal cells ofspinal cord multiply and theirdescendants migrate outward. Intheir new position, they aretransformed into neuroblasts andprimordial glia cells. The neuroblastsdevelop axons and dendrites. Theregion of the spinal cord containingthe central part of neuroblasts andglia cells constitutes the grey matterwhile the region of the spinal cordcontaining the axons constitutes thewhite matter.