Twinning PDF

TWINNING OR EMBRYONIC DUPLICATION Causes of multiple births Fertilization of separately ovulated egg Complete or partial separation cleavage/blastocyst blastomeres Duplication during gastrulation  CLASSIFICATION 1. Free or unattached to each other 2. Cojoined to each other 3. Symmetrical or...

TWINNING OR EMBRYONIC DUPLICATION Causes of multiple births Fertilization of separately ovulated egg Complete or partial separation cleavage/blastocyst blastomeres Duplication during gastrulation  CLASSIFICATION 1. Free or unattached to each other 2. Cojoined to each other 3. Symmetrical or asymmetrical Free or separate twins – are dizygotic twins (most common) developing independently with their own EEM. Free,symmetrical twins- are monozygotic derived from a single zygote which separates or duplicate at different stages ▪During late cleavage to blastocyst , bisection may lead to the development of two identical embryos with their own EEM ▪When z. pellucida ruptures causing escape of some blastomeres may develop into two embryos provided that part of the embryonic disc is bisected also ▪An instance when the blastodisc splits immediately prior to gastrulation, the embryos formed usually shares EEM Free, asymmetrical twins – may originate from a monozygotic or dizygotic embyros, one member is normal and the other is rudimentary surviving by sharing the blood supply of the normal one ▪Abnormal twin has its own amnion but has no recognizable body form consisting of skin, muscle teeth,etc; sometimes ,the craniofascial structures can be identified also ▪Abnormal twin is termed amorphous globosus, anidian,acardiac or holocardiac fetus ▪Acardiac twin is common in cow ▪Abnormal twin is different from mummified or lithopedion ( still birth) Cojoined or fused symmetrical twins – are monozygotic in origin and generally termed diplopagus but popularly known as siamese twin ▪Incomplete division of the embryo occur ▪sometime during the primitive streak stage and are identified according to the site of attachment such as: ▪Thoracopagus- fused at the thorax facing each other ▪Abdomonopagus- joined at the abdomen with often partially fused intestines ▪ Pygopagus- fused back to back at the pelvic or sacral region ▪ Cephalopagus – joined at the head region. Duplication at this region uses prefixes (di, tri, tetra) to describe the anomalies such as: ▪Dicephalus – two heads ▪Diprosopus –two faces ▪Dicaudatus – two tails ▪Tetrabrachius- 2 pairs of thoracic limbs ▪Tetrascelus – 2 pairs of pelvic limbs Cojoined, asymmetrical twins – are unequal in size, consisting of one normal individual, the autosite and an extra body part only, the parasite such as extra limbs attach at the back or at the schial region projecting caudally ▪Asymmetrical twinning usually happens when the specific organ forming regions are already established ( limb, heart,eye etc. fields) FERTILIZATION TO GASTRULATION Human prenatal development can be divided into 3 stages: pre- embryonic, embryonic, and fetal development. Fertilization usually occurs in the ampulla of the fallopian tube. The fertilized egg, referred to as the conceptus at this stage, immediately travels toward the uterus. During the journey, it undergoes several mitotic cell divisions, producing daughter cells, called blastomeres. This process is known as cleavage, because the cells divide without growing in volume. After about 3 days, the conceptus arrives in the uterus: it now contains about 16 cells and is called a morula. Cells of the morula are totipotent, they are capable of differentiating into all cell types, both embryonic and extra- embryonic. The morula continues to divide while floating freely in the uterus for several more days. During this time, it consumes nutrients stored in the egg cytoplasm, and “uterine milk” secreted by the endometrium. When there are about 100 cells, the cells start to arrange themselves around a fluid-filled cavity, forming a blastocyst. The blastocyst consists of a pluripotent inner cell mass, called the embryoblast, destined to be the embryo; and an outer shell, called the trophoblast, which nourishes the embryo. The trophoblast later becomes the chorion - the fetal portion of the placenta. The trophoblast secretes an enzyme that dissolves the membrane (zona pellucida) surrounding the conceptus, enabling it to “hatch”, ready to be implanted. At the end of the first week, the blastocyst becomes attached to the endometrium. At the site of contact, superficial cells of the trophoblast fuse together to form the syncytiotrophoblast, which grows into the endometrium and derives nutrition from it. The endometrium responds by growing over and eventually enveloping the blastocyst. The syncytiotrophoblast secretes the hormone named “human chorionic gonadotropin”, HCG (luteinizing hormone in animals), to instruct the corpus luteum to continue the production of progesterone. Progesterone is the hormone that maintains pregnancy. Among its many functions, progesterone stimulates the growth of nutrient-rich decidual cells to feed the early embryo. Around the middle of week 2, the embryoblast starts to form a two-layered embryonic disc - hypoblast and epiblast, at the mid-plane of blastocyst, with a cavity on either side.  This bilaminar disc sets the dorsal/ventral axis of the embryo, with the epiblast being the dorsal side. By the end of second week, the 2 layers grow around the cavity on their respective side and form the yolk sac (hypoblast) and amnion (epiblast), respectively. The yolk sac provides nutrients absorbed from the trophoblast, as well as gas exchange for the embryo during weeks 2 and 3, before the placenta takes over these functions at week 4. It is also the first site of embryonic blood cell production, before the liver. In week 3, the two-layered disc transforms into 3 germ layers, through the process of gastrulation, during which the cells lose their pluripotency and become committed to a smaller number of cell types. Gastrulation begins with the emergence of the primitive streak on epiblast surface. This event sets the cranial/caudal, or head/tail axis, of the embryo. A so- called primitive node at the cranial end of primitive streak produces signaling factors that control the movements of cells. Epiblast cells migrate toward and through the primitive streak into the embryo. Amnion - a thin membrane forming a closed sac about the embryo or fetus of a reptile, bird, or mammal and containing a watery fluid in which the embryo or fetus is immersed  The first set of cells displaces the hypoblast and forms the endoderm. The second set of cells fills in the space between the endoderm and epiblast, creating the mesoderm. The remaining epiblast cells form the ectoderm. Each of the 3 germ layers differentiates to become different organs and tissues. All organ systems are present in their primitive form by the end of the embryonic period, but most of them are not yet functional, or only partially functional. The yolk sac gives rise to the allantois, a primitive urinary bladder and part of the umbilical cord. The amnion grows to surround the embryo 4 by week 4. It is filled with amniotic fluid which protects the embryo from trauma and temperature fluctuations. The fluid also allows the fetus to move freely and prevents adhesion of body parts. The yolk sac, amnion, allantois and chorion are the 4 embryonic membranes. The embryos of reptiles, birds, and mammals produce 4 extraembryonic membranes - amnion, yolk sac, chorion and allantois. In birds and most reptiles, the embryo with its extraembryonic membranes develops within a shelled egg. The amnion protects the embryo in a sac filled with amniotic fluid.