Lecture 1 Embryology

What is Embryology The formation of basic structures of the body

A diploid cell  Has two sets of each of its chromosomes  In a human has 46 chromosomes (2n = 46)  Mouse 2n=40  Frog=26  Drosophila=8

Phases of embryonic development 7 steps: 1. gametogenesis 2. fertilization 3. cleavage 4. blastulation 5. gastrulation 6. neurulation 7. organogenesis

Gametes  Human being formed by union of……….  Each gamete has 23 ch (haploid=1N=22 somatic + 1 sex ch). Sperm: 22+y or 22+x Ovum: 22+x  Fertilized ovum (diploid=2N=44 somatic +2 sex ch)=22 homologous pairs+ heterologous pair.

Cell cycle

 In a cell in which DNA synthesis has occurred  All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

 Unlike somatic cells  Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes  intervening

Mitosis&meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6

Learning Objectives  Compare/contrast spermatogenesis & oogenesis  Describe where in the body each occurs  Identify the diploid and haploid cells in each  Recall how many functional cells each produces  Identify when meiosis I and II occur  Explain how a diploid zygote is produced

Gametogenesis  Production of the 2 cells required for generation of new individual

Spermatogenesis  Process of prod. of male gamete & modifying it to perform reprod. Function.  Occurs in the seminiferous tubules of the testes  Literally, the formation of sperm Takes 65 to 70 days in human.

Testis of rat

 his photograph compares two stages, stage 7 on the left and stage 12 on the right, illustrating some general features of the cycle.  As is usual for stages 1-8, the tubule at left contains two generations of spermatids, and one generation of primary spermatocytes. The nuclei in the older spermatids are quite small and have the mature sickle shape, while those of the younger spermatids are pale and round (you may be able to make out occasional red acrosomal caps). The nuclei of the single generation of primary spermatocytes are the prominent dark nuclei near the base of the epithelium.  In contrast, as is usual for stages 9-13, the tubule at right contains one generation of spermatids, and two generations of primary spermatocytes. The spermatid generation is in an intermediate phase of differentiation, and the large nuclei are in the process of shaping. The older primary spermatocytes have very large circular or ovoid nuclei, with prominent chromatin, and are in the pachytene phase of meiotic prophase (which lasts throughout stages 1-13). The younger primary spermatocytes have small, dark nuclei that lie near the basement membrane of the epithelium, and are in the zygotene phase of meiotic prophase (earlier stages 9-11 would be in leptotene phase).

Each vertical column shows the sequence of germ cell generations that would be observed in a histological section of a rat seminiferous tubule at that stage. I-XIV (the 14 stages). Spermatogonia: A (type A), In (intermediate), B (type B), R ("resting", time of final DNA replication), m (stages during which mitosis of spermatogonia occurs). Primary spermatocytes: L (leptotene), Z (zygotene), P (pachytene), Di (diakinesis). II (secondary spermatocyte) (phases of spermatid differentiation = spermiogenesis).

Differentiation of spermatids  production of flagellum and acrosome, shaping of the head  condensation of nuclear chromatin, formation of mitochondrial sheath and various supportive structures of the flagellum  loss of excess cytoplasm

 Mature spermatids are released into the lumen of the seminiferous tubule, within which they journey to the epididymis, where they will complete their functional differentiation into mature sperm.

Spermiogenesis  Converts immature spermatids to mature spermatozoan (in Epididymis)

Mitosis&meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6

Spermatogenesis Overview  Begins with one cell  Which becomes two  Finally four

N = # of sets of chromosomes 1N = haploid 2N = diploid C = amount of DNA 2C = amount of DNA in a normal cell 4C = amount of DNA in a cells right after all of the DNA is duplicated but before the cell divides 1C = amount of DNA in a gamete - half the amount in a normal, diploid, cell

Primary Spermatocytes  These are the basic cells, where process begins  Diploid (2n)=with two sets of chromosomes  Produced continuously by mitosis

Secondary Spermatocytes  These are haploid (n) = with a single set of chromosomes

Sperm cells  Also haploid, are produced when secondary spermatocytes divide

Oogenesis Overview  Begins with one cell  Which becomes two  Then finally, four

7 th Inning Stretch  Yahoo! LaunchLaunch

Answer  Spermatogenesis = 4 functioning cells

Question  What cell division process is used in spermatogenesis?

Answer  Both spermatogenesis and oogenesis employ meiosis  Spermatogenesis needs many, many parent cells—provided by mitosis

Reference  Gilbert, S.F. Developmental Biology Fifth edition p

 mitosis vvv  meiosis   vvvvv      Inculation GbrzYM&NR=1