Topic:“Development of Birds”

Ever since Aristotle first followed its 3-week development, the domestic chicken has been a favorite organism for embryological studies. It is accessible all year and is easily

raised. Moreover, at any particular temperature, its developmental stage can be accurately predicted. Thus, large numbers of embryos can be obtained at the same stage. The chick embryo can be surgically manipulated and, since it forms most of its organs in ways very

similarly to those of mammals, it has often served as a surrogate for human embryos.

Egg of Bird

All Birds have internal fertilization and are oviparous.

Sex Organs:

Male: Homogametic ZZ

Testis and penis/Cloacal Protuberance CP

Female: Heterogametic WZ

Ovary (ovaries)

Copulation:

Internal fertilization:

Ova (ovum) released via LH

Yolk=1/3 Lipid, ¼ Protein, Water + Associated compounds (Androgens, carotenoids)= Maternal Effects.

Chick:A chick is a domesticated fowl.

Kingdom: Animalia.

Phylum: Chordata.

Class: Aves.

Order: Galliformes.

Family: Phasianidae.

Genus: Gallus

Specie: G.gallusA chick emerges from a fertile egg after twenty-one (21) days of incubation.

The development of the chick begins in the single cell formed by the union of two parental cells, egg and sperm, in the process known as fertilization. In birds, fertilization occurs about 24 hours before the egg is laid. The newly formed single cell begins to divide into 2, then 4, 8, 16, 32 and so on. At the time of laying, hundreds of cells are grouped in a small, whitish spot (the blastoderm or germinal disc) that is easily seen on the surface of the yolk.

The Germinal Disk is visible on the surface of the Yolk.

When the egg is laid and cools, division of the cells ceases. After the egg is laid, cooling the egg does not result in the death of the embryo. It may resume its development after several days of rest if it is again heated by the hen or in an incubator.

Detail:

Structure of Egg:Shape: It is oval in shape.

Size: 3cm wide and 5cm long.

TYPE: The egg of chick is telolecithal, sub type of polylecithal.

Shell: It is made up of 95% calcium carbonate, white and porous and exchange of gases occur, soft and flexible in a freshly laid egg but soon become hard and brittle.

RELEASE: The egg is released from the ovary, it takes 24 hours to pass down the oviduct, before being laid.

FERTILIZATION: The fertilization is internal, upper part of the oviduct, resulting two polar bodies degenerate and disappear.

MEMBRANES:

Two types of membrane, shell membrane and vitelline membrane, separated at the broad end of the egg to enclose an air space.

Yolk Membrane:

The layer of yellow yolk is thicker than those of the white yolks. Yolk consist of phospholipids, lecithin. Solid yolk shows 60% fats and 70 % proteins. White yolk contains less fat and also less of fat soluble carotene to which the yellow color is formed. Yolk contains about 50% water.

Albumen: It has two types, Thick albumen and thin albumen.

Chalaza: Two ends of the egg are some fibrous, rope like twisted structures formed during journey of the oviduct.

Latebra: The central flask shaped area that is enclosed of white yolk is called latebra.

Germinal Disk/Blastodisk: Nucleus of the egg is surrounded by the negligible amount of yolk free cytoplasm called “Germinal Disk”.

“Development of Chick”

Cleavage or segmentation :

It starts with the cleavage. It is meroblastic means confined to small area of blastodisc. It takes 12-16 hours.

First cleavage forms two cells. Second cleavage occurs after 20 minutes of first cleavage and four cells are formed. This cleavage is irregular. It occurs in 4 hours and the end of cleavage 256 irregular cells are formed.

Blastulation: After cleavage, Blastulation occurs. Due to Blastulation, following structures are formed; Sub-germinal cavity: It is similar to blastocoel. Area Pellucida: It forms body properly. Area Opaca: It helps in the formation of extra embryonic membranes such as Yolk sac.

Gastrulation: Conversion of monoblastic egg into triploblastic gastrula is known as “Gastrulation”.

It involves the formation of Ectoderm, endoderm and mesoderm.

Primitive Streak Formation:

It is formed from the posterior marginal region cells. The streak first becomes visible as the epiblast (ectoderm) thickens at the posterior end.

Primitive Streak Formation 7-8 Hr after Fertilization: The epiblast is separated from the hypoblast by the blastocoel. The epiblast will give rise to Three Germ layers of the embryo while the hypoblast will give rise to extra embryonic structures.

o Primitive Streak Extension 15-16 Hr after Fertilization:

The primitive streak extends by convergent extension from the posterior to anterior pole. A depression forms along the primitive streak called as “Primitive Groove”.

o Formation of Henson’s node 19-22 Hr after Fertilization:

At this stage, ingression through the primitive streak of cells destined to become endoderm and mesoderm has begun.

o Henson’s Node:

It is formed by the cells of presumptive notochord and floor of neural tube.

o Formation of neural tube (Neurogenesis):

A narrow strip of blastoderm just in front of the primitive knot consists of presumptive notochordal cells responsible for the formation of Notochord.

o Meroblastic Somites:

On either side of notochord lies thick, solid, dorsal somatic cells. Somites are the blocks of mesoderm. They give rise to number of structures including skeletal muscles, bone and dermis of skin.

o Origin of coelom:

The ventral or lateral plate mesoderm splits into two layers. The outer somatic or parietal layer

lies next to ectoderm with which it forms the somatopleure . The inner splanchnic or

visceral layer in contact with endoderm forms the splanchnoplure. Space or cavity formed

between these two mesodermal layers is coelom or splanchnocoel.

ORGANOGENY:

“Formation of organs is called Organogeny”.

o Extra embryonic membranes/Foetel membrane yolk sac:

The chief embryonic food is yolk which is surrounded by the sac like investing membrane called yolk sac. Yolk sac is made up of splanchopluere.

Function: The function of yolk sac is to protect the yolk keep it in position, digestion and absorb it. Yolk sac serves as a primary organ of nutrition of embryo.

AMNION: It is made up of inner ectoderm, the space between amnion and embryo is filled with amniotic fluid.

Function: It protects the Embryo from mechanical jerks and prevent from desiccation.

Chorion: It is made up of an outer ectoderm, the cavity between amnion and chorion is called Chorionic cavity.

Function: It provides liquid medium for embryo.

Allantois: The bladder like structure is called allantois. It is made up of splanchnopleure endoderm inside and splanchnic mesoderm outside.

Allanto-Chorion: The mesodermal layers joined the allantois with chorion called Allanto-chorion, a compound layer is formed.

Function: It serves as respiratory organ, excretory organ and nutritional organ.

“33 Hours chick embryo” In the duration of 33 Hours, Heart is formed on the posterior of brain and two vitelline veins are entering in the heart from blood island that is supplying blood to heart. Optical vesicles are formed that indicates the eye formation starts. Two optical vesicles emerge out of brain and heart could be observed near the posterior of hind brain.

“72 Hours chick embryo” After 72 hours great increase in size of brain and head region and limb buds begin to form. Olfactory pits appear and sac like allantois appear which functions as the “embryonic lung” proving mechanism of exchanging respiratory gases. Somites also increase in number.

“96 Hours chick embryo”

Wing buds appears anteriorly, while leg buds appear posteriorly.Cerebral hemispheres are formed. Heart forms the heart chamber e.g. atrium and ventricles.

“Hatching” On the twentieth day of incubation the embryo becomes a chick as it breaks into the air cell and starts breathing air for the first time.Several changes take place during the 18th and 21st days. The chick draws what remains of the yolk into its body and "takes its lunch with it" (so-to-speak) when it hatches. Thus, the chick really doesn't need to be fed for the few days after it hatches. The chick's head is under its right wing with the tip of the beak pointed at the air shell. The large neck muscle contracts and forces the egg tooth through the air cell, and the chick takes it first breath. This is referred to as internal pipping. At this time, we may hear the chick peeping inside the shell.On the twenty-first day of incubation the chick finishes its escape from the shell. The egg tooth makes the initial break in the shell. This is referred to as external pipping. The hatching process can last from 4 to 12 hours before the chick breaks free from the shell. The chick, as it appears upon freeing itself from the shell, is wet and very tired. For the next several hours it will lie still and rest. A few hours later the chick, now dry and fluffy, will become extremely active and the egg tooth will dry and fall off.

References: http://www.sites.ext.vt.edu/virtualfarm/poultry/poultry_development.html https://en.wikipedia.org/wiki/Chicken https://www.google.com.pk/search?q=chick+embryo+after+40-

45+hours&rlz=1C1RUCY_enPK692PK692&espv=2&biw=1366&bih=599&source=lnms&tbm=isch&sa=X&ved=0ahUKEwieiqzX7KfNAhXH2BoKHWA4DQgQ_AUIBigB#imgrc=V4cQJPudeV3EYM%3A https://www.google.com.pk/search? q=chick+on+21st+day&rlz=1C1RUCY_enPK692PK692&espv=2&biw=1366&bih=599&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi8qJz68qfNAhWCuhoKHZPzCEsQ_AUIBygC#tbm=isch&q=hatching+of+chick&imgrc=8LLabqWzNia4SM%3A

Gilbert,S.F.,2008.Developmental Biology,Sinauer Associates,Sunderland, MA. http://www.enchantedlearning.com/subjects/birds/info/chicken/egg.shtml http://www.thepoultrysite.com/articles/1459/embryonic-development-day-by-day/