Chapter 53 Sila and Kharee Animal Develepment Chapter 53 Sila and Kharee

Fertilization Fertilization, the union of male and female gametes, is the first step in reproduction. Sperm must penetrate to the plasma membrane Saclike organelle named the acrosome is positioned between the plasma membrane and nucleus of sperm, digestive enzymes within create a hole in eggs membrane The egg is protected by coats zona pellucide (mammals) Jelly layer (urchin, frogs) Chorion (insects)

Membrane Fusion Activates the Egg The egg remain dormant until the sperm activates it Polyspermy: response to sperm fusion of additional sperm Sperm penetration of egg can also cause: Many eggs did not complete meiosis, therefore not a haploid yet, sperm penetration activates Triggers movement of egg cytoplasm Sharp increase of protein synthesis and metabolic activity in general

The Fusion of Nuclei Final Stage of Fertilization The haploid male nuclei and the haploid female form the diploid zygote Two nuclei migrate toward each other along a aster

Clevage and Blastula Stage Cleavage: The rapid division of the zygote into a larger and larger number of smaller cells. Blastomere: Each individual cell Animal Pole and Vegetal Pole

Blastula The blastula is a hollow mass of cells Outermost blastomeres join together through protein belt which create a seal that isolates the interior cell mass Cleavage patterns are diverse in every animal cell

Cleavage Patterns Eggs with moderate or little yolk go through Hoboblastic Cleavage (Complete) Echinoderms Annelids, Mollusks, and Flatworms Eggs with Dense Yolk go through Meroblastic Cleavage (incomplete) Fish, Reptiles, Birds Insects

Cleavage in Mammals Contain little yolk Holoblastic, forms a structure called a blastocyst A single layer of cells surrounds a blastocoel Inner cell mass (ICM) located at one pole of of blastocoel Trophoblast, the outer cells, part of them enter the maternal uterus lining and forms placenta

Gastrulation Gastrulation: Cells of blastula rearrange themselves to form the body plan Forms the three germ bilayers Converts the blastula into a bilaterally symmetrical embryo with a gut, visible anterior and posterior, and dorsal ventral axis.

How the Cells Move Lamellipdoa: crawl over neighbor cells Filopodia: feel out the surfaces of other cells and pull the cell forward Tightly attached cells move in sheets

Gastrulation in Chordates The movement of surface cells into the interior, by folding as a sheet ("invagination“) The cells of vegetal plate move inward to the blastocoel cavity The cells produce a tube (primitive gut) called archenteron and the opening (anus)

Gastrulation in Amphibians By rolling as a sheet ("involution") Cells from animal pole move to vegetal pole When on the dorsal lip, cells involute into interior Movement of the cells forms a new internal cavity Organogenesis then begins (explained later)

Amphibian Gastrulation

Gastrulation in Reptiles /Birds/ and Mammals The Gastrulation of all are similar by internalization/separation of cells from an epithelium ("ingression "), The embryo develops from flattened collection of cells forming two layers (endoderm and ectoderm) lower cells migrate out to line cavity to form yolk (ectoderm) mesoderm cells migrate to the interior

Three Germ Layers Endoderm: formed by cells that move into embryo to form the tube of a primitive gut Epidermis of skin, nervous system, sense organs Ectoderm: formed by the cells that remain on the exterior Skeleton, muscles, blood vessels, heart, blood, gonads, kidneys, dermis of skin Mesoderm: formed by the cells that move into the space between endoderm and ectoderm Lining of digestive and respiratory tracts, liver, pancreas, thymus, thyroid