Biology 4.2

Question 1

Descriptive Embryology Steps

Answer 1

Fertilization, cleavage, morula, blastula, gastrulation, organogenesis, extraembryonic membrane development

Question 2

Fertilization

Answer 2

First step of descriptive embryology. When sperm fertilizes oocyte, 2 nuclei fuse to make a zygote. In humans, zygote has 46 chromosomes, 23 maternal origin and 23 paternal origin

Question 3

Cleavage

Answer 3

Second step of descriptive embryology. Zygote undergoes many mitotic divisions without expanding cytoplasmic volume. Zygote remains same size while number of cells doubles with each division. New cells formed are called blastomeres and are smaller than previous. Cleavage patterns differ between species but consistent within animals of same species. Four types of cleavage:

• Determinate vs indeterminate cleavage
• holoblastic vs meroblastic cleavage

Question 4

Determinate vs indeterminate cleavage

Answer 4

Determinate cleavage makes blastomeres with developmental fate, each cell has pre-determined identity and lacks capacity to develop into a complete normal embryo upon separation from cleavage cluster, common for protosomes.

Indeterminate cleavage makes blastomeres that lack developmental fate, specialized later on in development, separating a single blastomere from cleavage cluster can produce a complete embryo, common for deuterosomes

Question 5

Holoblastic vs meroblastic cleavage

Answer 5

Determined by amount of yolk, which provides embryo with nourishments, in an egg. In zygote, dense yolk area is vegetal pole, area lacking yolk is animal pole.

Holoblastic cleavage occurs in species with little yolk, furrow completely penetrates the egg for complete divisions and make evenly divided embryos, occurs in humans, cleavage axes range from bilateral, radial (deuterostome), rotational, spiral (protosome)

Meroblastic cleavage occurs in species with abundance of yolk and clear polarity in terms of animal and vegetal poles, prevents cleavage furrow from completely penetrating egg for partial/incomplete divisions, making uneven blastomeres. Two major types: discoidal and superficial

Question 6

Morula

Answer 6

Third step of descriptive embryology. Zygote undergoes compaction to become tight ball of cells called morula. Allows for differentiation of internal cells and external cells, which allows for cellular differentiation into trophoblast (outer layer) and inner cell mass (ICM). Trophoblast becomes structure that allows for implantation and fetal portion of placenta. ICM differentiates to become embryo, amnion, yolk sac and allantois

Question 7

Blastula

Answer 7

Fourth step of descriptive embryology. As cell division continues, liquid fills centre of morula to push inner cell mass to one pole and form an internal cavity called blastocoel, indicating that zygote has become a blastula. Next, zygote goes to uterus and undergoes implantation upon tryphoblast-endometrium interaction. Inner cell mass differentiates into bilamaniar disc composed of 2 layers:
- hypoblast layer is bordered by blastocoel
- epiblast is found on opposing side
Bilaminar disc divides internal environment to 2 cavities that form the primitive yolk sac, bordered by hypoblastic layer, and amniotic cavity, bordered by epiblast

Question 8

Gastrulation

Answer 8

Fifth step of descriptive embryology. Invagination process on day 16 (humans) where the outer layer of epiblastic cells invaginate and flow into the region bordering the hypoblast and original epiblastic layer itself, invagination groove is called primitive streak. Regulated by hypoblastic layer and establishes the axes of the embryo and differentiates the epiblast into 3 embryonic germ layers. First wave of cells that undergo invagination migrate closest to hypoblast and form endoderm. The following cells spread out between endoderm and overlying epiblastic cells to become mesoderm, overlying cells become ectoderm. After gastrulation, blastula is then referred to as gastrula. Gastrulation changes shape of embryo to contain an internal cavity that is called archenteron with primitive streak as opening. Primitive streak also called blastopore. Archenteron is the first structure of gut development.

Question 9

Endoderm

Answer 9

Digestive system, liver, pancreas, lungs (inner layers)

Question 10

Mesoderm

Answer 10

Circulatory system, lungs (epithelial layers), musculoskeletal system

Question 11

Ectoderm

Answer 11

Hair, nails, skin, nervous system

Question 12

Developmental Patterns

Answer 12

Protostomes and deuterostomes are both coelomate organism types that differ in early embryonic development in 3 different stages: Early cleavage patterns, gastrulation-blastopore development and coelom development

Question 13

Early cleavage patterns

Answer 13

First stage of early embryonic development pattern differences between protosomes and deuterostomes. First few cleavages of zygote post-conception can occur in many different patterns.

• Protosome: determinate, holoblastic (spiral)
• Deuterostome: indeterminate, holoblastic (radial)

Question 14

Gastrulation-blastopore development

Answer 14

Second stage of early embryonic development pattern differences between protosomes and deuterostomes. Invagination of cells leads to formation of archenteron cavity where the blastopore is the opening.

• Protosome: blastospore first develops in mouth, then tunnels to secondarily form the anus
• Deuterostome: blastospore first develops into anus, then tunnels secondarily from the mouth

Question 15

Coelom development

Answer 15

Third stage of early embryonic development pattern differences between protosomes and deuterostomes. Coelom is fluid-filled cavity that cushions internal organs from outer wall of body. In lower level organisms, coelom acts as structural element. In higher level organisms, coelom allows for development and separation of organ systems which allow for increased physiological complexity.

• Protosome: undergo schizocoleous development of coelom where a mass of mesodermal cells split off and enter interior of gastrula to form coelom
• Deuterostome: undergo enterocoelic punching where outpourings of wall of archenteron pinch off to form mesodermal tissue that develops into coelom

Question 16

Organogenesis

Answer 16

Sixth step of descriptive embryology. Cellular differentiation becomes more complex, allowing for formation of various organs and tissues. Consists of neurulation and general organogenesis

Question 17

Neurulation

Answer 17

Part of organogenesis which is the sixth step of descriptive embryology. This is for chordates. It is when mesoderm forms rod of cells called notochord directly under primitive streaks, which induces overlying ectoderm to develop into a thickened neural plate extending along length of embryo. Neural plate invaginates and seals itself into a neural tube under the ectoderm, which eventually forms central nervous system. Some ectodermal cells become neural crest layer that becomes peripheral nervous system. Notochord becomes component of intervertebral disks. Surrounding mesodermal cells become axial skeleton and muscular components of vertebrate

Question 18

General organogenesis

Answer 18

Part of organogenesis which is the sixth step of descriptive embryology. It is when other mesodermal cells of embryo differentiate into groups of tissue called somites that act as segments along that notochord that migrate to create a general outline of the body and later develop into various bodily structures that would be found in that region.

Question 19

Extraembryonic membrane development

Answer 19

Seventh step of descriptive embryology. Only in birds, reptiles, amniotes (including humans). Structure and layers surrounding embryo that protect and nourish it. Consists of amnion, yolk sac, allantois and chorion

Question 20

Amnion

Answer 20

Part of extraembryonic membrane development which is seventh step of descriptive embryology. Innermost membrane that encloses amniotic cavity containing the embryo. Amniotic cavity is filled with amniotic fluid that cushions that embryo and absorbs mechanical impact, minimizing stress for the embryo.

Question 21

Yolk sac

Answer 21

Part of extraembryonic membrane development which is seventh step of descriptive embryology. Nutritional storage for embryo in birds and reptiles. In mammals, embryo receives nutrition directly from mother via placenta and umbilical cord. In placental mammals, yolk sac is preliminary circulatory system where blood cells develop, eventually being absorbed into the gut of the embryo.

Question 22

Allantois

Answer 22

Part of extraembryonic membrane development which is seventh step of descriptive embryology. Sac that branches off the initial archenteron, encircles embryo and forms a layer beneath the chorion. In birds and reptiles, acts as waste storage for uric acid but later fuses with chorion. In placental mammals, transports wastes to placenta and later develops into the umbilical cord which transports wastes, nutrients and gases between embryo and placenta

Question 23

Chorion

Answer 23

Part of extraembryonic membrane development which is seventh step of descriptive embryology. Outermembrane surrounding embryo. In birds and reptiles, acts as site for gas exchange. In mammals, aids in implantation into endometrium, the chorion and endometrium later form placenta to aid in gas, nutrient and water exchange between embryo and mother.