Development and Life History PDF

Development and life history 12 November 2024 12:56 Early embryology: - Stages of early embryology: ○ Zygote (fertilised egg) ○ Morula ○ Blastula ○ Gastrula ○ Neurula - Embryonic area - Extraembryonic area - supports the embryo (eg. Yolk). The...

Development and life history 12 November 2024 12:56 Early embryology: - Stages of early embryology: ○ Zygote (fertilised egg) ○ Morula ○ Blastula ○ Gastrula ○ Neurula - Embryonic area - Extraembryonic area - supports the embryo (eg. Yolk). The zygote (fertilised egg) is the union of two mature sex cells (gametes) that forms a fertilised egg (or zygote). - Zygotes are the earliest developmental stage. - Yolk delivered into the egg from the mother and accumulates. - Quantity of yolk varies with species. Yolk content in eggs: - Microlecithal - slight amount of yolk - Mesolecithal - moderate amount of yolk - Macrolecithal - Enormous amount of yolk Distribution of yolk in the egg: - Isolecithal - Yolk is evenly distributed - Telolecithal - Yolk concentrated at one pole of the egg - Vegetal pole - Pole of egg where most of the yolk resides - Animal pole - Pole opposite the yolk where the embryonic area is Oviparity - Laying eggs encapsulated in shells or other tertiary egg envelopes (exterior wrapping around the egg). - Parents may incubate eggs by nestling over the eggs to add warmth. Viviparity - Giving birth to embryos without shells or other tertiary egg envelopes. - Embryos develops (gestated) within the female. - Viviparity independently evolved over 100 times in vertebrates. Cleavage - In all vertebrate groups, cleavage converts a single-celled zygote into a multicellular, hollow blastula. - Gastrulation - Forming the endodermal tube. ○ 'Gut formation' - Neurulation - Forming the ectodermal tube, the neural tube. ○ 'Nerve formation' Three embryonic germ layers: - Ectoderm - Endoderm - Mesoderm ○ Each germ layer develops into distinct tissues or body regions. ○ Coelom forms within the mesoderm. Neural crest cells: - Key features of neural crest cells are their migratory ability and multipotency. - They move to distant sites and differentiate into many different cell types. - Neural crest cells only occur in vertebrates. lecture 7 Page 1 Development of tissues and organs: Tissue categories: - Epithelial tissue - Epithelia fall into one of two categories: ○ Membranes that line or cover cavities. ○ Glands that secrete products that act elsewhere in the body. - Connective tissue - Consists of cartilage, bone, fibrous connective tissue, adipose tissue and blood. - Muscle tissue - Nervous tissue The cartilage: The cartilage is hard but pliant material - mainly polysaccharide called chondroitin sulphates, which bind with ground substance proteins to form proteoglycans. The specialised cells called chondrocytes produce a large amount of extracellular chondroitin sulphate matric interspersed with collagenous or elastic protein fibres. - There are 3 types of cartilage - hyaline, fibrocartilage and elastic. ○ Hyaline cartilage is the most common type of cartilage in the body. It consists of short and dispersed collagen fibres and contains large amounts of proteoglycans. ▪ No fibres are visible when viewed under light microscopy. ▪ A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. ▪ Found in embryonic bones, nose, tips of ribs, tracheal rings and articular ends of long bones. ○ Fibrocartilage has thick bundles of collagen fibres dispersed through its matrix, giving mechanical resistance to tensile forces. It is found under conditions where tensile or warping loads are applied, like the intervertebral disks and pubic symphysis. ○ Elastic cartilage contains elastic protein fibres as well as collagen and proteoglycans, which make the cartilage springy and flexible. Elastic cartilage gives rigid support as well as elasticity. It can be found as internal support for ears and makes up the epiglottis. The bone: Bone is the hardest connective tissue. It is made up of hard and rigid material, mostly of collagen. Bone is a living, growing tissue. Bone's rigid extracellular matrix contains mostly collagen fibres embedded in a mineralised ground substance containing hydroxyapatite, a form of calcium phosphate. - Collagen is a protein that provides a soft framework, and calcium phosphate adds strength and hardens the framework. This combination of collagen and calcium makes bone strong and flexible enough to withstand stress. Bone cells: Bone cells are classified according to their role. - Osteoblasts are the bone cells responsible for producing new bone, or osteogenesis. ○ Osteoblasts synthesise and secrete the collagen matrix and calcium salts. - Osteoclasts are responsible for removing existing bone. ○ Osteoclasts are multinucleated and originate from white blood cells instead of from osteogenic stem cells. ○ Osteoclasts are continually breaking down old bone while osteoblasts are constantly forming new bone. - Osteocytes are formed from osteoblasts that get trapped within the calcified matrix that they secreted. ○ Osteocytes are the primary cell of mature bone and the most common type of bone cell. ○ Osteocytes maintain the mineral concentration of the matrix via secretion of enzymes. lecture 7 Page 2  ○ of enzymes. Types of bone: The two main types of bone tissues are compact and spongy bones. - Compact bone is the denser of the two types of bone tissue and formed of osteons. - Spongy bone, or cancellous bone, also contains osteocytes housed in lacunae but they are not arranged in concentric circles. Instead, they are found inside trabeculae, a lattice-like assemblage of beams, bars and rods that form a rigid framework. Trabeculae maximise strength at areas of stress, with each trabecula forming along lines of stress. The spaces of the trabeculae network provide balance to the dense and heavy compact bone by making bone lighter. ○ Marrow occupies the cavities and is lined with endosteum. Marrow also contains connective tissue fibres, blood vessels, nerve fibres and adipose tissue. Hemopoietic tissue (red marrow) produces red blood cells and some white blood cells. Growth and development of bones: The long bone has two parts, the diaphysis and the epiphysis. The diaphysis is the tubular shaft. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls of the diaphysis are composed of compact bone. The wider section at each of the bone is called the epiphysis, which is filled with spongy bone. The epiphysis meets the diaphysis at the metaphysis, which contains the epiphyseal plate, or the growth plate, a layer of hyaline cartilage in a growing bone. There are two forms of bone development: - endochondral bone development (or ossification) - in which bone develops from cartilage, eventually replacing it. - intramembranous bone development - in which bone is developed directly from mesenchyme tissue without a cartilage precursor. Endochondral Ossification: - Endochondral ossification starts with mesenchymal cells differentiating to chondrocytes that form the cartilaginous skeletal precursors of bone. - Next, the hyaline cartilage is surrounded by the perichondrium. - Then the hyaline cartilage in the core of the diaphysis is ossified by accumulation of inorganic salts. ○ Entombed chondrocytes die and blood vessels invade and erode calcified cartilage to form initial spaces of marrow - Then, osteoblasts appear in the core of the bone and primary centre of ossification appears. Old cartilage is replaced by bone, and the trabeculae form. - Cartilage replacement now moves to the metaphysis. - Epiphyseal plate is the last region of cartilage proliferation. ○ Fishes, amphibians and reptiles have indeterminate growth – they can continue to grow through life. ○ Birds & mammals have determinate growth and stop growing at maturity. In some lizards and birds, secondary centres of ossification arise in the epiphysis. At sexual maturity in mammals the epiphyses ossify completely and cartilage remains at joint surface as articular cartilage. Intramembranous ossification: In intramembranous ossification, bone directly develops from mesenchyme tissue without cartilage precursors. - This is the common way bone develops in dermal bone, like the skull, pectoral girdle and integument, sesamoid bone, which are associated with tendons, and perichondral bone, which develops early and retain ability to form bone in the lecture 7 Page 3  perichondral bone, which develops early and retain ability to form bone in the adult. - First, mesenchymal cells group into clusters and ossification centres form. The secreted osteoid traps osteoblasts, which then become osteocytes. - Then, the trabecular matrix and periosteum form. - Lastly, compact bone develops superficial to the trabecular bone and crowded blood vessels condense into red marrow. The coelom and body cavities: - In vertebrates, the coelom develops into body cavities and develops within the mesoderm. This is filled with coelomic fluid. - Body cavities: ○ Pericardial cavity ○ Pleuroperitoneal cavity ○ Separated by the transverse septum In amniotes, anterior portion of pleuroperitoneal cavity forms paired pleural cavities around each lung. In mammals, pleuroperitoneal membrane separates the pleural cavities from the peritoneal cavity. - Mammals have 4 cavities: pericardial cavity, paired pleural cavity and peritoneal cavity. - pleuroperitoneal membrane + transverse septum = diaphragm Development of body regions: - Vertebrates replace the notochord with a vertebral column. - Sclerotomes, segmented clusters of cells, gather around the notochord. - Differentiate into segmentally arranged vertebrae. - Vertebral column protects nerve cord and provide attachment sites for muscles. - Notochord persists only as a small core of the intervertebral disks. - Pharyngeal arches are the segmental structures of the pharynx. - Derivatives of the pharyngeal arches are phylogenetically conserved. - Mandibular arch develops into the jaws. - Hyoid arch supports the jaws. - Five gill-bearing arches. Development and life history 2 Embryonic development: - Later stages of embryonic development differ by vertebrate groups and species. - Some develop into a larva and other develop into a fetus. - All major features are developed by the fetal stage. - Continues to develop and grow. Metamorphosis - The radical and abrupt postembryonic changes to become an adult. - This is widespread in animals including vertebrates and the distinct stages include larval and adult. - Metamorphosis is controlled by thyroid hormones. Evolution of metamorphosis in vertebrates: - Metamorphosis may be ancestral to vertebrates. lecture 7 Page 4  - Metamorphosis may be ancestral to vertebrates. - Non-vertebrate chordates (tunicates and lancelets) undergo metamorphosis ○ Thyroid hormone induced metamorphosis in lancelets. - Non-jawed vertebrates (lampreys) undergo metamorphosis. Ontogeny - Development of an organism into an adult. It is an ongoing process throughout an individual's lifetime. Does ontogeny recapitulate phylogeny? Ernst Haeckel argued that embryonic development goes through the evolutionary sequence of the ancestors, thus ontogeny recapitulates (repeats) phylogeny. - This is known as the biogenetic law. Karl Ernst Baer argues that development proceeds from the general to the specific. - This law is of the general to the specific. The vertebrate body plan (Bauplan) is observed across vertebrates during the phylotypic stage of embryonic development. Morphological diversity of embryos converges towards this conserved body plan, then diverges again. - The 'hourglass model' Modern evolutionary developmental biology seems to be more in alignment with Von Baer's Law. Heterochrony - Is an ontogenetic shift in the timing of adult features appearing in descendant species. Paedomorphosis - When adults resemble the juveniles of ancestors - Humans - Progenesis - Neoteny Progenesis: - Early cessation of somatic development - Growth stops at an early age - Sexual maturity is attained at an early age - Paedomorphic adults Neoteny: - Features grow at a slower rate - Sexual maturity overtakes somatic maturity - Paedomorphic adults Peramorphosis - The development of new adult morphologies by extension of ancestral ontogenetic sequence. - Adding more steps to growth. lecture 7 Page 5