Biology 2401    Lecture Notes for Embryology Supplement    
Dr. Weis

EMBRYOLOGY: study of the Embryo

Development

I. Period of the Egg to Embryo from fertilization until 8 weeks

A. Fertilization

Begins with contact between sperm and the secondary oocyte and ends with the fusion of the nuclei of the sperm and ovum and the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote.

Results of Fertilization:

1. Restoration of the diploid number (2n), for humans 46 chromosomes
2. Species variation (1/2 of the chromosomes are from the mother, ˝
                                    from the father) so the zygote contains a new combination of chromosomes.
3. Sex determination    XX or XY, by the father's gamete
4. Initiation of cleavage
5. Embryonic development commences with fertilization

B. Cleavage: mitotic cell division of the zygote into daughter cells called blastomeres.  When the blastomere number is equal to 16, the zygote is referred to as a morula.  This phase ends with the formation of the blastocyst.

C. Blastocyst: cavities appear inside the morula and fluid will pass from the uterus to collect in these cavities.  As fluid increases, it separates the cells into two parts

1. The outer cell layers called Trophoblasts

a. inner layer secretes a hormone in humans called hCG
b. outer layer will give rise to the fetal portion of the placenta

2. The centrally located cells called the inner cell mass will reform to create the embryo
The fluid filled spaces fuse to form a single large space known as the blastocoele. The blastocyst will eventually attach to the endometrial epithelium (uterine lining) and the trophoblasts will proliferate and differentiate into two layers:

a.       inner cytotrophoblasts
b.      outer synctial trophoblasts

The synctial trophoblasts will grow into the endometrial epithelium by extending finger like processes in order to implant in to this lining. As the blastocyst is implanting, an early differentiation of the inner cell mass occurs.  A primitive endoderm called the HYPOBLAST appears on the surface of the inner cell mass, facing the blastocoele on the ventral surface.

Changes will continue to occur in the inner cell mass that produce a bilaminar embryonic disc composed of epiblast and hypoblast layers. The epiblast gives rise to all three germ layers of the embryo: Ectoderm, Mesoderm, and Endoderm.  The hypoblast represents the primitive endoderm but will be displaced to extra-embryonic regions.

As this bilaminar (two layered) disc forms, the amniotic cavity, yolk sac, connecting stalk, and the chorion develop.

II. The third week of development is characterized by the formation of the primitive streak and the three germ layers.

A.     Gastrulation is the process by which the bilaminar embryonic disc is converted to into a trilaminar (three layered) embryonic disc.

1.      Many cells of the epiblast detach themselves and migrate inwardly forming a loose network of tissues called mesoblasts.  These cells will form the layer between the epiblast and the hypoblast to become the embryonic mesoderm.
2.      Some mesoblastic cells invade the hypoblast and displace them laterally.  This newly formed layer is known as the embryonic endoderm.
3.      The cells that remain in the epiblast form the layer called the embryonic ectoderm.
Therefore, the epiblast is the source of embryonic ectoderm, mesoderm, and most if not all of the endoderm.

B.     Tissue Formation

            The cells of these three germ layers divided, migrate, and differentiate into tissues and then create the organs of the embryo.

1.      The ectoderm gives rise to

a.       the outer epithelial (stratified squamous epithelial of epidermis)
b.      Nervous system.

2.      The endoderm is the source of

a.       epithelial linings for most organs
b.      glandular epithelium

3. Mesoderm gives rise to

a. smooth muscle coats
b. connective tissue (blood, bone, cartilage, CT proper)
c. vessels
d. striated muscle
e. skeleton
f. reproductive and excretory organ muscles

C. A thickened band of epiblasts appear at the caudal end of the dorsal aspect of the embryonic disc.  This band called the primitive streak makes it possible to identify the embryo's cranial/caudal, dorsal/ventral surface and its right/left sides.

The primitive streak begins to produce mesoblastic cells destined to become mesoderm.  Due to the growth and migration of cells from the primitive streak, the flat circular embryonic disc will elongate and become pear shaped, with the expansion in the cranial portion.  Eventually, the primitive streak undergoes degenerative changes and becomes an insignificant structure in the sacro-coccygeal region of the embryo.

III. ORGANOGENESIS

First to occur is neutralization, the formation of the neural plate, neural folds and closure

            of the neural tube.   Second to develop is the cardiovascular system.

Development continues for structures of the head and limbs.

IV. The Fetal Period continues from the 9th week until birth and is concerned with the growth

and differentiation of tissues of organs that started to develop during the embryonic period.

V. Summary:

The three germ layers differentiate into various tissues and organs so that by the end of the embryonic period, the beginnings of ALL main organ systems have been established.  THEREFORE, the 4th – 8th week of development is the MOST CRUCIAL !!!! 
Disturbances during this period may give rise to major congenital malformations also known as birth defects.