lab embryology.pdf

The Embryonic Development of a Representative Vertebrate Comparative anatomy | Laboratory | Module 1 REPRODUCTION CHORDATES The process by which plants and animals give rise In general, they reproduce sexually. Chordate to of...

The Embryonic Development of a Representative Vertebrate Comparative anatomy | Laboratory | Module 1 REPRODUCTION CHORDATES The process by which plants and animals give rise In general, they reproduce sexually. Chordate to offspring by sexual or asexual process. development is a process involving unicellular to a multicellular and specialized organism. ASEXUAL REPRODUCTION Reproductive processes that do not involve the Includes: union of individuals or gametes (cell division, spore Gametogenesis formation, fission, budding). Fertilization Cleavage SEXUAL REPRODUCTION Chordate Development Involves gametes and sex cells. Gastrulation GAMETES Histological differentiation Are formed from primordial germ cells through Organogenesis mitosis and meiosis in the ovary (egg cells) and Parturition testis (sperm cells). GAMETOGENESIS EGG Production of gametes. Classified as to the distribution of yolk they contain. TYPES Classifications: Oogenesis – egg cells. Spermatogenesis – sperm cells. 1. Alecithal – eggs without yolk or with very PRIMORDIAL GERM small amount of yolk (e.g. invertebrates and Cells undergo mitosis to form diploid cells, which mammals). then undergo meiosis to form haploid cells. These 2. Meiolecithal – eggs contain a small amount mature cells are ready for fertilization. of yolk (e.g. Amphioxus). FERTILIZATION 3. Mesolecithal – eggs have moderate amount The union of male and female gametes to form a of yolk (e.g. amphibians). zygote. 4. Polylecithal – eggs contain enormous yolk (e.g. birds, reptiles, fishes). When the sperm cell (with half the required DISTRIBUTION OF YOLK chromosomes) fertilizes the female egg cell (with the ISOLECITHAL other half of the required chromosomes), it forms the Eggs have evenly distributed yolk throughout the zygote, a single cell with the correct number of cytoplasm (e.g. Amphioxus and mammals). chromosomes. TELOLECITHAL Eggs whose yolk tends to be more concentrated on About five hours after fertilization, the zygote enters one side than the other (e.g. fishes, amphibians, the isthmus where the new embryo starts to develop reptiles, and birds). by simple cell division. By the time the egg leaves the isthmus, the zygote, now called the blastoderm or embryo, comprises eight cells, and after four hours in the uterus, it has grown to 256 cells. MODULE 1 | 1  TYPES OF FERTILIZATION and some invertebrates (such as the EXTERNAL FERTILIZATION Madagascar hissing cockroach Occurs outside the body of an organism (i.e. Gromphadorhina portentosa). amphibians). Viviparity - The young develop within the female, receiving nourishment from the Occurs in aquatic environments where both eggs mother's blood through a placenta. The and sperm are released into the water. After the offspring develop in the female and are born sperm reaches the egg, fertilization takes place. alive. This occurs in most mammals, some Most external fertilization happens during spawning, cartilaginous fish, and a few reptiles, making where females release eggs and males release them viviparous. Internal fertilization protects sperm in the same area at the same time. the fertilized egg from dehydration on land. The embryo is isolated within the female, Release of reproductive material may be triggered limiting predation. Fewer offspring are by water temperature or daylight length. Nearly all produced, but survival rates are higher than fish spawn, as do crustaceans, mollusks, squid, for external fertilization. echinoderms, and amphibians. Spawning enhances the possibility of fertilization. INTERNAL EXTERNAL FERTILIZATION FERTILIZATION Aquatic external fertilization protects eggs from Occurs inside the Occurs outside drying out. Broadcast spawning can result in genetic female body. the female body. diversity, enhancing species survival. However, the Fewer gametes Large numbers of survival rate of eggs is low. are produced; gametes are male gametes are released into deposited inside water. INTERNAL FERTILIZATION the female using Internal fertilization occurs most often in land-based a copulatory Examples: Bony fish, animals, though some aquatic animals also use this organ. amphibians. method. Examples: Reptiles, birds, mammals. There are three ways offspring are produced following internal fertilization: Oviparity - Animals classified as oviparous. Fertilized eggs are laid outside the female's body and develop there, receiving nourishment from the yolk that is part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. Ovoviparity - Fertilized eggs are retained in the female, but the embryo obtains its FERTILIZATION (FROG) nourishment from the egg's yolk; the young Entrance of the sperm initiates several events: are fully developed when hatched. This occurs in some bony fish (such as the guppy 1. Meiosis II is completed. Lebistes reticulatus), some sharks, some 2. Cytoplasm rotates about 30° relative to the lizards, some snakes (such as the garter poles, forming the gray crescent opposite the snake Thamnophis sirtalis), some vipers, sperm entry point. In some amphibians 2  (including Xenopus), this is revealed by the appearance of a light-colored band, the gray crescent. This foretells the animal's future pattern: dorsal (D) and ventral (V) surfaces; anterior (A) and posterior (P); left and right sides. 3. Haploid sperm and egg nuclei fuse to form the diploid zygote. CLEAVAGE BASED ON YOLK DISTRIBUTION IN EGGS HOLOBLASTIC CLEAVAGE Eggs with low yolk content. A. Alecithal – Negligible yolk. B. Isolecithal – Evenly distributed yolk. C. Mesolecithal – Moderate and unevenly distributed yolk. FERTILIZATION (CHICKEN) 1. Cell division to create the embryo starts MEROBLASTIC CLEAVAGE about five hours after fertilization and Eggs with high yolk content. continues as the egg passes along the A. Telolecithal – Yolk concentrated at the oviduct and after it is laid. vegetal pole. 2. The hen’s egg takes 22 days to develop, B. Centrolecithal – Yolk concentrated at the including one day in the oviduct and 21 days center. in the incubator or nest. - In this type of the cleavage furrow bisects - The entire egg. Such a cleavage may be either equal or unequal. i. Equal holoblastic cleavage - In microlecithal and isolecithal eggs, cleavage leads to the formation of blastomeres of equal size. Eg: Amphioxus and placental mammals ii. Unequal holoblastic cleavage - In mesolecithal and telolocithal eggs,cleavage CLEAVAGE leads to the formation of blastomeres of A series of mitotic divisions setting the stage for unequal size. Among the blastomeres there development. Different cells receive different are many small sized micromeres and a few portions of the egg cytoplasm. large sized macromeres Morula Blastula In this type the cleavage furrows are restricted to the Blastoderm active cytoplasm found either in the animal pole Blastocoel (macrolecithal egg) or superficially surrounding the egg (centrolecithal egg). Meroblastic cleavage may be of two types. 3  A. Discoidal cleavage - Since the macrolecithal eggs contain plenty of yolk, the cytoplasm is restricted to the narrow region in the animal pole. Hence, cleavage furrows can be formed only in the disc-like animal pole region. Such a cleavage is called discoidal meroblastic cleavage. Eg: birds and reptiles. B. Superficial cleavage - In centrolecithal eggs, the cleavage is restricted to the peripheral cytoplasm of the egg. Eg: insects PLANES OF CLEAVAGE A. Meridional Plane: Cleavage along the animal-vegetal axis, dividing the egg into two equal halves. B. Vertical Plane: Cleavage on either side of the meridional plane. C. Equatorial Plane: Cleavage perpendicular to the main axis. D. Latitudinal Plane: Cleavage parallel to the equator. BASIS OF ARRANGEMENT OF BLASTOMERES IN CLEAVAGE A. Radial Cleavage: Cleavage at right angles, forming symmetrical tiers of cells. Examples: Deuterostomes (Echinodermata, Chordata). B. Spiral Cleavage: Oblique cleavage planes. Clockwise (dextral) or counterclockwise (sinistral). Examples: Mollusca, Helminthes, Annelids. C. Bilateral Cleavage: Establishes bilateral symmetry, with smaller and larger blastomeres on different sides. Examples: Amphibia, Tunicata, Cephalochordata. 4 D. Biradial Cleavage: First two cleavages are meridional, the third is vertical. Four central blastomeres are large, while four are small. FERTILIZED EGG The fertilized egg of the frog undergoes holoblastic unequal cleavage. The first cleavage plane passing from animal to vegetal pole bisects the fertilized egg into two blastomeres. The second plane at right angle with the first cleavage plane forming four blastomeres, while the third cleavage plane is latitudinal in relation to the first two cleavage planes. It is closer to the more pigmented area because the abundant amounts of cells of the animal pole are the micromeres, and the bigger cells of the vegetal pole are called the macromeres. 5