FORM & FUNCTION (Gastrulation) Flashcards
Gastrulation:
-process of formation of germ layers
-a single layered blastula is converted into a trilaminar structure containing the outer ectoderm, middle mesoderm, and the inner endoderm
Blastoderm layers (chicken):
-area opaca
-area pellucida
Area pellucida (chicken):
-gives rise to the upper epiblast and lower hypoblast
Lower hypoblast (chicken):
-becomes the extra-embryonic endoderm
Forming the ‘primitive streak’ (chicken):
-from thickening of the blastoderm by the convergence of cells in the surface of the blastoderm towards the midline
Primitive node or Hensen’s node:
-formed from an increased concentration of cells in the cranial end
Formation of primitive streak and primitive node:
-establish the cranial-caudal axis and the right and left sides
Primitive groove formed from:
-cells of the epiblast converging on the primitive streak ingress into space between epiblast and hypoblast
Migrating epiblast cells (mes-endoderm cells/intermediate cell type):
- Make contact with the hypoblast and form the endoderm
- Some continue to migrate and elongate (mesoderm/mesenchymal cells)
Endoderm formation:
-migrating cells from epiblast make contact with hypoblast, replaces the hypoblast and forms the embryonic endoderm
-ingression and cell migration downwards and inwards continues
Mesenchymal cells:
-the cells of the epiblast that migrate through the primitive streak, elongates and gain the capability to migrate and differentiate
-intra-embryonic mesoderm and extra-embryonic mesoderm
Intra-embryonic mesoderm:
-those that stay between the endoderm and epiblast
-will become the organism
Extra-embryonic mesoderm:
-mesenchymal cells that migrate further laterally between hypoblast and trophectoderm
-will become the extra embryonic membranes
What gives rise to the ectoderm:
-elongated, migratory epiblast cells (bottle cells, due to its shape)
Mammalian gastrulation and germ layer formation resembles:
-that of a chicken
Notochord:
-both the primitive streak and primitive node recede, and is replaced by a notochord
-prominent midline structure composed of epiblast cells which have migrated through the cranial most portion of the primitive nodes (primitive node)
-eventually spans the entire length of the embryo
-not retained in many species
Left-right symmetry in vertebrates:
-developed during gastrulation
-genes (lv) transcription factors and proteins are involved
-defects or absence of some of these elements cause developmental anomalies
Ex. mutations in Iv
lv:
-situs inversus viscerum
-left, nodal
Snail:
-gene for right side development
Gene expression on left side development:
-leftward flow
-Fgf-8 promotes nodal and lefty-2
>activates left side formation genes and inhibits right side formation genes (ex. snail)
mutations in lv results in:
-random placement of organs on either side of the left hand axis
*lethal anomaly
Twinning:
-two individuals developed in same pregnancy
-dizygotic and monozygotic
Dizygotic:
-2 oocytes, from 2 different ovarian follicles, fertilized by 2 different spermatozoa
Monozygotic:
Monozygotic:
-single oocyte fertilized by single spermatozoa
-2 ways
1. Twinning occurs at 2 blastomere stage, where each one giving rise to an embryo/individual with separate fetal membranes
2. Duplication of the inner cell mass
Incidence of twinning in domestic animals depends on species and breed:
-cattle: 2-3%
-sheep: 2-5%
-horses: up to 30%
*conjoined twins in cattle: 1 in 100,000 births
Production of gastrulation=gastrula
-a pear shaped embryo
>3 germ layers
>column of mesoderm
>notochord (oriented in cranial to caudal axis)
>relative to notochord, the embryo has a right and left side
