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Pre-Embryo Phase Or Germinal Phase (First 3 Weeks)

This phase of development is divided into following stages:-
  1. Ovulation
    During each ovarian cycle a single mature ovum is released from the ovary. It usually occurs 14 days after the onset of menstruation.
  2. Fertilization and implantation (0-7 days)
    Fertilization refers to fusion of male and female gametes (i.e. spermatozoon and ovum). It takes place in the middle segment (ampulla) of fallopian tube. Before fertilization, the ovum and sperms reach the ampulla for fertilization. Fusion of spermatocyte and ovum leads to formation of zygote. First week of development begins immediately after fertilization and includes:-
    1. Cleavage of zygote: Zygote (fertilized ovum) starts dividing immediately and large zygote is subdivided into smaller daughter cells called blastomeres. Blastomeres are still surrounded by Zonapellucida, Cleavage occurs in fallopian tube (uterine tube).
    2. Formation of morula: At about 16 cell stagetheblastomeres tightly align by the process of compaction to form a compact ban of cells called morula (mulberry). This process of compaction leads to segregation of cells into two groups (i) inner cells (inner cell mass), and (ii) outer cells (outer cell mass). Morula enters uterine cavity 4 days after fertilization.
    3. Formation of blastocyst: As the morula enters the uterine cavity, uterine fluid diffuse through zona pellucida and fills small intercellular gaps between blastomeres, and morula is converted to blastocyst Blastocyst consists of:-
      1. Zonapellucida: Outer covering.
      2. Embryoblast: A group of centrally located cells of inner cell mass and later give rise to tissues of embryo proper.
      3. Trophoblast: A thin outer layer of cells formed from outer cells mass and later give rise extra embryonic tissues.
      4. Blastocoel: Cavity of blastocyst:
        • The region of blastocyst containing embryoblast is known as embryonic pole and the opposite pole, theabembryonic pole. The trophoblasts overlying the embryoblast at embryonic pole is called polar trophoblast and that occupying the rest of wall called mural trophoblast. Between 5-6 days after fertilization, blastocyst hatches from zonapellucida, and this naked blastocyst is ready for implantation.
    4. Implantation: About 6-7 days after fertilization the blastocyst attaches to the endometrium, a process called as implantation or embedding. Normal site of implantation is posterior wall of the body of uterus close to fundus (at junction of fundus with body).
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Stage of bilaminar germ disc (2nd week of development)

  1. Changes in embryoblast
    1. As implantation is progressing, the embryoblast differentiates into two layers:-
      1. Epiblast: Upper layer composed of columnar cells. These cells give rise to embryonic cells.
      2. Hypoblast: Lower layer, composed of cuboidal (polyhedral) cells. These cells give rise to extra-embryonic tissues.
    2. Cells of epiblast and hypoblast are arranged in the form of a circular flattened disc, i.e. bilaminar germ disc (embryonic disc). Some of the cubical cells of hypoblast near the margin of embryonic disc become columnar to form prochordal plate. Prochordal plate determines the central axis of embryo (divides into right and left) and is an important organizer of head region and indicates the future site of mouth.
  2. Development of extraembryonic membranes and cavities
    1. After implantation of blastocyst in endometrium, trophoblast differentiates into two layers
      1. cytotrophoblast(inner or embryonic side), and
      2. Syncyitiotrophoblast (outer side).
    2. Amnion (a weak membrane) begins to form by delamination of cytotrophoblast. This layer is continuous with epiblast layer ofbilaminar disc, and encloses a space above the bilaminar disc called amniotic cavity.
    3. At the abembryonic pole, a layer offlattened cells derived from hypoblast begins to line the blastocoel on the inner surface of mural trophoblast. This membrane is known as Heuser's membrane or exocoelomicmembrane. This membrane is continous with hypoblast ofbilaminar disc and encloses the cavity called primary yolk sac. Amniotic cavity lies dorsal to the embryonic disc and primary yolk sac lies ventral to embryonic disc.
    4. By 11-12 days cells derived from cytotrophoblast fill the area between cytotrophoblast and outer surface of amnion and primary yolk sac. These cells form a loose connective tissue called extraembryonicmesoderm (primary mesoderm). Soon many cavities appear in extraembryonic mesoderm which coalesces to form a large cavity known as extraembryonic coelom (chorionic cavity). This cavity surrounds the primary yolk sac and amniotic cavity except where germ disc (bilaminar disc) is connected to trophoblastby connecting stalk. Extraembryonic coelom splits the extraembryonic mesoderm into 2 layers :-
      1. Somatopleuric (or parietal) extraembryonic mesoderm: lines inside of cytotrophoblast and outside of amnion.
      2. Splanchnopleuric (or visceral) extraembryonic mesoderm: lines outside of yolk sac.
    5. By the end of 2nd weak, lower part of primary yolk sac pinches off, resulting in a smaller sac next to embryonic disc (bilaminar disc) called secondary (definitive) yolk sac. The detached part of primary yolksac in the extraembryoniccoelom is called exocoelomic cyst.
    6. Somatopleuricextraembryonic mesoderm fuses with overlying cytotrophoblast to form a membrane called the chorion. The chorion forms the wall of extraembryonic coelom which is now called chorionic cavity (chorionic sac). The only place where extraembryonic mesoderm traverses the chorionic cavity is the connective stalk. With development of blood vessels this stalk will become umbilical cord. A portion of chorion together with adjacent endometrium will form placenta in subsequent weeks.


  1. Stage of trilaminar germ disc (3rd week of development)
    The stage of trilaminar disc formation is marked by changes during third week of development. The events during this period are:-
    1. Gastrulation
      Gastrulation is the morphogenic process of formation of three germ layers, i.e. ectoderm, mesoderm and endoderm. The embryo at this stage is called gastrula. The process of gastrulation begins with formation of primitive streak. Primitive streak is formed by proliferation of some of epiblastcellsnearthe caudal endofbilaminar germ disc (embryonic disc). The epiblastthrough the process of gastrulation is the source of all three germ layers in embryo:-
      1. Some cells of epiblast after invagination displace the hypoblast and form embryonic endoderm. Endoderm is the first germ layer to be formed.
      2. Some epiblast cells after invagination come to lie between the epiblast and hypoblast, and form mesoderm.
      3. Cells remaining in the epiblast than form ectoderm.
    2. Formation of notochord
      Notochord is a bud like structure formed by epiblast cells extending from cranial end of primitive streak to caudal end of prochordal plate, in between the ectoderm and endoderm. Significances of notochord includes following:-
      1. It defines the axis of embryo.
      2. It functions as the primary inductor, inducing the overlying ectoderm to develop into neural plate (theprimordium of CNS).
      3. It serves as the basis for development of axial skeleton. The notochord is an intricate structure around which vertebral column is formed and indicates future site of vertebral bodies. However, the notochord does not give rise to vertebral column, after development of vertebral bodies, the notochord degenerates and disappears, but parts of it persist as the nucleus pulposus of intervertebral disc.
    3. Migration of mesodermal cells
      Mesodermal cells derived from epiblastinvaginate through primitive groove and spread laterally and cranially between ectoderm and endoderm to form trilaminar disc, except following places where ectoderm and endoderm are fused and the embryonic disc remains a bilaminar structure:-
      1. The area of prochordal plate, which later forms the 'buccopharyngeal membrane' (future site of oral cavity).
      2. A circular area caudal to the primitive streak which forms the cloacal membrane (future site of anus).
      3. In median plane, where cranial to primitive streak notochord is present.
    4. Appearance of allantois
      On day 16, a diverticulum from posterior wall of yolk sac extends into the connecting stalk and is known as allantois. Allantois is associated with early blood formation and development of urinary bladder. After development of urinary bladder allantois becomes urachus which is converted into median umbilical ligament after birth. The blood vessels developing in allantois become umbilical vessels.
    5. Neurulation
      Neurulation refers to process of neural plate formation and its infolding to form neural tube. During neurulation, the embryo is referred to as neurula. Neural plate is' formed by the thickening of ectoderm overlying the notochord. It is the first appearance of nervous system. Neural plate formation isinduced by notochord. Neural groove appears in neural plate which is bounded on each side by neuralfolds. Neural tube is formed by fusion of neural folds in the midline dorsally. Neural tube is the primordium of CNS. Cranial part of neural tube forms brain and caudal part forms spinal cord.
    6. Subdivision of mesoderm (intraembryonic mesoderm)
      1. The intraembryonic mesoderm located on each side of notochord proliferate and is arranged in three columns:
        • Paraxial mesoderm (immediately lateral to notochord);
        • Intermediate mesoderm (just lateral to paraxial mesoderm); and
        • Lateral plate mesoderm (more laterally placed mesoderm).
      2. Small spaces appear in lateral plate mesoderm which later fuse to form intraembryonic coelom. This space divides the lateral plate mesoderm into: (i) an outer parietal layer (somatopleuric mesoderm) continous with mesoderm covering the amnion, and (ii) an inner visceral layer (splanchnopleuric mesoderm) continous with mesoderm covering the yolk sac. During 2nd month the intraembryonic coelom is divided into three cavities:
        • Pericardial cavity
        • Pleural cavity
        • Peritoneal cavity.
          Somatopleuric mesoderm forms the parietal layer of these cavities (i.e., parietal pericardium, parietal layer of pleura and parietal peritoneum) and splanchnopleuric mesoderm forms the visceral layer of these cavities.
      3. Paraxial mesoderm differentiates into somites. By the end of 20th day, the first pair of somiteshaveformed in neck region. After this, about 3 pairs of somites are formed per day and by the end of 5th week about 42-44 somite pairs are formed (4-occipital, 8-cervical, 12-thoracic, 5-lumbar, 5-sacral and 8-10 coccygeal). Somites are further differentiated into:-
        • Dermatomyotome:- Give rise to skeletal muscles and dermis.
        • Sclerotome:- Give rise to vertebral column.
      4. Sclerotomes which surround notochord starts projecting posteriorly (dorsally) to surround neural tube and forms.
        • Ventralsclerotomes:- Give rise to vertebral body and annulus fibrosus.
        • Lateralsclerotomes:- Give rise to vertebral arch (pedicle and lamina).
        • Dorsalsclerotomes:- Give rise to spinous process.
      5. The notochord forms the nucleus pulposus.

Molecular control of gastrulation

Gastrulation starts after the formation of primitive streak
  1. Nodal gene (a member oftransforming growth factor β: TGF β) initiates and maintains formation of primitive streak.
  2. After formation of primitive streak a number of genes regulate process of gastrulation:-
    1. Signaling molecules: Bone morphogenic proteins (MBPs), fibroblast growth factor (FGF), and transcription factor PIT-X 2.
    2. Primary organizer (primitive node) formation is induced by Wnt family (Wnt-3), FGF, homebox transcription factor mix.
    3. Head organizing centreformation is induced by LIM-l (main gene), OTX-l, OTX-2, HESX-l, secreted factor cerebrus and nodal gene.
    4. Trunk organization Centre formation is induced by 'T-box genes (Tbx-6 and brachury), FGF-9.
    5. Tail organizing centreformation is induced by Brachury, Wnt 5a and Wnt 5b.

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