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- Human embryogenesis refers to the development and formation of the human embryo.
- It encompasses the first eight weeks of development after fertilization, in which a single cell formed at fertilization turns into an organism with a multi-level body plan.
- It is characterized by the process of cell division and cellular differentiation that occurs during the early stages of development.
- The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth, and cellular differentiation.
- Human embryogenesis is completed in two stages:
(a) Germinal stage and
(b) Embryonic period proper stage
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- It extends from conception (fertilization) to the end of the second week of intrauterine life (IUL).
- The morphogenic events during this period include fertilization, transportation of zygote through the uterine tube, mitotic divisions/cleavage, implantation, and formation of primordial embryonic tissues.
- Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). It takes place in a section of the oviduct called the ampulla.
- The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of development commences.
- It takes the embryo 5 days to reach the lumen of the uterus.
- During its journey, the zygote undergoes mitotic divisions called cleavage divisions; these occur without the growth of daughter cells. These cells are now called blastomeres.
- At the 8-cell stage the embryo undergoes a peculiar process called compaction, leaving a portion of the blastomeres still facing the external environment.
- Compaction results in two cellular lineages: the outer trophoblast which forms the fetal component of the placenta; and the inner cell mass which forms the embryo proper and the extraembryonic membranes.
- These latter include the amniotic membrane of the amniotic cavity and the extraembryonic mesoderm.
- Once in the uterus, the embryo and the uterine lining recognize each other in a biochemical sense, permitting attachment of the embryo followed by a carefully controlled implantation.
- At implantation, the inner cell mass reorganizes into a two-layered embryo; the epithelial epiblast, which will form the embryo and amniotic membrane, and the hypoblast, which has an important role in the yolk sac formation and the orientation of the embryonic axes.
Embryonic period proper:
- It extends from the beginning of the third week to the end of the eighth week of IUL.
- It is also called the period of organogenesis.
- The morphogenic events during this period include differentiation of the germ layers into specific body organs, the formation of the placenta, umbilical cord, and extraembryonic membranes.
- The process of cellular movement, termed gastrulation, establishes the three primary germ layers. This occurs between days 14 and 19 post-conception.
- It is a series of rapid, complicated, but coordinated movements of cells from the surface of the bilaminar embryo into the interior.
- Gastrulation movements form the three germ layers:
– The ectoderm, which will develop into the skin and nervous system
– The mesoderm, which will develop into muscles, skeleton, connective tissue, blood, gonads, and kidneys
– The definitive endoderm, which will develop into the lining of the gut tube and respiratory system.
- Following gastrulation, the ectoderm gives rise to epithelial and neural tissue, and the gastrula is now referred to as the neurula.
- The neural plate that has formed as a thickened plate from the ectoderm continues to broaden and its ends start to fold upwards as neural folds.
- Neurulation refers to this folding process whereby the neural plate is transformed into the neural tube, and this takes place during the fourth week.
- They fold, along a shallow neural groove which has formed as a dividing median line in the neural plate.
- This deepens as the folds continue to gain height when they will meet and close together at the neural crest.
- The cells that migrate through the most cranial part of the primitive line from the paraxial mesoderm, which will give rise to the somitomeres that in the process of somitogenesis will differentiate into somites that will form the sclerotome, the syndrome, the myotome, and the dermatome to form cartilage and bone, tendons, dermis (skin), and muscle.
- The intermediate mesoderm gives rise to the urogenital tract and consists of cells that migrate from the middle region of the primitive line.
- Other cells migrate through the caudal part of the primitive line and form the lateral mesoderm, and those cells migrating by the most caudal part contribute to the extraembryonic mesoderm.
- The embryonic disc begins flat and round but eventually elongates to have a wider cephalic part and narrow-shaped caudal end. Cranial and caudal neuropores become progressively smaller until they close completely (by day 26) forming the neural tube.
- Organogenesis is the development of the organs that begins during the third to eighth week and continues until birth.
- The circulatory, excretory, and neurologic systems all begin to develop during embryogenesis.
- Hematopoietic stem cells give rise to all the blood cells develop from the mesoderm.
- The heart is the first functional organ to develop and starts to beat and pump blood at around 21 or 22 days.
- Vasculogenesis (the development of the circulatory system) begins. This starts on day 18 with cells in the splanchnopleuric mesoderm differentiating into angioblasts that develop into flattened endothelial cells.
- The digestive system starts to develop from the third week and by the twelfth week, the organs have correctly positioned themselves.
- The respiratory system develops from the lung bud, which appears in the ventral wall of the foregut about four weeks into development. The lung bud forms the trachea and two lateral growths known as the bronchial buds, which enlarge at the beginning of the fifth week to form the left and right main bronchi. These bronchi in turn form secondary (lobar) bronchi; three on the right and two on the left (reflecting the number of lung lobes). Tertiary bronchi form from secondary bronchi.
- The urogenital system begins to develop.
- The formation of the epidermis begins in the second month of development and it acquires its definitive arrangement at the end of the fourth month.
- Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen’s human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
- Sadler, T. W., & Langman, J. (2004). Langman’s medical embryology. Philadelphia, Pa: Lippincott Williams & Wilkins.
- Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
- Gilbert, S. F. (2000). Developmental biology. Sunderland, Mass: Sinauer Associates.