introduction to comparative anatomy

1.Introduction to Comparative Anatomy Lectured byBien Nillos, MD Reference: George C. Kents Comparative Anatomy of the Vertebrates

Study of the structure of vertebrates, of the functional significance of structure and of the variation in structure and function in geological time.

It is a study of historyof the speicies that preceded us on this planet, of the effects of mutations, of adaptations, of the struggle for compatibility with an ever-changing environment, of the invasion of new territory and of the extinction of aging species

King Philip Came Over From Great Spain

THE NOTOCHORD and VETERBRAL COLUMN

NOTOCHORD - flexible rodlike structure ofmesodermalcells that is the principal longitudinal structural element of chordates and of the early embryo of vertebrates, in both of which it plays an organizational role innervous system development . In later vertebrate development, it becomes part of thevertebral column

Nucleus Pulposus vestige/remnant of the notochord in mammals, found within the interverebral disks

2. PHARYNX: pouches, slits and arches

The pharynx is the region of the alimentary canal exhibiting pharyyngeal pouches in the embryo

Pharyngeal slits are permanent in adults that live in water and breathe by gills (maximum number is 8)

3. DORSAL Hollow Central nervous System

- Consists of brain and spinal cord.

HEAD cephalization

TRUNK contains coelom

TAIL persistent vs. vestigial

APPENDAGES pectoral and pelvic, bilaterally symmetrical

Phylogeny evolutionary history of a taxon (speciation)

Ontogeny history of an individual (embryo development)

"ontogeny recapitulates phylogeny,"- biogenetic law,Ernst Haeckel (1834-1919)

Many scientists are now questioning and in fact already saying that this theory is false and inaccurate.

English philosopherHerbert Spencerwas one of the most energetic promoters of evolutionary ideas to explain many phenomena. He compactly expressed the basis for a cultural recapitulation theory ofeducationin the following claim, published in 1861, five years before Haeckel first published on the subject

The maturationist theory of G. Stanley Hall was based on the premise that growing children would recapitulate evolutionary stages of development as they grew up and that there was a one-to-one correspondence between childhood stages and evolutionary history, and that it was counterproductive to push a child ahead of its development stage

The Austrian neurologistSigmund Freudalso held a favorable position towards Haeckel's doctrine. He was trained as a biologist under the influence of recapitulation theory at the time of its domination, and retained aLamarckianoutlook with justification from the recapitulation theory. He also distinguished between physical and mental recapitulation, in which the differences would become an essential argument for histheory of neuroses

the science which deals with the study of identifying, grouping, and naming organisms according to their established natural relationship.

Systematics taxonomy and the study of the evolutionary processes that give rise to new taxa

Convergent applied when two or more unrelated species occupying the same kind of environment, concurrently or millions of years apart, develop a similiar adaptive morphologic trait.

Parallel development of similar structures in recently related but isolated taxa whose common ancestor lacked such traits.

Microlecithal very little yolk (e.g. placental mammal)

Mesolecithal moderate amounts of yolk (e.g. amphibians)

Macrolecithal massive amounts of yolk (e.g. reptiles, birds)

Isolecithal yolk is evenly distributed

Telolecithal cytoplasm and yolk tend to accumulate at opposite poles

WHY PROTEIN?

Structural Purposes (muscle, bone, etc)

Regulatory Purposes (hormones)

Communication Purposes

Oviparous animals that spawn or lay their eggs

Viviparous animals that retain their eggs within the mothers body during embryonic development

Histotrophic nutrition nutrition by glandular secretions from maternal tissues

At fertilization, enzymes in the acrosome of the sperm help to penetrate the egg

requires that the sperm break through the plasma and vitelline membrane surrounding the egg

to prevent more than one sperm from penetrating the egg (polyspermy), the egg undergoes a cortical reaction to bring the sperm head into the interior of the egg and change the vitelline envelope to form the fertilization membrane

Just after fertilization the zygote (fertilized egg) undergoes cleavage (mitotic cell divisions) and becomes subdivided into smaller cells - the gross arrangement of cells differs greatly among vertebrates, depending on the amount of yolk in the egg

Holoblastic cleavage occurs when the cleavage furrows pass through the entire egg

cleavage can either be equal, where the resulting cells contain the same amount of yolk, or unequal, in which some cells contain more yolk than others:

- equal cleavage occurs in microlecithal eggs

- unequal cleavage occurs in mesolecithal eggs

cleavage results in the formation of a ball of cells (blastomeres) surrounding an internal cavity (blastocoel)

Meroblastic cleavage occurs more in macrolecithal eggs

cleavage takes place only in a disk at the animal pole

the cleavage furrows do not extend into the yolk

results in the formation of the blastodisk that lies on the top of the yolk

a collection of around 30 cells (blastomere), is created at about 96 hours. Because these cells arise only through the cleavage of the zygote and all are found inside the pellucid zone, which cannot expand, no growth is seen

On the 4th day after insemination the outermost cells of the morula that are still enclosed within the pellucid zone begin to join up with each other (so-called compaction). An epithelial cellular layer forms, thicker towards the outside, and its cells flatten out and become smaller.

1. Embryoblast

2.Pellucid zone

Trophoblast

Blastocyst cavity

The cells contact one another by means of tight junctions and gap junctions. A cavity forms in the interior of the blastocyst into which fluid flows (the so-called blastocyst cavity). The two to four innermost cells of the preceding morula develop into the so-called inner cell mass of the blastocyst. The actual embryo will develop solely from these cells (embryoblast). These cells are concentrated at one pole, the embryonic pole of the blastocyst.

What has thus been formed is an outer cell mass (the trophoblast), consisting of many flat cells, and the embryoblast, formed from just a few rounded cells. The ratio between the number of embryoblast cells to those making up the trophoblast amounts to roughly 1:10. From the trophoblast the infantile part of the placenta and the fetal membranes will arise

Around the end of the fifth day the embryo frees itself from the enveloping pellucid zone. Through a series of expansion-contraction cycles the embryo bursts the covering. This is supported by enzymes that dissolve the pellucid zone at the abembryonic pole.

The rhythmic expansions and contractions result in the embryo bulging out of and emerging from the rigid envelope.

Pellucid zone

Trophoblast (outer cell mass)

Hypoblast (part of the inner cell mass)

Blastocyst cavity

Epiblast (part of the inner cell mass)

Fallopian tube

Endometrium

Myometrium

Uterine cavity

Spermatozoon penetrates into the oocyte (conception), day 0

Two-cell stage, day 1

Four-cell stage, day 2

Eight-cell stage, day 3

Morula (16-32 cells), day 4

Free blastocyst (following hatching), day 6

While the fertilized oocyte develops into a morula and blastocyst and then hatches from the pellucid zone, it wanders from the ampulla through the fallopian tube into the uterine cavity, where it embeds itself in the endometrium at the end of the sixth day. The fertilized oocyte / embryo is transported via the movement of the cilia of the tubal epithelium and the contractions of its muscular layer.

characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers:

ectoderm, mesoderm, and endoderm.

the chordamesoderm that will go to form the notochord induces neural plate formation, which is the first stage in the formation of the neural tube

characterized in most vertebrates by three stages

- during the neural plate stage, the ectoderm on the dorsal side of the embryo overlying the notochord thickens to form the neural plate

- at the neural fold stage, the thickened ectoderm folds, leaving an elevated area along the neural groove. The neural fold is wider in the anterior portion of the vertebrate embryo, which is the region that will form the brain.

- during the neural tube stage, the neural folds move closer together and fuse - the neural groove becomes the cavity within the neural tube, which will later be capable of circulating cerebrospinal fluid that aids in the function of the central nervous system.

One derived characteristic found in vertebrates is the formation of neural crest cells

ectodermally derived

develop along the top of the neural tube as the neural folds close

most neural crest cells change into mesenchyme, an embryonic tissue that consists of star-shaped cells from all three germ layers

develop into the visceral skeleton (i.e. gill arches, some of which will develop into jaws), pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, Schwann cells that help protect neurons, and bony scales

After the production of the nerve tube, differentiation of the germ layers occurs rapidly, and organogenesis begins, in which the primary tissues differentiate into specific organs and tissues

Endoderm - gives rise to the epithelium of the alimentary tract, to structures derived from the pharyngeal pouches such as parathyroid glands, thymus gland, Eustachian tube and middle ear cavity (not the ossicles), and to structures that develop as an evagination of the gut, such as the thyroid gland, lungs or swim bladder, liver, gall bladder, pancreas, and urinary bladder.

Mesoderm - becomes organized into three regions: the epimere (dorsal mesoderm), mesomere (intermediate mesoderm), and hypomere (lateral mesoderm).

- Epimere: The somites constitute most of the dorsal mesoderm and have three regions:

dermatome - forms the dermis of the mid-dorsal skin

sclerotome gives rise to the vertebrae

myotome forms skeletal muscles other than those of the gill arches

- Mesomere: gives rise to the kidney tubules, excretory organs, and reproductive ducts.

- Hypomere: lateral-plate mesoderm is confined to the trunk and is divided into somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads and visceral muscles)

Ectoderm - gives rise to Neural tube

Epidermis and associated glands

Neural crest and its derivatives: migrate through the embryo, giving rise to a diversity of structures

Ectodermal placodes: localized thickenings that sink below the surface and give rise to sensory neurons and sensory structures: olfactory placodes, forming the olfactory sacs; lens placodes, for the lens of the eye; otic placodes, to become the membranous labyrinth; a group of placodes that contributes neurons to the sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, placodes that form the neuromasts of the cephalic and lateral line canals

the mammalian egg does contain some yolk, but it is microlecithal and isolecithal - requires that the embryo implant quickly in order to obtain more nutrients from the mother.

The endometrium consists of a single-layered prismatic epithelium with or without cilia (depending on how far along the menstruation cycle is) and its basal lamina, uterine glands, and a specialized, cell-rich connective tissue (stroma) containing a rich supply of blood vessels.

Menstruation

Proliferation

Secretion

Implantation window

Epithelium of the uterine mucosa

Hypoblast

Syncytiotrophoblast

Cytotrophoblast

Epiblast

Blastocyst cavity

Syncytiotrophoblast (ST)

Cytotrophoblast (CT)

Epiblast

Hypoblast

Blastocyst cavity

Maternal blood capillary

Amniotic cavity

Amnioblasts

Fibrin plug

Trophoblast lacunae

Multiplying hypoblast

Hypoblast growing ventrally

Eroded maternal capillaries

Extraembryonic reticulum

Heusers membrane

Amniotic cavity

Cytotrophoblast

Syncytiotrophoblast

Lacunae, filled with blood

Gametogenesis formation of Sperm (male) and Ovum (Female)

Testes produce the Sperm

primordial germ cellspermatogonia primary spermatocyte secondary spermatocytes spermatid spermatozoa

Sertoli cells (support cells) Interstitial cells or Leydig cells (produce hormone)

Ovarian Follicle Stage:primordial follicleprimary folliclesecondary follicletertiary folliclepreovulatory follicle

Fertilization usually occurs in first 1/3 of uterine tube (oviduct, Fallopian tube)

Fertilization can also occur outside uterine tube associated with In Vitro Fertilization (IVF, GIFT, ZIFT...) and ectopic pregnancy

The majority of fertilized eggs do not go on to form an embryo

Sperm Binding - zona pellucida protein ZP3 acts as receptor for sperm

Acrosome Reaction - exyocytosis of acrosome contents (Calcium mediated) MBoC - Figure 20-31. The acrosome reaction that occurs when a mammalian sperm fertilizes an egg

enzymes to digest the zona pellucida

exposes sperm surface proteins to bind ZP2

Membrane Fusion - between sperm and egg, allows sperm nuclei passage into egg cytoplasm

Membrane Depolarization - caused by sperm membrane fusion, primary block to polyspermy

Cortical Reaction - IP3 pathway elevates intracellular Calcium, exocytosis of cortical granules MBoC - Figure 20-32. How the cortical reaction in a mouse egg is thought to prevent additional sperm from entering the egg

enzyme alters ZP3 so it will no longer bind sperm plasma membrane

Meiosis 2 - completion of 2nd meiotic division

forms second polar body (a third polar body may be formed by meiotic division of the first polar body)

Week 1: Pre-implantation

Week 2: Implantation

Week 3: Gastrulation

Week 4: Neurulation

http://embryology.med.unsw.edu.au/

http://www.shsu.edu/~bio_mlt/Chap5.html

http://www.embryology.ch/indexen.html

"Let us so live that when we come to die even the undertaker will be sorry. Mark Twain

introduction to comparative anatomy

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