HUMAN EMBRYOLOGY~ BY MAHEK NOOR DAUDPOTA~

HUMAN EMBRYOLOGY

•       Human embryo-genesis is the process of CELL DIVISION  and cellular differentiation of the embryo that occurs during the early stages of development.

•       In biological term, human development entails growth from a one-celled ZYGOTE to an adult HUMAN BEING.

•       Fertilization occurs when the SPERM  cell successfully enters and fuse with an EGG CELL (Ovum).

•       The genetic material of the sperm and the egg then combine to from a single cell called ZYGOTE and the germinal stage of prental development commence.

•       Embryogenesis cover the first eight week of development at the beginning of the ninth week the embryo is termed  a FETUS.

The human embryology is the study of this development during first eight week after fertilization. The normal period  of gestation (Pregnancy) is nine month or 38 weeks

•       The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the UTERUS.

•       The germinal stage takes around 10 days. During this stage, the zygote being to divide, in a process called CLEAVAGE.

•       A BLASTOCYST is then formed and implanted in uterus.

•       Embryogenesis continues with the next stage gastrulation, when the three germ layers of embryo from in the process called Histogenesis, and processes of Neurulation and Organogenesis follow.

•       In comparison to the embryo, the fetus has more recognizable external  features and more complete set of developing organ. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression cell growth and  cellular differentiation

•       A nearly identical process occurs in species, specially among chordates.

 

 •       Step 1: a zygote is the single cell formed when an egg and a sperm cell fuse; the fusion is known as fertilization.

•       Step 2: the first 12 to 24 hours after a zygote is formed are spent in cleavage – very rapid cell division

 

 Blastulation And Cell Differentiation

•       Step 3: during blastulation, the mass of cells from a hollow ball.

•       Step 4: cell begin to differentiate and from cavities.

 •       Step 5: During gastrulation the three germ layers form; the cell mass is now as gastrula.

•       Step 5a: the primitive streak forms.

Step 6: The notochord is formed

Neurulation

•       Step 6a: tubes from, making neurula.

•       Step 6b: the notochord induces the formation of the neural plate

•       Step 6c: the neural plate folds in on itself to make the neural tube and neural crest.

•       Step 7: the mesoderm has five distinct categories.

Major Developmental Stages Of Embryonic Period (1-8^thweek)

Time period (Week)	Major Events/characteristics
1st week	Fertilization, cleavage, morula formed (3days), blastocyst superficially implanted, zona pellucida disappears, inner cell mass(embryoblast)& outer cell mass(trophoblast) present. A cuboidal layer of hypoblast is formed.
2nd week	Blastocyst enlarged, completion of implantation, formation of amniotic cavity, yolk sac, extra-embryonic mesoderm, extra-embryonic coelom that later become chorionic cavity. Bilaminar disc (ectoderm, endoderm) is formed along with syncytiotrophoblast and cytotrophoblast. Primary chorionic villi appear.
3rd week	Appearance of:- trilaminar disc(ectoderm,mesoderm,endoderm), primitive streak(15 day), somites, interembryonic mesoderm, notochord neural tube,body cavities cardiovascular system
4th week	Body is narrow, tubular & C shaped limb buds appear, appearance of four pair of branchial arches, placenta begins to form, heart begins to beat, lens placodes and otic placodes formed, head & tail folds present . Three primary brain vesicles also formed

5th week	Head increases greatly in size, face formed, limb buds shows limbs, & upper limb become paddle shaped
6th week	Head dominant. Oral and nasal cavities confluent. Curvature of embryo diminished. Digital rays, footplate, finger & toes recognizable
7th week	Yolk sac is reduced to yolk stalk and limb differentiate rapidly.
8th week	Head nearly as large as rest of the body. Genital tubercle formed but sex difference is not obvious. Facial features are distinct. Eyes directed more anteriorly. Neck is established, digits of hands and feet seprated along with retrogression of tail.

Major Developmental Stages Of Embryonic Period (9-38^th weeks)

Time period (Week)	Major Events/characteristics
9-12th  week	Rapid growth in fetal length, head still realtively large, eyes look anteriorly, eyelids fused, lanugo & nails present. Primary ossification centers appear.
13-16th  week	Further rapid growth in fetal length, head still relatively large, eyes widely separated but eyelids fused. External ears on side of head. Fetus look human and sex recognition possible.
17-20th  week	Lanugo covers entire body. Venix caseosa present. Hairs present on head mother Quickening brown fat forms. Eyebrows and eye lashes present.
21-25th  week	Skin wrinkled and red, head still relatively large, face child like, fingers & toe nail develop. Interalveolar walls of lungs secrete surfactant(24th week)
26-29th week	Skin wrinkles lost due to subcutaneous fats, eyelids open, hairs on head longer, CNS matured and controlled breathing movement primitive alveoli formed.
30-34th week	Skin is pink & smooth, langugo hairs has disappeared from face. Vernix caseosa thick. Nails reach end of fingers.
35-38th week	Fetus fully developed, lanugo hairs disappear, nails project beyond ends of finger and toes. Testis reachs scrotum.
266 days from conception	Birth