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Fertilization and Activation Chapter 8 Principles of Development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Contact and Recognition Between Egg and Sperm Fig. 8.4 Cortical Reaction • 1) Thousands of enzyme-rich cortical granules below the egg membrane fuse with the membrane. • 2) The cortical granules release contents between the membrane and vitelline envelope. • 3) This lifts the envelope and forms a moat. • 4) One cortical granule enzyme causes the vitelline envelope to harden, becoming the fertilization membrane. Contact and Recognition Between Egg and Sperm • Marine organisms release enormous numbers of sperm in the ocean to fertilize eggs. • Many marine eggs release a chemotactic molecule to attract sperm of the same species. • Sea urchin sperm first penetrate the jelly layer before contacting the vitelline envelope. • Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope. • In the marine environment, many species may be spawning at the same time. • Similar recognition proteins are found on sperm of vertebrate species. Prevention of Polyspermy • A fertilization cone forms where the sperm contacts the vitelline membrane. • Important changes in the egg surface block entrance to any additional sperm. • In the sea urchin, an electrical potential rapidly spreads across the membrane; this is the “fast block.” • This is followed by the cortical reaction. Fig. 8.6 Fusion of Pronuclei and Egg Activation • After sperm and egg membranes have fused, the sperm disconnects from its flagellum. • The enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus. • Fusion forms a diploid nucleus. – Nuclear fission takes 12 minutes in sea urchins; about 12 hours in mammals. – The fertilized egg is now properly called a zygote. • Fertilization initiates reorganization of cytoplasm and repositions determinants that begin development and cleavage. 1 Cleavage and Early Development Blastomeres • The embryo undergoes cleavage to convert the large cytoplasmic mass into small maneuverable cells. • No cell growth occurs, only subdivision until cells reach regular somatic cell size. • At the end of cleavage, polychaete worms have 1000 cells, amphioxus has 9000, and frogs have 700,000. • Polarity—a polar axis—establishes the direction of cleavage and differentiation. Types of Cleavage • Isolecithal yolk describes eggs with very little yolk and the yolk is distributed evenly. • In such eggs, cleavage is holoblastic. • The cleavage furrow extends completely through the egg. • Isolecithal eggs are widespread and seen in: echinoderms, tunicates, cephalochordates, molluscs and mammals. • Cleavage is slowed in the yolkrich vegetal pole. Types of Cleavage • Telolecithal eggs have much yolk concentrated at the vegetal pole. – Actively dividing cytoplasm is confined to a narrow discshaped mass on the yolk. – Cleavage is partial or meroblastic; the furrow does not cut through the heavy yolk. – Birds, reptiles, most fishes and a few amphibians have telolecithal eggs. Patterns of Cleavage • The pattern of cleavage is affected by – quantity and distribution of yolk present, and – genes controlling the symmetry of cleavage. • There are four principal types of cleavage. Types of Cleavage • Mesolecithal eggs have a moderate amount of yolk concentrated in the vegetal pole. – The animal pole is opposite the vegetal pole and contains cytoplasm and very little yolk. – These eggs cleave holoblastically, but cleavage is retarded in yolk-rich vegetal – The cleavage furrow progresses much more slowly through the vegetal pole; thus cleavage is faster in the animal region. – Amphibians have mesolecithal eggs. Types of Cleavage • Centrolecithal eggs have a large mass of centrally located yolk. – Cytoplasmic cleavage is limited to a surface layer of yolk-free cytoplasm; yolk-rich inner cytoplasm is uncleaved. – They have meroblastic cleavage. – Insects and many other arthropods have centrolecithal eggs. – Yolk is therefore an impediment to cleavage. 2 Amount of Yolk Affects Developmental Mode Types of Development • • • In most animals, a mother does not directly nourish embryonic development but has provisioned the egg with yolk. The amount of yolk is related not only to cleavage pattern, but also to whether a larval stage occurs during development. Animals in which the zygote is telolecithal generally have direct development. In direct development is characteristic of animals where the larval stage is between embryo and adult. • – – – – • Cleavage Affected by Different Inherited Patterns • Different cleavage patterns are characteristic of different phylogenetic lineages. • Isolecithal eggs demonstrate four major patterns: – Radial Cleavage – Spiral Cleavage – Bilateral Cleavage – Rotational Cleavage Spiral Cleavage • Spiral cleavage proceeds in a sequence oblique to the animal-vegetal axis. • Cells produced pack tightly in the adjacent furrows, like soap bubbles. • Spiral cleavage is found in annelids, nemerteans, turbellarians, all molluscs except cephalopods, some brachiopods, echiurans and some other Prostomia. Metamorphosis is a change from larval to adult body form. Mammalian zygotes bypass the larval stage. A placental attachment to the mother provides ongoing nourishment. Direct development can occur when there is enough yolk to support growth as juveniles; this occurs in reptiles and birds. Species with isolecithal or mesolecithal zygotes generally have indirect development. Radial Cleavage • Embryonic cells are arranged in a radial symmetry around the animal-vegetal axis. • In sea stars, cleavage begins by two identical daughter cells cut through the animal vegetal axis. • The next cleavage runs parallel to the animal vegetal axis and cuts the blastomeres in half. • The next cleavage is perpendicular to the animal vegetal axis and forms two tiers of 4 cells each. • Amphibian embryos have similar cleavage with slower furrowing in the yolk region. • Radial cleavage is characteristic of the Deuterostomia, including echinoderms, hemichordates and chordates. Bilateral Cleavage • Prior to fertilization, the egg is defined by unequal cytoplasmic components. • The first cleavage furrow passes through the animal vegetal axis dividing the asymmetrically divided cytoplasm between the two blastomeres. • This first cleavage pattern determines the future right and left side. • The half-embryo formed on one side is the mirror image of the half embryo on the other side. • Ascidians (tunicates) demonstrate this cleavage pattern. 3 Discoidal Cleavage • Telolecithal eggs divide by discoidal cleavage. • There is a large mass of yolk in each egg; cleavage is confined to a small disc of cytoplasm. • Early cleavage furrows carve the disc into a single layer of cells called the blastoderm. Blastulation • The blastula is the resulting cluster of cells regardless of cleavage pattern. • In mammals, this is called the blastocyst. • Often, cells arrange themselves around a central fluid-filled blastocoel. • At this stage, cell number ranges from a few hundred to several thousand. • The embryo has not increased in size beyond the size of the zygote, but each nucleus has a full set of DNA. Sea Star Gastrulation • Gastrulation begins with vegetal area flattening to form the vegetal plate. • Invagination is a bending inward of the vegetal plate one-third into the blastocoel. • The archenteron is the new cavity formed by this invagination. • The archenteron is the primitive gut; the blastopore is the opening to the outside. • In deuterostomia, the blastopore becomes the anus, and the mouth forms secondarily. • The archenteron elongates toward the animal pole and expands into two pouch-like coelomic vesicles Rotational Cleavage • The first cleavage plane is aligned with the animal vegetal axis. • However, in the second cleavage, one blastomere divides meridionally while the other divides equatorially, rotated 90 degrees to the first. • Early divisions may be asynchronous and possess odd numbers of cells below the 2-4-8-16... series that would occur with synchronous division. • After the third division, cells form a tightly packed cluster stabilized by outer cells with tight junctions, the trophoblast. • The trophoblast will form the embryonic portion of the placenta. • Cells that give rise to the embryo are the inner cell mass. • This type of cleavage is present in mammals and is slower than in other animal groups. Gastrulation and the Formation of Germ Layers • Gastrulation converts the spherical blastula into a complex structure with three layers. – Ectoderm covers the embryo. – Mesoderm and endoderm are on the interior. – The new positions and cell neighbors establish the embryonic body plan. • Patterns of gastrulation vary enormously depending on the amount of yolk. – The yolk impedes gastrulation. – Gastrulation is simple in non-yolky embryos, complex in yolk-laden eggs. Germ Layers • The outer ectoderm will give rise to epithelium and the nervous system. • The endoderm gives rise to the epithelial lining of the digestive tube. • The mesoderm will form the muscular system, reproductive system, peritoneum and the sea star’s endoskeleton and water-vascular system. 4 Gastrulation in Prostomia • The blastopore becomes the mouth and the anus forms secondarily. • The mesoderm forms differently, arising from the lip of the blastopore and proliferating between the walls of the archenteron and outer body wall. • These mesodermal precursors arise from the large 4d cell at the 29 to 64 cell stage. Fig. 8.13 Two versus Three Germ Layers • Cnidaria and Ctenophora have only two germ layers (endoderm and ectoderm) and are diploblastic. • Other metazoa have three germ layers and are triploblastic. Formation of the Coelom The coelom is a true body cavity that contains the viscera; it is formed in one of two methods. – Schizocoelous formation forms the coelom from splitting of mesodermal bands originating from blastopore region and growth between ectoderm and endoderm. – Enterocoelous formation forms the coelom from pouches of the archenteron. – Protostomes develop by the schizocoelous method. – Deuterostomes, except for vertebrates, follow the enterocoelous plan. – Vertebrates form a coelom by schizocoelous formation; this evolved anew to accommodate large stores of yolk. Development of Systems and Organs Germ Layers • Germ layers should not be confused with germ cells (eggs and sperm). • Germ layers do not alone determine differentiation but rather the position of embryonic cells. Derivatives of Ectoderm: Nervous System and Nerve Growth – Just above the notochord, the ectoderm thickens to form a neural plate. Edges of the neural plate fold up to create an elongated, hollow neural tube. – The anterior end of neural tube enlarges and forms the brain and cranial nerves. – The posterior end forms the spinal cord and spinal motor nerves. – Neural crest cells pinch off from the neural tube. 5 Nervous System (continued) Nervous Tissue development Neural crest cells form many structures. • In 1907, Ross Harrison cultured nerve cells; each axon grows from one cell. • Additional research revealed that a nerve axon grows in response to guidance molecules secreted into its path. – They become portions of cranial nerves, pigment cells, cartilage, bone, ganglia of the autonomic system, medulla of the adrenal gland, and parts of other endocrine glands. – It is unique to vertebrates and was important in evolution of the vertebrate head and jaws. Derivatives of Endoderm: Digestive Tube and Survival of Gill Arches • During gastrulation, the archenteron forms as the primitive gut. • This endodermal cavity eventually produces the digestive tract, lining of pharynx and lungs, most of the liver and pancreas, thyroid and parathyroid glands and thymus. • The alimentary canal develops from primitive gut; ends are lined with ectoderm. • Lungs, liver and pancreas arise from the foregut. Fig. 8.28 Endoderm derivatives During development, endodermally-lined pharyngeal pouches interact with overlying ectoderm to form gill arches. – In fish, gill arches become gills. • Gill arches remain as necessary primordia for a variety of other structures in terrestrial vertebrates. – The first arch and its endoderm-lined pouch form the upper and lower jaws. – Second, third and fourth gill pouches become the tonsils, parathyroids and thymus. Derivatives of Mesoderm: Support, Movement and the Beating Heart • With an increase in size and complexity, mesodermally derived structures take up a greater proportion. • Muscles arise from mesoderm along each side of the neural tube. – The mesoderm divides into a linear series of somites (38 in humans). – The splitting, fusion and migration of somites produce the: • axial skeleton, • dermis of dorsal skin, and • muscles of the back, body wall and limbs. • Limbs begin as buds from the side of the body; projections become fingers and toes. 6 Mesoderm gives rise to the embryonic heart • Guided by underlying endoderm, two clusters of precardiac mesodermal cells move to either side of the gut. • These clusters differentiate into a pair of double-walled tubes that fuse into a single thin tube. • The primitive heart begins beating on the second day of the 21-day incubation period; there are no blood or vessels at this time. • Twitching becomes rhythmical as the ventricle and atrium develops; each chamber has a faster intrinsic beat. 7