Download Primitive Streak

Embryology
2nd Stage
Dr. Sameer Ahmed
*Fertilization : Is the union of gametes (ovum
and sperm) to from a single-celled
zygote (fertilized egg) which eventually leads to the development of an embryo.
*A series of dynamic and complex events are triggered following sperm–
oocyte interaction that sequentially leads to fertilization and the formation of a
zygote.
Fertilization Events
Sperm–Ova Association
Fertilization begins with gamete fusion (zygote formation). The fusion of a
spermatozoa with a secondary oocyte takes place in the uterine tube, near the ovary.
**At the beginning, a spermatozoa binds to a specific glycoprotein on the zona
pellucida that surrounds the oocyte ((this recognition process precludes union with
foreign sperm)).
**The spermatozoa releases degradative enzymes (acrosomal reaction) that
allows the sperm cell to penetrate the zona pellucida.
Acrosomal Reaction
1) The acrosomal reaction normally takes place in the ampulla of the fallopian
tube (site of fertilization in human).
2) The sperm cell acquires a "hyperactive motility pattern" by which its flagellum
produces vigorous whip-like movements that propel the sperm through the cervical
canal and uterine cavity.
3) Glycoproteins on the outer surface of the sperm then bind with glycoproteins on
the zona pellucida of the ovum.
4) The first stage is the penetration of corona radiata, by releasing hyaluronidase from
the acrosome to digest cumulus cells surrounding the oocyte.
5) After reaching the zona pellucida the actual acrosome reaction begins. Acrosin
digests the zona pellucida.
6) Part of the sperm cell membrane then fuses with the ova cell membrane, and the
contents of the head sink into the ova.
**Spermatozoa and oocyte plasma membranes fuse to form a zygote (the
secondary oocyte completes meiosis).
**After fertilization the oocyte precludes fusion with other sperm the zona
pellucida undergo denaturing via enzymes released by exocytosis from oocyte
cytoplasmic granules.
**Male & female haploid pronuclei make contact, lose their nuclear
membranes, and begin mitosis (mitosis begins 12 hours after sperm fusion; DNA
synthesis takes place before mitosis)
*Cleavage: Increasing the cell number by cell divisions (mitosis) during the early
stage of embryogenesis immediately after fertilization. Each daughter cell of the
cleavage process is termed a blastomere.
Blastomere: is a type of cell which produced by cleavage of the zygote after
fertilization. Thereafter, and due to the continuous development the blastomeres
differentiate into inner & outer cells masses.
Stages of Cleavage
Morula Stage (small mulberry):
Is a solid ball of blastomeres, within a zona pellucida. A morula typically
consists of 16 to 64 blastomeres = four to six cell divisions.
Blastomeres become compacted
cells packed on the inside differentiate from those along the surface of the morula:
— outer blastomeres become flattened and form tight junctions (resulting in reduced
permeability to fluids); they develop the capacity to secrete fluid (internally); they are
destined to become trophoblasts which form the chorion & amnion (fetal
membranes) ::: Trophoblasts: the cells distributing around cavity of a blastcyst
— inner blastomeres form gap junctions to maximize intercellular communication;
they are destined to become inner cell mass (embryo blast) which forms the embryo
(plus two fetal membranes).
Blastula Stage (Blastocyst):
Blastula, a ball of cells with a central cavity, one of the result of development
the embryo is an accumulation of fluid inside the central cavity, which signals to
formation of the Blastocyst or Blastula Stage. The fluid is called blastocoele
(blastocyst cavity).
Gastrula Stage or Gastrulation:
Gastrulation or germ layer formation is a stage of embryological development
during which the single layered blastula is converted in to trilaminar structure
(trilaminar embryonic disc) that gives rise to three germ layers: Ectoderm,
Mesoderm, and Endoderm. (In a gastrula [Gr.= little stomach] one can see evidence
of primitive gut formation.) which ultimately give rise to specific tissues and organs.
Gastrulation events includes the following sequence, beginning with a blastocyst:
— A thickened embryonic disc becomes evident at the blastocyst surface, due to cell
proliferation of the inner cell mass cells. Trophoblast cells overlaying the inner cell
mass degenerate in domestic mammals (in some mammals, e.g., mouse and human,
trophoblast cells overlaying the inner cell mass separate and, instead of degenerating,
become amnionic wall.)
— From the inner cell mass, cells proliferate, break loose (delaminate), and migrate
to form a new cell layer inside the trophoblast layer. The new layer of cells is called
the hypoblast (endoderm) ; it forms a yolk sac. The remaining inner cell mass may
henceforth be called epiblast (ectoderm).
— On the epiblast surface, a primitive streak forms as differential cell growth
generates a pair of ridges separated by a depression. [NOTE: The primitive streak
defines the longitudinal axis of the embryo and indicates the start of germ layer
formation.]
— The separation of the hypoblast layer from the epiblast establishes a space
(coelom/celom) deep to the primitive streak. Subsequently, the coelom is temporarily
filled by mesoderm that undergoes cavitation to restablish the coelom that gives rise
to body cavities.
— Epiblast cell proliferation along primitive streak ridges becomes the source of a
cellular migration through the streak depression. The migrating cells form endoderm
& mesoderm layers.
The Germinal Layers
An adult, multicellular animal typically possesses a concentric arrangement of
tissues of the body. These adult tissues are derived from three embryonic cell layers
called germinal layers; the outer layer is the Ectoderm, the middle layer is the
Mesoderm, and the innermost layer is the Endoderm.
Endoderm : Is the innermost of the three primary germ layers, or masses of
cells (lying within ectoderm and mesoderm), which appears early in the development
of an animal embryo. The endoderm consists at first of flattened cells, which
subsequently become columnar. The endoderm subsequently gives rise to
the epithelium (tissue that covers or lines a structure) of the
the stomach
(except mouth and anus), the liver, the pancreas, the urinary bladder, the vagina (in
females), the urethra, the trachea, the bronchi, the lungs, the pharynx, the thyroid, the
parathyroid, the tonsils and the intestines.
Mesoderm : The middle of the three primary germ layers, or masses of cells
(lying between the ectoderm and endoderm), which appears early in the development
of an animal embryo. Which during embryogenesis subsequently gives rise
to muscle, connective
tissue, cartilage, bone,
notochord, blood,
bone
marrow,
lymphoid tissue, and to the epithelia (surface, or lining, tissues) of blood vessels,
lymphatic vessels, body cavities, kidneys, ureters, gonads (sex organs), genital ducts
and adrenal cortex.
Ectoderm : The outermost of the three primary germ layers, or masses of
cells, which appears early in the development of an animal embryo. It emerges and
originates from the outer layer of germ cells. In vertebrates, ectoderm subsequently
gives rise to hair, skin (epidermis skin cells), nails or hooves, and the lens of the eye,
the epithelia (surface, or lining, tissues) of sense organs, the nasal cavity, the sinuses,
the mouth (including tooth enamel), and the anal canal, and nervous tissue, including
the pituitary body.
Primitive Streak Is a transient structure whose formation on week 2 of human
development marks the start of gastrulation. The process in which the inner cell
mass in converted into the trilaminar embryonic disc, which is comprised of the
three germ layers (ectoderm, mesoderm and endoderm). The primitive streak
originates from the anterior epiblast, and appears as an elongating groove (primitive
groove) on the dorsal mid-sagittal surface of the epiblast, along the anterior-posterior
axis of the embryo. The rostro-caudal and medial-lateral axes of the embryo are
defined by the primitive streak. The rounded primitive node, or Hensen's node, is
situated at the cranial tip of the primitive streak, and contains a depression called
the primitive pit. The primitive pit is continuous with the primitive groove.