Answers-1, BIO 3220, Early Development


1. Define gamete.
Gamete – sex cell

2. Compare microlecithal, mesolecithal, and macrolecithal eggs. What does lecithal mean?
Microlecithal – small amount of yolk; found in amphioxus and eutherians
Mesolecithal – medium amount of yolk; found in lampreys, dipnoans, chondosteans and amphibians
Macrolecithal – large amount of yolk; found in most fish, reptiles, birds, monotremes
Lecithal – yolk

3. Compare isolecithal and telolecithal eggs.
Isolecithal – even yolk distribution
Telolecithal – uneven yolk distribution

4. Define animal and vegetal pole.
Animal pole – the region of the egg with the highest metabolic activity
Vegetal pole – the region of the egg with the highest concentration of yolk

5. Describe the various membranes/enclosures around different eggs.
Vitelline membrane – a thin non-cellular membrane secreted by the oocyte and follicular cells that surround an oocyte and that will form the fertilization membrane if sperm penetration occurs
Jelly – protective covering of eggs; for example, amphibian eggs
Capsule – outermost encapsulating structure of the egg, consisting of one or more membranes, the protective shell
Shell – hard protective covering of eggs
Albumin – the egg white of bird eggs formed in the oviduct before the addition of the shell
Corona Radiata – a layer of protective follicle cells derived from the cumulus oophorus surrounding the zona pellucida of an oocyte after ovulation
Zona Pellucida – a translucent non-cellular membrane surrounding a mammalian egg through which sperm must penetrate in order for fertilization to occur

6. Distinguish and give and example of oviparous, viviparous, and ovoviviparous animals.
Oviparous – producing eggs that develop and hatch outside the body of the female, as in many fish, birds, and many amphibians
Viviparous – bringing forth living young, rather than laying eggs; producing live young from within the body of the parent female, as in most mammals
Ovoviviparous – retaining the eggs within the body of the female in a brood chamber in which the development of the embryo takes place, as in Squalus

7. Define zygote.
Zygote – Cell formed from the union of an egg and a sperm; fertilized egg; joined gametes

8. Compare cleavage, blastula formation, and gastrulation in microlecithal, mesolecithal, and macrolecithal eggs.
Microlecithal eggs – cleavage is equal; blastula is a hollow ball of cells with a blastocoel; gastrulation process is involution
Mesolecithal eggs – cleavage is unequal; blastocoel is displaced into animal pole; yolk impedes mitotic process; more mitosis at animal pole; gastrulation process is epiboly
Macrolecithal eggs –only mitotic division at animal pole; blastoderm forms at animal pole; gastrulation process is delamination

9. Name the 3 germ layers and list tissues derived from each of the layers.
Endoderm – first forms as outgrowth of inner cell mass in blastocyst in mammals; grows down to surround the blastocoel which then becomes the yolk sack; gives rise to digestive, liver, lungs, pancreas, thyroid gland, other glands
Ectoderm – cells of inner cell mass thicken and begin to multiply forming first the neural tube; gives rise to CNS, sense organs, mammary glands, sweat glands, skin, hair, hooves
Mesoderm – separate from the ectoderm and fills space between endoderm and ectoderm; gives rise to circulatory, skeletal, muscle, reproductive tracts, kidneys, urinary ducts

10. Explain the process of neurulation and organogenesis.
Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar. Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube. The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis. These cells become the neural crest cells.
During organogenesis considerable cell interactions and rearrangements occur which produces the tissues and organs of the body. Many cells undergo migrations from their place of origin during organogenesis and many organs contain cells from more than one germ layer.

11. Define the following terms –
Involution – enfolding; tucking in
Epiboly – streaming in; ex. cells move into middle of cell in gastrulation
Chordomesoderm – cells that form notochord during gastrulation
Epimere = dorsal mesoderm – the dorsal portion of the mesoderm of a chordate embryo that gives rise to the dermatone, myotome, and sclerotome
Somite – block of dorsal mesodermal cells adjacent to the notochord during vertebrate organogenesis. These transient structures define the segmental pattern of the embryo, and subsequently give rise to vertebrae and ribs, dermis of the back, and skeletal muscles of the back, body wall and limbs
Mesomere = intermediate mesoderm – layer of the mesoderm that gives rise to kidney tubules and associated ducts
Hypomere = lateral plate mesoderm – layer of the mesoderm that gives rise to the somatic, splanchnic, somatopleure, splanchnnopleure, and coelom layers
Somatic mesoderm – layer of the lateral plate mesoderm (hypomere) that contributes to bone of the girdles and limbs, muscles of body wall
Splanchnic mesoderm – layer of the lateral plate mesoderm (hypomere) that contributes to muscles of the heart, blood vessels and other visceral structures
Somatopleure – layer of the lateral plate mesoderm (hypomere) that forms the muscles and connective tissue of body wall
Splanchnnopleure – layer of the lateral plate mesoderm (hypomere) that forms the muscles and connective tissue of the gut tube
Coelom – layer of the lateral plate mesoderm (hypomere) that forms the body cavity
Primitive streak – thickening of the epiblast cell layer caused my movement of mesodermal cells into the blastocoel; this structure is characteristic of avian, reptilian and mammalian gastrulation
Mesenchyme – mesodermal tissue that forms connective tissue and blood and smooth muscles; mesoderm “on the move,” detaches and moves to another location
Notochord – a flexible rodlike structure in embryos of higher vertebrates, from which the spinal column develops
Blastoderm – the layer of cells formed by the cleavage of a fertilized mammalian egg, which later divides into the three germ layers from which the embryo develops
Delamination – the process by which the blastoderm forms 2 layers, the epiblast and the hypoblast; the epiblast becomes the ectoderm, the hypoblast becomes the endoderm
Archenteron – the central cavity of the gastrula, which ultimately becomes the intestinal or digestive cavity
Dermatome – the part of a mesodermal somite from which the dermis develops
Myotome – the segment of a somite in a vertebrate embryo that differentiates into skeletal muscle
Sclerotome – the portion of a somite that proliferates mesenchyme which migrates about the notochord to form the axial skeleton and ribs

12. List the amniote classes.
Reptiles, birds and mammals

13. Define body stalk and umbilical cord.
Body stalk – a band of mesoderm that connects the yolk and the embryo
Umbilical cord – a cord that connects the fetus to the maternal placenta in viviparous mammals

14. Name the main extraembryonic membranes and discuss their functions.
Body stalk – connects the yolk and the embryo
Yolk sac – in birds, to nourish embryo; in mammals atrophies, but source of blood cells and primordial germ cells
Amnion – non-vascular, fluid-filled protective cushion
Chorion – outermost membrane of embryo; attachment to mother
Allantois – for gas exchange and waste receptacle
Placenta – allows for better waste removal and nutrient uptake; primarily a yolk sac in marsupials