Embryogenesis

Embryogenesis is the process of embryonic development occurring in the first eight weeks after fertilization. After implantation of the blastocyst in the endometrium, the embryo consists of the embryoblast and the trophoblast. While the embryoblast further develops into different structures of the body, the trophoblast is mainly involved in the development of the placenta. The amniotic cavity, yolk sac, extraembryonic mesoderm, and the chorionic cavity develop during the second week. In weeks 3 and 4, the bilaminar disc differentiates into a trilaminar embryonic disc through the process of gastrulation. A number of structures develop from the three germ layers. The nervous system also develops during weeks 3 and 4 through the process of neurulation. Weeks 5–8 are mainly characterized by organogenesis and continued differentiation of embryonic tissue.

Embryonic development

This table is based on time since fertilization (i.e., developmental age), instead of time since first day of last menstrual period (i.e., gestational age).

The embryo is extremely susceptible to teratogens from week 3 to week 8, when the process of organogenesis occurs.

The developmental events from week 2 to week 4 can be remembered as follows: At 2 weeks there are 2 layers (bilaminar disc), 3 weeks there are 3 layers (trilaminar disc) and 4 weeks there are 4 limb buds and 4 heart chambers present.

Embryoblast

In the 2^nd week of embryonic development (days 8–14), the embryoblast differentiates into two layers (epiblast and hypoblast), termed the bilaminar disc. After formation of the amniotic cavity and yolk sac, the bilaminar disc is sandwiched between them.

• Embryoblast → bilaminar disc
  • Epiblast: columnar cells adjacent to amnioblasts → form the amniotic cavity → forms the embryo (begins the 3^rd week of embryonic development)
  • Hypoblast
    • Cuboidal cells adjacent to the blastocyst cavity (blastocoel)
    • Forms yolk sac, lining of chorionic cavity
  • Fusion of epiblast and hypoblast → prechordal plate → mouth

All three germ layers (ectoderm, mesoderm, and endoderm), as well as the amniotic cavity and therefore the entire embryonic tissue, arise from the epiblast. The extraembryonic mesoderm and the yolk sac are derived from the hypoblast.

The bilaminar disc forms the dividing layer between the yolk sac and amniotic cavity.

Trophoblast

• The trophoblast is the layer of cells that surrounds the blastocyst.
• During week 2, the trophoblast divides into two layers, the cytotrophoblast and the syncytiotrophoblast.
• They form the embryonic component of the placenta.

Cytotrophoblast

• The inner layer of the chorionic villi; mitotically active
• Function: cells migrate outward and fuse to become the syncytiotrophoblast

Cytotrophobasts make “cytos” (cells); Syncytiotrophoblasts synthesize hormones.

Syncytiotrophoblast

• The outer layer of the chorionic villi; mitotically inactive
• Function: invades the endometrium to create lacunae (spaces that fill with maternal blood)
  • Secretes beta-hCG to maintain the corpus luteum (which increases progesterone secretion by corpus luteum during the first trimester) and the decidua (starts when implantation of the blastocyst occurs)
  • Invades the endometrium to create lacunae (spaces that fill with maternal blood)
    • Proteolytic enzymes secreted by the syncytiotrophoblast break down the extracellular matrix
    • Formation of vacuoles within the syncytiotrophoblast, fusion of vacuoles to form lacunae (lacunar stage)
    • Invasion of syncytiotrophoblast into maternal capillaries (sinusoids)
    • Maternal sinusoids and lacunae fuse; maternal blood flows through the trophoblast → uteroplacental circulation
• Does not express MHC class I on the surface of its cell and is, therefore, less susceptible to the mother's immune cells attack.

Formation of the chorionic cavity

• Starting point: The amniotic cavity and primary yolk sac are surrounded by trophoblast cells.
• Sequence of events
  • The trophoblast grows significantly faster than the surrounding structures → a gap forms between it and the amniotic cavity.
  • The gap is lined by the extraembryonic mesoderm.
  • Renewed gap formation in the extraembryonic mesoderm results in the formation of a new cavity
  • The extraembryonic mesoderm splits into two layers (somatopleuric and splanchnopleuric mesoderm) but remains attached at a connecting stalk.
    • Somatopleuric (parietal) mesoderm: lines the trophoblast
    • Splanchnopleuric (visceral) mesoderm: surrounds the amniotic cavity and yolk sac with an intermediate embryonic disc
    • Extraembryonic coelom (chorionic cavity)
      • Cavity that is formed between the somatopleuric and splanchnopleuric layers → precursor of the gestational sac
      • Lined by the chorion (outermost cell layer surrounding the embryo)
    • Connecting stalk: A derivative of extraembryonic mesoderm that connects the amniotic cavity and primitive yolk sac to the chorion; precursor of the umbilical cord

The extraembryonic coelom is also called the chorionic cavity, which is lined by the chorion.

Gastrulation is the formation of the trilaminar embryonic disc or gastrula through the migration of epiblast cells. Epiblast cells migrate through the primitive streak between the epiblast and hypoblast layers and form an intermediate cell layer called the intraembryonic mesoderm. The hypoblast is replaced by epiblast cells, from which the endoderm arises. The original epiblast becomes the ectoderm.

• Origin: primitive streak, an elongated accumulation of epiblasts
• Result: formation of the trilaminar embryonic disc (mesoderm, endoderm, and ectoderm)
• Process: occurs in weeks 3–4
  • Mesoderm development (intraembryonic mesoderm)
    • Epiblast cells detach from the cellular complex below the primitive streak → formation of the primitive pit
    • Mesoderm cells migrate through primitive pit
    • Proliferation of cells on both sides of the primitive pit and formation of the mesoderm, a new layer between the epiblast and the hypoblast
  • Endoderm development
    • Epiblast cells migrate from the primitive streak and the primitive node to the hypoblast layer.
    • Hypoblast cells are invaded by epiblast cells and form an additional germ layer called the endoderm.
  • Ectoderm development: differentiation of remaining epiblast cells into ectoderm after migration of cells for endoderm, mesoderm, and notochord

All embryonic tissue originates from the epiblast.

• Definition: development of the notochord, a rodlike structure between the ectoderm and endoderm that is essential for the development of the nervous system and primitive skeletal structures
• Location: the notochord migrates along the primitive streak (the future craniocaudal axis), and ends at the prechordal plate. It is part of the axial mesoderm
• Process: occurs weeks 3–4 (at the same time as development of the trilaminar embryonic disc)
  • Epiblast cells migrate from the primitive nodes and extend cranially to form the notochordal process.
  • Notochordal process margins converge until they merge into the rod-shaped notochord at the end of week 4
• Derivatives
  • The notochord degenerates during the course of embryogenesis.
  • The nuclei pulposi of the intervertebral discs are remnants of the notochord.

Neurulation is the formation of the neural tube and neural crests, which are the precursors to the central and peripheral nervous systems. During this process, the surface ectoderm is also formed, which gives rise to the epidermis.

• Definition: formation of the neural tube and neural crests via neural plate folding
• Process
  • The notochord secretes signaling molecules that trigger differentiation of the medial ectoderm (located directly above the notochord) into neural cells.
  • This region is termed the neural plate.
• Sequence
  • The central area of the neural plate decreases (invagination) and becomes the neural groove.
  • The neural folds are found on both longitudinal sides of the neural groove.
  • The neural folds converge (week 3) and convert the neural groove into the neural tube.
  • The dorsal-most part of the neural tube forms the specialized neural crest.
  • The neural tube communicates with the amniotic cavity through cranial and caudal openings (neuropore).
    • Anterior neuropore: closes on ∼ days 24–25
    • Posterior neuropore: closes on ∼ days 26–27
  • Fusion of the opposite sides of the neural folds results in the formation of the surface ectoderm.
  • Neural fold fusion leads to several cells dissociating from the epithelial cluster, which deposit between the neural tube and surface ectoderm as the neural crest cells.
• Derivatives
  • Neural tube derivatives
    • Central nervous system (brain and spinal cord), including neurons and glial cells (except microglia)
    • Retina
  • Neural crest derivatives
    • Peripheral nervous system neurons (in cranial, dorsal root, and autonomic ganglia) and glia (satellite cells and Schwann cells)
    • Melanocytes
    • Adrenal medulla
    • Leptomeninges (pia and arachnoid)
    • Odontoblasts
    • Enterochromaffin cells
    • Aorticopulmonary septum (spiral membrane)
    • Endocardial cushions (partially derived also from mesoderm)
    • Cranial bones
  • Surface ectoderm: forms the epidermis, adenohypophysis, olfactory epithelial cells and sensory organs of the ear
  • Notochord: differentiates into nucleus pulposus of the intervertebral disk

The entire nervous system develops from the ectoderm.

Neural tube defects are one of the most common CNS malformations and develop as a result of incomplete closure of the neural tube (e.g., spina bifida, anencephaly).

Neural crest derivatives (Cranial bones, Adrenal medulla, Leptomeninges, Melanocytes, Enterochromaffin cells, Tracheal cartilage, PNS ganglia, Odontoblasts, Spiral membrane, and Endocardial cushions): Embryos have CALMEST POSE.

The branchial apparatus is an embryological structure with five paired arches composed of mesodermal and neural crest cells bound externally by an ectodermal cleft and internally by an endodermal pouch, which differentiate into various head and neck structures. The branchial apparatus is externally visible below the developing brain of a 4-week-old embryo. The fifth arch regresses in utero and does not contribute to the development of any head and neck structures.

• Pharyngeal groove: ectodermal origin
• Pharyngeal arch
  • Central mesenchymal core, which contains a nerve, an artery, and cartilage
  • Derived from the neural crest and mesoderm
• Pharyngeal pouch: endodermal origin

Derivatives from the outer to the inner layer (Groove: ectoderm; Arch: mesoderm and neural crest; Pouch: endoderm): GAP

Pharyngeal arches

Sensory and motor nerves do not derive from pharyngeal arches, but from the neural crest and neuroectoderm respectively.

Pharyngeal pouch

• Derivatives
  • First pharyngeal pouch
    • Eustachian tube
    • Tympanic cavity
    • Mastoid air cells (ear structures covered by endoderm)
  • Second pharyngeal pouch: epithelial crypts of palatine tonsil, supratonsillar fossa
  • Third pharyngeal pouch
    • Dorsal wings: inferior parathyroid glands
    • Ventral wings: thymus
  • Fourth pharyngeal pouch
    • Dorsal wings: superior parathyroid glands
    • Ventral wings: ultimobranchial body, parafollicular cells (C cells) of the thyroid

The inferior parathyroid glands arise cranially (3^rd pouch) but migrate caudally (lower poles of the thyroid gland). The superior parathyroid glands arise caudally (4^th pouch) but migrate cranially (superior poles of the thyroid gland).

Pharyngeal grooves

• Derivatives
  • The first pharyngeal groove develops into the external auditory meatus, the auditory canal, and the external aspect of the tympanic membrane.
  • The second to fourth pharyngeal grooves are obliterated in utero by the rapid growth of the second pharyngeal arch, and they do not differentiate into any of the head and neck structures.
• Cervical sinus
  • The second pharyngeal arch; develops faster than the others → overlap and merging with the third and fourth arches; → grooves lose their connection with the amnion → formation of cervical sinus.
  • The cervical sinus is obliterated during later development due to the proliferation of the 2^nd arch mesenchyme
  • Failure of the cervical sinus to obliterate completely results in branchial cleft sinus, which can lead to lateral cervical fistula formation
    • Located anterior to the sternocleidomastoid muscles
    • In contrast to a thyroglossal duct cyst, the swelling caused by the branchial cleft sinus does not move with swallowing.

A lateral cervical fistula is prone to infection and is a clear indication for operative treatment.

The aortic arches are blood vessels that run in between the pharyngeal pouches and form the major head and neck arteries. The arches develop in craniocaudal order, with the first two arches obliterating early and the fifth either never developing or also obliterating without giving rise to a vessel.

1^st is maximal (maxillary artery); Second for Stapedial; C is the 3^rd letter of the alphabet (Common Carotid, proximal internal Carotid); 4^th arch and 4 limbs (systemic arch).

Morphogenesis is the process by which the shape of an organism is generated. The embryo undergoes folding, resulting in transformation of the flat, embryonic disc into an embryo that approaches the human form during the course of the pregnancy. During the folding processes, the abdominal cavity, the abdominal wall, and the gut tube are formed. At the cranial and caudal embryonic poles, there is an area devoid of the mesoderm where the endoderm and ectoderm come into direct contact with one another, called the buccopharyngeal or cloacal membrane. The mouth and anus will later form in these areas.

Body axis determination

• Dorsoventral body axis: formed as the bilaminar disc develops
  • Dorsal body axis: epiblast
  • Ventral body axis: hypoblast
• Craniocaudal body axis
  • Formed by the extension of the primitive streak
  • Cranial pole: primitive nodes
• Right-left differentiation: The positioning of unpaired chest and abdominal organs is determined by asymmetric expression of signaling molecules in the lateral plate mesoderm.

Situs inversus is a very rare congenital condition in which the chest and abdominal organs are reversed or mirrored. It is usually considered a benign condition, but can also present as part of a syndrome, e.g., Kartagener syndrome.

Craniocaudal folding

• Process: The cranial and caudal embryonic poles curl, resulting in curving of the embryonic disc.
• Result
  • The embryo develops a C form.
    • Caudal end: tail fold
    • Cranial end: head fold
  • Constriction of the yolk sac

Lateral folding ^[1]

• Process
  • Ventral outgrowth of the lateral margins of the embryonic disc
  • The ectoderm and the overlying lateral plate mesoderm on both sides converge.
  • Adhesion of both sides results in the formation of a lateral and anterior body wall as well as a cavity, the intraembryonic coelom
  • The endoderm and the overlying lateral plate mesoderm on both sides converge → gut tube formation
• Result
  • Intraembryonic coelom
    • Pericardial cavity
    • Pleural cavity
    • Peritoneal cavity
  • Gut tube
    • Anterior end of the foregut
    • Foregut
    • Midgut
    • Hindgut
  • Further constriction of the yolk sac

The midgut stays connected to yolk sac remnants via the vitelline duct (omphalomesenteric duct). This duct is obliterated during the course of embryogenesis. Persistence of this duct most commonly results in Meckel diverticulum but could also cause retroumbilical cysts and fistulae.

Buccopharyngeal and cloacal membrane formation

Process

• Buccopharyngeal membrane: region without mesoderm at the cranial embryonic pole
  • Stomodeum formation as a result of deep depression by the buccopharyngeal membrane due to expansion of the brain
  • The buccopharyngeal membrane ruptures shortly after folding → development of a link between the stomodeum (and therefore also the amniotic cavity) and the gut tube
• Cloacal membrane: region without mesoderm at the caudal embryonic pole
  • Proctodeum formation as a result of inward folding of the ectoderm
  • The cloacal membrane tears significantly later than the buccopharyngeal membrane → a link develops between the amniotic cavity and urogenital tract/anal canal

Results

• Formation of future body orifices
• The embryo is connected to its external environment (e.g., amniotic fluid) via the body orifices.

Abnormalities of morphogenesis

Differentiation of the mesoderm

Axial mesoderm

• Location: central region of the mesoderm
• Components: notochord and prechordal mesoderm

Paraxial mesoderm

• Location: tube-shaped area of the mesoderm surrounding the notochord
• Components: somites
• Process
  • The paraxial mesoderm becomes divided into segmented round cell clusters (somites) along the neural tube (week 4)
  • Up to the beginning of week 5, an initial 42–44 somite pairs are formed in a craniocaudal direction.
  • Degeneration of several somite pairs, so that 35–37 somite pairs remain
  • As primitive segments, somites determine body segmentation
• Somite segments
  • Sclerotome: medial migration of cells toward the notochord and fusion of both sclerotomes of a somite pair
  • Dermomyotome
    • Dermatome: migration toward the surface ectoderm
    • Myotome: subdivision into the dorsal epaxial and ventral hypaxial skeletal muscles in the lateral and anterior regions of the thorax and abdomen

Intermediate mesoderm

• Location: lateral to the paraxial mesoderm
• Components: urogenital fold, consisting of the nephrogenic ridge and genital ridge

Lateral plate mesoderm

• Location: lateral to the intermediate mesoderm
• Sequence of events
  • In the second week of development, small folds develop in lateral aspect of the mesoderm.
  • These folds split horizontally to form two components.
    • Somatic (parietal) mesoderm: the dorsal layer that underlies the ectoderm and differentiates into the lining of the pleural, pericardial, and peritoneal cavities.
    • Splanchnic (visceral) mesoderm: the ventral layer that overlies the endoderm and differentiates into the visceral lining of internal organs.
  • The space between these components is called the coelom
  • The two coeloms from either side fuse at the end of the lateral folding of the embryo to form one large cavity, the intraembryonic coelom, which will differentiate into the thoracic and abdominal cavities (see “Morphogenesis” above).

Mesenchyme ≠ mesoderm: The mesoderm is one of the three germinal layers that differentiates into different tissues. The mesenchyme is embryonic connective tissue that develops from the mesoderm and other germ layers.

Fate mapping

A fate map is used to determine the origin of a cell lineage, e.g., a germ layer. The following table provides an overview of the various tissue types and structures that arise from the three germ layers.

Ectoderm: ec-sternal layer; Mesoderm: middle layer; Endoderm: en-ternal layer.