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