Topic 2 Human Reproductive Anatomy: Cleavage

Question 1

• The cleavage stage involves rapid cell division of the zygote without cell growth. Each cell is a blastomere, which has less cytoplasm than the original zygote. The transition from fertilization to cleavage is caused by mitosis promoting factor, which is now being transcribed

Answer 1

Note

Question 2

• An egg has an upper, animal pole and lower, vegetal pole. Depending on the species, the vegetal pole can contain more yolk material, which is denser than the cytoplasm and settles at the bottom. In general, the vegetal pole differentiates into extra-embryonic membranes that protect and nourish the embryo. Polarity is critical in setting up body axes.
• Note the grey crescent in frogs, which is a region of non-pigmented cytoplasm formed at the opposite side of sperm entry. The grey crescent is a marker of the future dorsal side that is always bisected by the first cleavage plane.

Answer 2

1. Embryo Polarity

Question 3

• Early cleavages are polar, and divide the egg into segments that stretch from pole to pole, like the segments of an orange. Others are parallel with the equator. Note that in frogs, the horizontal cleavage is closer to the animal pole.

Answer 3

2. Polar and equatorial cleavages

Question 4

• Radial cleavage occurs in deuterostomes. Radial cleavage forms indeterminate cells at animal and vegetal poles that are aligned together, with top cells directly above bottom cells. In protostomes, spiral cleavage occurs, and determinate cells are formed on top and are shifted relative to those below.
• Recall that the first opening in protostomes forms the mouth, while the first opening in deuterostomes forms the anus.

Answer 4

3. Radial and spiral cleavages

Question 5

• In indeterminate cleavage, blastomeres can individually complete normal development if separated. In determinate cleavage, blastomeres cannot develop into a complete embryo if separated; each is differentiated into part of the embryo. Thus, if blastomeres of determinate cleavages are separated from each other, they will die/arrest

Answer 5

4. Indeterminate and determinate cleavages

Question 6

• There are two cleavage types: holoblastic and meroblastic. In holoblastic, cleavage is complete (passes all the way through the zygote), and occurs in most vertebrate ancestors and descendants. In meroblastic, cleavage is only partial, and occurs in birds, reptiles, fish, monotremes, and mollusks.

Answer 6

Note

Question 7

• Successive cleavage results in a solid ball of ~8 cells, where the first 8 cells are totipotent, meaning the cells are capable of giving rise to any cell type or embryo

Answer 7

5. Morula

Question 8

• Cell division continues, and liquid fills the morula and pushes cells outward to form a circular cavity surrounded by a single layer of cells. The blastocoel is the fluid filled cavity.
  a. Note that there are ~128 cells at the blastula stage
  b. In humans, the blastula is called the blastocyst when it implants into the endometrium, and this blastocyst contains an inner cell mass. Frogs and sea urchins have similar blastula to one another, but frogs have a built up vegetal hemisphere
  c. Note that all embryos have an animal and vegetal pole, but amphibians, reptiles, fish, and birds are distinct in that their yolk, and therefore nutrients, is not evenly distributed.
  d. Blastula cells are pluripotent, and can develop into any cell type but not a complete organism like morula cells can

Answer 8

6. Blastula

Question 9

• The formation of
  the gastrula occurs with the invagination of a group of cells into the blastula, forming a two-layered embryo with an opening from the outside into a center cavity. The gastrula forms ~14 days post fertilization. Actin filament contractions changing the shape of migrating cells causes the invagination of the blastula tissue. Three features associated with the gastrula are as follows:

Answer 9

7. Gastrula

Question 10

• Ectoderm, mesoderm, and endoderm. The third layer is formed between the outer and inner layer of the invaginated embryo. These germ layers give rise to all subsequent tissues

Answer 10

Three germ layers of Gastrula

Question 11

• gives rise to the following:
• Nervous system (brain and spinal cord)
• Integument (epidermis, hair, epithelium of nose, mouth, and anal canal)
• Sensory structures (lens of eye, retina)
• Neural tube
• Via neural crest cells, gives rise to teeth, jaws, and bones of face and skull
• Adrenal medulla

Answer 11

1. Ectoderm

Question 12

• Gives rise to the following:
• Musculoskeletal system
• Circulatory/lymphatic system
• Excretory system
• Gonads
• Connective tissue
• Portions of digestive and respiratory system
• Notochord
• Somites
• Kidney
• Dermis of skin
• Adrenal cortex
• Some primitive animals like sponges and cnidaria will develop mesoglea, a noncellular layer, instead of the mesoderm

Answer 12

2. Mesoderm

Question 13

• Gives rise to the following:
• Epithelial lining of digestive and respiratory tract
• Parts of liver
• Pancreas
• Gall bladder
• Thyroid and parathyroid
• Thymus
• Urinary bladder lining

Answer 13

3. Endoderm

Question 14

• The center cavity formed by gastrulation that is completely surrounded by endoderm cells and gives rise to the gut

Answer 14

Archenteron of Gastrula

Question 15

• Opening into the archenteron, becomes the mouth in protostomes or the anus in deuterostomes

Answer 15

Blastopore of Gastrula

Question 16

• In birds, reptiles, and humans (collectively, amniotes), membranes develop outside of the embryo proper. Note that amphibians are not amniotes.

Answer 16

8. Extra-embryonic membrane development

Question 17

a. Chorion
b. Allantois
c. Amnion
d. Yolk Sac
e. Organogenesis

Answer 17

Types of Extra-embryonic membrane development

Question 18

i. Birds and reptiles - functions as a membrane for gas exchange
ii. Mammals - chorion implants into endometrium, and later, the chorion and maternal tissue (which is modified endometrial tissue called deciduas basalis) form the placenta. The placenta is a blend of maternal and embryonic tissue across which gases, nutrients, and wastes are exchanged.

Answer 18

a. Chorion

Question 19

• Sac that buds off from archenteron that eventually encircles the embryo, forming below the chorion.
  i. Birds and reptiles - initially stores waste products as uric acid, and later fuses with the chorion to form a membrane for gas exchange with blood vessels beneath it
  ii. Mammals - allantois functions to transport waste products to placenta, and eventually forms the umbilical cord between the embryo and placenta. The umbilical cord transports gases, nutrients, and wastes. In adults, the allantois becomes the urinary bladder. The allantois is only found in reptiles, birds, and mammals.

Answer 19

b. Allantois

Question 20

• Encloses the amniotic cavity, and is a fluid-filled cavity that cushions the developing embryo, much like the coelom that cushions internal organs in coelomates. Amphibians do not have an amnion.

Answer 20

c. Amnion

Question 21

i. Birds and reptiles – digests enclosed yolk, and blood vessels transfer nutrients to embryo
ii. Mammals - is empty and contains no yolk as the umbilical cord and placenta deliver nutrients instead. Major function involves aiding in the formation of developing RBC’s

Answer 21

d. Yolk Sac

Question 22

• Cells continue to divide after gastrulation → differentiate into specific tissues and organs. Once gastrulation is complete, evidence of cell differentiation can be observed. In chordates:
  i. Notochord - cells along the dorsal surface of the mesoderm layer form the notochord, a stiff cartilaginous rod that provides support in lower chordates. Vertebrae of higher chordates are formed from nearby cells in mesoderm.
  ii. Neural tube - in ectoderm layer directly above the notochord, a layer of cells forms the neural plate. When the plate indents, the neural groove forms, which rolls up into a cylinder forming the neural tube. The neural tube develops into the CNS, and additional cells roll off the top of the neural tube and form neural crest cells, which form teeth, bones, muscles of skull, pigment cells in skin, and nerve tissue.

Answer 22

e. Organogenesis