EARLY EMBRYOLOGY

Why do we need knowledge of embryology to study anatomy?

The fact is that all of those structures in all organisms are derived from a single cell that was formed by the union of two gametes.
Every organism you see came originally from a single cell, which divided and differentiated to form more complex structures. Thus, everything that we will study during the remaining part of the semester is dependent on the complicated embryological process.
Reproduction starts with two cells, the sperm and egg - haploid cells formed through the process of meiosis which are specially designed for their own specific purpose.
Sperm are:
extremely small cells that lack most of their cytoplasm
designed to travel through an aquatic medium (either internal or external) to reach the egg cell
travel by movements of one or more flagella that propel it toward the egg
all sperm consist of three basic pieces:
    - the head, which contains the genetic material and is capped by the acrosome (cap) at the apex that contains enzymes needed for the sperm to penetrate the egg
    - the middle piece contains the primary power source of mitochondria that fuel the movements of the tail piece.
Eggs are:
designed more for providing nutrient sources to the developing young than for movement
contains yolk that consists of lipids and protein for nutrients, along with enzymes needed to initiate development
primarily classified by the amount and the distribution of yolk in the egg:
    - microlecithal eggs (characteristic of the protochordates and eutherian mammals) have a very small amount of yolk, and the young hatch quickly as a result
    - mesolecithal eggs (characteristic of lampreys and amphibians) have an intermediate amount of yolk, and the young hatch at a later stage of development
    - macrolecithal eggs (characteristic of fishes, reptiles, birds and monotremes) have a large amount of yolk, and the young hatch at an even later stage
    - isolecithal: distribution of yolk can be even through the egg
    - telolecithal: yolk concentrated in one part of the egg - the area with less yolk and prominent haploid nucleus as the animal pole and the area with more yolk as the vegetal pole
At fertilization, enzymes in the acrosome of the sperm help to penetrate the egg
requires that the sperm break through the plasma and vitelline membrane surrounding the egg
to prevent more than one sperm from penetrating the egg (polyspermy), the egg undergoes a cortical reaction to bring the sperm head into the interior of the egg and change the vitelline envelope to form the fertilization membrane
Just after fertilization the zygote (fertilized egg) undergoes cleavage (mitotic cell divisions) and becomes subdivided into smaller cells - the gross arrangement of cells differs greatly among vertebrates, depending on the amount of yolk in the egg:
Holoblastic cleavage occurs when the cleavage furrows pass through the entire egg
cleavage can either be equal, where the resulting cells contain the same amount of yolk, or unequal, in which some cells contain more yolk than others:
    - equal cleavage occurs in microlecithal eggs
    - unequal cleavage occurs in mesolecithal eggs
cleavage results in the formation of a ball of cells (blastomeres) surrounding an internal cavity (blastocoel)
Meroblastic cleavage occurs more in macrolecithal eggs
cleavage takes place only in a disk at the animal pole
the cleavage furrows do not extend into the yolk
results in the formation of the blastodisk that lies on the top of the yolk
Gastrulation is characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers: ectoderm, mesoderm, and endoderm.

The cells migrate inward at the blastopore, which forms, or is close to, the location of the anus in the adult
the ectoderm forms the outer tube of the embryo
the endoderm is an inner tube that forms the alimentary canal and all its derivative organs
the mesoderm lies between these two layers.
At the end of gastrulation, the embryo is bilaterally symmetrical, with three discrete cell layers, and rudiments of the notochord and neural tube.

This blastopore-to-anus developmental pathway is found in Chordata, Hemichordata, Echinodermata (starfish, sea urchins, sea cucumbers, etc.), uniting these groups into a monophyletic group called the Deuterostomes. The plesiomorphic condition, found in the Protostomes, is for the blastopore to become the mouth.

Gastrulation in Amphioxus
begins by the flattening of the blastula, loss of the blastocoel, and formation of the archenteron - the embryonic gut cavity that is lined with endoderm. After flattening, two cell layers can be distinguished - ectoderm and endoderm.
chordamesoderm, or the longitudinal mid-dorsal group of mesoderm cells, moves into the roof of the archenteron during gastrulation and gives rise to the notochord
after flattening, the process of folding continues to further form the archenteron as well as the blastopore, or external opening of the gastrula
further differentiation of cells occurs through the process of budding off of mesodermal cells to:
    - form pouches that will later become organs
    - within these pouches are spaces that will become the body cavity or coelom
    - notochord formation proceeds with the condensing of the chordamesoderm into the notochord
    - the neural tube then forms from pinching of the ectoderm over the notochord

*Note: One important thing to realize at this point is that in your text, as well as in other diagrams of embryonic development, cells are color-coded to help you recognize where they are from:
    ectoderm is generally blue
    endoderm yellow
    mesoderm pink
    cells of the notochord are usually represented as green

Gastrulation in amphibians
amphibian gastrulation is changed slightly due to the larger amount of yolk contained within the egg - the cells at the animal pole are retained for the formation of the embryo while the yolk-filled cells at the vegetal pole are used more by the embryo as an energy source
gastrulation is initiated by invagination of cells to form the dorsal lip of the blastopore
cell movements cause the pushing out of the ectoderm and inward movement of the endoderm and yolk-filled cells
the archenteron is formed and the blastocoel is slowly filled with cells and lost
the yolk-filled cells of the vegetal pole remain for a short time to fill the space between the dorsal and ventral lip of the blastopore, thus forming the yolk plug
the mesoderm gradually differentiates from the rest of the cells of the gastrula, as do the chordamesoderm cells which go on to form the notochord
the gastrula then progresses into the neural tube formation stage, called neurulation

Gastrulation in birds
with birds, we begin to see the meroblastic type of development of the embryos, significantly more yolk than in the previous examples, and the movement of cells is different as the cells lie more in sheets rather than in a ball.
two processes lead to cell movement in the chick embryo:
    - delamination: sheets of cells split into separate layers
    - ingression: individual surface cells migrate to the interior of the embryo
during delamination, two layers of cells form (the hypoblast and the epiblast) with a cavity in between that is comparable to the blastocoel in amphibians - separation of these two layers results in the formation of two regions of the blastodisk, the area opaca and the area pellucida
during ingression the primitive streak forms (a longitudinal thickening of cells along the blastoderm of large-yolked eggs) through which prospective chordamesoderm and mesoderm cells move inward - cells of the hypoblast are replaced by endodermal cells
the primitive streak lengthens along the surface of the yolk through ingression - the embryo grows longer and occupies more of the area pellucida
after gastrulation, the process of neurulation, or formation of the neural tube and associated structures, occurs

Neurulation
occurs at or near the end of gastrulation and transforms the gastrula into a neurula by establishing the central nervous system
the ectoderm gives rise to neural folds flanking a neural groove along an axis from the blastopore toward the future head - these sink into the dorsum of the embryo and the folds meet mid-dorsally, forming a neural tube, of which the anterior part becomes the brain and the rest, the spinal cord
a population of mesodermal cells called chordamesoderm aggregates to form the notochord - chordamesoderm generally induces (embryonic induction) the neural tube to form (if the chordamesoderm is removed experimentally, the neural tube will not form)

During neurulation
the chordamesoderm that will go to form the notochord induces neural plate formation, which is the first stage in the formation of the neural tube.
characterized in most vertebrates by three stages
    - during the neural plate stage, the ectoderm on the dorsal side of the embryo overlying the notochord thickens to form the neural plate
    - at the neural fold stage, the thickened ectoderm folds, leaving an elevated area along the neural groove. The neural fold is wider in the anterior portion of the vertebrate embryo, which is the region that will form the brain.
    - during the neural tube stage, the neural folds move closer together and fuse - the neural groove becomes the cavity within the neural tube, which will later be capable of circulating cerebrospinal fluid that aids in the function of the central nervous system.

One derived characteristic found in vertebrates is the formation of neural crest cells
ectodermally derived
develop along the top of the neural tube as the neural folds close
most neural crest cells change into mesenchyme, an embryonic tissue that consists of star-shaped cells from all three germ layers
develop into the visceral skeleton (i.e. gill arches, some of which will develop into jaws), pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, Schwann cells that help protect neurons, and bony scales

Differentiation and derivation - Organogenesis:
After the production of the nerve tube, differentiation of the germ layers occurs rapidly, and organogenesis begins, in which the primary tissues differentiate into specific organs and tissues (Fig. 5.17).
* Endoderm - Endoderm gives rise to the epithelium of the alimentary tract, to structures derived from the pharyngeal pouches such as parathyroid glands, thymus gland, Eustachian tube and middle ear cavity (not the ossicles), and to structures that develop as an evagination of the gut, such as the thyroid gland, lungs or swim bladder, liver, gall bladder, pancreas, and urinary bladder.
* Mesoderm - becomes organized into three regions: the epimere (dorsal mesoderm), mesomere (intermediate mesoderm), and hypomere (lateral mesoderm).
    - Epimere: The somites constitute most of the dorsal mesoderm and have three regions:

dermatome - forms the dermis of the mid-dorsal skin
sclerotome gives rise to the vertebrae
myotome forms skeletal muscles other than those of the gill arches
    - Mesomere: gives rise to the kidney tubules, excretory organs, and reproductive ducts.
    - Hypomere: lateral-plate mesoderm is confined to the trunk and is divided into somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads and visceral muscles)
* Ectoderm - gives rise to:
Neural tube
Epidermis and associated glands
Neural crest and its derivatives: migrate through the embryo, giving rise to a diversity of structures
Ectodermal placodes: localized thickenings that sink below the surface and give rise to sensory neurons and sensory structures: olfactory placodes, forming the olfactory sacs; lens placodes, for the lens of the eye; otic placodes, to become the membranous labyrinth; a group of placodes that contributes neurons to the sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, placodes that form the neuromasts of the cephalic and lateral line canals

Sources of energy during development and extraembryonic membranes
The cleidoic egg is an important derived trait among many vertebrates and enabled tetrapods to be independent of water. In just considering the macro- and mesolecithal species, we know that both contain a moderate to large amount of yolk for the embryo to use as an energy source. In the cleidoic egg, water and oxygen are obtained through diffusion.

Extraembryonic membranes vary in complexity among the vertebrates:
Yolk sac
- forms around the yolk, and connects to the embryo via the yolk stalk to provide nutritional support during development
- amniotes and anamniotes differ in their yolk sacs
    fish have a trilaminar yolk sac, with an extraembryonic coelom that surrounds the yolk
    birds and reptiles possess a bilaminar yolk sac (consisting only of endoderm and splanchnic mesoderm).
Shell membrane/shell
- is formed only in the cleidoic egg of amniotes
- surrounds the embryo, yolk and albumins (egg white) and protects it
- provides a surface for diffusion of oxygen
    Allantois - acts as a compartment for storage of nitrogenous excretory products such as uric acid, and may remain after birth or hatching as the urinary bladder.
    Amnion - surrounds the embryo, and is filled with amniotic fluid to cushion the embryo
    Chorion - surrounds the amnion and yolk sac

Mammalian developmental modifications
the mammalian egg does contain some yolk, but it is microlecithal and isolecithal - requires that the embryo implant quickly in order to obtain more nutrients from the mother.
early cleavage in mammalian embryos followed by the blastocyst stage, of which the outer layer of cells is called the trophoblast. The inner cell mass of the blastocyst will go on to form the embryo.
during implantation in the uterus, the placenta is formed, which is a structure for physiological exchange between the fetus and the mother. The placenta consists of both a maternal contribution (endometrium of the uterus) and fetal contribution (trophoblast), which is believed to be used as an immunological barrier that prevents rejection of the fetus (and its paternal chromosomes) by the mother. The shape of the placenta varies depending on the species.
the inner cell mass of the blastocyst develops into the blastodisk (similar to that in chickens). Early stages of development of the mammalian embryo, such as primitive streak stage, neurulation and germ layer differentiation, are similar to that occurring in chickens and reptiles.
the primary difference found in mammals is the development of the umbilical cord - contains allantois and yolk sac as well as circulatory system structures that connect the embryo to the placenta.

Ontogeny and Phylogeny
The notion of a parallel between the stages of development (ontogeny) and the evolutionary history of adults (phylogeny) predates the acceptance of evolution. It was thought that there was a Scala Naturae, a "Ladder of Nature" or "Scale of Being" for living things, which could be arranged in a sequence, as if on the rungs of a ladder. The highest rung was viewed as a stage of perfection. Likewise, it was generally noted that the ontogeny of an individual consisted of a series of stepwise stages, and it was natural to assume a connection between the two.

Carl Von Baer: made a number of general conclusions about development called Von Baer's laws:

1. In development from the egg the general characters appear before the special characters.
2. From the more general characters the less general and finally the special characters are developed.
3. During its development an animal departs more and more from the form of other animals.
4. The young stages in the development of an animal are not like the adult stages of other animals lower down on the scale, but are like the young stages of those animals.
In other words, a chick embryo would be recognizable at an early stage as Vertebrata, but not any particular subtaxon. Later, it would be recognizable as Aves, and finally, it would be recognizable as a Gallus domesticus. Therefore, the ontogenetic stages do not run parallel to the sequence of taxa on the scale of being.

Ernst Haeckel: believed that the adult stages of the chain of ancestors are repeated during the ontogeny of the descendants, but that these stages are crowded back into the earlier stages of ontogeny. Thus, ontogeny is an abbreviated version of phylogeny. Haeckel claimed that the gill slits of human embryos were literally the same structures of ancestral adult fishes, that were pushed back into the early ontogeny of humans by an acceleration of development in lineages. In other words, the sequence of ontogenies was condensed, and new features were added by terminal addition.

Von Baer, in contrast, argued that the gills slits are not the adult stages of ancestors; rather they are simply a stage common to the early ontogeny of all vertebrates. That is, evolution proceeds from "undifferentiated homogeneity to differentiated heterogeneity"; from the general to the specific. Von Baer's theory requires only that organisms differentiate; Haeckel's requires a change in developmental timing.

Biogenetic Law
Both of these ideas preceded Darwin's theory of evolution and were re-read in the light of Darwin. Although Darwin favored Von Baer, Haeckel's ideas became more accepted. Haeckel's theory came to be known as the theory of recapitulation or the biogenetic law - Ontogeny recapitulates Phylogeny. This was an attractive idea, because it gave biologists a way of reading phylogeny directly from ontogeny.

The biogenetic law eventually lost popularity with the rise of experimental embryology and Mendelian genetics. Embryology showed that many varieties of change in developmental timing were possible, and that different parts of the organism might differ in rates of development; Mendelian genetics showed that genes could effect changes at any stage of development, and that terminal addition was not the only possibility.

 

 
Definitions 
Acrosome - cap at the apex of a sperm head that contains enzymes needed for the sperm to penetrate the egg

Allantois - extraembryonic membrane that develops as an outgrowth of the hindgut. Serves for respiration and excretion in reptile and bird embryos, contributes to the placenta in eutherians, and forms the urinary bladder and part of the urethra in adult amniotes

Archenteron - the embryonic gut cavity that is lined with endoderm

Blastocoel - a cavity of the blastula that becomes obliterated during gastrulation and mesoderm formation

Blastodisk - the disk of cells formed during cleavage that lies on top of the yolk of large-yolked eggs of fishes, reptiles and birds, and on the top of the yolk sac of mammals

Blastopore - external opening of the gastrula

Blastula - ball of cells formed during cleavage, usually containing a blastocoel

Chordamesoderm - the longitudinal middorsal group of meosdermal cells that moves into the roof of the archenteron during gastrulation and gives rise to the notochord

Delamination - downward movement of cells to form a new layer near the yolk

Dermatome - the lateral portion of a somite which will form the dermis of the skin

Fate map - shows the cell areas of blastulas that will subsequently give rise to particular kinds of cells

Holoblastic cleavage - cleavage furrows pass through the entire egg

Ingression - the longitudinal movement of cells along the surface of the yolk

Mesenchyme - an embryonic tissue that consists of star-shaped wandering cells that gives rise to most adult tissues

Myotome - a muscle segment, usually applied to embryonic segments

Nephric ridge - the region of the mesoderm between the somite and lateral plate that gives rise to the kidneys and gonads

Neural crest - a pair of ridges of ectodermal cells that develop along the top of the neural tube as the neural folds close; this derived character of vertebrates gives rise to many of their distinctive features, including visceral skeleton, pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, and certain bony scales

Neural tube - the tube formed in the embryo by the joining of the pair of neural folds; the precursor of the brain and spinal cord

Neuroectoderm - the portion of the ectoderm that gives rise to the neural tube and neural crest.

Primitive streak - a longitudinal thickening of cells along the blastoderm of large-yolked eggs through which prospective chordamesoderm and mesoderm cells move inward

Sclerotome - the medial portion of a somite that forms the vertebrae

Somatic - descriptive of structures that develop into the body wall or appendages as opposed to those in the gut tube, such as the somatic muscles, somatic skeleton

Splanchnic - descriptive of structures that supply the gut

Trophoblast - outer layer of the mammalian blastocyst; initiates placenta formation