1. Embryology Notes

2. 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

3. 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

5. 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

6. Morula

7. 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

8. 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

11. 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

13. Protostomes – mollusks, annelids, and arthropods
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

15. 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)

16. 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

17. 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

18. Larva Pupa Emerging Adult Adult

19. 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

20. Mechanisms of Development
The 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)

21. 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

22. Induction
The 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

23. Differentiation
Cells 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)

24. 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