mammals development

Question 1

amniotes

Answer 1

amniotes also include mammals, which are egg-laying (few) or placental

Question 2

mammal cleavage

Answer 2

-mammals exhibit rotational holoblastic cleavage
-second cleavage division characterized by orthogonal division planes, one meridional and one equatorial
-blastomere pairs appear to rotate relative to each other
-cell divines are asynchronous
-activation of zygotic gene expression begins early (relative to number of cleavage divisions), and many zygotic genes are required for progression of cleavage

Question 3

mammal cleavage timeline

Answer 3

cleavage divisions are relatively slow, occurring 12-24 hrs apart

Question 4

blastomere compaction

Answer 4

-around the 8 cell stage, loosely arranged blastomeres form close contacts with each other and begin to undergo compaction
-this coincides with the expression of adhesion molecules such as E-cadherin
-outside cells form tight junctions that “seal” the embryo and inside cells form gap junctions that allow for exchange of small molecules

Question 5

morula to blastocyst

Answer 5

• at 16 cells, the embryo is a morula with clear inner and outer cells
• outer morula cells become trophoblast cells, and inner morula become inner cell mass cells: these are both characteristic of the blastocyst

Question 6

trophoblast

Answer 6

a layer of tissue on the outside of a mammalian blastula, supplying the embryo with nourishment and later forming the major part of the placenta.

Question 7

morula

Answer 7

a solid ball of cells resulting from division of a fertilized ovum, and from which a blastula is formed.

Question 8

inner cell mass cells

Answer 8

The inner cell mass of the blastocyst is composed of two kinds of cells: those that will become the mature organism (the epiblast), and those that will develop into the placenta, the chorion, and the amniotic membranes

Question 9

developmental adaptations for internal embryonic development

Answer 9

• mammalian eggs don’t rely on yolk, get nutrients directly from mother –> adaptation required to restructure lower oviduct into uterus
  -embryos evolved trophoblast to interact with mother
  -trophoblast forms chorion induces uterus to form decide, both parts of placenta
  -the ICM will form epiblast for the embryo, the amnion, and the allantois
  -the ICM will also form hypoblast cell types that create a yolk sac

Question 10

human gastrulation similar to chick

Answer 10

• the ICM is initially localized to one side of the blastocoel
• the ICM then forms two layers; epiblast and hypoblast
  -hypoblast cells migrate away and line the blastocoel; this forms the fluid-filled yolk sac
  -epiblast cells for a second layer of epiblast cells that form the amnion; fluid collects between the amnion and epiblast to form the amniotic cavity
• these events result in two distinct embryonic cavities separated by a bilaminar germ disc (two layer blastodisc) composed of epiblast and hypoblast
  -this arrangement is like the chick blastodisc, except with a yolk sac instead of yolk
  -like in the chick blastodisc, gastrulation begins in the posterior with a primitive groove and node that extends anteriorly
  -the process of ingression, EMT, and migration are largely conserved

Question 11

how are regional embryonic identities established?

Answer 11

• established by conserved homeotic genes
  -the homeotic (Hox) genes that impart segment identity in flies are conserved in mammals
  -the Hox genes exist in mammals in four sets (Hoxa to Hoxd)
• as in flies, gene expression in the embryo correlates with gene position within the cluster
  -note that timing of positional gene expression is different in mammals; anterior develops before posterior, so expression varies accordingly

Question 12

how genes specify vertebral identity along the A/P axis

Answer 12

• the mouse and chick have a similar number of overall vertebrae, but diff proportions of cervical and thoracic vertebrae
• in both animals, the cervical/thoracic boundary is established by end and beginning of Hox5 and Hox6, respectively
• thus, identity codes are likely the same, with positional information being slightly different
  -examination of the thoracic/lumbar boundary supports this model, as it coincides with the Hox9/10 transition in both animals

Question 13

Homeotic transformation of vertebral identity in mouse

Answer 13

• hox gene mutations in mice demonstrate homeosis and the anteriorization rule
  -when Hox10 is mutated (knocked-out), posterior lumbar vertebrae develop ribs like anterior thoracic vertebrae
  -when Hox11 is mutated, posterior sacral vertebrae develop as anterior lumbar vertebrae

Question 14

blastodisc

Answer 14

a blastula having the form of a disk of cells on top of the yolk in the eggs of reptiles and birds.