Lecture 19: Embryological Development Weeks 2-8

Embryological Development Week 1

• Ovulation occurs at time T = 0
• Fertilization occurs at approximately T ≈ 12-24 hours
• The 2-cell stage occurs at T ≈ 30 hours, involving the first cell division
• The 4-cell stage occurs at T ≈ 40 hours, involving the second cell division
• The morula (16-cell stage) forms at approximately T ≈ 3 days
• The blastocyst emerges in its early stage at approximately T ≈ 4-5 days
• At the time of attachment and implantation, the uterine mucosa is in the secretory phase
• The endometrium consists of three layers: compact, spongy, and basal
• The blastocyst first attaches to the compact layer and then invades the spongy layer
• Trophoblast cells attach to the uterine epithelium via L-selectin
• Integrin-laminin binding promotes stronger attachment
• Trophoblast cells invade the uterine epithelium via integrin-fibronectin binding which promotes migration
• By 10-12 days post-fertilization, the embryo is fully embedded in the endometrium, with maternal endometrial epithelium cells covering it
• The endometrial stroma undergoes the decidual reaction in response to the implanting embryo
• Stromal cells accumulate glycogen and lipids, swell, and become decidual cells, providing nutrition to the early embryo via the trophoblast
• Maternal leukocytes infiltrate endometrial stroma and secrete interleukin-2 to prevent maternal recognition of the implanting embryo as foreign

Embryological Development Week 2

• Marked by the "week of twos" which consists of 2 layers of the embryonic disk: Epiblast and Hypoblast
• Further development leads to the formation of 2 trophoblast layers: Cytotrophoblast and Syncytiotrophoblast
• Also leads to the 2 extraembryonic membrane layers: extraembryonic somatic mesoderm and extraembryonic splanchnic mesoderm
• The embryoblast (inner cell mass) forms two layers of cells which creates the bilaminar disk
• Hypoblast (yellow) is adjacent to the epiblast and blastocyst cavity
• Epiblast (blue) is adjacent to the hypoblast and cytotrophoblast (innermost trophoblast cells)
• Amnioblast (blue) forms from the epiblast cells
• Amnioblast cells are adjacent to the cytotrophoblast cells
• A small cavity appears between the epiblast cells and amnioblast cells
• This small cavity becomes the amniotic cavity
• Hypoblast cells line the innermost trophoblast cells (cytotrophoblasts) to encase the blastocyst cavity and form the primitive yolk sac
• The trophoblast forms two layers consisting of Cytotrophoblast (mitotic mononucleated cells) and Syncytiotrophoblast (cells without individual plasma membranes)
• Trophoblast cells will contribute to the chorion that surrounds the embryo and is a fetal component of the placenta
• Syncytiotrophoblast is highly invasive and Penetrates into maternal stroma and engulfs decidual cells and also invades maternal capillaries (sinusoids)
• Lacunae (vacuoles) form in syncytiotrophoblast
• This enables the Lacunae to form intercommunicating network (lacunar stage)
• Maternal blood enters lacunae network to form the uteroplacental circulation
• Syncytiotrophoblast produces hCG to maintain corpus luteum activity (estrogen & progesterone)
• Extraembryonic mesoderm forms between the primitive yolk sac and the cytotrophoblast
• A space (chorionic cavity) forms within the extraembryonic mesoderm separating the extraembryonic mesoderm into two layers consistisng of Splanchnic mesoderm (adjacent to the yolk sac) and Somatic mesoderm (adjacent to cytotrophoblast & amnioblast).
• Somatic mesoderm contributes to the fetal component of the placenta; it forms the chorion

Embryological Development Week 3

• Gastrulation begins with the formation of the Primitive streak
• Primitive streak is a linear indentation (groove) that forms on the surface of the epiblast
• The primitive node forms at the cephalic end of the primitive streak
• Epiblast cells at the primitive streak detach and migrate into the area between the epiblast and hypoblast
• This process is termed invagination
• There are now three embryonic layers of cells consisting of ectoderm, endoderm, and intraembryonic mesoderm
• Epiblast cell movement is controlled by Fibroblast growth factor 8 (FGF8) which is secreted by primitive streak cells
• FGF8 downregulates E-cadherin surface expression, reducing cell-cell contact strength and regulates Brachyury expression to induce invaginated cells to become embryonic mesoderm
• Some invaginating cells displace hypoblast cells and become embryonic endoderm
• Remaining epiblast cells become embryonic ectoderm
• Epiblast is the source of all germ layers
• The oropharyngeal and cloacal membranes are ectoderm cells adjacent to endoderm cells, without intervening mesoderm cells
• Oropharyngeal becomes the oral cavity, Cloacal becomes the anus & urethra
• Invaginated epiblast cells become mesoderm cells and spread/migrate laterally and toward the cephalic/cranial end
• The cephalic/cranial region expands the most in size
• The epiblast cells that invaginated (mesoderm cells) at the primitive node migrate cranially (cephalic end) along the midline and become the prechordal plate
• This is adjacent to the oropharyngeal membrane
• Prechordal plate forms the forebrain
• Some prenotochordal cells intercalate with hypoblast cells, while others migrate on top. Together, these two layers form the notochordal plate at the midline
• The cells of the notochordal plate detach from the endoderm/hypoblast, replicate, forming a separate entity, which defines the notochord
• The notochord is a rod-shaped midline structure
• The primitive streak is a linear indentation.
• Epiblast cells invaginate along the streak to form mesoderm
• Invaginated cells (mesoderm) form in cranial to caudal order consisting of notochord, paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, and extraembryonic mesoderm
• Epiblast cells that invaginate through the primitive streak near the primitive node become mesoderm, migrating to the area cranial to the oropharyngeal membrane (cardiogenic area)
• These cells form cardiogenic mesoderm
• The cardiogenic area is a horseshoe (crescent) shaped region where the heart will form
• The primitive streak diminishes in size and degenerates by the end of the fourth week

Embryological Development Weeks 2 - 3

• Bilaminar transforms to trilaminar
• The primitive streak (see Chapter 4) further transforms into mesoderm, embryonic ectoderm and extraembryonic mesoderm
• The amniotic ectoderm leads to the formation of ectoderm of the amnion
• The embryonic endoderm leads to the formation of the embryonic ectoderm
• The endoderm of the umbilical vesicle leads to the formation of extraembryonic mesoderm

Embryological Development Weeks 3 - 8

• Known as the Period of organogenesis
• Tissues and organs are generated from each of the three germ layers, which also generates main organ systems that develop concurrently
• The external body becomes recognizable
• The three germ layers consist of the ectoderm, mesoderm, and endoderm
• During ectoderm development, notochord and prenotochord mesoderm induce overlying ectoderm to form the neural plate
• Neural plate cells will become neuroectoderm
• Neurulation describes how the neural plate forms the neural tube
• Lateral edges of the neural plate elevate to form neural fold and groove
• Neural folds move toward the midline
• Neural folds meet at the midline and fuse to form the neural tube
• After fusion, ectoderm cells cover the neural tube
• Fusion begins near the middle of the embryo (cervical level, 5th somite) and continues cranially and caudally
• Cranial (anterior) neuropore closes around day 25, caudal (posterior) closes around day 28
• As neural folds fuse, cells separate from the ectoderm epithelium and migrate into the mesoderm underneath the epithelium
• These are neural crest cells that differentiate into numerous cell types (nerve ganglia, melanocytes, glia, odontoblasts, etc.)
• Ectoderm produces the central and peripheral nervous system, sensory epithelium of the eye, ear, and nose, Epidermis including skin, hair, nails, Subcutaneous and mammary glands, Pituitary gland and Tooth enamel
• Near the Notochord (midline) the Mesoderm cells proliferate and form the paraxial mesoderm and adjacent to the paraxial mesoderm the intermediate mesoderm forms
• Mesoderm that is most lateral remains thin and forms the lateral plate mesoderm

Mesoderm Development

• The paraxial mesoderm organizes into segments that become neuromeres found cranially and somites
• Neuromeres will form mesenchyme in the head and Somites will form skeletal muscle and form the axial skeleton
• Paraxial mesoderm is initially unsegmented tissue, becoming segmented first in the cephalic regions then caudally
• Segmentation is formed by the 'segmentation clock' via a number of genes (retinoic acid, FGF8, Wnts) that are expressed in a cyclic manner
• By the end of the fifth week, 42-44 pairs of somites are present
• The somites consist of 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal
• First occipital and last 5-7 coccygeal somites later disappear, giving a final count of approximately 35 pairs of somites
• The somite is at first a clump of cells then arranges into a donut shape creating a lumen
• Each somite is induced by surrounding tissues to form all 3 components consisting of Sclerotome, Dermatome and Myotome
• Sclerotome, myotome & dermatome formation is regulated by different molecular signals
• Shh and Noggin induce sclerotome being identified by PAX1 experession, Wnt & Shh induce dorsomedial myotome Myf5 and Wnt & BMP4 induce ventrolateral myotome myoD = NT-3 induces dermatome PAX3
• The medial and ventral cells of the somite become the sclerotome, which will form the vertebrae, ribs, and tendon
• The dorsal-medial cells and the ventral-lateral cells of the somite become muscle cell precursors (myotome)
• Ventral-lateral muscle precursors will form most of the body wall and limb musculature, dorsal-medial muscle precursors will form back and body wall muscles
• Intermediate mesoderm forms the urogenital structures (gonads and urinary system)
• The parietal layer will line the intraembryonic cavity and form the lateral body wall folds (along with the ectoderm), closing the ventral body wall to form the dermis of skin, bones & connective tissue of limbs
• Mesoderm produces axial and limb muscles along with the axial and limb skeleton, dermis of skin and the genitourinary system.
• Other systems produced by mesoderm include the heart, Hematopoietic system and the wall of the gastrointestinal system, parietal pleura and visceral peritoneum

Blood Vessel & Tube Development

• Mesoderm gives rise to the vascular system through vasculogenesis
• Blood islands appear in extraembryonic and intraembryonic mesoderm, via mesoderm cells induced to form hemangioblasts
• The hemangioblasts will form vessels and blood cells and later the liver will be the major embryonic hematopoietic organ
• Vessels also form via angiogenesis (sprouting from existing vessels)
• Within hematopoiesis, hemangioblasts give rise to hematopoietic stem cells and hematopoiesis begins within the mesoderm around the yolk sac in week 3
• Intraembryonic hematopoiesis begins in mesoderm associated with the dorsal aorta, in the aorta/genital ridge/mesonephros (AGM) region
• AGM cells migrate to the liver and by 5-6 weeks, the liver is the prominent site, followed by spleen and bone marrow to produce Hematopoiess
• The heart tube forms in the cardiogenic area
• The cardiovascular system is the first organ system to reach a functional state (starts to beat around day 21-22)
• Epiblast cells invaginate through the primitive streak near the primitive node becoming mesoderm and migrate to the area cranial to the oropharyngeal membrane (cardiogenic area) to form cardiogenic mesoderm
• The cardiogenic area (heart field) is a horseshoe (crescent) shaped region where the heart will form and is cranial to the oropharyngeal membrane and to the neural folds, lying in the splanchnic layer of lateral plate mesoderm
• There are distinct mesoderm populations forming a primary and a secondary heart field, enabling the heart formation

Cardiac Development

• Mesoderm cells migrate cranially from the primitive streak to form crescent-shaped PHF
• PHF cells area specified from lateral to medial into the atria, left ventricle, and right ventricle
• Pharyngeal mesoderm cells and neural crest cells (from cranial neural folds) form crescent-shaped SHF, which is medial to PHF
• SHF cells are specified to become the right ventricle,along with the outflow tract (conus cordis and truncus arteriosus)
• Myoblasts and blood islands form in Primary Heart Fields, where they form two endocardial tubes
• The embryo folds cranially-caudally and also laterally, merging the paired endocardial tubes (except at the caudalmost end)
• The central portion of the fused tubes expands and these tubes form the future ventricles and outflow tract
• Cranial – Caudal embryo folding changes the position of the developing heart dramatically
• The gastrointestinal tract experiences folding of the embryo to create a 'Purse string' ventral portion of the embryo to separate the endoderm of the gut from the yolk sac proper
• Endoderm develops to gastrointestinal tract and Lungs
• Endoderm produces epithelial lining of the gastrointestinal and respiratory tracts along with the urinary bladder and the tympanic cavity, as well as thyroid, parathyroid and liver

Folding & Development

• A flat trilaminar disc transforms into a cylindrical shaped embryo in conjunction with rapid growth
• The embryo experiences folding, specifically in the fourth week in which the Head and tail move ventrally and the lateral body walls move ventrally
• During 4th week, The Oropharyngeal membrane breaks down and in the 7th week the cloacal membrane also breaks down
• Crown-Rump Length (CRL) is the measurement of the embryo to determine size since Little variability in size occurs in the 1st trimester
• This serves as the optimal method to estimate age until end of 1st trimester Gestational age marks pregnancy from the first day of the LNMP (last normal menstrual period) where the embryonic age begins at fertilization and approximately 2 weeks after LNMP

Week 4 - 8 Development

• Major changes in body form occur during week 4, where at the beginning, the embryo is almost straight and has four to 12 somites
• By the end of this week, the embryo is slightly curved because of head and tail folds
• The neural tube is formed opposite the somites, but it is widely open at rostral and caudal neuropores
• On/Around day 24, the first two pharyngeal arches are visible as the first known arch (mandibular arch) and the second arch.
• During heart looping (cardiac loop) completion produces a large ventral prominence, and pumps blood
• Three pairs of pharyngeal arches are visible by day 26
• The Rostral neuropore closes and the Forebrain produces a prominent elevation of head
• Folding gives the embryo a C-shaped curvature
• Upper limb buds are recognizable by day 26 or 27 as small swellings on the ventrolateral body walls to form Otic pits and lens placodes that are visible on sides of head
• An additional Fourth pair of pharyngeal arches and lower limb buds are visible by end of fourth week, ultimately forming a long tail-like caudal eminence that is a characteristic feature
• On the fifth week, the second pharyngeal arch overgrows the third and fourth arches, forming a lateral ectodermal depression on each side called the cervical sinus
• Mesonephric ridges indicate site of mesonephric kidneys as temporary excretory organs
• Around the sixth week, embryos show reflex response to touch and the Upper limbs begin to show regional differentiation as the elbows and large handplates develop
• Primordia of digits (fingers) digital rays, begin to develop in the handplates, indicating formation of digits and major septa of the heart form
• Intestines enter umbilical cord also occurs around week 6
• During week eight (50-56d) the digits of the hand are separated but noticeably webbed and notches are now clearly visible between digital rays of feet
• The caudal eminence is still present but stubby
• At the end of the eight week, the embryo as distinct human characteristics
• All major systems are developed and the hhead is disproportionately large
• Eyelids are obvious and closing; by end of eighth week, they begin to unite by epithelial fusion
• The neck region is established with intestines still in the proximal portion of the umbilical cord
• Auricles of external ears begin to assume their final shape and external genitalia are present, yet not distinctive enough to permit accurate sexual identification