Topic 2 Human Reproductive Anatomy: Factors that Influence Development

Question 1

1. Influence of egg cytoplasm
2. Embryonic induction
3. Homeotic (Hox) genes
4. Apoptosis

Answer 1

Factors

Question 2

• Cytoplasmic material is distributed unequally in the egg (think gray crescent in frogs and yolk in bird eggs), which results in embryonic axes, such as animal and vegetal poles. When cleavages divide the egg, daughter cells have different quality of cytoplasmic substances, or cytoplasmic determinants. These determinants are unique substances that influence subsequent development of each daughter cell

Answer 2

1. Influence of egg cytoplasm

Question 3

• Influence of one cell/group of cells over neighboring cells. Organizers (controller cells) secrete chemicals that diffuse among neighboring cells, and influence their development. The dorsal lip of the blastopore, functioning as a primary organizer, induces notochord development in nearby cells.
  i. Example within human development - the lens of the eye is formed from the ectoderm of the head. The optic vesicle (part of the brain) touching a portion of the ectoderm of the head induces lens formation.

Answer 3

2. Embryonic induction

Question 4

• Control development by turning on/off other genes that code for substances that directly affect development of body segments. An experiment in fruit flies found that mutant homeotic genes resulted in wrong body parts in wrong places.
  i. Homeobox (unique DNA segments of 180 nucleotides) - identifies a particular class of genes that control development (encodes homeodomain of protein that can bind DNA). The homeobox sequence is highly preserved across species.
  ii. Embryonic lethals - mutations that affect a process as fundamental as segmentation, and cause death at embryo/larval stage

Answer 4

3. Homeotic (Hox) genes

Question 5

• Programmed cell death that is part of normal cell development. Apoptosis is essential for development of the nervous system, operation of immune system, and destruction of tissue (webbing) between fingers and toes.
  i. Damaged cells also undergo apoptosis; if not, cancer may develop. The process is regulated by protein activity, rather than at the transcriptional/translational level. Apoptosis proteins are present but inactive in normal cells.
  ii. Characteristics of apoptosis - changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation. There is no cellular rupturing, and no inflammatory response

Answer 5

4. Apoptosis

Question 6

• In mammals, mitochondria play an important role in apoptosis, which typically affects single cells.
• A cell is said to be determined if its final form cannot be changed. Cytoplasmic influences become diminished with each successive cell division, and the cells become determined later rather than sooner. Cells can be traced during development to build a lineage map.
• Cell differentiation is the process by which cells become specialized in structure and function. Recent research has shown that even fully differentiated cells can be altered under proper conditions. These are referred to as induced pluripotent stem cells. Induction occurs when one cell type affects the direction of differentiation of another cell type. In this case, cytoplasmic determinants still play a role. In the example of a frog, it takes just four days for cell division, differentiation, and morphogenesis, which describes the shaping of an organism, to transform a fertilized frog egg into a tadpole. Morphogenesis can be traced back to changes in shape, motility, and other characteristics of the cells that make up various parts of the embryo.

Answer 6

Note