Embryology ppt

Editor's Notes

• #3: Customary to divide human development into pre and post birth periods Most developmental changes happen during the embryonic and fetal periods – important changes also occur after birth – later periods of development – infancy, childhood, adolescence, early adulthood – the weight of the brain triples between birth and 16 years. Most developmental changes are complete by the age of 25. TRIMESTER: period of 3 calendar months during a pregnancy – obstetricians – divide the 9-month period of gestation into 3 trimesters – most critical stage of development = first trimester = 13 weeks Difficult to say when fertilization/ conception occurred – process =in vivo (inside the body) - GESTATIONAL AGE: Fetus age – calculate from the presumed first day of the last normal menstrual cycle. 2 weeks longer than fertilization age – Oocyte is not fertilised till ~ 2 weeks after the preceding menstrual cycle. SPERM: Male germ cell – produced in the testes – 300 million sperm cells are expelled from the urethra during ejaculation - A sperm can survive for up to 48 hours EGG: produced in the ovaries – mature = secondary oocyte/mature oocyte - The formation of primary oocytes, from which ova are developed, is complete before birth. About 2,000,000 primary oocytes are present at birth, but by adolescence only about 40,000 remain. Over the reproductive period about 400 of these pass through maturation to ovulation. The maturation of ova occurs in follicles within the ovary, under hormonal control. Picture = The zona pellucida is seen as a thick clear girdle surrounded by the cells of the corona radiata. The zona pellucida is essential for oocyte survival and fertilization. In non-mammalian animals, the zona pellucida plays an important role in preventing breeding of different species, especially in species that fertilize outside of the body - fish. The zona pellucida is commonly used to control wildlife population problems by immunocontraception. When the zona pellucida of one animal species is injected into the bloodstream of another, it results in sterility of the second species due to immune response. FERTILIZATION: As soon as the oocyte from a female is penetrated by a sperm from a male – the ova is charged negative on the inside relative to the outside (like most other body cells)-as soon as the sperm enters – immediate depolarisation – resulting zero – keep other sperm cells out for first few minutes – after which a protective shell has formed Ends when a zygote is created – takes about 24 hrs 300 million sperm!! - Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization. ZYGOTE: union of oocyte + sperm during fertilization The most visible changes =3rd to 8th week – look at timetable!! Most terms have latin / Greek origins Understanding where the terms originate from – add clarity & serve as a memory key SPERM: Numerous sperms are expelled from the urethra during ejaculation
• #4: DEF: The process of forming and developing specialised generative cells-gametes-to prepare them for fertilisation-by: reducing the chromosome number by half + altering the shape of the cells Sperm + oocyte = male + female gametes = highly specialised sex cells-history differs/timing-sequence is the same Half the amount of chromosomes than body cells = 24 – zygote – totipotent just visible to the unaided eye=tiny speck-unicellular – divide many times – multicellular human being – cell division, migration, growth, differentiation Meiosis – special type of cell division - reduce the number of chromosomes = maturation process:2 meiotic cell divisions!!! Spermatogeneis = males Oogenesis = females
• #13: In the case of male structures, all four cells will eventually develop into typical sperm cells. In the case of female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell which is usually much larger than a typical sperm cell.
• #14: 2nd phase = similar to ordinary mitosis-except-chromosome number of cell entering the division is haploid Divide each of the haploid cells
• #15: The zygote, formed by fusion of the two haploid gametes, is the first form of a new human being. Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information came from the mother's egg, and the other half from a single sperm. While traveling down through the fallopian tube – divides several times. STAGE 2 - The zygote undergoes a rapid series of mitotic divisions called cleavage (Picture =first stage). As division proceeds two cells are followed by four then eight etc - about three days after fertilization the zygote forms a ball of cells known as the morula L – morus – mulberry – looks like = solid mass of 12-32 blastomeres = early embryonic cells – cells change their shape to cling to each other tightly – compaction – after about five days – post fertilisation - the morula enters the uterus and develops a central cavity. At this stage the structure is termed a blastocyst. Embryo will hatch from its ZONA PELLUCIDA – (prev. slide – ovum) 5 days after fertilisation = glycoprotein membrane - degenerates and decomposes – replaced by underlying trophoblastic cells.
• #16: At six days the blastocyst – enter the uterine/fallopian tube = fluid-filled - attaches to the mucous membrane of the uterus / endometrium to begin the process of implantation – completed by end of second week – hormones, cytokines + many growth factors are involved. Cell division continues rapidly until the blastocyst is formed into outer syncytiotrophoblast and inner cytotrophoblast layers. The cells of the syncytiotrophoblast invade the uterine epithelium. By seven days after fertilization the blastocyst is securely attached to the endometrium and the inner cell mass has divided to form the hypoblast. The inner cell mass and hypoblast will together form the embryonic disc which gives rise to the germ layers of the embryo. Gastrula – Gr. Gaster – stomach – Process – gastrulation: blastocyst – gastrula – week 3 – 3 * germ layered embryonic disc – ectoderm + mesoderm + endoderm – differentiate – tissue and organs – muscular tissue and stomach. Morning after pill = large doses of estrogen for several days , starting shortly after unprotected sex – usually will not prevent fertilisation – prevent implantation of the blastocyst into the endometrium. – large amount of estrogen – disturb the balance between estrogen and progesterone – needed to prepare the endometrium for implantation = abortion pill – R486 – interfere with the hormonal environment – need for the implanting embryo.
• #18: End – all major organs have formed – minimal function – only cardiovascular system – end of period look human Growth = somatic cell division Morphology = the branch of biol that deals with structures – development of different shapes and sizes – of different snails – complex interactions in an orderly sequence – cells move – interact with others – form tissue and organs Differentiation – the physiological processes mature – when complete – specialised organs that can perform specialised functions Rapid differentiation – teratogens – produce/raise the incidence of congenital anomalies during this time greatly
• #25: To establish body form FOLD- because of rapid growth of the embryo – length – growth rate at the sides fail to keep pace with the rate of growth in the long axis SIMULTANEOUS FOLDING – at the caudal and cranial – move ventrally - and sides
• #26: The ends of the embryo folds ventrally – produce head and tail folds – the cranial and caudal regions move b as the embryo elongates HEAD: Beginning of week 4 cranial region have thickened by now – form Brain primordium BRAIN: Forebrain – will grow dorsally into amniotic cavity – grow cranially beyond the oropharyngeal membrane – overhang the developing heart. HEART: