Lecture Fourteen - Animal development II Flashcards
What is organogenesis?
Various regions of the three embryonic germ layers develop into the rudiments of organs during the process of organogenesis.
The first structures to take shape in vertebrate organogenesis are the notochord and neural tube.
The notochord, a rod-like structure, is formed from dorsal mesoderm.
As notochord elongates, it stretches embryo along anterior-posterior axis.
The dorsal ectoderm above notochord becomes neural plate.
Neural plate folds inwards to form the neural tube which will eventually become the brain and spinal chord.
Strips of mesoderm on both sides of notochord form blocks called somites which will later give rise to vertebrae and muscles associated with axial skeleton.
Lateral to the somites, the mesoderm splits into two layers to form the lining of the body cavity (I.e. coelom).
Morphogenesis and cell differentiation continue to refine the organs…
Show and explain how organogenesis works in frogs.
Early in vertebrate organogenesis, the notochord forms from mesoderm and the neural plate forms from ectoderm.
The neural plate soon curves inwards forming the neural tube (future brain and spinal chord).
Mesoderm lateral to the notochords form somites (future axial muscles and vertebrae). Lateral to somites, mesoderm splits to form coelom (body cavity lining).
How does early and late organogenesis work in birds?
Early organogenesis:
The archenteron forms when lateral folds pinch the embryo away from the yolk. The embryo remains open to the yolk, attached by the yolk stalk. The notochord, neural tube, and somites subsequently form much as they do in the frog.
Late organogenesis:
Rudiments of most major organs have already formed in this chick embryo.
What do the three germ layers give rise to?
Ectoderm:
- Epidermis of skin and its derivatives (including sweat glands, hair follicles)
- Epithelial lining of mouth and rectum
- Sense receptors in epidermis
- Cornea and lens of eye
- Nervous system
- Adrenal medulla
- Tooth enamel
- Epithelium or pineal and pituitary glands
Mesoderm:
Notochord
• Skeletal system
- Muscular system
- Muscular layer of stomach, intestine etc.
- Excretory system
- Circulatory and lymphatic systems
- Reproductive system (except germ cells)
- Dermis of skin
- Lining of body cavity
- Adrenal cortex
Endoderm:
- Epithelial lining of digestive tract
- Epithelial lining of respiratory system
- Lining of urethra, urinary bladder, and reproductive system
- Liver
- Pancreas
- Thymus
- Thyroid and parathyroid glands
What structres have developed to to allow vertebrates to reporduce in dry environments?
All vertebrate embryos require an aqueous environment for development.
- Shelled egg of birds, reptiles and some mammals (monotremes).
- Uterus of marsupial and eutherian (placental) mammals.
Within the shell or uterus, the embryo is surrounded by fluid within a sac formed by a membrane called the amnion.
Reptiles, birds and mammals are therefore called amniotes.
The three germ layers in these organisms also give rise to the four extraembryonic membranes that surround the developing embryo:
Amnion protects the embryo in a fluid-filled cavity that prevents dehydration and cushions mechanical shock.
Allantois functions as a disposal sac for certain metabolic wastes. Also functions with chorion as a respiratory organ.
Chorion and the membrane of the allantois exchange gases between the embryo and the surrounding air. Oxygen and carbon dioxide diffuse freely across the egg’s shell.
Yolk sac expands over the yolk, a stockpile of nutrients stored in the egg. Blood vessels in the yolk sac membrane transport nutrients from the yolk into the embryo. Other nutrients are stored in the albumen (the “egg white”).
What occurs during mammalian development?
Eggs of placental memmals are small and contain few nutrients.
Exhibit holoblastic cleavage.
Show no obvious polarity.
Gastrulation and organogenesis resembe the process in birds and other reptiles.
Extraembryonic membrane in mammals are homologous to those of birds and other reptiles and have similar functions.
Describe the process of early human development.
Fertilisation gives rise to an embryo.
Embryo secretes human chronic gonadotrophins (HGC) which maintains secretion of progesterones and estrogens to help maintin lining of uterus.
Fertilisation occurs relatively far from uterus (near where oviduct, or fallopian tube receives eggs from ovaries).
After fertilisation, zygote divides several times as it travels through remainder of oviduct.
After reaching uterus, the embryo (now at blastocyst stage) implants into the inner lining of uterus (endometrium), where development continues.
Gestation:
Gestation involves three trimesters. Organogenesis occurs in first trimester.
Fetus grows rapidly in second trimester. HCG levels at this stage decline and placenta starts to secrete its own progesterone.
Final trimester involves rapid growth.
Parturition (fancy word for birthing):
Birth occurs though a series of strong unterine contractions.
Three stages:
1) Cervic dilaties.
2) Infant expelled head first through the vagina.
3) Placenta detaches from the uterus and is expelled shortly after the infant.
What is the process of partutition?
1) Cervix dilation.
2) Infant expelled head first through the vagina.
3) Placenta detaches from the uterus and is expelled shortly after the infant.
What does the developmental fate of cells depend on?
Their history and induction siganals.
Histroy:
During early cleavage, embryonic cells must somehow become different from one another…
Cytoplasmic determinants are present in the unfertilised eggs of many animals.
Uneven distribution of determinants help specify body axis and influence the expression of genes that affect developmental fate of cells.
Fate of embryonic cells, however, can be affected not only by the distribution of cytoplasmic determinants but also by how their distribution is affected by the zygote’s characteristic pattern of cleavage
This is illustrated by studies of amphibian development…
In amniotes, environmental differences in position of the cells (e.g. internally or on the outside surface of embryo) determine their different fates.
In mammals, this occurs around the 16-cell stage. Before that, the cells are totipotent.
Totipotency means that the cells still have the potential to become any kind of cell in the adult body (in some animals, only zygote is totipotent). In all species, as development proceeds, cells become less potent.
Inductive signals:
Once embryonic cell division creates cells that differ from each another, the cells begin to influence each others fate by induction.
Induction brought about by either chemical signals or, if cells are next to each other, cell-surface contact.
The response to an inductive signal is the switching on of genes that make the receiving cells differentiate into specific tissue.
This was demonstrated by experiments on gastrula…
What did the experiment with salamaners show, and what did it involve?
Control: Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres.
Experiments: Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere.
The two blastomeres were then separated and allowed to
Results:
Blastomeres that receive half or all of the gray crescent develop into normal embryos, but a blastomere that receives none of the gray crescent gives rise to an abnormal embryo without dorsal structures (“belly piece”).
Conclusions:
Cytoplasmic determinants localized in the gray crescent critical for normal development.
What experiment showed how inductive signals effected development?
Spemann and Mangold showed that the dorsal lip of the blastopore functions as an organiser of the embryo.
The organiser initiates a chain of inductions forming the notochord, neural tube and other organs.
Experiment:
Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the ventral side of the early gastrula of a nonpigmented newt.
Results:
During subsequent development, the recipient embryo formed a second notochord and neural tube in the region of the transplant, and eventually most of a second embryo.
Examination of the interior of the double embryo revealed that the secondary structures were formed in part from the host tissue.
Conclusions:
The transplanted dorsal lip was able to induce cells in a different region of the recipient to form structures different from their normal fate. In effect, the dorsal lip ‘organised’ the later development of an entire embryo.
This experiment implies that inductive signals play a major role in pattern formation (i.e. arrangement of organs and tissues in the right places in three dimansional space).
E.g. Embryonic cells within a limb bud of chick responds to positional information indicating location along three aces.
