Biology, 9th ed,Sylvia Mader Chapter 09 The Cell Cycle

Differentiation and development Development involves: 1) Cell division and differentiation Differentiation is the creation of different types of cells 2)  Changes in anatomical structures Anatomical changes include gradual modification of physical and physiological characteristics Development begins at fertilization

Stages of development Development can be divided into: Prenatal and postnatal development Prenatal development begins at fertilization and ends with birth Prenatal development includes: Embryological development Changes occurring during the first two months after fertilization Fetal development Begins at the start of the ninth week and continues until birth Postnatal development Commences at birth and continues to maturity

Fertilization (conception) Fertilization is fusion of two haploid gametes (egg and sperm) each with 23 chromosome to produce a zygote that contains 46 chromosomes Fertilization occurs in the uterine tubes Within a day of ovulation Spermatozoa cannot fertilize an ovum until after capacitation

Fertilization Steps Step 1 Ovulation At ovulation the oocyte is in metaphase of meiosis II Both the occyte and the polar body is surrounded by corona radiate Step 2 Fertilization Oocyte is surrounded by the sperms Acrosomal enzyme from several sperms creates gaps in corona radiata One sperm makes contact with the oocyte membrane Sperm and oocyte fused The process of meiosis will complete

Fertilization

The First Trimester The first trimester is the period of embryological and early fetal development Four processes occur during the first trimeter 1) Cleavage 2) Implantation 3) Placentation 4) Embryogenesis

The First Trimester Cleavage Cleavage - cleavage is the first cell division The first cleavage occurs at 32 hours Second cleavage occurs at 60 hour Third cleavage occurs at 72 hours Several divisions later the embryo becomes a solid mass of cells known as morula Molurla becomes a blastocyst (also known as blastula) Blastocyte includes: Trophoblast – outer layer of cells that give rise to the chorion and placenta Inner cell mass – cluster of cells at one end of blastocyst Blastocoel – hallow fluid-filled inner cavity

Cleavage and Blastocyst Formation

The First Trimester Implantation and Placentation Implantation Attachment of blastocyst into the uterine endometrium The uterus is prepared for implantation by the hormone progesterone (see reproductive and endocrine lectures) Occurs about 7 days after fertilization Placentation Blood vessels form around blastocyst and placenta develops The placenta is a complex organ that permits exchange between the maternal and embryonic circulatory systems

The First Trimester Gastrulation After implantation, cell migration transforms blastula (single cell layer) to gastrula (3 layered structure) Gastrulation is the generation of 3 distinct germ (cell) layers Endoderm Mesoderm Ectoderm

The Inner Cell Mass and Gastrulation

Germ layers Gastrulation The three germ layers are: By day 12 surface cells move toward the primitive streak A third germ layer forms The three germ layers are: Ectoderm – superficial cells that did not migrate Will become integument (epidermis, hair, nail, epithelium, nose, mouth and anal canal), lense of eye, ear and nervous system Endoderm – cells facing the blastocoele Will become liver, pancreas, thyroid, bladder, lungs, epithelium of digestive and respiratory systems. Mesoderm – migrating cells between ectoderm and endoderm Will become musculskeletal system, circulatory system, excretory system, gonads, connective tissue of digestive and respiratory systems, smooth and cardiac muscle.

Germ layers

Germ layers Organs and glands are consist of or include more than one germ layer Adrenal cortex is derived from mesoderm Adrenal medulla is derived from ectoderm

Neurulation By the end of gastrulation, regions of the germ layers begin to develop into a rudimentary nervous system Neurulation is the process of rudimentary nervous system formation Notochord – a rod of mesodermal cells that develop along the axis. Notochord cause ectoderm to bend inward and form a groove Ectoderms grow on either side of the groove and then fuse together to form a neural tube Neural tube becomes central nervous system (Brain and spinal cord)

Fetal Respiration Growing fetus receives oxygen and eliminates carbon dioxide through a specialized circulatory system The two components of this system are: Placenta Umbilical cord Placenta and umbilical cord are outgrowths of the 4 extra-embryonic membrane (Amnion, Chorion, Allantois and yolk sac) that are formed during the development Chorion extends villi into uterine wall and these villi become closely associated with endometrial cells Chorionic villi forms intricate branching network for maternal blood Umbilical cord connects fetus to placenta

Extraembryonic Membranes Four extraembryonic membranes: Yolk sac Amnion Allantois Chorion

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Extraembryonic Membranes and Placenta Formation

Embryo Anatomy Yolk sac Amnion Allantois Chorion Important site of blood cell formation Amnion Encloses fluid that surrounds and cushions developing embryo Allantois Eventually becomes bladder Chorion Placenta formation begins for chorion Chorion is a membrane that completely surrounds amnion

A Three-Dimensional View of Placental Structure

Hormones of the placenta Trophoblast secretes hormones to maintain pregnancy Human chorionic gonadotropin (hCG) – Prevents disintegration of corpus luteum At the ovaries corpus luteum produce progesterone Estrogens – in addition to having role in secondary sexual characteristics it is involved in the thickening of the endometrium Progesterone - enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus Human Placental Lactogen (hPL) - decreases maternal insulin sensitivity, and therefore raises maternal blood glucose levels, whilst decreasing maternal glucose utilization, which helps ensure adequate fetal nutrition Placental prolactin- stimulates the mammary glands to produce milk Increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and increases the production of milk. However, the high levels of progesterone during pregnancy stop ejection of milk. After childbirth progesterone decreases and milk ejection initiates. Relaxin

Second and Third Trimesters Second trimester Organ systems increase in complexity Third trimester Many organ systems become fully functional Fetus undergoes largest weight change At end of gestation fetus and uterus push maternal organs out of position

The Second and Third Trimesters

Growth of the Uterus and Fetus

Growth of the Uterus and Fetus

Developing fetus totally dependent on maternal organs Maternal adaptations include increased Respiratory rate Tidal volume Blood volume Nutrient and vitamin uptake Glomerular filtration rate

Structural and Functional Changes in the Uterus Progesterone inhibits uterine muscle contraction Opposed by estrogens, oxytocin and prostaglandins Multiple factors interact to produce labor contractions in uterine wall

Factors Involved in the Initiation of Labor and Delivery

Labor and Delivery Goal of labor is parturition Stages of labor Dilation The cervix dilates and fetus moves toward cervical canal Expulsion The cervix completes dilation and fetus emerges Placental Ejection of the placenta