Embryo Stages of Development Organogenesis And Folding Notes

General Embryology And Development Of Face Oral And Paraoral Structures

‘Embryology’ deals with different stages of development of human body in the womb of the mother (intrauterine life) starting from the fertilization of the ovum till the birth of the baby.

Period Of Embryo Development

The duration of human pregnancy is about 266 days from the day of Fertilization. This period is called gestation period or intrauterine life of the embryo.

Read And Learn More: General Embryology Important Notes

The entire gestation period can be subdivided into

1. Embryonic period
2. Fetal period

Embryonic period

• From the 1st week of intrauterine life to 8th week of intrauterine life.
• Includes
  • Division of zygote, formation of morula and blastocyst
  • Implantation of blastocyst
  • Differentiation of trophoblast
  • Formation of chorionic villi, bilaminar and trilaminar germ discs
  • Differentiation of individual germ layers into different tissues and Organs of the body

Some authors term the first 3 weeks of intrauterine life as the germinal period and the 3rd to 8th week as the embryonic period.

Fetal period:

• From the 9th week of intrauterine life till the termination of pregnancy.
• Significant growth of the fetus occurs at a rapid pace.

Embryonic Period

Embryonic Period First week of development:

The zygote undergoes series of mitotic divisions from the two-celled stage.

• By the 3rd day of intrauterine life, it consists of 16 cells and is called the morula.
• At this stage, the cells (blastomeres) can be identified in two groups:
  • Inner cell mass or embryoblast/embryo proper
  • Outer cell mass or trophoblast

Blastocyst formation:

Embryonic Period On the 4th day of intrauterine life:

• Morula enters into the uterine cavity.
• The uterine fluid enters through the zona pellucida and occupies the spaces between the cells of the inner cell mass. The fluid-filled intercellular spaces unite to form a larger cavity called the blastocoele (blastocyst cavity).
• Formation of the blastocoele pushes the embryoblast to a side. This stage of embryo is called blastocyst.
• When fully formed, the blastocyst consists of
• Embryoblast: A clump of inner cell mass fixed to a pole (embryonic pole) Of the blastocyst
• Trophoblast: An outer layer of flattened cells forming the epithelial wall of the blastocyst

Implantation:

• Implantation is the process by which the blastocyst attaches itself to the uterine endometrium.
• Normal site of implantation is the posterior wall of uterine fundus.
• At about 5th or 6th day of intrauterine life, as the blastocyst is formed, zona pellucida disappears and trophoblast begins to penetrate between the epithelial cells of uterine endometrium.
• By the end of the 1st week, blastocyst begins to implant in the uterine Endometrium.

Sometimes blastocyst is embedded in abnormal sites; extrauterine or intrauterine.

• Extrauterine implantation; also called ectopic pregnancy, can take place in uterine tube, ovary or abdominal cavity.
• Intrauterine abnormal implantation can take place at the lower part of the uterine cavity overlapping internal os.

Embryonic Period Second week of development:

By 8th day (beginning of 2nd week)

• The blastocyst is partly embedded in the endometrial stroma.
• Trophoblast starts differentiating into two layers:
  • Inner layer of cells with single nucleus called cytotrophoblast
  • Outer multinucleated zone without distinct cell boundaries called Syncytiotrophoblast
• Cells of embryoblast also start differentiating into two layers to form a bilaminar germ disc.
  • A layer of small flattened or cuboidal embryoblast cells close to the Blastocyst cavity is called hypoblast.
  • A layer of tall columnar cells close to the amniotic cavity is called Epiblast. (amniotic cavity is a cavity that appears within the epiblast.)

Embryonic Period By 9th–11th day:

• The blastocyst gradually gets embedded in the uterine endometrium.
• Syncytiotrophoblast proliferates rapidly at the expense of endometrial stroma due to which vacuoles appear in the syncytiotrophoblast which gradually fuse to form large lacunae. This is called the lacunar stage.
• At the same time, at the abembryonic pole, flattened cells originating from hypoblast form a thin membrane called exocoelomic (heuser’s) membrane. This membrane lines the inner surface of cytotrophoblast and together with hypoblast it forms the lining of exocoelomic cavity or primitive yolk sac.

Embryonic Period By 12th day:

The blastocyst is fully embedded in endometrial stroma.

• Syncytiotrophoblast is characterized by lacunar spaces that connect With each other.
• Cells of syncytiotrophoblast start penetrating deeper into the endometrium and erode the maternal capillaries (sinusoids).
• The lacunae now become continuous with sinusoids and maternal blood enters lacunar system and starts flowing through the trophoblastic system, establishing uteroplacental circulation.
• On the other hand, fresh cells are produced by the cells lining the yolk sac which occupy the space between inner surface of cytotrophoblast and the outer surface of exocoelomic cavity (primary yolk sac) and this mass of cells is called extraembryonic mesoderm.
• Soon, a cavity develops within extraembryonic mesoderm and is called extraembryonic coelom (chorionic cavity).
• Extraembryonic coelom gradually surrounds the primitive yolk sac and amniotic cavity except in the region of connecting stalk where latter connects the bilaminar germ disc to the trophoblast.

The cells of the connecting stalk are derived from the extraembryonic mesoderm. Subsequently, with the development of fetal blood vessels in it, the connecting stalk develops into umbilical cord.

Embryonic Period Around the 13th day:

Formation of secondary yolk sac or definitive yolk sac.

• Cells originating from hypoblast move along the inner side of exocoelomic membrane and start proliferating to form a cavity which is smaller than primitive yolk sac.
• Extraembryonic coelom expands and forms a large cavity which can Now be called chorionic cavity.

By the 13th day, one of the notable changes seen in the trophoblast is the initiation of the development of fetal component of placenta.

Embryonic Period Formation of prochordal plate:

• At the beginning of 3rd week, embryo is in the form of bilaminar disc.
• At about 18th day, the disc becomes pear-shaped with broad cephalic (cranial) end and narrow caudal end. A circular area near the cephalic end of embryonic disc becomes distinct from the remaining part of the disc. This area is called rochordal plate.
• In the region of prochordal plate, cuboidal cells of hypoblast become columnar and are firmly fused with the overlying epiblast cells.
• Appearance of prochordal plate helps in determining the central axis and Cephalic (head) end of the embryo.
• At a later stage, the prochordal plate differentiates into buccopharyngeal membrane which is the opening of the future oral cavity.

Embryonic Period Gastrulation:

Gastrulation is a process that converts the bilaminar germ disc into a trilaminar germ disc. It occurs during the 3rd week of intrauterine life.

At the end of this process, the following three definitive germ layers of the embryo are established:

1. Ectoderm
2. Intraembryonic mesoderm
3. Endoderm

• Beginning of the gastrulation is marked by the formation of the primitive streak on the surface of the epiblast.
• Primitive streak is a narrow midline groove with raised lateral edges.
• At the cephalic end of the primitive streak, a slightly elevated area called primitive node (hensen’s node) is formed.
• A small depression called primitive pit appears in the primitive node.
• At about 15th day, cells of the epiblast start migrating actively towards the primitive streak.
• As cells reach the streak, they become elongated or flaskshaped and get separated from the epiblast to pass into the space between the epiblast and hypoblast.
• Some of the invaginating cells displace the cells of hypoblast to form a new layer of cells creating the definitive endoderm.
• Some of the invaginating epiblast cells pass bilaterally through the primitive streak and come to lie between the epiblast and the newly created definitive endoderm to form a new (third) layer called intraembryonic mesoderm.
• The remaining cells of the epiblast form the definitive ectoderm.
• As increased number of cells move between the epiblast and hypoblast, they start to spread in lateral and cephalic direction and migrate beyond the margin of the embryonic disc to come in contact with the extraembryonic mesoderm.
• The cells of the intraembryonic mesoderm which travel in the cephalic direction pass on the either side of the prochordal plate, whereas in the region of prochordal plate, there is no mesoderm. Here, ectoderm and endoderm are firmly fused with each other.
• Cells of intraembryonic mesoderm which travel in the caudal direction enter the connecting stalk which is by now confined to the caudal end of the embryonic disc. As these cells pass towards connecting stalk, they leave an area caudal to primitive streak, where ectoderm and endoderm remain in contact without any mesoderm in between. This midline circular area at the caudal end of the embryonic disc is called cloacal membrane.

Notochord:

Notochord is a craniocaudally directed midline structure which extends from cranial end of the primitive streak to caudal end of the prochordal plate.

Formation of notochord:

• Notochord is formed between the ectoderm and the endoderm in the midline of the embryo.
• During gastrulation, the cells of the epiblast which invaginate through the primitive pit and move in the midline in cephalic direction until they reach prochordal plate are called prenotochordal cells.
• These cells form a solid cord between ectoderm and endoderm called the Notochordal process.
• The primitive pit extends its cavity into the notochordal process to form a tube called notochordal canal.
• Cells of the floor of canal merge with the cells of endoderm in that region and subsequently degenerate to communicate with the yolk sac.
• On the other end, the canal communicates with amniotic cavity through Primitive pit.
• At this stage, for a short period, the amniotic cavity communicates with the yolk sac through this neurenteric canal. Remaining parts of the walls of notochordal canal (other than the floor) become flattened to form notochordal plate.
• Notochordal plate again becomes curved to form a tube which is soon converted into a solid rod by proliferation of cells to form definitive notochord.

Functions of notochord:

• Notochord acts as a guide for the formation of axial skeleton of the embryo.
• It also induces the formation of neural tube.

Fate of notochord:

During development, a major part of the notochord disappears.

• In adults, some parts of notochord remain in the form of nucleus pulposus of intervertebral disc and apical ligament of dens.

Period Of Organogenesis

• The duration of 3rd to 8th weeks of gestation is called period of organogenesis.
• During this period, each of the three germ layers gives rise to a number of specific organs and tissues.
• By the end of the 8th week, major organ systems of the embryo are formed.