Embryology

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Animal Development

Early Views of development:

Preformation. Belief that animals formed from a progressively enlarging preformed infant.

Successive generations were progressively contained in each generation.

Epigenesis. Idea that animals develop from a simple structure an a series of stages. Early microscopy supported this, however ignored sperm & egg fusion.

Sperm & Egg

Acrosomal Reaction

Describes the process of fertilization of egg by sperm for aquatic animals.
Sperm has special proteins on its tip which attach to special receptor sites on egg.

Specificity prevents fertilization by another species

Once attached, enzymes are released from the tip of the sperm cell called an acrosome

These enzymes function to penetrate an outer jelly coat and membrane to allow sperm nucleus to enter the ovum.

Cortical reaction

Functions to prevent polyspermy (multiple fertilization)
When ovum is fertilized, calcium ions are released into cytoplasm

Stimulate corticle granules (calcium filled vesicles) to fuse forming a fertilization membrane preventing further fertilizations

Acrosomal & Cortical Reactions

Activation of egg

Once fertilized, egg metabolism is stimulated (probably through internal pH changes)

Mammal Fertilization

Some important differences exist

Sperm must pass through a series of cells surrounding/protecting egg called follicle cells
Sperm must penetrate a tough protein matrix called the zona pellucida.
Zona pellucida then is stimulated to become impenetrable once fertilization occurs.

Mammal Fertilization

Cleavage

Refers to a rapid succession of divisions following fertilization

Two types of cleavage;

Spiral - Cells divide in a diagonal/corkscrew orientation
Radial - Cells divide on 2 planes perpendicular to each other

Often fertilized zygote omits G₁ & G₂ phases of mitosis
Results in numerous cells called blastomeres
Original zygote develops into two regions or poles separated by a region called the grey crescent.

First is called the animal pole

Will develop into organism's tissue

Second is called vegetal pole

Will develop into yolk

Early stages of frog zygote cleavage

Division contiues until embryo develops into a ball of cells termed a Morula.

Consists of 16-64 cells

morula

Cleavage continues until embryo enlarges an develops a hollow center.

Known as a Blastula
Contains a hollow portion called a blastocoel.

Blastula

GASTRULATION

As cleavage continues, a portion of the blastula invaginates from the vegetal pole forming a Gastrula

New cavity is formed called an Archenteron. This is the organism's primitive gut
New opening formed is called a blastopore

Will determine what general classification the developing animal will become

In protostomes the blastopore will become the mouth
In deuterostomes the blastopore will become the anus

Results in an embryo with 3 distinct layers of cells which will develop into specific tissues

Ectoderm - Will form skin, nervous tissue etc…
Endoderm - Digestive tract and many other organs
Mesoderm - Blood and other organs…

Organogenesis

Refers to rudimentary organ formation from germ layers
Earliest for chordates in the formation of a notochord.

Forms from ectoderm above archenterons
Tissue on the ectoderm invaginates inwards forming a neural plate
Ectoderm layers connect above forming a neural crest, and isolating a neural tube
Also form structures called somites running along its axis

Will eventually form ribs, vertebra and associated muscles and structures

Early Human Development

Human cleavage is relatively slow in comparison to other animals ~30 hours for first division ~60 for the second.
- Probably due to the fact that fertilization occurs in fallopian tubes of the female and zygote will not receive nutrition until it reaches the uterine wall.
During transit cell undergoes compaction.
- Tightly adhere to each other.
When cell reaches ~100 cells it is called a blastocyst (not blastula), which contains an Inner Cell Mass. This is surrounded by a layer of cells called the trophoblast.
The embryo implants itself in the uterine lining with the help of enzymes secreted by the trophoblast.
Trophoblast will develop into the fetal portion of the placenta.
Inner cell mass will form embryo

Human Development:

Weeks 1-6:

Embryo develops rudimentary organs from the various dermal layers
Placenta begins development.

Weeks 6-8:

Embryo develops a recognizably human appearance
Fully functioning placenta has developed.
- Placenta is the only disposable human organ.
- Allows nutrients, gasses, and wastes to be transported from mother to fetus, or vice versa.
- Circulatory systems of mother and fetus pass parallel to each other. Materials are exchanged by diffusion. Blood systems do not connect.

Childbirth:

Signaled by the hypothalamus of the fetus. Starts a chemical chain reaction which stimulates contractions of the mother’s uterus.
Pressure on the uterus stimulates the cervix to release oxytocin, which stimulates further uterine contractions.
This results in a classic positive feedback control, in which each contraction stimulates further contractions of increasing strength.
The cervix gradually dilates to the extent that the fetus’ body can pass through, and through the vaginal canal.
Childbirth completes with the delivery of the placenta

Stages of Birth

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