Developmental Biology

Developmental Biology

1. II B.Sc.(Zoology) Developmental Biology Topics: Planes and Types of Cleavage Pattern, Fate Map, Blastulation and Gastrulation in Amphioxus and Frog and Organogenesis E-Learning Study Material Prepared by Dr. M. PAVUNRAJ

2. Cleavage Cleavage is initiated by the appearance of a grooves or constriction called cleavage furrow . The furrow appears first at one point of the eggs. For example, in Amphioxus, the first furrow appears at the animal pole. The furrow then deepens and extends downward on both side.

3. Difference Between an Animal Pole and a Vegetal Pole The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. In some cases, the animal pole is thought to differentiate into the later embryo itself, forming the three primary  germ layers and participating in  gastrulation . The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal pole above it. In some cases, the vegetal pole is thought to differentiate into the extraembryonic membranes  that protect and nourish the developing embryo, such as the placenta  in mammals and the  chorion  in birds.

4. The two ends meet at the vegetal pole. The furrow then extends inwards radially , finally constricting the egg into two blastomeres. The cleavage furrows divide the egg at different angles or planes. There are four main planes of cleavage. They are as follows: a. Meridional Plane If the cleavage furrow bisects both the poles of the egg passing through the polar axis*; the clevage plane is said to be meridional (Fig 8.5A). [*Polar axis is the imaginary line passing from the centre of animal pole to that of vegetal pole]

5. b. Vertical Plane It resembles the meridional plane because the furrow tends to pass from the animal pole to the vegetal pole. But it does not pass through the median axis of the egg; it appears on one side of the axis (Fig 8.5D). c. Equatorial Plane The equatorial plane of cleavage bisects the egg at right angles to the median axis and half way between the animal and vegetal poles. This type of cleavage plane is exhibited by Sea urchin (Fig. 8.5B).

6. d. Latitudinal Plane Latitudinal plane cuts the egg at right angles to the median axis; but it passes either above (near the animal pole) or below (near vegetal pole) the equator of the egg (Fig. 8.5C) PATTERNS OF CLEAVAGE There are mainly two types of cleavage. They are holoblastic cleavage and meroblastic cleavage.

7. Holoblastic Cleavage In holoblastic cleavage, the entire egg divides, It is otherwise called total or complete cleavage. In holoblastic cleavage, when the blastomeres are equal in size, the cleavage is said to be equal. When the blastomeres are unequal, the cleavage is called unequal Meroblastic Cleavage In meroblastic cleavage, only a portion of the egg divides. It is otherwise called partial of incomplete cleavage. It is characteristic of telolecithal and megalecithal eggs.

8. FATE MAP IN AMPHIOXUS Fate map is a chart showing the fate of each cell of any embryo. It shows the positions of the various presumptive organs in the early embryo itself. The fate map is essential for the correct interpretation of gastrulation . In amphioxus, the presumptive organ forming areas can be marked clearly in the blastula stage. 1. All the micromers develops into the endodermal lining of alimentary canal. Hence, the macromeres are said to be presumptive endoderm cells. They are all situated at the vegetal pole in the blastula.

9. 2. The micromeres at the animal pole develops into skin epidermis. Hence, the micromeres are called presumptive epidermal ectoderm. 3. There is a crescent-like area on one side of the blastula just above the equator. The cells or this area develops into the mesoderm. 4. Opposite to the mesoderm cells, there is another creascent like areas, the cells of which develops into the notochord. 5. Between the notochordal cells and the epidermal ectoderm, there is creascent like areas which develops into the nervous system. Hence, these cells are clled neurectoderm cells.

10. Fate Map in Frog

11. Blastulation and Gastrulation in Frog

12. A  germ layer  is a primary layer of  cells  that forms during  embryonic development .  The three germ layers in  vertebrates  are particularly pronounced; however, all eumetazoans  ( animals  more complex than the  sponge ) produce two or three primary germ layers. Some animals, like  cnidarians , produce two germ layers (the  ectoderm and   endoderm ) making them  diploblastic . Other animals such as  chordates  produce a third layer (the  mesoderm ) between these two layers, making them  triploblastic . Germ layers eventually give rise to all of an animal’s  tissues  and  organs  through the process of  organogenesis . Organogenesis - Germ layer

13. Gastrulation of a diploblast :  The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

14. Endoderm The  endoderm  is one of the germ layers formed during animal  embryonic development . Cells migrating inward along the archenteron  form the inner layer of the  gastrula , which develops into the  endoderm . The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the  digestive tract  except part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. The endoderm forms: the  pharynx , the  esophagus , the  stomach , the  small intestine , the  colon , the  liver , the  pancreas , the  bladder , the  epithelial  parts of the  trachea  and  bronchi , the  lungs , the  thyroid , and the  parathyroid .

15. The  endoderm  produces tissue within the  lungs ,  thyroid , and  pancreas .

16. Mesoderm The  mesoderm  germ layer forms in the  embryos  of  triploblastic   animals . During  gastrulation , some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm . The formation of a mesoderm leads to the development of a  coelom . Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks. The mesoderm has several components which develop into tissues:  intermediate mesoderm ,  paraxial mesoderm , lateral plate mesoderm , and chorda -mesoderm. The chorda -mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, and wall of the human body. Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of  differentiation . The mesoderm forms: muscle ( smooth  and  striated ),  bone ,  cartilage ,  connective tissue ,  adipose tissue ,  circulatory system ,  lymphatic system ,  dermis ,  genitourinary system ,  serous membranes ,  spleen  and  notochord .

17. The  mesoderm  aids in the production of cardiac muscle ,  skeletal muscle ,  smooth muscle , tissues within the  kidneys , and  red blood cells .

18. Ectoderm The  ectoderm  generates the outer layer of the embryo, and it forms from the embryo's  epiblast .  The  ectoderm  develops into the  surface ectoderm ,  neural crest , and the  neural tube . The surface ectoderm develops into:  epidermis ,  hair ,  nails ,  lens of the eye ,  sebaceous glands ,  cornea ,  tooth enamel , the epithelium of the  mouth  and  nose . The neural crest of the ectoderm develops into:  peripheral nervous system ,  adrenal medulla ,  melanocytes , facial cartilage, dentin  of teeth. The neural tube of the ectoderm develops into:  brain ,  spinal cord ,  posterior pituitary ,  motor neurons ,  retina . Note: The anterior pituitary develops from the ectodermal tissue of  Rathke's pouch .

19. The  ectoderm  produces tissues within the  epidermis , aids in the formation of neurons  within the brain, and constructs melanocytes .

20. Neural crest Because of its great importance, the neural crest is sometimes considered a fourth germ layer.  It is, however, derived from the ectoderm.

21. Development of Brain The  mammalian   central nervous system  (CNS) is derived from the  ectoderm —the outermost  tissue layer  of the embryo. In the third week of  human embryonic development  the  neuroectoderm  appears and forms the  neural plate  along the dorsal side of the embryo. The neural plate is the source of the majority of neurons and glial cells of the CNS. A groove forms along the long axis of the neural plate and, by week four of development, the neural plate wraps in on itself to give rise to the  neural tube , which is filled with  cerebrospinal fluid  (CSF). As the embryo develops, the anterior part of the neural tube forms three  brain vesicles , which become the primary anatomical regions of the brain: the  forebrain  ( prosencephalon ),  midbrain  ( mesencephalon ), and  hindbrain  ( rhombencephalon ). These simple, early vesicles enlarge and further divide into the  telencephalon  (future  cerebral cortex  and  basal ganglia ),  diencephalon  (future  thalamus  and  hypothalamus ),  mesencephalon  (future  colliculi ), metencephalon  (future  pons  and  cerebellum ), and  myelencephalon  (future  medulla ). [  The CSF-filled central chamber is continuous from the telencephalon to the spinal cord, and constitutes the developing  ventricular system  of the CNS. Because the neural tube gives rise to the brain and spinal cord any mutations at this stage in development can lead to fatal deformities like  anencephaly  or lifelong disabilities like  spina bifida . During this time, the walls of the neural tube contain  neural stem cells , which drive brain growth as they divide many times. Gradually some of the cells stop dividing and differentiate into  neurons  and  glial cells , which are the main cellular components of the CNS. The newly generated neurons  migrate  to different parts of the developing brain to self-organize into different brain structures. Once the neurons have reached their regional positions, they extend  axons  and  dendrites , which allow them to communicate with other neurons via  synapses . Synaptic communication between neurons leads to the establishment of functional  neural circuits  that mediate sensory and motor processing, and underlie behaviour.

22. Development of Eye Eye formation in the human  embryo  begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the  neuroepithelium , surface  ectoderm , and the extracellular mesenchyme which consists of both the  neural crest  and  mesoderm . Neuroepithelium forms the  retina ,  ciliary body ,  iris , and  optic nerves . Surface ectoderm forms the  lens ,  corneal epithelium   and eyelid . The extracellular mesenchyme forms the  sclera , the corneal  endothelium  and  stroma ,  blood vessels ,  muscles , and vitreous . The  eye  begins to develop as a pair of  optic vesicles  on each side of the forebrain at the end of the 4th week of pregnancy. Optic vesicles are outgrowings of the brain which make contact with the surface  ectoderm  and this contact induces changes necessary for further development of the eye. Through a groove at the bottom of the optic vesicle known as  choroid fissure  the blood vessels enter the eye. Several layers such as the  neural tube ,  neural crest ,  surface ectoderm , and  mesoderm  contribute to the development of the eye.

23. Eye development is initiated by the master control gene  PAX6 , a homeobox gene with known homologues in humans ( aniridia ), mice (small eye), and  Drosophila  (eyeless). The PAX6 gene locus is a transcription factor for the various genes and growth factors involved in eye formation. Eye morphogenesis begins with the  evagination , or outgrowth, of the optic grooves or sulci . These two grooves in the neural folds transform into  optic vesicles  with the closure of the neural tube.  The optic vesicles then develop into the  optic cup  with the inner layer forming the retina and the outer portion forming the retinal pigment epithelium. The middle portion of the optic cup develops into the ciliary body and iris.  During the  invagination  of the optic cup, the ectoderm begins to thicken and form the  lens placode , which eventually separates from the ectoderm to form the  lens vesicle  at the open end of the optic cup. Further differentiation and mechanical rearrangement of cells in and around the optic cup gives rise to the fully developed eye.

24. Development of Heart Heart development  (also known as  cardiogenesis ) refers to the  prenatal development  of the human  heart . This begins with the formation of two  endocardial tubes  which merge to form the  tubular heart , also called the  primitive heart tube , that loops and  septates  into the four  chambers  and paired  arterial  trunks that form the adult heart. The heart is the first functional organ in vertebrate embryos, and in the human, beats spontaneously by week 4 of development . The tubular heart quickly differentiates into the  truncus arteriosus ,  bulbus cordis ,  primitive ventricle ,  primitive atrium , and the  sinus venosus . The truncus arteriosus splits into the  ascending aorta  and  pulmonary artery . The bulbus cordis forms part of the ventricles. The sinus venosus connects to the  fetal circulation . The heart tube elongates on the right side, looping and becoming the first visual sign of left-right asymmetry of the body.  Septa  form within the atria and ventricles to separate the left and right sides of the heart.