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Embryology Notes. Embryology. Development of a multicellular animal begins with fertilization : fusion of sperm with egg forming zygote Zygote immediately begins to develop – triggered by contact between cell membranes. Fertilization Process. - PowerPoint PPT Presentation
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Embryology Notes
EmbryologyDevelopment of a multicellular animal begins
with fertilization:
• fusion of sperm with egg forming zygote
• Zygote immediately begins to develop – triggered by contact between cell membranes
Fertilization Process• Sperm cell comes into contact with jelly coat
surrounding egg cell – Zona pellucida• Acrosome (vesicle in head of sperm) releases
enzymes that act on jelly coat and membrane of egg
• Sperm forms a tube that pushes through jelly coat
• Sperm pronucleus moves into egg cell• Egg’s membrane becomes impermeable to other
sperm – cortical reaction• Development is initiated
Embryological Development• Zygote begins rapid series of mitotic divisions
immediately after fertilization – cleavage
• During early cleavage, nuclei cycle rapidly between DNA replication (S phase) and mitosis – forms a solid ball of cells called a morula
Morula
• Newly formed cells (blastomeres) begin to pump Na+ into center of morula
• Results in the diffusion of water which creates a fluid-filled cavity – blastocoel
• Embryo is now a blastula
Gastrulation• An indentation forms on one side of the blastula –
blastopore• Blastula cells migrate in a continuous sheet
through blastopore – gastrulation• Forms three embryonic tissue layers – gastrula
– Ectoderm – forms skin and nervous system– Endoderm – lines digestive tract and forms associated
organs (liver and pancreas)– Mesoderm – forms most organs and tissues (kidney,
heart, muscles, inner layer of skin)• Blastocoel disappears as a new cavity forms –
archenteron – will become the cavity of digestive tract and blastopore becomes anus
Organogenesis• Ectoderm, endoderm, and mesoderm, rearrange
themselves into organs• Nervous system – derived from ectoderm• Soon after gastrulation, ectoderm is divided into 2
components: epidermis and neural plate• Sheet of ectodermal cells lying along midline of
embryo dorsal to newly formed digestive tract and developing notochord bends inward – neurulation
• Forms long groove extending the length of embryo• Folds that border groove move toward each other
and fuse, converting groove into long tube beneath surface of back
• Neural tube becomes detached from epidermis and differentiates into spinal cord and brain
• There are some differences between the early stages of embryological development in protostomes and deuterostomes – Development of coelom from mesoderm follows
a different pattern– Blastopore becomes the mouth in protostomes
and becomes the anus in deuterostomes• Protostomes – mollusks, annelids, and
arthropods• Deuterostomes – echinoderms and
chordates
Post Embryonic Development• Degree of development at birth varies
between species• Some are self sufficient – fish, reptiles,
amphibians• Some need limited amount of care (chicks,
ducks)• Some are helpless and totally dependent
(humans, mammals, many birds)
• All organisms undergo major developmental changes after birth
• Growth usually begins slowly, becomes rapid for a time and then slows or stops
• Growth does not occur at same rate or same time in all parts of body
Larval Development and Metamorphosis• Larval stages – many animals go through stages
that bear little resemblance to adult• Metamorphosis – developmental changes that
convert immature animal into adult form• Involves extensive cell division and differentiation• Stage 1 – egg Stage 2 – larva• Stage 3 – pupa
– After larval development, enter an inactive stage (pupa)– Enclosed in a case or cocoon, old larval tissues are
destroyed and new tissues and organs develop from small groups of cells – imaginal discs
• Stage 4 - adult• Complete metamorphosis – all stages including
pupal stage• Incomplete metamorphosis – has a nymph stage
Larva Pupa Emerging Adult Adult
Aging and Death• Development does not end with mature adult
– continues until death• Aging – complex of developmental changes
through time that ultimately leads to deterioration and death
• Factors contributing to aging:– Replacement of damaged tissue by connective
tissue – becomes a burden on other cells– Changing hormonal balance – may disturb
function of variety of tissues– Cells tend to accumulate metabolic wastes as
they get older
Mechanisms of DevelopmentThe developmental fate of cells is based on two general
principles:1. The cytoplasmic makeup is not the same throughout the
unfertilized egg (differences in concentration of proteins and mRNAs)• Helps to establish polarity (animal/vegetal poles)• Local differences influence the expression of genes• In many species, first few divisions result in totipotent
cells – retain zygote’s potential to form all parts of the animal
• Determination – results in progressive reduction of potency – occurs when cytoplasmic environment affects gene expression (cells begin to “remember” what they are supposed to be)
2. Cell to cell interactions create more differences during morphogenesis• Cell interactions may elicit changes in gene
expression that occur only among neighboring cells
• May be accomplished by the transmission of chemical signals or by membrane interactions if cells are in physical contact
InductionThe ability of one group of cells to influence
the development of an adjacent group of cells• Interactions between neighboring cells
are crucial during and after gastrulation in the origin of most organs
• Resulting effect is to switch on sets of genes that cause cells to differentiate into specific tissues
DifferentiationCells begin to specialize in structure and
function – cell has alterations to cellular structure and has tissue-specific proteins• Cells become experts at making certain
proteins• All cells of an organism have genomic
equivalence (have the same genes)
• Genes that are not used are turned off (usually permanently)
Conclusion:• Nuclei change in some way as they prepare
for differentiation• This change in not always irreversible
implying that the nucleus of a differentiated cell has all the genes required for making all other parts of organism
• Cells of body differ in structure and function because they express different parts of the common genome