Animal Development II: Gastrulation and Organogenesis Flashcards
Gastrulation
the cells in the blastula rearrange themselves to form 3 layers of cells and form the body plan
- embryo is called a gastrula
3 Important Outcomes of Gastrulation
- Formation of the embryonic tissues (germ layers) that later differentiate into specialized tissues and organ systems
- Formation of the embryonic gut, the archenteron
- Appearance of the major body axes. The information specifying the body axes was already present during cleavage as a result of cytoplasmic determinants/yolk polarity but do not become VISIBLE until AFTER gastrulation
Gastrulation Process: Triploblasts
- one group of cells moves into the blastocoel, the interior of the embryo, through an invagination called the blastopore to form the ENDODERM
- another group of cells moves to surround the embryo, forming the ECTODERM
- a third group of cells moves into the locations in between the outer + inner cell layers to form the MESODERM
- the endodermal cells continue through the interior of the embryo; this tack is the archenteron
- In protostomes, the blastopore becomes the embryo’s mouth. In deuterostomes, the blastopore becomes the embryo’s anus
Gastrulation Process: Diploblasts
lack mesodermal cells; have radial symmetry and have far fewer tissue types than triploblasts due to the lack of a mesoderm
Ectoderm
give rise to the nervous system, skin epidermis, and the epithelial lining of the mouth and rectum
Mesoderm
give rise to muscle and skeletal cells, the circulatory system, and reproductive organs
Endoderm
give rise to many internal organs, the epithelial lining, and digestive + respiratory systems
Tissues
groups of similar cells that work together on a specific task
Organs
structures made up of 2+ tissues to carry out a particular function
Organ System
groups of organs with related functions
Triploblasts Tissues
triploblastic organisms have 4 primary adult tissues that arise from the 3 embryonic germ layers:
1. Epithelial (lining of organs, skin surface)
2. Connective (fat + padding, bone, tendon)
3. Muscle (cardiac, smooth, skeletal)
4. Nervous (brain, spinal cord, nerves)
Epithelial Tissue
tightly packed sheets of cells that cover surfaces and line body cavities. the tight packing lets them act as barriers to the movement of fluids + potentially harmful microbes
- are “polarized”, meaning they have a top + bottom. the apical side faces inside the cavity/outside of a structure and is exposed to fluid or air. the basal side faces the underlying cells
- EX: outer layer of skin + internal lining of the small intestine
Connective Tissue
cells suspended in some kind of extracellular matrix that is usually made of protein fibers like collagen/fibrin in a solid, liquid, or jelly-like ground substance
- supports and connects other tissues
- LOOSE: most common type of CT that supports organs + blood vessels AND links epithelial tissues to the muscles underneath
- DENSE: AKA fibrous, is found in tendons + ligaments and connects muscle to bones and bones to each other
- Other types include adipose tissues (body fat), bone, cartilage, and blood
Muscle Tissue
essential for keeping the body upright, allowing movement, pumping blood, and pushing food through the digestive tract. muscle cells (AKA muscle fibers) contain the proteins actin and myosin that allow them to contract
- SKELETAL: attached to bones by tendons + allows you to control movement
- CARDIAC: found in the walls of the heart (not under voluntary control)
- SMOOTH: found in the walls of blood vessels, digestive tract, uterus, urinary bladder, and other internal structures (not under voluntary control)
Nervous Tissue
involved in sensing stimuli (external + internal cues) and processing + transmitting information
- NEURONS: basic function unit of the nervous system; generate electrical signals called action potentials that allow the neurons to convey information across long distances
- GLIA: supports neuronal function
Extra-Embryonic Tissues in Amniotes
The terrestrially-adapted amniotic egg is THE defining characteristic of amniotes (reptiles, birds, mammals). eggs contain four additional extra-embryonic tissues that are PRODUCED from the embryo but are not PART of the embryo:
1. Amnion
2. Chorion
3. Allantois
4. Yolk Sac
Amnion
Inner amniotic membrane surrounds the embryo itself, enclosing the aqueous environment the embryo develops in to protect it from mechanical shock and to support hydration
- in mammals, it encloses the fluid-filled cavity and surrounds the developing fetus with an aqueous environment
Chorion
surrounds the embryo and yolk sac, facilitates exchange of CO2 and O2 between the embryo and egg’s external environment
- in mammals, it separates the fetal and maternal sides of the placenta
Allantois
stores nitrogenous wastes produced by the embryo + facilitate respiration in combination with the chorion
- in mammals, it is part of the mammalian umbilical cord
Yolk Sac
encloses the nutrient-rich yolk and transports nutrients from the yolk to the embryo (is not the yolk itself, but is the membrane that surrounds the yolk)
- in mammals, it consists of blood vessels that are part of the mammalian umbilical cord
Mammals and Extra-Embryonic Tissues
- most mammals do not lay eggs but produce these tissues that function as part of the placenta and umbilical cord
- pregnancy in placental mammals is the result of INTERNALIZATION AND INCORPORATION of the amniotic egg into the uterus
- results in direct nourishment for the embryo inside the egg rather than laying it outside of the body with a predefined yolk amount
Organogenesis
germ layers created in gastrulation lead to the development of different organs in the animal body
Organogenesis: Nervous System
in vertebrates, one of the primary steps during organogenesis is the formation of the NERVOUS SYSTEM
- originates from ectodermal tissues
- during the formation of the neural system, induction causes some cells at the edge of the ectoderm to become epidermis cells. the remaining cells in the center form the neural plate that forms the NS
Notochord
rod-shaped mesodermal structure under the neural plate; signals the neural plate cells to fold over to form a tube called the neural tube
- disappears later in development, and the neural tube gives rise to the brain and spinal chord
