Chick Embryology and Histology - Term Paper Example

CHICK EMBRYOLOGY AND HISTOLOGY   Name Instructor’s Name Course Date Stages in chick embryo development Major Milestones After incubation starts, the pointed thick layer of cells becomes observable on the caudal or the tail end of the embryo. The pointed area is known as the primitive steak. The backbone and the head of the embryo develop from the primitive steak. Blood appears on the embryo and later develops into the blood system. Blood islands starts linking to form the vascular system while the heart is form in a different location. After the 44 hours the vascular system and the heart link thus the chicks heart’s starts beating .Two different circulatory systems are formed; the embryonic system and the vitelline system. After three days the beak appears as well as the limb buds and the chick’s legs. The chick’s whole body rotates at an angle of 90 degrees and lies on the left side of the egg yolk. Parts of the digestive system such as the tongue, mouth and nasal pits develop during the fourth day as the heart continues to enlarge. At the end of the fourth day one can identify most of the embryo’s parts of the body needed to sustain life of the chick after it has been hatched. After seven days, digits appear on the wings and feet of the chick and the heart is totally enclosed in the thoracic cavity. On the tenth day of incubation feathers can be seen as the beak becomes harder. After 14 days the claws of the chick is formed and the embryo moves into position ready for hatching. On the 20th day the beak of the chick pierces the air cell and the pulmonary respiration begins. On the 21st day of incubation, the chick starts coming out from the shell. During hatching the chick pushes its beak through the egg shell as the allantois dries up. How the chick embryo does resemble human embryo? During the early development of the embryo, both the human and chick embryo form blastula, gastrula morula and primary germ layers. Early embryogenesis of a chick and a human being are similar in that they both form the mesoderm (which later becomes the blood system) and ectoderm which later become the digestive organs, bladder and lungs. Both the chick and human embryo have the pharyngeal arches which develop on parts of the head and neck. Differences The differences can be classified morphologically and ontologically. Morphologically the chicken embryo has feather tissue and claws in their feet as compared to the human embryo which has nails on their feet and hands. Also the chick has a beak as compared to human embryo which has a nose. Ontogenically the chick grows within an egg shell while human embryo grows in the mother’s womb. The digestive system of a chick requires the growth of the proventriculus, ventriculus and gizzard while in human embryo none of the above parts are needed. In human beings the neural tube fuses from a central region of the embryo and moves outwards. In chicks the neural folds join together in an anterior to posterior fashion. The egg yolk of the chick provides nutrients to the chick as compared to human fetus which gets the nutrients from the mothers’ body since it is attached to the mother’s body. The chick embryo takes duration of 21 days to completely develop while human fetus takes nine months. The chick develops the three germ layers between 2-3 days while in human embryo they take around two weeks. Formation of the Primitive streak Primitive streak formation is the most important event in chicken embryo development. The primitive streak is formed when cells accumulate in the epiblast in the posterior pole of the chick embryo.After 6-7 hours of incubation the primitive streak appears as a short conical thickening and 0.3-0.5 mm broad. After 12 to 13 hours of incubation, the streak spreads out from the posterior margin to the center of the pellucid area. The streak broadens and flares out to touch the opaque part. After 18-19 hours the primitive its optimal length and primitive pit, primitive groove and the Hensen’s node are formed. The pellucuda section acquires a pear shape. Formation of the Notochord/head process After 19-22 hours, the notochord becomes visible as a condensed mesoderm rod which elongates from anterior edge of the Hensen’s node. (23-25 hours) The blastoderm anterior to the head process folds making the embryo end to acquire a proper shape (Langman 1981). At this stage the somites have not yet appeared on the mesoderm adjacent to the notochord (Langman 1981). Somitogenesis After 23-26 hours of incubation the neural folds are visible near the head region and the first somite if formed. The pour somites are formed after 26-29 hours as the neural folds meet at the midbrain level. Blood stains become visible in the posterior half of the blastoderm. Between 29-33 hours the revensomites are formed and the optic vesicles are visible. The paired heart primodia start to fuse. (33-38 hours.) The first somite starts to disperse and 3 primary brain vesicles can be clearly seen and the heart bends slightly to the right. After 40-45 hrs.some of the neuromeres of hindbrain becomes distinct as thirteen somites are formed. The anterior neuropore closes and the optic vesicles are constricted to the right as the heart turns to the right. At the 16th somite the chick head turns to the left and the anterior neuropore closes. At this stage the primary optic vesicles are well formed. The auditory pit is deep and wide open as the heart has an S-shape. (48-52 hours.) The head turns partly to the left as the Cranial and cervical flexures form wide curves. The distinct development of the telencephalon is visible. The amnion head fold covers the forebrain and the midbrain. The lens-placode develops as the optic vesicle starts to invaginate.At stage fifteen the lateral body folds elongate the anterior end of the wing level. The mesoderm condenses as the limb areas flatten. The axes of the fore brain and hindbrain develop into an acute angle. At stage 16 the lateral body folds elongate towards 17-20 somites. The wings are lifted from the blastoderm and the leg promordinm is still flat. The egg amnion extends towards the 10-18 somites as the flexures are accentuated. The tail bud at this stage is still short and the 3rd cleft maintains its oval shape.At stage 17 the lateral body folds extend around the body circumference. The wing and leg buds are lifted from the blastoderm by infolding of the body folds. The cervical flexure is sharply bent as compared in the previous stages. At this stage the allantois is not yet formed.Stage 18 the limb buds become larger than the wing buds .The amnion closes as the trunk flexure shifts to the lumbar region. The rotation extends to posterionbody part thus the leg buds change their position from the horizontal plane. The tail bud tumes to the right at 90 degrees angle to the posterior trunk axis.At the 19th stage the legs are larger as compared to the wing buds. The trunk flexure almost disappears due to body rotation. The posterior trunk straightens to the tail base. The tip of curved tail bud points forward. The 1st visceral cleft develops into a shallow furrow as the 2nd arch projects above the surface. At stage 20 the eye pigment becomes light grey. The bent tail region starts to extend towards the lumbo-sacral section and the rotation comes to an end. The 2nd arch project above the surface as the 4th arch remains below the surface. At stage 21 the wing and leg buds acquire an assymetrical shape. The posterior and anterior contours meet at the baseline at an angle of 90 degrees. The trunk dorsal contour slightly bends as the 2nd arch extends above the surface overlapping the third arch. The eye pigmentation continues to become fainter. At stage 21 the anterior and posterior contour are parallel at the base level (see the 7th plate) as the eye pigmentation becomes more distinct. The dorsal contour of the trunk becomes curved as the allantois extends towards the chick head covering the forebrain. After four days both the 3rd and 4th arch sink under the surface. The 3rd visceral cleft develops into an elongated groove as the 4th cleft reduces into a small pit. At stage 25 the joints of the elbow and knees become more distinct. The visceral arches meet the nasal groove wall (view plate 7 ). The 3rd and 4thcleft decreases their size to smaller round pits.Stage 26 the digital plate contour becomes rounded as the middle Protuberances project above the surface.Stage 27 the 1st toe projects above the tibia and the maxillar contour process curves into a broken line. At this stage one can bare recognize the chick beak Gastrulation in the Chick The embryo forms from one layer of cell known as the epiblast. During gastrulation, cells from the epiblast move through a line in the middle of the blastoderm where they separate and move towards the yolk. The epiblast cells pile up forming the primitive steak. The epiblast cells from the primitive steak go through a transition known as mesenchymal transition. Primitive steak cells form the mesoderm, the hypoblast cells form the endoderm and the epiblast layer forms the ectoderm. After formation of the mesoderm the lateral folds form the three primary germ layers. The endoderm invagination forms the archenteron. The hypoblast ensures primitive streak is formed before gastrulation starts. The hypoblast separates from the endoderm to form extraembryonic membranes of the embryo. Organogenesis in the chick: During organogenesis the neural tube and notochord start to take shape. The notochord is formed from the dorsal mesoderm above the archenteron (Langman 1981).. The neural tube grows from dorsal ectoderm cells known as neural plate. The neural plate rolls into a tube forming the spinal cord. Mesoderm cells detach into discrete blocks known as somites. Somite cells later develop to a backbone and muscles related with the axial skeleton (Langman 1981).. The mesoderm adjacent to the somites separates into layers forming the body cavity lining known as the coelom. The ectoderm forms the epidermis and related glands, it also forms the nervous system (refer to the figure below) and eye structures such as eye lenses. The mesoderm grows into the notochord, muscles and the circulatory system. The endoderm forms the digestive tract lining, liver and the pancreas (Langman 1981). The archenteron folds form the lungs.   The Formation of the Neural Tube Formation of the neural tube involves two ways, which are the primary neuralation and secondary neuralation (Langman 1981). During Primary neuralation the cells surrounding the neural plate proliferate, invaginate and tweak off from the surface forming a hollow tube (Langman 1981). During secondary neuralation, the neural tube arises from a solid cord of cells which sinks into the embryo forming a hollow tube. Primary Neuralation The ectoderm is divided into 3 groups of cell: which are the internally position neural tube (forms the brain and spinal cord), externally positioned skin epidermis and neural crest cells. Neural crest cells are formed in the region connecting the neural tube and the epidermis; they later migrate to other areas and generate glia and peripheral neurons. After the neural plate has been formed, the edges of the chick embryo thicken and travel upwards forming the neural folds as the neural grooves appear at the center of the neural plate dividing the right and left regions of the embryo. Neural folds move towards the embryo midline and eventually fuse to form neural tube under the ectoderm. The cells located on the dorsal part of the neural tube form the neural crest cells. Neurulation occurs differently in different parts of the chick body. The head, tail and trunk form their own part of the neural tube. The head and trunk go through primary neurulation variants thus dividing the whole process of secondary neuralation into four different stages. These stages are discussed below. Formation and Shaping Of the Neural Plate Neurulation process starts when the dorsal mesoderm alerts the ectodermal cells to elongate into columnar neural plate cells. The elongated shape of the cells differentiates them from the pre-epidermal cells bordering them. The intrinsic activities of the epidermal and neural plate parts lead to shaping of the neural plate. The neural plate elongates along the anterior-posterior axis and narrows itself by bending so as to form a neural tube. Neural Plate Bending This involves the formation of the hinge locations where the neural tube communicates with the neighboring tissues. In these sections presumptive epidermal cells follow the neural plate lateral edges and transfers the cells towards the midline. Cells at the middle of the neural plate are known as medial hinge point (MHP) cells .MHP cells originate from parts of the neural plate. These cells are anchored in the notochord forming a hinge. Cells on the lateral to the MHP cells undergo no changes. After a short while, a furrow is formed (from two hinge sections) close to the neural plate connection. These hinge sections are known as dorsolateral hinge points (DLHPs). DLHPs elongate and acquire a wedge shape. After furrowing of the platethe neural plate bends around the DLHPs .The hinge points are similar to a pivot which controls rotation of nearby cells. The ectoderm surface of the chick is pushed towards the middle line of the embryo by extrinsic forces thus bending the neural plate. Anchoring of the neural plate and presumptive epidermis movement is important is important since it ensures the neural tube invaginates into the embryo. The movement of presumptive epidermis towards the central region and neural tube furrowing leads to the formation of the neural folds. Closure of the Neural Tube The paired neural folds come together near the dorsal midline thus closing the neural tube (Roberts 1986). The neural folds follow each other and cells from the 2 folds are combined. In chicks the neural crest does not transfer from the dorsal region until the neural tube has been closed at that region as compared to human beings whereby the crest cells migrate and the neural folds elevate(Roberts 1986).. Figure 12.5 shows neurulation of a chick embryo after 24 hours. Neurulation in the head is advanced while in the tail of the embryo gastrulation is still taking place. Division of the neural tube into regions occurs as result of changes in the neural tube shape. Where the brain is formed (cephalic end) the wall of the neural tube thickens and becomes broad. The brain compartments can be defined from constrictions and swelling of the neural tube. The neural tube tapers off towards the tail of the chick embryo thus forming the anterior neuropore and posterior neuropore. figure 12.5 neural tube development and mesoderm Secondary Neurulation Secondary neurulation involves development of the medullary cord and hollowing of this cord into a neural tube. In chicks secondary nerulation occurs in the lumbar neural tube and the tail vertebrae (Roberts 1986).. Conclusion The importance of the above experiment was to get a better understanding of chick embryology and how its process. Also to understand the differences and similarities of chick and human embryology; how the embryology process between the two are related. Plate 7 Bibliography Langman J. Medical embryology (Fourth Edition) 1981; Williams & Wilkins, Baltimore.pp 384. Larsen WJ. Human embryology 1993; Churchill Livingstone, New York.pp 479 Moore KL. Before we are born: basic embryology and birth defects (Third Edition) 1989; Saunders, Philadelphia. Pp, 306. O’Rahilly RA. Colour atlas of human embryology 1975; Saunders, Philadelphia. Biology a functional approach4th edition M B V Roberts Nelson 1986 V Hamburger, H L Hamilton (1951) A series of normal stages in the development of the chick embryo. 1951. Dev. Dyn.: 1992, 195(4); 231-72 Read More